Endoscopes are used routinely in modern medicine for in-vivo imaging of luminal organs. Technical advances in the micro-electro-mechanical system (MEMS) and optical fields have enabled the further miniaturization of endoscopes, resulting in the ability to image previously inaccessible small-caliber luminal organs, enabling the early detection of lesions and other abnormalities in these tissues. The development of scanning fiber endoscopes supports the fabrication of small cantilever-based imaging devices without compromising the image resolution. The size of an endoscope is highly dependent on the actuation and scanning method used to illuminate the target image area. Different actuation methods used in the design of small-sized cantilever-based endoscopes are reviewed in this paper along with their working principles, advantages and disadvantages, generated scanning patterns, and applications. An endoscope is an imaging device made up of a long and thin tube that can be inserted into the hollow openings of the body to image the inner sections in real time and in a less invasive manner. Advances in fiber optic systems led to the development of flexible endoscopes, enabling high-resolution images of narrow sections of the body and reducing the number of biopsies required for a specific diagnosis, with applications such as cancer detection, microvascular oxygen tension measurement, chronic mesenteric ischemia, subcellular molecular interactions, etc. Earlier developed standard white light endoscopes (WLEs) had limited ability to differentiate metaplasia from dysplasia. Such limitations were surpassed by enhancing the image contrast through the use of dyes in chromoendoscopy or applying digital filters in narrow band imaging (NBI) ,2. The iOne of the recently advanced imaging devices was based on confocal laser endoscopy (CLE). Two variations of CLE that are used commercially in medical applications are the so-called e-CLE, which is an integrated confocal endoscope developed by Pentax Medical , and the probe-based confocal microscope (p-CLE) developed by Mauna Kea Technologies  . OpticalThe advancement in optics along with the development of micro-electro-mechanical systems (MEMS) and microfabrication techniques led to the fabrication of sub-millimeter-sized flexible endoscopes that can image the narrow cavities in the body, providing information about early-stage pre-cancerous tissues. The optimized final design of an endoscope is often the result of an optical design optimized using special software-based computer simulations . The rapIt is possible to obtain high-spatial-resolution images with flexible endoscopes having sub-millimeter diameters by scanning the laser light at the proximal end and capturing the image on a temporal basis, i.e., acquiring one pixel at a time. Seibel et al. from the University of Washington developed such a cantilever-based scanning fiber endoscope (SFE), where a single-mode fiber was excited at resonance to scan the light beam on the target area, and an outer ring of optical fibers captured the backscattered light . Since tA large number of state-of-the-art reviews on endoscopic imaging devices are available in the literature which are mainly focused on different imaging modalities. The review work in  providedThe paper is organized as follows: V, defined as:λ is the wavelength of the light, and NA is the numerical aperture of the fiber [NA is calculated from the refractive index of the core and cladding material of the fiber as [The key element used in cantilever-based endoscopic devices is an optical fiber acting as a waveguide through which light is transmitted using the principle of total internal reflection at the interface of two different dielectric media. The inner cylindrical portion is called the core, while the outer region is named the cladding. Both materials are characterized by a slight change in refractive index; it is from this that the name of step-index fibers comes. There are different classifications of optical fibers available based on normalized wave number he fiber . NA is cfiber as :(2)NA=n2V-parameter determines the number of spatial modes of the electromagnetic wave that can propagate within the fiber. An optical fiber is defined as a single mode fiber (SMF) if V < 2.405.The V-parameter, the total number of allowed modes N through the fiber can be approximated as [For a large mated as :(3)N≈V2/Such fibers are called multimode fibers (MMF).Other than simple step-index fibers, there are other fibers having different refractive index profiles with radius and they are increasingly finding use in communication fields to avoid the dispersion of optical waves during propagation. Among these fibers, the most commonly used ones are fibers having a double-step-shaped profile or a nearly parabolic variation of index designated double-clad fibers (DCF) and gradient-index (GRIN) fibers, respectively. In GRIN fibers, light propagates due to the profile of the refractive index instead of the total internal reflection and follows a sinusoidal path .Different kinds of optical imaging modalities used in endoscopic imaging devices are described in detail in this section. These technologies are compared with a CCD/CMOS camera in Fiber bundles comprise thousands of step-index (or GRIN) fibers contained within a very small area, ranging from a few micrometers to millimeters in range. A fiber bundle is used to carry the illumination light from the proximal end to the distal end, and vice versa, of a device. In a coherent fiber bundle, fibers at both ends are placed at the same relative position, so that the image is transmitted from one end to the other without any distortion. In miniaturized scanners, a coherent fiber bundle is predominantly used due to the perfect alignment of fibers, which facilitates the decoding of the signal . As the core, the cross-sectional core density or pixel density is given by [Every core of the step-index fiber in the bundle acts as a pixel of the imaged data. Thus, the resolution of the image captured using a fiber bundle depends on the core size of the fiber and the core-to-core distance between the fibers . For a cgiven by :(4)Core cfbA, neglecting the space occupied by outer protective jacket and sheath, the image resolution can be obtained as [From the core density and the active area covered by the fibers ained as :(5)ImageThis equation indicates a major drawback of using fiber bundles for miniaturized devices as the resolution will be poor due to the honeycomb effect generated by the space between the consecutive fiber cores representing the non-imaged area .Another disadvantage of using fiber bundles is the crosstalk phenomenon. The cladding of the step-index fibers constituting the bundle is rendered to a thin layer around the core during the fabrication of a flexible fiber bundle. A very thin cladding surface enables the leakage of the light as it travels through the core, reducing the contrast and the resolution of the image generated. It is possible to reduce the effect of crosstalk by increasing the thickness of the cladding section, enlarging contrast between the refractive indices of the core and cladding material, and/or by reducing the number of modes propagating through the fiber. However, these approaches make the device rigid, and the honeycomb effect will be more intensified, thus degrading the resolution ,24.It is possible to refine the resolution of the obtained images by post-processing the data using certain algorithms. Using specific transformation models based on the point spread function of the cores of each fiber from the bundle, it is possible to smooth the light gradient, reducing the pixilation effect given by the honeycomb pattern ,26,27. TIn endoscopes that use fiber bundles, a pair of mirrors placed at the proximal end of the device permit the illumination of a single fiber from the bundle at a time. The fast-axis scanning (axis with high scanning frequency) is performed using the resonant scanner (mirror surface vibrated at resonance using one of the actuators described later in the paper), while a galvo scanner  is used for the slow-axis scanning. A schematic diagram of a confocal micro-endoscope developed using such a technique is shown in Sung et al. developed an early fiber-optic-based confocal reflectance microscope to image epithelial cells and tissues in real time, showing the cell morphology and tissue architecture without the use of any fluorescent stains. The image guide contained 30,000 fibers and the device had an overall diameter of 7 mm and rigid length of 22 mm to image human tissue with a lateral and axial resolution  of 2 µm and 10 µm, respectively . KnittelA single-mode fiber (SMF), characterized by having a step-shaped index of refraction change from the core of the fiber to the cladding surface, permits the propagation of only one mode of light through the fiber. A single spatial mode enables a diffraction-limited spot to be projected on the sample plane, resulting in a high-resolution image with the use of an SMF. Due to this property and high flexibility, such fibers find use in miniaturized optical scanners.An SMF may find a use in a scanning fiber optic microscope, acting as a spatial filter  or as a The SFE described earlier uses an SMF vibrated in resonance to scan the light beam across the target tissue surface, and a peripheric ring of fibers detects the time-multiplexed backscattered light. In this case, the sample resolution depends on the scanning motion and sampling rate, which are not fixed a priori during fabrication. In an SFE, the smallest resolvable feature is determined by its point spread function. A wider tip displacement will provide a higher FOV and higher image resolution in terms of the number of pixels in the scanned area, as described in In contrast to an SMF, a multimode fiber can transmit a large number of spatial modes at the same time. These fibers have core sizes much larger than the single-mode fibers, usually in the range of 50 µm–2000 µm. Multimode fibers can be classified into step-index multimode fibers and graded-index (GRIN) multimode fibers on the basis of the change in refractive index from the core to the cladding, which can be sharp or gradual, respectively .A multimode fiber can be considered as an alternative to a fiber bundle and supports the miniaturization of optical devices. As each fiber in the fiber bundle represents a pixel for the acquired image, each pixel can be represented by a propagating mode in the fiber. Thus, it is possible to increase the pixel density of a device by up to 1–2 orders of magnitude by replacing the fiber bundle with a multimode fiber . A side-The main limitation of using a multimode fiber in an imaging device is modal dispersion, which causes multipath artifacts. Several methods have been explored to provide an image without image artifacts. For example, Papadopoulos et al. used a digital phase conjugation technique to generate a sharp focus point. In this technique, the phase of the distorted wavefront was calculated and an unmodulated beam of this phase was propagated in a backward direction to cancel out the distortions and to generate the original signal . Some otThe modal dispersion effect is avoided using GRIN fibers, where the refractive index change along the section of the fiber equalizes the travel time of different modes. Thus, different spatial modes propagate at similar velocities. Sato et al. used a GRIN fiber for the fabrication of a single-fiber endoscope used for reflectance imaging. However, this device had some problems related to nonuniform image quality, background distortion, etc. . High-quDouble-clad fibers consisting of a central core and two outer cladding layers are another type of frequently used fibers in endoscopes. These fibers possess the unique feature of allowing the propagation of both single-mode and multimode light through