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First published online April 18, 2008
Journal of Experimental Biology 211, 1402-1413 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.009241

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Mechanics and energetics of level walking with powered ankle exoskeletons

Gregory S. Sawicki^1,2,3,* and Daniel P. Ferris^1,2,4,5

¹ Human Neuromechanics Laboratory, University of Michigan-Ann Arbor, Ann Arbor, MI 48109, USA
² Department of Movement Science, University of Michigan-Ann Arbor, Ann Arbor, MI 48109, USA
³ Department of Mechanical Engineering, University of Michigan-Ann Arbor, Ann Arbor, MI 48109, USA
⁴ Department of Biomedical Engineering, University of Michigan-Ann Arbor, Ann Arbor, MI 48109, USA
⁵ Department of Physical Medicine and Rehabilitation, University of Michigan-Ann Arbor, Ann Arbor, MI 48109, USA

^* Author for correspondence (e-mail: gsawicki{at}umich.edu)

Accepted 19 February 2008

Robotic lower limb exoskeletons that can alter joint mechanical power output are novel tools for studying the relationship between the mechanics and energetics of human locomotion. We built pneumatically powered ankle exoskeletons controlled by the user's own soleus electromyography (i.e. proportional myoelectric control) to determine whether mechanical assistance at the ankle joint could reduce the metabolic cost of level, steady-speed human walking. We hypothesized that subjects would reduce their net metabolic power in proportion to the average positive mechanical power delivered by the bilateral ankle exoskeletons. Nine healthy individuals completed three 30 min sessions walking at 1.25 m s^–1 while wearing the exoskeletons. Over the three sessions, subjects' net metabolic energy expenditure during powered walking progressed from +7% to –10% of that during unpowered walking. With practice, subjects significantly reduced soleus muscle activity (by 28% root mean square EMG, P<0.0001) and negative exoskeleton mechanical power (–0.09 W kg^–1 at the beginning of session 1 and –0.03 W kg^–1 at the end of session 3; P=0.005). Ankle joint kinematics returned to similar patterns to those observed during unpowered walking. At the end of the third session, the powered exoskeletons delivered 63% of the average ankle joint positive mechanical power and 22% of the total positive mechanical power generated by all of the joints summed (ankle, knee and hip) during unpowered walking. Decreases in total joint positive mechanical power due to powered ankle assistance (22%) were not proportional to reductions in net metabolic power (10%). The `apparent efficiency' of the ankle joint muscle–tendon system during human walking (0.61) was much greater than reported values of the `muscular efficiency' of positive mechanical work for human muscle (0.10–0.34). High ankle joint `apparent efficiency' suggests that recoiling Achilles' tendon contributes a significant amount of ankle joint positive power during the push-off phase of walking in humans.

Key words: locomotion, walking, metabolic cost, exoskeletons, ankle, human, efficiency, inverse dynamics, joint power

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