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

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Abstract
We 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–1), 100% (1.25 m s–1), 120% (1.50 m s–1) and 140% (1.75 m s–1) of their preferred step length (L*) at 1.25 m s–1. 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–1) at longer step lengths for a constant step frequency (i.e. preferred at 1.25 m s–1). As step length increased from 0.8 L* to 1.4 L*, exoskeletons delivered ∼25% more average positive mechanical power (P=0.01; +0.20±0.02 W kg–1 to +0.25±0.02 W kg–1, respectively). The exoskeletons reduced net metabolic power by more at longer step lengths (P=0.002;– 0.21±0.06 W kg–1 at 0.8 L* and– 0.70±0.12 W kg–1 at 1.4 L*). For every 1 J of exoskeleton positive mechanical work subjects saved 0.72 J of metabolic energy (`apparent efficiency'=1.39) at 0.8 L* and 2.6 J of metabolic energy (`apparent efficiency'=0.38) at 1.4 L*. 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.