In situ observation of adjustment of sarcomere length in skeletal muscle under sustained stretch.

Adjustment of sarcomere length to sustained stretch was studied in situ by the light diffraction method applied to the extensor digitorum longus muscle of the mouse. The muscle was stretched maximally (with the ankle and toes in full plantar flexion) and immobilized by a plaster cast: the sarcomere length immediately after stretch was 2.98 microns. After 1, 3, 5, 7, and 14 days of immobilization, the sarcomere length was 2.94, 2.86, 2.83, 2.77 and 2.78 microns, respectively. This shows how a muscle under sustained stretch adapts itself to a new length and explains why the efficiency of muscle function is maintained after limb lengthening.