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

A new method has been 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 fibres remained at constant length. The afferent endings of muscle spindles were used to detect extension of the muscle fibres. In this null situation, when the spindles did not 'see' any movement, all of the applied movement was assumed to have been taken up in the tendinous components, and measurements of the movement and force allowed the stiffness to be calculated. Precautions were taken to avoid the effects of fusimotor stimulation. The stiffness of the entire tendinous component increased with increasing muscle force by approximately 2 N/mm per Newton mean force from 2 N/mm at low force to about 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 external tendon. Measurements of the dimensions of the tendon allowed Young's modulus for the tendon to be calculated. It increased from about 250 N/mm2 at 2.5 N to about 450 N/mm2 at 10 N mean force. Measurements of dissected muscles allowed comparisons to be made between the stiffness of the external tendon and the stiffness of the entire tendinous component in the muscles. Scaling of the stiffness of the external part of 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.