Changes in tendon compliance and muscle energetics of in vivo human skeletal muscle

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The study of animal locomotion has fascinated scientists for centuries. Locomotion involves repetitive muscle contractions and stretching-shortening cycles of tendons. It has generally been accepted by scientists for generations that the primary role of tendons during locomotion is the storage and release of elastic energy. This storage and return of energy has been thought to keep the energy cost of locomotion low, across a range of species, including humans. During locomotion, the tendons are stretched, storing elastic energy; a portion of this energy can be returned, thus contributing to a low energy cost. Previously, it has been suggested the storage and return of elastic energy by tendons provide a significant portion of the energy for locomotion; however, the muscle energy cost required to store this elastic energy has, until recently, been completely ignored. Furthermore, changes in tendon properties with exercise, disuse, injury and its relation to metabolic cost has not been explored. Elucidating a greater understanding of muscle-tendon function, and accompanying energy cost during locomotion has important scientific implications for human motion, mobility, functional tolerance and athletic performance. Here, I propose to study the interaction between muscle energy cost and the energy returned from elastic tendons in vivo. I further propose a new paradigm in the role of tendons in keeping metabolic energy low: tendons, in-series with contracting muscles serve to reduce muscle energy cost by reducing the magnitude and velocity of muscle shortening. The proposed experiments will provide a greater understanding of the impact of in vivo muscle shortening on the metabolic energy cost of locomotion and the role of tendons in doing so. As such, my proposed research program aims to offer new insights into the energetics and mechanics of human locomotion.