Fig. 6.

Internal cycling of energy in a muscle–tendon unit. (A) Muscle force, (B) fascicle power and (C) fascicle length in a fixed-end contraction (i.e. the muscle–tendon unit is isometric) illustrate the possible negative consequences of internal energy cycling. The mechanical work produced by the muscle is loaded into the tendon (positive power phase) and then released back into the muscle fascicles (negative power phase). No external work is performed, but fascicles produce and absorb work. This type of futile energy cycling could occur in vivo if the timing and magnitude of muscle activity is poorly matched to the kinetics and kinematics of movement or relative to the timing of release of a catch mechanism. Measurements were taken on frog plantaris muscle, using sonomicrometry to measure fascicle length and a servomotor to measure muscle force (Azizi and Roberts, 2010).