the fiber. The single-mode light travels through the central core, while the multimode light is transmitted through the inner cladding material. Such a fiber is principally used in fluorescence imaging devices having single-mode illumination and multimode signal collection. Thus, the advantages of single-mode illumination and multimode collection are combined in these fibers ,46,47.It is possible to combine OCT and fluorescence imaging in a single endoscope using a DCF. In this case, OCT illumination and fluorescence excitation light is projected on the sample through the core of the DCF. The backscattered OCT signal is collected through the core, and the fluorescence emission from the sample is collected through the inner cladding of the fiber. The OCT source light and fluorescence excitation light are combined using a wavelength division multiplexer (WDM) before sending it through the core of DCF. The recollected light is separated using a DCF coupler, where the recollected OCT signal from the core of the DCF is submitted to an SMF, while the fluorescence signal from the inner cladding is forwarded to an MMF ,49.Buenconsejo et al. developed a device that combined narrowband reflectance, OCT, and autofluorescence imaging in a single-fiber endoscope using a DCF. This device worked analogously to other OCT devices, except for the difference that the red/green/blue (RGB) light was emitted from the central core, while the collection of the reflected light was performed through the inner cladding. The separation of the various light signals from three modalities was done using an additional WDM .In a single-fiber-based micro-endoscope, the light beam can be steered at either the proximal or distal tip of the fiber. The possibility of using a light beam for a proximal scan allows the separation of large-sized beam scanning devices from the distal end of the endoscopic device used to monitor the target sample. Thus, it is possible to develop small-sized endoscopic devices that can image the deep tissue systems within the body. Proximal scanning is usually performed using side-viewing imaging probes, where a drive mechanism rotates the fiber to scan the light beam along the circumference of the target sample . On the In lens scanners, light is deflected due to a non-planar interface between the air and the lens. The lateral motion of the lens perpendicular to the direction of the incident beam in these scanners causes variation in the refraction angle, allowing light scanning.Wu et al. developed a paired-angle rotating scanning OCT (PARS-OCT) probe to image the gill structure of a Xenopus laevis tadpole, where the beam steering the distal end of the probe was obtained by the rotary motion of the two angle polished GRIN lenses. A schematic diagram of this system is shown in In small cantilevered optical scanners, the image is obtained by scanning the light beam at the distal end of the device, as stated earlier. In most of the earlier endoscopes using CFBs to transport the light to the tissue surface, the beam is scanned using micromirrors placed at the proximal end of the device. In these so-called proximal scanners, the large-dimensioned scanning components can be separated from the distal end of the endoscopic device. Thus, it is possible to fabricate very compact-sized scanning devices. However, as the beam sweeps light across the CFB, a portion of the light enters through the cladding of the fibers as well, which results in poor contrast in the image. On the other hand, the scanning device is placed at the distal end of the endoscopic scanner to illuminate the light on the target sample in distal scanners. These single-fiber-based endoscopes require distal scanning to sweep the light across the target sample .Among the distal scanners, it is possible to have two different configurations of devices based on the scanning direction . In a siThe cantilever-based endoscopic scanners belong to the category of forward-view imaging devices. In such devices, an optical fiber is fixed at a distance of a few millimeters from its distal end. The free end of the fiber acts as a cantilever beam, which is vibrated, using certain actuators described later in the paper, to illuminate the target tissue area. The backscattered reflected light is used to reconstruct the image of the area using certain image processing algorithms. These cantilevered optical fibers can be vibrated in resonance, or at a frequency different from their resonant frequency.E, ρ, R, and L being the Young’s modulus, density, radius, and length of the cantilever beam, respectively [Almost all the cantilevered-fiber optic endoscopes can be considered as cylindrical-shaped beams. The first resonant frequency  of a cylindrical-shaped cantilevered beam (where one side is rigidly blocked for any movement and the other end is free to move) is given by:ectively . From thw and the applied load f, assuming small deformations in the beam such that the planes perpendicular to the x-y axis do not bend after the deformation. The equation describing the deflection w of the beam in the y direction, in time (t) and along the length (x), can be derived considering the force and moment equilibrium of an infinitesimal element dx of the beam as in y direction yields:V is the shear force and f the total applied external force per unit length, while the term on the right-hand side describes the inertial force of the element, with A(x) being the cross-section of the beam. Similarly, the equilibrium of moment acting on the element can be written as:M being the bending moment related to beam deflection w and flexural stiffness EI(x) of the cantilever beam, where E is the Young’s modulus, and I(x) is the cross-sectional area moment of inertia [M is given by:The deflection of a cantilevered beam in the transverse direction can be obtained considering the Euler–Bernoulli beam. The Euler–Bernoulli beam theory describes the relationship between the beam deflection  inertia . M Simplifying and neglecting higher order terms in Equation (8), and combining it with (7) and (9), gives:f = 0. For beams with a uniform cross-section, Equation (10) can be further simplified by having A(x) = A, and I(x) = I. Thus,The beam deformation under free vibration can be attained by considering The beam deflection can be solved using this equation with four boundary conditions and two initial conditions. The initial conditions are the specified initial deflection For a cantilever beam, the boundary conditions are the zero bending moment and the shear force at the free end, and no deflection and slope at the fixed end. In other words,w = X(x)T(t). This approach permits the separation of Equation (11) into two sub-equations, which can be solved separately to yield temporal and spatial results. The total solution can be obtained by combining the two results. As stated above, the temporal solution depends upon the initial conditions, which vary from case to case. Given the boundary conditions, the spatial part yields:Equation (11) can be solved by separating variables as in s 0.7341 . The firIn scanning fiber endoscopes, cantilevered fibers are usually excited at resonance to scan the light beam. The main advantage of using the driving frequency equal to the resonant frequency consists in obtaining a higher tip displacement of the free end of the fiber, which results in high-resolution images (higher number of pixels in the FOV). It is possible to excite the cantilevered fiber using a variety of micro-actuators, such as piezoelectric ,79,80,81Depending upon the actuation technique used to excite the cantilever beam, it is possible to observe the development of 2D motion of the fiber tip by exciting the fiber along one direction. The so-called whirling motion, causing the fiber tip to follow an elliptical-shaped pattern instead of its linear motion, is caused by the cross-coupling of the motion between the planes perpendicular to the beam axes. It is possible to obtain a stable whirling motion within a small frequency range ,92. The Some actuation methods such as electrothermal actuators are unable to generate motion at very high frequency. It is difficult to generate resonant scanners characterized with a low resonant frequency as they require long and slender beams, compromising their mechanical stability. In addition, the fiber tip displacement occurs symmetrically to the optical axis and is difficult to offset in resonant scanners. In such cases, the cantilevered optical fibers are excited at a frequency different from their resonant frequency. Even though there is less tip displacement for a given excitation power, it is possible to achieve beam scanning at low frequency and offset the center of the scan by adding a bias voltage .L, a, q, E, I, g being the length of the cantilevered portion of the fiber, length ratio of the fixed end, mass per unit length of the fiber, Young’s modulus, the moment of the inertia of the fiber, and acceleration of the gravity, respectively [In such scanners, the deflection of the distal tip of the fiber ectively .Zhang et al. proposed a similar scanner where a 45-mm-long fiber was electrothermally actuated using a micro-electro-mechanical (MEMS) actuator operating at no more than 6 V . A similThe miniature size of MEMS devices, along with their light weight and stable performance characteristics, makes them attractive for micro and nano applications, among which are endoscopic optical devices. On the basis of the working principle, MEMS actuators can be subdivided into piezoelectric, electrostatic, electrothermal, electromagnetic, and shape memory alloy actuators. The piezoelectric actuators are widely used in endoscopic catheter design due to their compact size, low power consumption, and large output force. On the other hand, the actuation displacement is limited in such devices. Electrostatic actuators are the second most used actuation method in medical scanning devices due to their fast response and ease of fabrication. However, it is difficult to produce such devices at very small dimensions, which limits their use in systems requiring a distal actuation. Electrothermal actuators generate high actuation displacement and force, but the elevated working temperature and low working frequency limit their use in some cases. Electromagnetic and shape memory alloy actuators find limited applications in cantilevered fiber optic endoscopic scanners . All theThe working principle of piezoelectric actuators is based on the so-called piezoelectric effect. Piezoelectric materials have the ability to change the material polarization in the presence of a mechanical stress and conversely generate strain or force in the presence of an external electric field. Among the various crystalline, ceramic, and polymeric materials, aluminum nitride (AlN) and lead zirconate titanate (PZT) are most frequently used in MEMS devices . Piezoelε is the strain tensor, E is the electric field vector, σ is the stress tensor, d is the piezoelectric tensor (vector of strain coefficients), and c is the elastic tensor. In the case of no external force, the second component on the right-hand side of Equation (19) becomes zero. The piezoelectric strain depends upon the direction of the mechanical and electrical fields [The relationship between the electric field applied to the material and the mechanical deformation exhibited by the material is nonlinear due to the presence of hysteresis and drift. For a small variation in electric field, the material behavior is almost linear and can be described by:l fields .q) is related to the endpoint displacement (x) and the applied voltage (pV) by:C0 and emT being the capacitance of the piezoelectric element and the electromechanical coupling factor, respectively [Piezoelectric materials often show a nonlinear hysteresis behavior, which causes the relationship curve between the displacement exhibited by the