The Effects of Adding Mass to the Legs on the Energetics and Biomechanics of Walking

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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

Section: Discussionmentioning

confidence: 99%

Mechanics and energetics of level walking with powered ankle exoskeletons

Sawicki

Ferris

2008

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“…However, our findings imply that the reduction of muscle work may not necessarily result in metabolic energy savings, and suggest that the energetic benefits of long, compliant tendons may largely be due to the reduction in muscle fibre length they allow over evolutionary time. Shorter muscle fibres not only reduce the cost of force generation due to a reduction in muscle volume to cross-sectional area ratio (Roberts et al, 1998), but also concentrate muscle mass proximally within the limb, thereby reducing inertia and therefore swing costs (Steudel, 1990;Marsh et al, 2006;Browning et al, 2007;Tickle et al, 2010). Hence, we propose that the energetic benefits of short muscle fibres, rather than reduced muscle work, drove the evolution of long, compliant tendons in the distal limbs of cursorial species.…”

Section: Implications For Our Understanding Of the Energetic Benefitsmentioning

confidence: 99%

The energetic benefits of tendon springs in running: is the reduction of muscle work important?

Holt

Roberts

Askew

2014

The distal muscle-tendon units of cursorial species are commonly composed of short muscle fibres and long, compliant tendons. It is assumed that the ability of these tendons to store and return mechanical energy over the course of a stride, thus avoiding the cyclic absorption and regeneration of mechanical energy by active muscle, offers some metabolic energy savings during running. However, this assumption has not been tested directly. We used muscle ergometry and myothermic measurements to determine the cost of force production in muscles acting isometrically, as they could if mechanical energy was stored and returned by tendon, and undergoing active stretch-shorten cycles, as they would if mechanical energy was absorbed and regenerated by muscle. We found no detectable difference in the cost of force production in isometric cycles compared with stretch-shorten cycles. This result suggests that replacing muscle stretch-shorten work with tendon elastic energy storage and recovery does not reduce the cost of force production. This calls into question the assumption that reduction of muscle work drove the evolution of long distal tendons. We propose that the energetic benefits of tendons are derived primarily from their effect on muscle and limb architecture rather than their ability to reduce the cyclic work of muscle.

“…Taxa with more lightweight and distally light legs should possess lower locomotor costs and may attain higher speeds (Lull, 1904;Gregory, 1912;Howell, 1944;Smith and Savage, 1956;Gray, 1968;Gambaryan, 1974;Hildebrand, 1984Hildebrand, , 1985Hildebrand, , 1988Hildebrand and Hurley, 1985;Steudel, 1990;Wickler et al, 2004;Raichlen, 2005Raichlen, , 2006Browning et al, 2007). In contrast, taxa with more massive and distally heavy legs should incur higher locomotor costs or might be restricted to slower speeds of terrestrial locomotion (e.g.…”

Section: Introductionmentioning

confidence: 99%

Morphology and motion: hindlimb proportions and swing phase kinematics in terrestrially locomoting charadriiform birds

Kilbourne

Andrada

Fischer

et al. 2016

Differing limb proportions in terms of length and mass, as well as differences in mass being concentrated proximally or distally, influence the limb's moment of inertia (MOI), which represents its resistance to being swung. Limb morphology -including limb segment proportions -thus probably has direct relevance for the metabolic cost of swinging the limb during locomotion. However, it remains largely unexplored how differences in limb proportions influence limb kinematics during swing phase. To test whether differences in limb proportions are associated with differences in swing phase kinematics, we collected hindlimb kinematic data from three species of charadriiform birds differing widely in their hindlimb proportions: lapwings, oystercatchers and avocets. Using these three species, we tested for differences in maximum joint flexion, maximum joint extension and range of motion (RoM), in addition to differences in maximum segment angular velocity and excursion. We found that the taxa with greater limb MOI -oystercatchers and avocets -flex their limbs more than lapwings. However, we found no consistent differences in joint extension and RoM among species. Likewise, we found no consistent differences in limb segment angular velocity and excursion, indicating that differences in limb inertia in these three avian species do not necessarily underlie the rate or extent of limb segment movements. The observed increased limb flexion among these taxa with distally heavy limbs resulted in reduced MOI of the limb when compared with a neutral pose. A trade-off between exerting force to actively flex the limb and potential savings by a reduction of MOI is skewed towards reducing the limb's MOI as a result of MOI being in part a function of the radius of gyration squared. Increased limb flexion is a likely means to lower the cost of swinging the limbs.