The effect of mono- versus multi-segment musculoskeletal models of the foot on simulated triceps surae lengths in pathological and healthy gait.
BACKGROUND
Estimating muscle-tendon complex (MTC) lengths is important for planning of soft tissue surgery and evaluating outcomes, e.g. in children with cerebral palsy (CP). Conventional musculoskeletal models often represent the foot as one rigid segment, called a mono-segment foot model (mono-SFM). However, a multi-segment foot model (multi-SFM) might provide better estimates of triceps surae MTC lengths, especially in patients with foot deformities.


RESEARCH QUESTION
What is the effect of a mono- versus a multi-SFM on simulated ankle angles and triceps surae MTC lengths during gait in typically developing subjects and in children with CP with equinus, cavovarus or planovalgus foot deformities?


METHODS
50 subjects were included, 10 non-affected adults, 10 typically developing children, and 30 children with spastic CP and foot deformities. During walking trials, marker trajectories were collected for two marker models, including a mono- and multi-segment foot; respectively Newington gait model and Oxford foot model. Two musculoskeletal lower body models were constructed in OpenSim with either a mono- or multi-SFM based on the corresponding marker models. Normalized triceps surae MTC lengths (soleus, gastrocnemius medialis and lateralis) and ankle angles were calculated and compared between models using statistical parametric mapping RM-ANOVAs. Root mean square error values between simulated MTC lengths were compared using Wilcoxon signed-rank and rank-sum tests.


RESULTS
Mono-SFM simulated significantly more ankle dorsiflexion (7.5 ± 1.2°) and longer triceps surae lengths (difference; soleus:2.6 ± 0.29 %, gastrocnemius medialis:1.7 ± 0.2 %, gastrocnemius lateralis:1.8 ± 0.2%) than a multi-SFM. Differences between models were larger in children with CP compared to typically developing children and larger in the stance compared to the swing phase of gait. Largest differences were found in children with CP presenting with planovalgus (4.8 %) or cavovarus (3.8 %) foot deformities.


SIGNIFICANCE
It is advisable to use a multi-SFM in musculoskeletal models when simulating triceps surae MTC lengths, especially in individuals with planovalgus or cavovarus foot deformities.
['Zandbergen|Marit A|MA|', 'Schallig|Wouter|W|', 'Stebbins|Julie A|JA|', 'Harlaar|Jaap|J|', 'van der Krogt|Marjolein M|MM|']
D000328:Adult D000843:Ankle Joint D001696:Biomechanical Phenomena D002547:Cerebral Palsy D002648:Child D005260:Female D005528:Foot D005529:Foot Bones D005532:Foot Deformities, Congenital D005684:Gait D020233:Gait Disorders, Neurologic D006801:Humans D008297:Male D008953:Models, Anatomic D018482:Muscle, Skeletal D016059:Range of Motion, Articular D013710:Tendons D016138:Walking
Cerebral palsy Foot model OpenSim Biomechanics Gastrocnemius muscle