material and the applied electric field to be different in ascending and descending directions. Various models have been proposed to describe this hysteresis phenomena. Bahadur and Mills proposed a hysteresis model to characterize the symmetric and asymmetric rate-dependent hysteresis. The output charge  or using two different piezo elements (bimorph actuators). In either case, piezo elements are connected to a base material. There is an expansion or contraction of the piezo material in the presence of an electric current, which provides the bending motion of the actuator. The schematic diagram showing the working principle of a piezo bending actuator is shown in Tekpinar et al. developed a piezoelectric fiber scanner where two planar piezo bimorph cantilever actuators were placed perpendicular to each other to generate the 2D motion of an optical fiber connected to them. In this case, the dimensions of the cantilevered fiber and the mode optimization allowed the fiber tip to interchangeably follow a raster, spiral, or Lissajous scanning pattern by simply changing the actuation parameters. A schematic diagram of such a model is shown in Rivera et al. developed a compact multiphoton endoscope (with an outer diameter of 3 mm) where two bimorph piezoelectric actuators were used to excite a DCF. Two bimorph structures were placed in such a configuration to have perpendicular bending axes. A raster scanning pattern was generated by exciting the fiber in two directions. The fast-scanning motion was obtained by exciting one of the actuators at the resonant frequency of the extending cantilevered DCF, and slow axis scanning was performed by exciting the other actuator at a frequency much lower than the resonance frequency. The mechanical assembly of the described endoscope is illustrated in It is possible to enhance the cantilever fiber deflection with low power consumption by using very thin piezoelectric ceramic layers. Recent developments in the field showed the ability to form piezoelectric layers with thicknesses at submicron levels. A variety of methods were implemented to deposit the thin film of these materials. Such methods included arc discharged reactive ion-plating, epitaxial process, sol–gel spin-coating, and sputtering .In the tubular structure, a thin ceramic sheet is bent into a cylindrical shape and can experience axial, radial, or bending motion. The tube shrinks radially and axially in the presence of a voltage difference between the inner and outer electrodes of the tube. In these actuators, the load can be mounted either on the curved surface for radial displacement or on the rims for axial displacement. These devices are very rapidly responsive, but the generated force is limited. Usually, the displacement produced by a piezoelectric device is very small. Thus, it is possible to stack up various piezoelectric disks or tubes to amplify the generated displacement .As described earlier, piezoelectric actuators are most commonly used in cantilever-based endoscopic probes, especially the tubular piezoelectric actuators. In this case, the tube structure is divided into four electrodes and placed near the blocked end of the cantilever fiber. The base excitation of the fiber along a certain direction is obtained by applying voltage to two opposite electrodes. Seibel et al. used this configuration to obtain a 2D displacement of the fiber tip. In this small-sized endoscope, the drive voltage at two pairs of electrodes had an increasing amplitude and a phase shift of 90°, which resulted in a spiral pattern followed by the fiber tip . The schA similar fiber optic scanner using a tubular piezoelectric actuator, packaged within a 2 mm housing tube, was developed by Liang et al. as a two-photon and second harmonic endoscope. This novel endo-microscope enabled label-free histological imaging of tissue structures with subcellular resolution. The schematic diagram of the developed two-photon endoscope is illustrated in Vilches et al. developed a fiber scanner for OCT where a piezoelectric tubular actuator provided the base excitation motion to an optical fiber having a GRIN lens attached to its free end. In this configuration, schematized in The working principle of an electrothermal actuator is based on the Joule effect. The electric resistivity causes an increase in temperature in the presence of the current flow through the actuator. The amount of heat generated in a material is directly proportional to the material’s resistivity, current, and the length of the actuator, while it is inversely proportional to the cross-sectional area of the device. Generated heat causes thermal expansion and consequently the deformation of the material.T) in space and time can be obtained using the conservation of energy [w, and height h), the partial differential equation (PDE) describing the heat transfer along the length x becomes:ρ, pc, ρr, pk being the density, specific heat, resistivity, and the thermal conductivity of the material characterizing the actuator, respectively. csh and cfh are the convection coefficients for the side walls and the faces of the actuator element, respectively. λ is the coefficient describing the heat loss. xε and σ are the surface emissivity and the Stefan–Boltzmann constant for radiation heat transfer, respectively, while pT and aT are the substrate and ambient temperature, respectively. TR is the thermal resistance between the actuator surface and the substrate material. J is the current density along the actuator given by the current passing through it per unit section of the actuator material. S is the shape factor and is a function of total heat flux defined as:vt is the air gap between the actuator material and the substrate [In electrothermal actuators, the cross-section is usually much smaller than the length of the actuator to make it more resistive and, consequently, cause higher temperature variations for a given input power. Thus, the temperature along the actuator can be calculated using a one-dimensional model. A correction factor can be included in the equation to consider this approximation. The nonlinear partial differential equation describing the temperature variation  can be neglected ,110. TheThe temperature profile along the actuator can be determined knowing the initial temperature of the actuator and the two boundary conditions.On the basis of configuration, electrothermal actuators can be divided into hot-and-cold arm, chevron, and bimorph actuators.These actuators are also called U-shaped actuators, folded beam actuators, heatuators, or pseudo bimorph actuators. As the name implies, the structure of the actuator is made up of at least one hot arm and one cold arm. The actuator is usually made up of a homogenous material with folded arms in a U-shaped pattern that are constrained by anchors. Usually, the anchor surfaces are characterized by having a large surface area, required to ensure heat dissipation. Two arms of the actuator characterized by different cross-sections are connected in series to an electric circuit. The current flows through the structure with different current densities within the two arms. Therefore, more heat is produced within the thin arm through the Joule heating principle compared to that of the wide arm. This differential thermal expansion of the material causes the thin arm to expand more and bend towards the wide section, generating the bending moment ,111.At the base of the cold arm, there is a thin section flexure arm which helps the bending deflection at the tip of the actuator in the shape of an arc in the actuator plane. The length of the flexure arm plays an important role in the value of the tip deflection. In the original model proposed by Guckel at al., the length of the flexure arm and the wide arm were equal to half of the length of the thin arm . Huang aL, a, A, r, α, I are the actuator length, ratio of flexure arm length to hot arm length, cross-section area of the flexure and hot arm, center gap between the hot arm and the flexure component, coefficient of thermal expansion, and the moment of inertia of the hot arm (flexure arm), respectively. ΔnetT is the net temperature difference defined as the temperature which would cause the expansion of the hot arm alone and is the same as the net expansion in the real actuator case, where a small expansion of the cold arm corresponds to a decrease in the flexure component and results in a decrease in the net expansion between the two arms [The temperature along the beam can be obtained by unfolding the beam and applying Equation (23). In the case of no external load acting on the tip, the lateral deflection of the tip at the free end of these actuators can be described by:two arms .Hot-and-cold arm actuators are used widely in MEMS devices. There are large variations in the geometry of the actuators to achieve asymmetric thermal expansion. It is possible to obtain the in- plane deflection by changing the length of the arms instead of the cross-section , or usinLara-Castro et al. designed an array of four electrothermal actuators based on the hot-and-cold arm configuration to obtain out-of-plane displacement. Four actuators are used to control the rotation of a MEMS mirror for endoscopic OCT purposes . Some otChevron actuators are also called bent-beam actuators or V-shaped actuators and are the other type of in-plane electrothermal actuator, with a slightly different working principle. In this case, the in-plane displacement of the tip of the actuator is obtained from the total thermal expansion of the components instead of a differential expansion .In a V-shaped electrothermal actuator, two symmetrical slanted beams are connected at a certain angle to a central shuttle beam at the apex to the base with anchors. The current passing through the actuator causes the thermal expansion of both slanted beams due to the Joule heating principle. As the movement of the beam is constrained by the anchors and the central shuttle, the thermal expansion causes a compression force and a bending moment, which gives rise to the lateral displacement of the shuttle beam .θ is the initial tilt angle of the arm beam, l is the length of the single actuator arm, and l’ is the elongation due to thermal expansion [Similar to the U-shaped beams, the temperature distribution along the arms of the actuator can be obtained using Equation (23). Enikov et al. described the analytical model for V-shaped thermal actuators. The analysis of the beam deformation was considered by taking into account the buckling effect in the beam due to the axial thermal load and transversely applied force, if any. The numerical solution of the thermoelastic buckling model of the beam led to the tip deflection of the beam or the central shuttle beam . Howeverxpansion .