The behaviour of a mammalian muscle during sinusoidal stretching.

1. Tension changes have been recorded in tetanized and passive cat soleus muscles during sinusoidal stretching at frequencies between 0.25 and 12.5 c/s.2. During sinusoidal stretching the muscle tension varied in an approximately sinusoidal manner. The tension record showed an angular advance on muscle length. A method of measuring mean angular advance in nonlinear systems is described.3. In tetanized muscle, resistance to lengthening increased with increasing frequency of stretching. With increasing frequency the elastic stiffness increased more rapidly than the viscous stiffness, so that at the higher frequencies the angular separation between length and tension became smaller.4. With increasing stretch amplitude the elastic stiffness of the tetanized muscle declined, and the mean angular advance of tension became larger.5. In passive muscle the viscous stiffness increased at the higher frequencies, and the mean angular advance of tension on length was correspondingly increased.6. The functional significance of these findings and their relation to the findings of other workers are discussed.