The musculoskeletal system of humans is not tuned to maximize the economy of locomotion

TLDR

Humans exhibit energetically optimal walking and running speeds, and this pattern may have evolved to broaden sustainable running ranges or improve non‑endurance motor performance. The study aimed to determine whether individual locomotor muscles have optimal speeds and whether these muscles are more finely tuned to walking than running speeds. Electromyography of 13 back and leg muscles across various walking and running speeds was used to compute each muscle’s cumulative activity per kilometer. Each muscle’s activity was minimized at specific speeds, yet no muscle was tuned to a particular walking or running speed, indicating that human locomotor muscles do not maximize locomotion economy.

Abstract

Humans are known to have energetically optimal walking and running speeds at which the cost to travel a given distance is minimized. We hypothesized that "optimal" walking and running speeds would also exist at the level of individual locomotor muscles. Additionally, because humans are 60-70% more economical when they walk than when they run, we predicted that the different muscles would exhibit a greater degree of tuning to the energetically optimal speed during walking than during running. To test these hypotheses, we used electromyography to measure the activity of 13 muscles of the back and legs over a range of walking and running speeds in human subjects and calculated the cumulative activity required from each muscle to traverse a kilometer. We found that activity of each of these muscles was minimized at specific walking and running speeds but the different muscles were not tuned to a particular speed in either gait. Although humans are clearly highly specialized for terrestrial locomotion compared with other great apes, the results of this study indicate that our locomotor muscles are not tuned to specific walking or running speeds and, therefore, do not maximize the economy of locomotion. This pattern may have evolved in response to selection to broaden the range of sustainable running speeds, to improve performance in motor behaviors not related to endurance locomotion, or in response to selection for both.