θ. The opposite changes increase the exhibited force. Moreover, it is possible to amplify the motion of the shuttle beam by connecting the two bent-beam actuators in cascade. In the cascaded configuration, two V-shaped electrothermal actuators are anchored to the substrate and connected together with secondary V-shaped beams. The current can be passed either through the primary units only or through all the structural components [These actuators provide certain advantages over the bent-beam actuators described earlier, such as rectilinear displacement, larger exhibited force at the tip, and lower power consumption . The dismponents . Similarmponents . It is emponents .Another variation in chevron actuators consists of changing the geometry of the actuator to obtain a wide range of output properties. Among these, the most frequently used are the electrothermal actuators with Z-shaped patterned arms. In this configuration, the thermal expansion of the beams is blocked due to symmetry constraints, leading to the bending of the beams and thus the in-plane displacement of the central shuttle element. Z-shaped actuators permit smaller feature sizes and larger displacement compared to the V-shaped electrothermal actuators ,125. AnoChevron actuators find use in some cantilever-based optical scanners. Kaur et al. developed a sub-millimeter-sized cantilevered fiber optical scanner that can find use as a forward-viewing endoscopic probe. In this design, shown in A third class of electrothermal MEMS actuators is bimorph or bi-material type actuators. In this kind of actuator, two or more materials with different thermal expansion coefficients are stacked on top of each other. The different thermal expansion causes the actuator to bend or curl due to the induced strain generated by the Joule heating during actuation, which results in an out-of-plane motion .L being the length of the cantilevered bi-metallic beam, and k being the curvature, which is:r is the radius of curvature, b, t, E, α are the width, thickness, Young’s modulus of elasticity, and the thermal expansion coefficient, respectively, of the two materials characterizing the actuator. ΔT is the change in temperature due to Joule heating [The basic design of a bimorph electrothermal actuator consists of a cantilever-shaped micro-actuator fabricated using two layers of different materials connected to each other. The bending direction of the actuator tip during actuation will be dictated by the material with the higher thermal expansion coefficient compared to the one with the lower thermal expansion coefficient. The mathematical model for the tip deflection of such a micro-actuator is described by Chu et al. . Assumin heating .As with other electrothermal actuators, it is possible to place the different bimorph actuators in a cascaded configuration to amplify the obtainable tip displacement. In such structures, the various bimorphs are placed together in a serpentine direction, which causes the tip deflection from each bimorph to be added in series, yielding the higher overall tip displacement. It is possible to use different geometries for the bimorph structures to adapt according to the required spacing limitations in the microdevices. Large numbers of bimorph structures can be connected in a vertical cascaded form to generate a large out-of-plane displacement. Many bimorph structures can also be placed in parallel to lift the high load mirror surface .2 bimorph actuators were placed in a configuration, shown in the schematic of Bimorph actuators are the most frequently used electrothermal actuators and find use in a large number of scanning mirror MEMS devices. Zhang et al. developed a cantilevered fiber scanner excited non-resonantly using a MEMS stage. This platform, placed at a certain distance from the fixed end of the fiber, was connected to the fixed surface using three-segment bimorph actuators at four edges. Three segments of the Al-SiOThe working principle that governs the motion in these actuators is the so-called electromagnetic principle, where the conversion from electric/magnetic energy to mechanical energy takes place by means of a magnetic field. Similar to an electrostatic actuator, there are stationary and moving parts, named the stator and the rotor, respectively. Depending on whether the magnetic field is generated by the static or rotor component, there are two different configurations available for these actuators. Both configurations are described below in detail.µ0 is the permeability constant, N is the number of turns, I is the current, r is the mean radius, and z is the distance along central axis [In this configuration of MEMS electromagnetic actuators, a bulk magnet is placed inside an electric coil. When the current flows inside the wire coil, it generates a magnetic field. The intensity of the generated magnetic field depends upon the current passing through the coil, the radius of the coil surface, and the distance from the coil. For a circular coil, the generated magnetic field is given by the Biot–Savart equation:ral axis .M is placed inside an external magnetic field Hext created by one or more electric coils at angle α. The external field applies a torque on the moveable magnet given by:magV being the magnetic volume. Using a soft magnetic material, the generated torque TH rotates M moving it away from the equilibrium position (easy axis) by an angle θ. An anisotropy magnetic torque Ta will be generated inside the magnet, tending to realign it to its initial position.In the moving magnet configuration, a permanent magnet with net magnetization vector aK is the magnetic-anisotropy constant. An opposite torque Ta is exerted on the easy axis and thus on the magnet itself [Hext remains constant at an angle γ (α = γ at the beginning) from the easy axis, the torque becomes:t itself . If the M = sM (saturated magnetization), and θ = 0. The magnetization of soft magnets changes with the applied external magnetic field and sM.In a permanent magnet, M is:The external magnetization vector acting along the direction of M induces the poles at the end of magnet, which generates a demagnetized field Hd in the opposite direction of Ha:MN is the shape anisotropy coefficient. The net field (Hi) inside the sample changes to the sum of the applied and demagnetized field. The sample moves to reduce Hi and cause the magnetization vector to be:Thus, the change in TH rotates M from easy axis and is balanced by the anisotropy torque Ta and tends to align M and vibrate the magnetic component [In equilibrium, the field torque omponent .Joos et al. developed an OCT probe for imaging based on this technique. In this probe, an electromagnetic coil was placed at the outer surface in the center, in which a magnet was placed carrying a thin-walled 28-gauge tube. An SMF fiber was contained in a “S”-shaped 34-gauge stainless-steel tube placed within the 28-gauge tube, as in the schematic shown in Sun et al. developed a cantilevered fiber scanner for medical endoscopic applications, where an SMF with a collimating lens was excited at resonance using an electromagnetic actuator working on this principle. The researchers fixed a soft cylindrical magnet to an optical fiber using a 1 mm diameter polyimide pipe, and a tilted coil was fabricated using a microfabrication lithography technique . The schA similar probe, schematized in B is the external magnetic field, I is the current, L is the length of the conductor, and θ is the angle between the direction of the current and magnetic field [In the moving coil configuration, an electric coil is fabricated on the scanner and is placed inside a static magnetic field created by external magnets. When the current flows through the coil in the presence of an external magnetic field, a force is exerted on the coil, designated as the Lorentz force. The force generated on the coil is given by:ic field . The forN turns, the generated magnetic torque on the coil is:nr being the distance of the nth coil turn from the center [In the case of a coil with e center .As in the previous case, the actuator (coil surface) deforms due to the generated torque, and a restorative torque will arise in the coil to bring it to its initial state, causing the vibration of the moving coil. This technique is frequently used to actuate micromirror surfaces ,137 and Magnetic materials are characterized by a special property which allows them to change their dimensions in the presence of a magnetic field. This effect is called magnetostriction. The material can undergo a change in dimension until it reaches the value of saturation magnetostriction, which depends on the magnetization and, therefore, on the applied magnetic field .Bourouina et al. developed a 2D optical scanner based on the magnetostrictive effect. In this case, a silicon cantilever was coated with a magnetostrictive film. Due to the uniaxial nature of the magnetostrictive material, bending and torsion vibrations were generated simultaneously in the presence of an AC magnetic field generated by the electric coils placed in its surroundings. Later, a piezoresistive detector was incorporated in the device to measure the bending and torsional vibrations ,140.A slightly different fiber optic scanner was developed by a group of researchers from the University of Texas. In this design, an optical fiber was coated with a ferromagnetic gel which experienced a bending motion in the presence of an external magnetic field generated by a magnet placed at the outer surface. The schematic configuration of this device is shown in A is the electrode area, ε is the dielectric constant of the air, V is the total voltage difference applied to the plates, and g is the air-gap distance [An electrostatic actuator includes at least two pairs of electrodes attached to two plates separated by a gap. One of these plates is fixed by anchors and is named the stator, while the other plate is able to move and is designated the shuttle. In the presence of a voltage difference between the two plates, an attractive electrostatic force generates among them, causing the movement of the shuttle plate towards the stator. The amount of the electrostatic force generated between the two components depends on the gap and the dielectric constant of the media separating the two plates. The generated electrostatic force is given by:distance . The maxIn a parallel plate configuration, two electrodes are placed parallel to each other in an interdigitated finger configuration. In optical scanners, the moving electrode is mostly represented by a polysilicon mirror used to deflect the light.Another variation of a parallel plate actuator is a system where the moving electrode has a rotational degree of freedom. The application of the voltage to the electrodes in this case causes the rotation of the moving electrode with a tilt angle obtained from the equilibrium between the electrostatic torque generated by the electrostatic force and the restoring torque. The large deflection angle in this case requires a large air gap between the electrodes. The maximum deflection of the rotating electrode should be less than one third of the air gap to avoid the pull-in phenomenon .Most of the torsional electrostatic actuators are divided into small-sized scanner arrays causing large tilting angles with small air gaps. There are some systems developed with tapered electrodes to allow large tilting angles.One of the main drawbacks of electrostatic actuators is that a large driving voltage is required to obtain moderate deflection angles. It is possible to partially overcome this by using tapered electrodes instead of the parallel-shaped ones .In electrostatic comb actuators, multiple plates are connected to make interdigitated static and mobile rows. Such a configuration enables an increase in the interaction area between the two electrodes, and, consequently, high electrostatic forces are generated. As in the parallel plate configuration, the out-of-plane motion of the mobile mirror structure can be obtained by making a vertical offset between the torsional support and the driving arm .In vertical comb drives, the moving comb motion is out-of-plane with the motion of the fixed comb, which avoids the pull-in phenomenon. Moreover, the deflecting mirror can be decoupled from the actuating part, permitting a large possible deflection of the mirror itself. The higher electrostatic torque generated by the comb structure leads to the possibility of higher driving frequencies and thus a higher scan speed .Vertical comb drives are used frequently to actuate micromirrors ,147,148.A group of researchers at Fraunhofer University studied the design optimization for comb drive micro-actuators. It was more convenient to place the electrodes in a star-shaped pattern to obtain a higher deflection of the mirror surface .Both types of actuators are mainly used to actuate micromirrors . Munce eShape memory alloys (SMAs) are unique metallic alloys having the ability to return to their original shape after being deformed