Stiffness of cat soleus muscle and tendon during activation of part of muscle

Experiments have been carried out on the soleus muscle and its tendon in the anesthetized cat. Measurements of isometric tension and muscle stiffness were made during contraction of whole or part of the muscle in response to stimulation of ventral root filaments. In an attempt to determine the distribution of tension in different portions of the tendon during activation of only part of the muscle, the free tendon of insertion was split longitudinally into two halves and a strain gauge attached to each piece. From a large number of measurements, it was found that the mean fraction of tension recorded in one-half of the tendon remained about the same, over a wide range of tensions. However, the scatter of values, which increased as the portion of muscle contracting was reduced, was greater than expected if muscle fibers were randomly distributed throughout the muscle. Measurements of muscle and tendon stiffness were made from length and tension changes during stretch of the actively contracting muscle. Ventral root stimulation that engaged 20% or more of the muscle yielded a value for tendon compliance (0.09 mm/N), which was the same as for stimulating the whole muscle. This result suggested that for contraction of portions as small as 20% of the muscle, fibers were effectively attached to the whole tendon, indicating that tendinous attachments of individual muscle fibers ran independent of one another over only a short distance and were bound together over most of their remaining course. It was concluded that groups of muscle fibers selected by stimulation of ventral root filaments are not entirely randomly distributed throughout the muscle. However, for groups representing larger fractions of the total tension, (greater than 20%) the distribution is uniform enough and the connections between their tendinous attachments firm enough for the force applied by such a group to act through a tendon compliance, which is the same as that seen by the whole muscle.