We examined healthy human subjects wearing robotic ankle exoskeletons to study the metabolic cost of ankle muscle–tendon work during uphill walking. The exoskeletons were powered by artificial pneumatic muscles and controlled by the user’s soleus electromyography. We hypothesized that as the demand for net positive external mechanical work increased with surface gradient, the positive work delivered by ankle exoskeletons would produce greater reductions in users’ metabolic cost. Nine human subjects walked at 1.25ms–1 on gradients of 0%, 5%, 10% and 15%. We compared rates of O2 consumption and CO2 production, exoskeleton mechanics, joint kinematics, and surface electromyography between unpowered and powered exoskeleton conditions. On steeper inclines, ankle exoskeletons delivered more average positive mechanical power (P<0.0001; +0.37±0.03Wkg–1 at 15% grade and +0.23±0.02Wkg–1 at 0% grade) and reduced subjects’ net metabolic power by more (P<0.0001; –0.98±0.12Wkg–1 at 15% grade and –0.45±0.07Wkg–1 at 0% grade). Soleus muscle activity was reduced by 16–25% when wearing powered exoskeletons on all surface gradients (P<0.0008). The ‘apparent efficiency’ of ankle muscle–tendon mechanical work decreased from 0.53 on level ground to 0.38 on 15% grade. This suggests a decreased contribution from previously stored Achilles’ tendon elastic energy and an increased contribution from actively shortening ankle plantar flexor muscle fibers to ankle muscle–tendon positive work during walking on steep uphill inclines. Although exoskeletons delivered 61% more mechanical work at the ankle up a 15% grade compared with level walking, relative reductions in net metabolic power were similar across surface gradients (10–13%). These results suggest a shift in the relative distribution of mechanical power output to more proximal (knee and hip) joints during inclined walking.

(далее…)

Категория: Научные статьи | Нет комментариев »

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% (1.00ms–1), 100% (1.25ms–1), 120% (1.50ms–1) and 140% (1.75ms–1) of their preferred step length (L*) at 1.25ms–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 (Wkg–1) at longer step lengths for a constant step frequency (i.e. preferred at 1.25ms–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±0.02Wkg–1 to +0.25±0.02Wkg–1, respectively). The exoskeletons reduced net metabolic power by more at longer step lengths (P=0.002; –0.21±0.06Wkg–1 at 0.8 L* and –0.70±0.12Wkg–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.

(далее…)

Категория: Научные статьи | Нет комментариев »