The relation between intrinsic and steady-state stiffnesses in active human muscle

Measurements of the responses to oscillatory perturbations of the elbow flexors and extensors in normal adult male human subjects show that these muscles behave much like elastic springs at frequencies above 10 Hz, with a stiffness independent of frequency and a small phase shift between muscle moment and forearm displacement. The stiffness of this ´spring´, however, is only 40% to 50% of the muscle stiffness measured at the onset of an oscillatory perturbation. These observations do not support the traditional A.V. Hill model of muscle.<>