The series elastic shock absorber: Tendon elasticity modulates energy dissipation by muscle during burst deceleration

Abstract

During downhill running, manoeuvring, negotiation of obstacles and landingsfrom a jump, mechanical energy is dissipated via active lengthening oflimb muscles. Tendon compliance provides a ‘shock-absorber’ mechanism thatrapidly absorbs mechanical energy and releases it more slowly as the recoil ofthe tendon does work to stretch muscle fascicles. By lowering the rate of muscularenergy dissipation, tendon compliance likely reduces the risk of muscleinjury that can result from rapid and forceful muscle lengthening. Here, weexamine how muscle–tendon mechanics are modulated in response tochanges in demand for energy dissipation. We measured lateral gastrocnemius(LG) muscle activity, force and fascicle length, as well as leg jointkinematics and ground-reaction force, as turkeys performed drop-landingsfrom three heights (0.5–1.5 m centre-of-mass elevation). Negative work bythe LG muscle–tendon unit during landing increased with drop height,mainly owing to greater muscle recruitment and force as drop heightincreased. Although muscle strain did not increase with landing height,ankle flexion increased owing to increased tendon strain at higher muscleforces. Measurements of the length–tension relationship of the muscle indicatedthat the muscle reached peak force at shorter and likely safer operatinglengths as drop height increased. Our results indicate that tendon complianceis important to the modulation of energy dissipation by activemuscle with changes in demand and may provide a mechanism for rapidadjustment of function during deceleration tasks of unpredictable intensity.