Mechanics and energetics of level walking with powered ankle exoskeletons

Abstract

SUMMARYRobotic 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. Supplementary material available online at

Enhancing performance during inclined loaded walking with a powered ankle–foot exoskeleton

“…Although we did not measure exoskeleton mechanical power during the exercise test, the values of ~0.12 and ~0.13 W•kg -1 per leg that were collected on a separate day are in line with the results of other studies on exoskeleton walking (Malcolm et al 2013;Mooney et al 2014;Sawicki and Ferris 2008, 2009a, 2009b. The muscular efficieny of positive joint power during steep uphill locomotion is assumed to be ~0.25 (Margaria 1976 Much attention is paid to loaded walking in a military context as this can determine mission success (Knapik et al 2012).…”

“…However, we see a role for biomechanists and physiologists in studying the human-exoskeleton interaction without being concerned about technical or practical limitations. While this allows to study more fundamental topics (Sawicki and Ferris 2008, 2009a, 2009b, specific knowledge that results from these studies, e.g. on optimal actuation timing (Malcolm et al 2013), can also be used in the development of autonomous exoskeletons (Mooney et al 2014).…”

“…While most research is focused on technical enhancements, a quantitative evaluation of the effectiveness is often missing (Dollar and Herr 2008). The metabolic energy expenditure, often calculated as metabolic power (W•kg -1 ) based on oxygen consumption and carbon dioxide using a standard equation (Brockway 1987) and body weight normalization, is a key value in the evaluation of several exoskeleton devices (Galle et al 2013a;Malcolm et al 2013;Mooney et al 2014;Norris et al 2007a;Sawicki and Ferris 2008, 2009a, 2009bWehner et al 2013). Regardless of the functional goal of the device, reducing the metabolic power will improve the usability of the exoskeleton (Ferris et al 2007) and can therefore be considered a prime outcome when evaluating exoskeleton effectiveness, that can even be used to drive kinematic behavior with exoskeletons (Collins and Jackson 2013).…”

“…Walking with powered exoskeletons with pneumatic muscles that assist plantarflexion during push-off results in reductions in metabolic power of 10 to 17% compared to walking with an unpowered exoskeleton (without plantarflexion assistance) (Galle et al 2013a;Malcolm et al 2013;Norris et al 2007a; Sawicki and Ferris 2008, 2009a, 2009b. Despite the increased weight of the device, Malcolm et al (2013) were the first to report a 6% reduction in metabolic power for powered exoskeleton walking compared to walking with normal shoes if the actuation timing of the exoskeleton was optimal.…”

“…However, we are not aware of any succesfull attempts in increasing walking performance during maximal exercise intensities as current research is mainly focused on submaximal intensities (Galle et al 2013a;Malcolm et al 2013;Mooney et al 2014;Norris et al 2007a;Sawicki and Ferris 2008, 2009a, 2009b. Studying higher intensities is also useful for applications in specific populations (e.g.…”