Passive muscle-tendon amplitude may not reflect skeletal muscle functional excursion

To quantify the gain in muscle mobility with progressive release of surrounding connective-tissue structures and to compare this property with the known architecture of each muscle.

Each of 5 different muscle tendon units (extensor carpi radialis brevis, extensor carpi radialis longus, flexor carpi ulnaris, flexor digitorum superficialis, pronator teres) was released from its insertion and secured into the jaws of a clamp attached to a servomotor that could be operated under length or force control to simulate the load placed on the tendon by a surgical assistant. A constant load of 5 N was applied to the tendon while the muscle-tendon unit was released surgically from the surrounding tissue in 1-cm increments. Mobility was plotted against release distance and analyzed by linear regression to yield mobility gain, the slope of the regression equation. One-way analysis of variance was used to compare mobility gain among muscles.

In contrast to previous results from the brachioradialis muscle in which the mobility gain was large and highly nonlinear, mobility gain was small, consistent, and linear for all muscles studied. The smallest mobility gain was for the flexor digitorum superficialis and was highly linear. The largest gain was for the pronator teres and again was highly linear. In general, the mobility gain for the extensor carpi radialis brevis was similar to that of the extensor carpi radial longus. The flexor carpi ulnaris muscle was difficult to mobilize, and its gain was modest. There was no significant correlation between mobility gain of the forearm muscles during progressive release and the length of their fibers.

The small mobility and complete lack of correlation with fiber length provide strong evidence that mobility gain does not accurately reflect muscle excursion as it is typically described. This calls into question the general practice of tensioning muscles by first passively extending the muscle and then choosing the attachment length as a particular portion of that passive relationship.