Elastic properties of the cat soleus tendon and their functional importance.

Abstract

A new method was used to measure the stiffness of the entire tendinous component of the soleus muscle of the cat. During sinusoidal stretching of the muscle-tendon combination, the motor nerves were stimulated repetitively in such a way that the force of contraction offset the movement, and the muscle fibers remained at constant length. The afferent endings of muscle spindles were used to detect extension of the muscle fibers. In this null situation, when the spindles did not register any movement, all of the applied movement may have been taken up in the tendinous components; measurements of movement and force allowed the stiffness to be calculated. Precautions were taken to avoid the effects of fusimotor stimulation. The stiffness of the entiure tendinous component inceased with increasing muscle force by .apprx. 2 N/mm per N mean force from 2 N/mm at low force to .apprx. 25 N/mm at 11 N; the method could not be used for larger forces. Independent measurements of the stiffness of the external part of the tendon were made by both static and dynamic methods. The entire tendinous component was much less stiff than the extermal tendon. Measurements of the dimensions of the tendon allowed Young''s modulus for the tendon to be calculated. It increased from .apprx. 250 N/mm2 at 2.5 N to .apprx. 450 N/mm2 at 10 N mean force. Measurements of dissected muscles allowed comparisons to be made between the stiffness of external tendon and the stiffness of the entire tendinous component in these muscles. Scaling of the stiffness of the external part of the tendon to the tendon to the length of the entire tendinous component gave a value of stiffness which was similar to that measured by the spindle null method. The compliance of tendons has implications for the control of movement which are discussed.