Contribution of series elasticity in human cyclic heel-raise exercise

Contribution of series elasticity of the human mm. triceps surae in cyclic heel-raise exercise (similar to hopping but the feet do not leave the floor) was examined via computer modeling and simulation. A two-dimensional skeletal model of the human body was constructed. Upright posture was maintained throughout the simulation to prevent the model from falling. A mathematical representation of the mm. triceps surae was implemented in the skeletal model. The muscle was activated by the neural activation input signal with a time resolution of 0.050 sec. Cyclic heel-raise exercises of cycle duration ranging from 0.300 sec to 0.900 sec, corresponding to the motion frequency of 200 to 66.7 cycles/min, were generated using an optimization approach. The goal of the numerical optimization was to generate cyclic motions with as much range of motion as possible. As a result, realistic heel-raise motions were generated with the range of motion between 0.0023 m (cycle duration = 0.300 sec) and 0.0414 m (cycle duration = 0.900 sec). It was found that contribution of the series elasticity in positive mechanical work output of the muscle-tendon complex during the pushoff phase (from the lowest position to the termination of a cycle) increased as motion frequency increased (3% at 66.7 cycles/min to 47% at 200 cycles/min). Relatively higher muscle activation was found during the downward moving phase when the motion frequency was higher. These tendencies are consistent with the findings reported in preceding studies involving experimental animals as well as human participants. It is suggested that series elasticity plays an integral role in the generation of cyclic human motions.