plastically. The deformation recovery is usually obtained by increasing the temperature of the material, which releases the state of stress.SMA material is available in either a wire or a sheet shape. Frequently used SMA actuators take the form of a coil structure as they can provide a larger stroke as compared to a straight wire per unit length. The shear modulus and the spring constant of a SMA coil/spring depend on the composition, temperature, strain, and shape memory treatment applied to the material. In the design of an SMA actuator, the material is deformed at a low temperature and thermally treated to remember its shape. The shape recovery of the coil is obtained by increasing the temperature via the Joule effect by passing a current through the wire .SMA coil actuators show a one-way shape memory effect. Thus, a bias spring or a second SMA coil spring is combined with an SMA coil actuator to obtain a two-way actuation. When a current is passed through the SMA coil, it tends to return to its original shape, which exerts a force on the second spring/coil, permitting motion in one direction. When the current is stopped, the SMA coil cools down and it is re-deformed by the force applied by a bias spring or activating the second coil . SMA coils are largely used in endoscopes to bend the distal tip for a long time. Maeda et al. fabricated a 2 mm diameter endoscope head where an actuation ring connected to two SMA springs guided the bending motion of the endoscope tip which contained the optical guide fibers by pulling or releasing the pull wire connected to it. The schematic diagram showing the structure of the described design is shown in Haga et al. used three SMA coil actuators at equilateral triangle locations between two links to form a joint. The bending motion generated by these actuators allowed the snake-like movement of the central working channel of the endoscope, i.e., guided the endoscope .A similar endoscope was designed by Makishi et al., where the active bending motion of the endoscopic tip, containing a CCD imager, was obtained using three SMA coil actuators . The endOptical endoscopic imaging enabled the visualization of cellular and subcellular structures in real time, enabling early interventions and improved diagnostic yield from fewer biopsies. Moreover, the less invasive nature of the imaging device results in reduced tissue trauma, low risk of complications and operational costs, and fast recovery times. In cantilever fiber endoscopes, an optical fiber is rigidly fixed to an actuator’s substrate surface, leaving a few millimeters of free end at its distal end. Most often, this free end is vibrated at resonance to obtain a large tip displacement, which in these imaging devices is directly related to the resolution of the obtained image. Various imaging applications, such as confocal endo-microscopy, OCT, photoacoustic imaging, etc., can be found using such endoscopes.Frequently used MEMS actuators in fiber optic endoscopic devices comprise piezoelectric, electrothermal, electromagnetic, electrostatic, and shape memory alloy actuators. Piezoelectric actuators are available in plate or tubular structure, among which the latter is largely used as it provides the base excitation to the fiber held in the center along two directions. By linearly increasing the amplitude of sinusoidal voltages at two pairs of electrodes with a 180° phase shift, one can obtain a spiral scanning pattern.Electrothermal actuators are available in three different types including hot-and-cold arm, chevron, and bimorph actuators. In a hot-and-cold arm actuator, the different geometries of the two legs produce asymmetric heat generation and, consequently, an asymmetric thermal expansion, generating a bending motion at the actuator tip. Similarly, in bimorph actuators, the different material properties cause the bending of the actuator tip. These techniques can be combined with other actuation methods to obtain a 2D Lissajous scanning pattern. Chevron actuators have symmetrical legs and provide linear 1D motion at the tip. Electrostatic actuators are available in parallel plate and comb drive configurations. These actuators find limited use in fiberoptic scanners due to their large dimensions but are frequently used in scanners having proximal scanning devices such as mirrors.The shape recovery property of shape memory alloy materials makes them an excellent alternative for providing a large actuation force in compact dimensions.Various imaging endoscopic devices using these actuators are described in this paper. The performance of some recently developed fiber optic cantilever-based scanners is summarized and compared with the clinically available endoscopes in As stated earlier, scanning fiber endoscopes were introduced recently in the field. Most of the devices mentioned in this paper find limited use in clinical application but are more in the transition from research to clinical phase. The only commercially available cantilever-based endoscopes for clinical use are those developed by Pentax  and MaunThe choice of an endoscopic device depends on the target imaging region, ease of use, and cost of the device. The cost of an endoscopic device comprises fabrication and operating cost. The operating costs mainly consist in decontamination and sterilization, which can be performed at high temperature such as autoclaving or hot air oven or at low temperature using chemical agents. The fabrication cost of an endoscope highly depends on the cost of the laser source, actuation, and detection mechanisms. Among the various actuators studied in this paper, electrothermal actuators are economic ones. However, the high working temperature can limit their usage at a high frequency. Otherwise, piezoelectric tubular actuators are cost-effective in terms of possible bidirectional actuation at high actuation frequency. The development of MEMS actuators enabled batch production of miniaturized actuators, reducing the fabrication costs to great extent and permitting the fabrication of disposable endoscopic scanners.The actuation and scanning devices for cantilever-based endoscopic probes are described in this review paper along with their design and working principles. The size of an actuator is an important factor in a cantilevered endoscopic device. The developments in the MEMS field permit the mass production of small-sized actuators, enabling the fabrication of small-sized imaging probes. The endoscopic technology is aiming towards the design of low-cost disposable imaging devices as the reprocessing and sanitation of an endoscopic device is a complex and expensive process. The fabrication of small-sized actuation mechanisms helps to reduce the price of device. Moreover, small-sized probes can be advanced further to image the small body cavities, enabling early detection of pre-cancerous surfaces and helping in the diagnostic procedure to control the lesion at a preliminary stage.


Fit to Work is a formalized working dog foundational physical fitness assessment and development program. The Penn Vet Working Dog Center developed this program to address the needs of working dog handlers, trainers, and programs for simple, effective, and efficient methods to develop and assess working dog physical fitness. Fit to Work focuses on the foundational fitness modalities of strength, stability, mobility, and proprioception. The Penn Vet Working Dog Center piloted and refined this program over 3 months in a closed population of 31 working dogs in training. Fit to Work consists of posture development and maintenance, warm-up and cool-down routines, training exercises, and assessment methods. To simplify implementation for dogs and personnel, the foundational training program incorporates a discrete number of exercises, standardized progression steps, defined criteria for progression, and a reduced emphasis on learned behaviors. Fit to Work also enables safe and progressive assessment of foundational fitness through a tiered and inexpensive process. Future research will focus on validation of training and assessment methods, development of assessment standards, and correlation of physical fitness with operational performance. Canine physical fitness is a recognized requirement for safe and effective performance of working dogs. Current definitions of canine physical fitness are vague and are frequently tied to a specific type of activity . Extrapolating from human literature, physical fitness is defined by The American College for Sports Medicine as “a set of attributes that people have or achieve” . The AmeBasic canine fitness requires a foundational fitness program coupled with a scientifically based and repeatable assessment process. Foundational fitness addresses mobility, stability , strength, and proprioception required to perform basic physical tasks such as running, jumping (up and down), navigating unstable surfaces, and quickly or abruptly changing direction. The definition for each foundational fitness component can be found in This canine foundational fitness program is designed to be incorporated into an established working dog training program . The dog's career training develops the baseline cardiovascular endurance and skill related components of career-specific fitness. However, additional career-specific fitness training should be pursued to optimize career performance. A canine foundational fitness program for working dogs is designed to incorporate exercises that require minimal additional training of the dog and handler, include clear incremental exercise progressions, utilize easily accessible and low-cost equipment, and should also be applicable to a wide variety of working dog careers.One of the biggest challenges in evaluating the current level of canine fitness and the impact of any intervention, such as a fitness program, is the establishment of a formalized, repeatable method for assessment. To date, there are no peer-reviewed, published studies of systematic foundational programs with assessments specifically for working dogs. The methods for creating a foundational fitness assessment should produce scientifically valid, reliable, and reproducible canine fitness data –10. CleaThe Fit to Work (FTW) foundational exercises and associated assessment method presented here are designed to create a formalized, highly reproducible, and inexpensive method to create the first stage of a balanced fitness program for working dogs. The goals of the program are to meet the foundational fitness requirements of working dogs that are not currently addressed in the traditional training disciplines. The program was designed with the realistic expectations and time constraints of the working dog training and utilization worlds. To address the gap in knowledge and lack of additional training time, the FTW program contains a discrete number of exercises, standardized progression steps, and defined criteria for progression. The FTW program significantly reduces the requirement for learned behaviors to shorten the learning curve and enable a novice working dog handler to develop their dog's physical fitness. We will present each of the components of the program, their description, the purpose of the exercise, the known or anticipated role in performance and injury prevention, a recommended progression of difficulty, a brief summary of contraindications, and suggested training approaches.All dogs included in the implementation below are owned by the Penn Vet Working Dog Center, School of Veterinary Medicine, University of Pennsylvania. The Fit to Work (FTW) program is included in the Penn Vet Working Dog Center (PVWDC) Puppy Foundation Program protocol 804547 approved by The University of Pennsylvania Institutional Care and Use Committee.n = 15), German Shepherd Dog (n = 8), Belgian Malinois (n = 3), Dutch Shepherd (n = 3), Small Munsterlander (n = 1), and Doberman Pinscher (n = 1) breeds. The dogs were assigned to 5 full-time trainers who each had responsibility for 3–8 dogs. The training team was assisted by ~20 part-time undergraduate interns and ~30 adult volunteers.The FTW program was piloted on a closed population of 31 dogs in training for careers in search and rescue, law enforcement, single-purpose detection, and medical detection. Each dog was fostered on nights and weekends by an individual or family who did not perform any FTW activities. The dogs ranged in age from 2 months to 6 years of age and were of the Labrador Retriever  or by increasing the speed of execution. The range and thus the intensity of the Four-Position Cookie Stretch may be increased by extending the dog's nose closer to each targeted position or by increasing the duration at each position.While many ways exist to perform or train the Warm-up and Cool-down exercises, we have found the following to work well for our population of working dogs. To train the Paws-up on an arm, have the dog sit, kneel next to them, scoop your forearm under their forelimbs, and use a toy or food reward to lure them into a stand as you slowly rise. Use the toy or food reward to maintain them in the Paws-up position for the desired duration. Common technique errors include rewarding too high above the dog's head which overextends the dog's neck and allowing the dog to rest a significant portion of their weight on your forearm.To train the Figure-8, start with your feet approximately twice your shoulder width distance apart. Use a “Touch” command or lure the dog between your legs, then with alternating hands direct the dog in one direction , then back through your legs and around the other leg in the opposite direction . Common technique errors include inadequate rate of reinforcement, giving improperly timed commands and imprecise luring, all of which prevent the dog from sharply turning around your legs. Good form also requires good handler form, standing up straight is the goal.To train the Four-Position Cookie Stretch, lure the dog in a standing position so that its spine is aligned perpendicular to your spine. Gently place your opposite hand under their thorax or abdomen to maintain their position against you. Lure their nose away from your body, in a plane parallel to their spine to their hip, mark, and reward. Then lure their nose to their hip and then gradually move it as close to their shoulder as possible, while maintaining it in the plane parallel to their spine, mark, and reward. Lure their nose to their hip and then move the lure distally until their nose is as close to their rear foot as possible, mark, and reward. Finally, lure their nose between their forelimbs and as close to their chest as possible, mark, and reward. A common technique error is not maintaining the dog's position against your body which allows them to decrease the intensity of the exercise.To perform the Posture Sit, the dog should sit with its coxofemoral joint, stifle, tarsus, and hindlimb digits in the same straight sagittal plane . The forTo perform the Posture Down, the dog should lay down so that the ipsilateral right and left limbs are within the same sagittal plane, often referred to as a “sphinx” position . To achiPerforming the Posture Sit and Posture Down primarily targets stabilizer muscles of the spine  and abdominal engagement . The primary muscle groups of the Posture Sit and Posture Down are the cervical spine extensor muscle (splenius), trunk extensor muscles—epaxials , latissimus dorsi, hip stabilizers , and abdominal muscle (rectus abdominis). Proprioception of spine, shoulder, elbow, lumbosacral, coxofemoral joint, and stifle are secondarily developed during both the Posture Sit and Posture Down.Performing the Posture Sit and Posture Down strengthens the dog's core stability musculature and supports optimal biomechanical alignment by creating a proprioceptive memory of correct posture , 24, 25.Engaging in Posture Sits and Posture Downs may prevent repetitive stress on the spine resulting in kyphosis and susceptibility to traumatic or alignment-related injuries . AdditioTeaching correct posture development in both the sit and down positions when the dog is a puppy (8 weeks) and maintained throughout the dog's life is highly encouraged. For healthy dogs, progression in the Posture Sit and Posture Down is achieved by increasing the duration of holding the correct position and then destabilizing the surface on which the dog is performing the exercise. For beginner dogs, the duration may be as short as 1–2 s. The dog is expected to maintain a 30 s hold in the correct position before progressing to an unstable surface. An unstable surface should be a flat platform with the destabilization component located underneath the platform. Performing this exercise on a flat surface allows the trainer to evaluate if the dog is not remaining in a proper position and/or is consistently favoring one side over another. If the exercise is performed on two unstable surfaces such as balance discs alone, the dog can make subtle changes in body posture that may result in asymmetric muscle development and lack of engagement of the smaller secondary musculature.The Posture Sit and Posture Down are safe for healthy dogs of all ages. Caution in progression of the Posture Sit and Posture Down should be taken with suspected spine, hip, or stifle abnormalities. While critical to the rehabilitation process of many musculoskeletal injuries, the Posture Sit and Posture Down should only be performed for these patients under the guidance of the dog's veterinarian.To train the Posture Sit and Posture Down, have the dog perform a correct (square) sit or (sphinx) down position as described above. Placing the dog on a narrow platform or between two objects will encourage the dog to align all limbs in the sagittal plane. Once in the proper square sit or sphinx down, a treat or toy lure at a dog's resting nose level height should be utilized to encourage the dog to move forward slightly [2–7 cm (1–3 in) depending on the dog's size]. The dog should be lured until the spine is straight and the stifles are dorsal to or just cranial to the hindlimb digits. If the treat or toy lure placement is too high, the dog will attempt to stand. If the treat or toy lure placement is too low in the Posture Down, the dog will lift its hocks or attempt to crawl. If the treat or toy lure placement is too low in the Posture Sit, the dog will attempt to lay down or will round its back. Time the delivery of the reward to the dog based on the dog's progression level and deliver the reward while the dog is holding the correct posture position.To perform the Pivot, the dog places its forepaws on an elevated stable object  and steps laterally (sidesteps) with its hindpaws around the object both clockwise and counterclockwise . The PivThe Pivot primarily develops the dog's hip, stifle, and tarsus stability. Core stability , hindlimb proprioception, and hip, stifle, and tarsus extension are secondarily developed. Hip stability is provided by the hip abductors  and adductors . Stifle and tarsus stability may also benefit from the improved control over the lower extremity provided by the stable hip during movement .The Pivot strengthens the dog's hip stability musculature and supports optimal biomechanical alignment of the hip, stifle, and tarsus when the dog is moving and stationary . A stablStable hips developed by the Pivot may prevent traumatic  or alignFor mature dogs, the Pivot is usually trained on a 20 cm (8 in) high and 40 cm (16 in) square object  , althougWithout guidance from a veterinarian, the Pivot is not recommended for dogs with suspected spine, hip, stifle, tarsus, or hindpaw abnormalities. With supervised rehabilitation, the Pivot can be adapted to the dog's injury or condition.While there are many ways to perform or train the Pivot, the following works well for our population of working dogs. First, have the dog place their forepaws on the elevated object, stand next to the dog's flank, provide the verbal cue to “Step,” provide gentle body pressure by moving into the dog, and then mark and reward any lateral movement of the hind foot while the forelimbs remain on the object. Hold the reward in the right hand when performing the Pivot clockwise and in the left hand when performing the Pivot counterclockwise. Work toward having the dog moving at least a quarter rotation ahead of you and rewarding after each quarter rotation. A well-trained dog should perform the Pivot in a position 180° from the handler and be able to complete a 30 s interval prior to reward. Common technique errors include relying solely on body pressure after an initial learning period, using the hand closest to the dog to reward, rewarding too far above or away from the dog's mouth or too infrequently, or allowing the dog to move too quickly and skip with or cross their hind feet. A soft high-value reward  assists our population of working dogs to rapidly swallow the reward and resume movement.To perform the Plank, the dog stands with its forepaws on one stable object  and its hindpaws on a second stable object of equal height. Place (or cue the dog to place) their forepaws on the rear edge of the front object and their hindpaws on the front edge of the rear object. Adjust the distance between the objects until both the dog's distal forelimbs (elbow to carpus) and metatarsals (rear pastern) are vertical , Line 5.The Plank primarily develops a dog's core stability (predominantly resistance to thoracic and lumbar spine extension). Isometric elbow and carpus extension are secondarily developed. Core stability in the Plank is predominantly provided by the rectus abdominis, external abdominal oblique, iliopsoas, and psoas minor muscles. In humans, the spinal extension muscles have also been shown to play a role , 35.The Plank strengthens the muscles that provide stability to a dog's spine. This increased stability may enhance the dog's ability to generate whole-body power –38, run Training movements that develop a dog's spinal stability may increase the likelihood of maintaining optimal biomechanical alignment when gravity or the motion of the dog's body cause thoracic or lumbar spine hyperextension. A dog that repeatedly hurdles obstacles , spends prolonged time in a forepaw-elevated position , traverses unstable surfaces that predispose it to a fall,  encounters powerful force to the spine , or is at risk of spine injury or intervertebral disc disease may benefit from training the Plank.The Plank is usually trained in intervals of up to 30 s at a particular level. The dog is given 45 s in which to accumulate 30 s of proper Plank. The 30 s duration time is paused when the dog moves out of the proper position  and the timer is resumed when the proper position is achieved again. The interval is stopped when the dog accumulates 30 s, the 45 s time elapses, or the dog steps off either object.Once a dog is able to accumulate 30 s of proper Plank within the 45 s window, the objects can be moved to the next level . This prWithout guidance from a veterinarian, the Plank is not recommended for dogs with suspected spine or hip abnormalities. With supervised rehabilitation, the Plank can be adapted to the dog's injury or condition.While there are many ways to perform or train the Plank, the following works well for our population of working dogs. Position the stable objects  at the starting position (Level 0), and have the dog step onto the rear object and walk across to the front object. For some dogs, placing an additional object between the front and back objects will facilitate the dog walking across the gap. An extra object may be used on top of the front object to assist the dog with maintaining its forepaws on the inside edge . Gently The Chipmunk is also known as the “beg,” “sit pretty,” “sit up,” or “sit erect.” To perform the final correct position, the