The mechanical response of the active human triceps brachii muscle to very rapid stretch and shortening.

Very rapid, small amplitude, ramp-and-hold rotations were imposed on the braced forearms of three normal adult male subjects who were isometrically contracting their elbow extensors. By carefully accounting for inertial and viscoelastic coupling effects in the experimental system it was possible to compute the time course of the muscle-moment evoked by these mechanical perturbations. The muscle-moment responses, and their dependence on rotation amplitude and direction, as well as tonic contraction level, are described. These responses are also compared to the predictions of a simple muscle model which we have proposed previously on the basis of frequency-response tests. The results indicate that: at a given tonic contraction level, triceps may be stiffer in an isometric state than in an oscillatory steady state, and high frequency fluctuations in the myoelectric activity are very ineffective in generating corresponding muscle-force fluctuations.