Adaptation of skeletal muscle in limb lengthening: a light diffraction study on the sarcomere length in situ.

Adaptation of skeletal muscle during limb lengthening was assessed by measurement of the length of sarcomeres in situ. An external fixator was applied to the rabbit radius and ulna to elongate the forelimb by 3.5 mm, while allowing all the joints to be free. At regular intervals after the operation, the extensor digitorum lateralis muscle of the fifth digit was exposed in situ and the length of the sarcomeres was measured by a laser diffraction technique. The sarcomeres, which had stretched to 3.51 microns immediately after elongation of the bone, became shorter with the passage of time. On postoperative day 9, the length was 3.10 microns, which was similar to the length of the unstretched muscle. These results indicated structural adaptation of the muscle to a new length and could explain why the efficiency of muscle function is maintained after limb lengthening. When these findings are combined with our previous results, it appears that stretch-induced changes in sarcomere length are common in immobilized and nonimmobilized muscles.