dog must have a square sit as described in the Posture Sit exercise. Once in a sit, the dog's forelimbs are suspended off the ground with the carpi suspended between elbow and shoulder height, the dog maintains a straight spine, and holds their head in a neutral position facing forward .The Chipmunk primarily develops the dog's core stability . Secondarily, isometric shoulder and elbow contraction and minor concentric contraction of quadriceps muscles are involved. The primary stability muscles engaged during the Chipmunk are the rectus abdominis, external abdominal oblique, iliopsoas, transversospinalis, longissimus, and iliocostalis muscles.The Chipmunk strengthens the muscles that provide stability to a dog's spine . Along wTraining exercises that enhance a dog's balance, proprioception, and core strength may protect them during uncontrolled movements which result in body misalignment, spinal hyperextension or compression, such as jumping, apprehension training, or ladder climbing , 43.A Chipmunk should only be attempted once the dog has achieved consistent proper Posture Sits and Posture Downs. Achievement of the correct Posture Sit and Posture Down ensures that the dog has sufficient body awareness and muscle development of the epaxial and abdominal muscles. As described in the training section below, the dog should gradually be taught to engage its core in short training sessions before expecting the final Chipmunk position. Once a dog can achieve the proper form of a Chipmunk for a duration of 30 s , it can The Chipmunk is not recommended for dogs with suspected spine , hip, or stifle abnormalities, unless under direct supervision of a veterinarian.Training sessions for the Chipmunk should be short, and the dog should not perform more than 3–6 repetitions during any single session. To begin to train the Chipmunk, place the dog in a Posture Sit and place a lure just above the dog's head to encourage lifting of the forelimbs. Reward as soon as the forelimbs leave the ground [<15 cm (6 in)] . Once thTo perform the Squat, the dog places its forepaws on an elevated object, sits onto a restricted area platform, and then returns to the starting position . The SquThe Squat primarily develops the dog's hip, stifle, and tarsus extension strength . Core stForceful hip, stifle, and tarsus extension developed from performing the Squat may increase the dog's ability to jump, sprint, and change direction. Dogs needing to jump up, onto an elevated surface, over an object, or across a gap may benefit from the Squat. Dogs rising from a down or sitting position or those rapidly accelerating while sprinting can benefit from the Squat. Dogs doing single-leg movements, climbing stairs, or rapidly changing direction may also perform better as a result of incorporating the Squat into their fitness plan .Strong hips, stifles, and tarsi developed by the Squat may be less prone to injury . IncreasFor mature dogs, the Squat is usually trained on a 20 cm (8 in) high stable object  , althougWithout guidance from a veterinarian, the Squat is not recommended for dogs with suspected spine, hip, stifle, or tarsus abnormalities. With supervised rehabilitation, the Squat may be adapted to the dog's injury or condition.A restricted area platform typically 40 cm (16 in) square  is used to maintain the forepaws on the elevated object and the hindpaws at a consistent distance from the object. While there are many ways to perform or train the Squat, the following works well for our population of working dogs. Have the dog step onto the platform and then onto the elevated object. Provide the verbal cue to “Sit” and use a food reward or toy to guide the dog's nose backward. Mark and reward any flexion of the hips, stifles, or tarsi. Progress toward a full sit with both forepaws on the elevated object and the majority of the hindlimb below the hock on the platform. Use a “Touch” command or lure the dog to return to the starting position. A soft high-value reward  assists our population of working dogs to rapidly return to the starting position, swallow the reward, and begin the next repetition.To perform the Back-up exercise, beginning at a walk, the dog moves in a backwards or reverse motion by picking up and pushing off the ground with each foot to propel themselves backwards. The dog's spine should remain parallel with the floor during the entire movement .Back-up is utilized for proprioception of the hindlimbs and primary targeting of the biceps brachii and quadriceps muscles , 24. ThePerforming the Back-up exercise focuses on the extensors of the hindlimb while complementing the flexion focused Squat exercise , 49, 52.Training of secondary muscle groups, activation of secondary gait neuromuscular pathways, and enhancing hindlimb proprioception may protect a dog from injury , 48, 49.The Back-up is not recommended for dogs with suspected musculoskeletal abnormalities. The Back-up should be used with caution in dogs with diagnosed lumbosacral pain, hip dysplasia or osteoarthritis, or cruciate ligament disease.Progression for the Back-up can be broken down into several stages . The firThere are several ways to train the Back-up exercise. One method is to place the dog between two objects such as a wall and chair. The handler then steps in front of the dog with a lure in front of the dog's nose. The handler takes a step toward the dog and the dog will respond by stepping backwards. During initial training of the Back-up, the dog should be rewarded after each step backwards. Once the dog understands the movement, the reward frequency can be decreased. Moving toward the dog too quickly or placing the lure too low causes the dog to round its spine. Placing the lure too high causes the dog to sit. As a general principle for training the Back-up, a dog should take 2–3 steps backwards for every step forward by the handler.The Foundational Fitness Assessment (FFA) objectively measures a dog's fitness across the foundational fitness components. We developed this assessment to aid in measuring a dog's current fitness, identifying any change in fitness after completing a training program, adjusting training programs, and comparing the fitness of dogs in similar age ranges, breeds, or careers. Measuring a dog's current fitness was the primary impetus for the development of this assessment. We wanted a way to objectively assess the fitness of the dogs in the PVWDC training program in order to establish a baseline before they began the foundational training. We also desired a formalized assessment to determine if the training program we developed was making a difference in a dog's fitness. This kind of assessment will now allow us to compare different training program styles, methods, durations, frequency, and equipment. We hypothesized that dogs would become more fit during the course of a training program, and a formalized assessment would allow us to adjust that training program to maximize their fitness development. Finally, we wanted to be able to objectively compare the fitness of dogs. This will eventually allow us to develop age, breed, and career-specific standards and scoring.We prioritized durability, accuracy, and simplicity when we designed the FFA. In this setting, a durable assessment is one that is easily implemented in a non-professional setting, involves inexpensive and readily available equipment, and is quick to conduct. Accuracy amongst evaluators (inter-rater and intra-rater reliability) and between assessments (inter-assessment and intra-assessment) are also important. Finally, we wanted a simple test that either does not require new behaviors or utilizes behaviors that could be rapidly learned. Our end state is an assessment that could work for most dogs, by most evaluators, and in most environments.As a result of our experience during the pilot implementation, the FFA is divided into two levels. Level One consists of two tests that evaluate core stability and whole-body power generation (predominantly hindlimb extension strength) without the requirement for learned behaviors. Level Two utilizes two behaviors that take ~4 weeks to learn but that provide an independent assessment of hindlimb extension strength and hindlimb stability. The two levels may be used separately or together, and future work will focus on validating their independent and consecutive use.The Foundational Fitness Assessment-Level One (FFA-L1) consists of the Sprint Test (ST) and the Progressive Plank Test (PPLT). This assessment requires minimal and inexpensive equipment and takes ~15–20 min per dog to complete. While the order of the FFA-L1 requires further exploration, we propose that the PPLT should be completed after the ST so that core stability fatigue does not affect the maximal sprint. Some dogs may require the PPLT before the ST for behavioral reasons. Regardless of the order, the dog should be given at least 5 min to recover between tests.The ST assesses a dog's ability to generate whole-body power during the acceleration phase  of sprinting. Sprinting is predominantly a hindlimb extension movement , so the The dog must start from a down position (chest and tarsi on the ground) with its entire body behind the starting line. Light restraint may be used to minimize the obedience requirements. Accurately measure the 25 m (82 ft) course and place a narrow but conspicuous marker  to define the finish line. Any method may be used to motivate the dog to sprint maximally, but the motivation should be located at least 10 m beyond the finish line to encourage maximal effort for the duration of the test. A toy reward may be held by the handler and a “Come” or “Here” command given. A toy may be thrown beyond the finish line, but this method is not preferred as some dogs will decrease their effort to track the toy in flight. For a dog trained to a bite sleeve, a decoy running away from the finish line may be used.Due to the need for precise measurement and the inherent margin of error with manual timing the ST must be recorded using a video camera capable of at least 30 frames per second (accurate to 0.033 s) and preferably 60 frames per second (accurate to 0.017 s) . The camThe PPLT assesses a dog's trunk muscle endurance in a safe and objective manner. The plank (or prone bridge) is reliably used to assess human core muscle endurance , 55–59. The Foundational Fitness Assessment-Level Two (FFA-L2) consists of the Progressive Pivot Test (PPT) and the Progressive Squat Test (PST). This assessment requires ~4–6 weeks of prior training, minimal and inexpensive equipment and takes ~15–20 min per dog. While the order of the FFA-L2 requires further exploration, we propose that the PPT should be conducted before the PST so that fatigue from the Squat does not affect hip stability. The dog should be given at least 5 min to recover after completion of the final PPT interval before beginning the PST.The PPT assesses a dog's hip stability in a safe, objective, and specific manner. To perform the PPT, the dog completes up to 3 complete Pivot rotations in under 30 s in both directions at successively higher levels until the dog's maximum level is reached. The dog has 45 s of total time in which to complete 30 s of active Pivot. The 30 s active time is paused if the dog steps off the object or if there is a handler or reward issue. The interval ends when the dog successfully completes the 3 rotations, the 30 s of active time elapses, or the 45 s of total time elapses. After completion of a level in one direction, the dog is given 30 s of rest before attempting that level in the opposite direction.If the dog successfully completes the level, the object height is increased to the next level. If the dog successfully completes a level in one direction but does not complete the level in the other direction, the object height is not increased. This process is continued until either the dog fails to complete a level or the