Evidence for muscle attachment at relatively long lengths in tendon transfer surgery

Sarcomere length was measured intraoperatively during 22 tendon transfers about the wrist primarily involving the flexor carpi ulnaris (15), but also including the extensor carpi radialis longus (4) and brachioradialis (3). Muscle tension during transfer was chosen based on traditional guidelines suggested for optimal function. After these criteria were used, it was determined that sarcomere lengths were consistently much longer than optimal lengths, even to the point of resulting in zero active tension generation. Average sarcomere length after transfer was 3.78 ± 0.52 μm (mean ± SD), which was significantly different than optimal sarcomere length (2.8 μm) in human skeletal muscle. Muscles were predicted to generate on average-on the basis of the 3.78-μm sarcomere length-only 28% of maximum active force. This appeared to be due to the fact that passive tension in upper-extremity skeletal muscles becomes important at only relatively long lengths. It is suggested that the use of passive tension as the major factor guiding intraoperative decision making results in overstretch of the muscle- tendon unit and accompanying low active force generation.