Reducing the metabolic cost of walking with an ankle exoskeleton: interaction between actuation timing and power.
BACKGROUND
Powered ankle-foot exoskeletons can reduce the metabolic cost of human walking to below normal levels, but optimal assistance properties remain unclear. The purpose of this study was to test the effects of different assistance timing and power characteristics in an experiment with a tethered ankle-foot exoskeleton.


METHODS
Ten healthy female subjects walked on a treadmill with bilateral ankle-foot exoskeletons in 10 different assistance conditions. Artificial pneumatic muscles assisted plantarflexion during ankle push-off using one of four actuation onset timings (36, 42, 48 and 54% of the stride) and three power levels (average positive exoskeleton power over a stride, summed for both legs, of 0.2, 0.4 and 0.5 W∙kg-1). We compared metabolic rate, kinematics and electromyography (EMG) between conditions.


RESULTS
Optimal assistance was achieved with an onset of 42% stride and average power of 0.4 W∙kg-1, leading to 21% reduction in metabolic cost compared to walking with the exoskeleton deactivated and 12% reduction compared to normal walking without the exoskeleton. With suboptimal timing or power, the exoskeleton still reduced metabolic cost, but substantially less so. The relationship between timing, power and metabolic rate was well-characterized by a two-dimensional quadratic function. The assistive mechanisms leading to these improvements included reducing muscular activity in the ankle plantarflexors and assisting leg swing initiation.


CONCLUSIONS
These results emphasize the importance of optimizing exoskeleton actuation properties when assisting or augmenting human locomotion. Our optimal assistance onset timing and average power levels could be used for other exoskeletons to improve assistance and resulting benefits.
['Galle|Samuel|S|0000-0002-1928-7764', 'Malcolm|Philippe|P|', 'Collins|Steven Hartley|SH|', 'De Clercq|Dirk|D|']
D000842:Ankle D001696:Biomechanical Phenomena D001701:Bionics D004576:Electromyography D000067190:Exoskeleton Device D005260:Female D005528:Foot D005684:Gait D064368:Healthy Volunteers D006801:Humans D007866:Leg D008660:Metabolism D018482:Muscle, Skeletal D011474:Prosthesis Design D012371:Robotics D016138:Walking D055815:Young Adult
Metabolic cost Optimal assistance Augmentation Human locomotion Lower-limb exoskeletons