object height is raised to the dog's maximum level. Then, the dog completes a final 2 min maximum effort in each direction. The PPT score is expressed as the final level and number of rotations (rounded down to the nearest quarter rotation) in the clockwise (expressed first) and counterclockwise directions. Examples are 3–2.75/1.5  and M-10.25/12 (ten and one-quarter rotations clockwise and twelve rotations counterclockwise at this dog's maximum level).The PST assesses a dog's hip, stifle, and tarsus extension strength and endurance in a safe, objective, and specific manner. To perform the PST, the dog completes up to 7 Squats in under 30 s at successively higher levels until the dog's maximum level is reached. The dog has 45 s of total time in which to complete 30 s of active Squats. The 30 s active time is paused if the dog steps off the object or platform or if there is a handler or reward issue. The interval ends when the dog successfully completes the 7 Squats, the 30 s of active time elapses, or the 45 s of total time elapses.If the dog successfully completes the level, the object height is increased to the next level. After successful completion of a level, the dog is given 30 s of rest before attempting the next level. This process is continued until either the dog fails to complete a level or the object height is raised to the dog's maximum level. Then, the dog completes a final 2 min maximum effort. The PST score is expressed as the final level and number of Squats completed at that level. Examples include 2–5 (5 Squats at Level 2) and M-14 (14 Squats at this dog's maximum level).Our goal is to assess the foundational fitness of a sufficient number of dogs in order to develop both a scoring system and age, breed, and career-specific standards. We aim to develop a bell curve of results for each assessment. Then, we propose a scoring system where results clustered near the mean receive an average score while results above the mean receive higher scores and those below the mean receive lower scores. See We also propose further work to explore the relationship between foundational fitness results, injury, and objective career-specific performance measures. We are interested in identifying the effect a dog's fitness has on the likelihood it will experience an injury, the type of injury (acute or degenerative), the severity of injury, and the duration of recovery. We hypothesize that more fit dogs will be injured less frequently, experience fewer degenerative injuries, be injured less severely, and recover faster than their less fit counterparts. We also believe a dog's fitness is integral to its ability to perform its job or sport. While performance in some dog activities is easy to measure , other dog activities are harder to measure or have more poorly defined performance metrics . We hypothesize that more fit dogs will perform better at their activities whether those activities are predominantly physical or less physically-focused.Our aim was to develop a formalized method to develop and assess foundational fitness in working dogs. We implemented the FTW program in the PVWDC population over an ~3-month period consisting of training personnel, familiarizing dogs, initial assessment, and regular training. Our pilot results are summarized below.The FTW program was safe in this group of dogs, under the conditions tested. We assessed 31 dogs on two occasions without injury or negative effect on training. The same group of dogs conducted ~600 foundational training sessions during the familiarization and regular training periods. We identified several minor abrasions from contact with the concrete blocks but no other injuries. The formalized Cool-down routine with its paw, pad, and nail check conducted after every training session facilitated identification and treatment of injuries before the dog left for the day. Without this system, injuries sustained during FTW or other training could have been overlooked or identified at home by a non-professional foster without medical resources.The PVWDC veterinarians also experienced the common phenomenon in human athletic performance where the athletic trainer identifies an athlete's musculoskeletal issues before a medical professional does. On numerous occasions we had a non-trainer individual conduct FTW and then bring us a data-driven issue. Comments such as “I was just doing Pivot with Ivey, and both times she did 2–2.5 rotations clockwise but only 1–1.5 rotations counterclockwise. Can you come take a look?” resulted in a targeted musculoskeletal examination, identification of the issue , treatment, and rapid return to full performance.Finally, we believe FTW to be safe for young dogs. The primary concerns with fitness training in these dogs are the effects of excessive force on epiphyseal plates and repetitive motion on the skeleton and joints –63. The FTW appears to be an accessible method to develop and assess fitness in working dogs. Dogs of diverse breeds, temperaments, and ranging in age from 2 months to 3 years learned to perform each of the movements. Dogs were also appropriately challenged by the program. After initial familiarization, dogs who tested well on the Plank were then advanced to higher levels while dogs who scored lower on the Plank were provided lower progressions appropriate for them.Puppies started learning proprioception and height-adjusted hip stability during their first week at the PVWDC, and dogs that had trained fitness for years in our program adjusted as well. We found that dogs that started FTW earlier (before 6 months of age), were exposed to the exercises more frequently, and practiced the exercises outside of the foundational training sessions rapidly learned the exercises and progressed. While dogs over 6 months of age and dogs that were only exposed to the exercises during the foundational training sessions took longer to learn the exercises, the limited number of exercises, repetition, and formalized criteria and progressions assisted their learning.FTW is also an accessible program for the people involved in assessing and developing working dog fitness. The feedback from trainers, interns, and volunteers on the foundational fitness training program centered on simplicity and ease of progression. Our personnel found the reduced number of movements and formalized structure simple to implement. The clarified criterion and standardized levels made determining when and how to progress the difficulty of a movement easy.The trainers, interns, and volunteers universally adopted the revised FTW program. The feedback we received identified an increase in perceived relevance to a working dog's future career as a major factor in this adoption. Also, the significantly reduced training requirements increased the confidence of trainers with less marker  training experience. We consistently observed trainers taking 2–3 weeks to become comfortable enough to teach an intern who then took the same length of time before teaching a volunteer. It took ~4–6 weeks for the FTW knowledge to pass through four generations (FTW expert to trainer to intern to volunteer) and make a novice competent to perform the training.We developed a formalized method to train and assess foundational fitness modalities for working dogs. The PVWDC FTW program incorporates posture development and frequent reinforcement of this behavior, warm-up and cool-down routines to prepare for and recover from training, methods to efficiently train foundational fitness, and a two-level format for assessing foundational fitness. We implemented this program in a working dog training facility and demonstrated the program's safety and ease of implementation.Some limitations to this initial implementation are the young age of some of the dogs in our population, the prior exposure of our dogs and personnel to fitness training, the unique structure of the PVWDC, and the short duration of the implementation period. The young age of some of the dogs in our program allowed us to determine optimal methods for early familiarization with fitness exercises but limited our ability to see how more mature dogs would fare. In addition, PVWDC has a culture of fitness, and our personnel were accustomed to performing fitness training with our dogs. These factors likely shortened the learning curve for both our dogs and our personnel. While not completely unique among working dog programs, our pattern of bringing dogs into our program at 8 weeks of age and training with them on a daily basis until 12–24 months of age provided more and earlier contact time than some programs. Finally, the formalized and quantitative methods described in this paper represent an implementation period of ~4 months. The number of refinements accomplished in that time indicate more are likely as we continue to train working dog fitness.The aim of this paper was to initially describe these pilot techniques to assess and train foundational fitness for working dogs. Thus, further prospective studies are needed to validate the assessment components and training exercises. Also, this initial implementation allowed us to define initial progression levels for some exercises . The progression for other exercises  is more generally described. Further work remains to be done to define the optimal method, sequence, and rate to progress each exercise.Formalized canine fitness programs are rare. Even fewer are suitable for the functional fitness requirements and temperaments of working dogs or the time and logistical constraints of a kennel or training facility. In contrast, the human fitness realm has numerous methods to assess and train fitness for tactical athletes in comparable organizations , 69–73. We anticipate this work will serve to add momentum to the growing field of canine performance medicine. One gap deserving future research is the quantification of muscle activity for various fitness exercises. While these investigations have been started with rehabilitation movements , much woIn addition to the exercises themselves, we believe research needs to be done to provide evidence for the optimum programming and periodization methods. For training sessions, the frequency, timing throughout the day, and timing relative to work or other training need to be explored. The proper order of exercises within the session along with the optimum combination of sets, repetitions, and intensities needs to be determined. The canine fitness realm also needs safe and effective methods for developing strength and power similar to the barbell and kettlebell for human fitness.Finally, formalized canine fitness must grow beyond these foundational fitness roots. Various working and sporting dog careers and disciplines have specific fitness requirements that should be layered on top of foundational fitness. Dogs need speed, power, endurance, and agility to perform in these careers, and their handlers and trainers need evidence-based training methods and assessments to help them improve those modalities.We believe multidisciplinary collaboration is the key to unlocking progress toward filling these gaps. Working dog handlers, experienced trainers, and canine performance-oriented veterinarians should partner with a diverse array of similarly-oriented scientists to solve these challenges. The PVWDC is eager to collaborate with like-minded individuals, kennels, programs, and organizations.The animal study was reviewed and approved by the University of Pennsylvania Institutional Animal Care and Use Committee.BF and MR developed the method, conducted the pilot implementation, and participated in manuscript preparation. CO developed the method and participated in manuscript preparation. All authors contributed to the article and approved the submitted version.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer LW declared a past co-authorship with one of the authors CO to the handling editor.