Powered ankle exoskeletons reveal the metabolic cost of plantar flexor mechanical work during walking with longer steps at constant step frequency

Abstract

SUMMARYWe examined the metabolic cost of plantar flexor muscle-tendon mechanical work during human walking. Nine healthy subjects walked at constant step frequency on a motorized treadmill at speeds corresponding to 80% (1.00 m s . In each condition subjects donned robotic ankle exoskeletons on both legs. The exoskeletons were powered by artificial pneumatic muscles and controlled using soleus electromyography (i.e. proportional myoelectric control). We measured subjects' metabolic energy expenditure and exoskeleton mechanics during both unpowered and powered walking to test the hypothesis that ankle plantarflexion requires more net metabolic power (W kg Declining ankle muscle-tendon 'apparent efficiency' suggests an increase in ankle plantar flexor muscle work relative to Achilles' tendon elastic energy recoil during walking with longer steps. However, previously stored elastic energy in Achilles' tendon still probably contributes up to 34% of ankle muscle-tendon positive work even at the longest step lengths we tested. Across the range of step lengths we studied, the human ankle muscle-tendon system performed 34-40% of the total lower-limb positive mechanical work but accounted for only 7-26% of the net metabolic cost of walking. 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.…”