A macroscopic description of stress relaxation in a muscle subject to stepwise deformation

Stress relaxation regimes arising in a muscle subject to stepwise deformation are described on the basis of a recent phenomenological model of fully activated muscle tissue which is presented in the form of a second-order constitutive equation. It is shown that this model reproduces the qualitative form of relaxation curves observed experimentally. Relations between rheological parameters which correspond to different types of stress relaxation are found for the case where the jump duration is much smaller than the relaxation times of the sample. As illustrated by the simplest model of a slow length jump (linear deformation), the qualitative form of the stress relaxation depends on the jump duration in this case. This effect can lead to rough errors in determination of rheological and molecular parameters of muscle tissue in mechanical experiments in which the relation between jump duration and relaxation times is not controlled.