We have established that the sarcolemma of frog skeletal muscle is so firmly tied in at each sarcomere level near the M line, as well as near the Z line, that it is thrown into folds or festoons when the fibers shorten. The attachment is not broken even when the fibers shorten to 25% of optimum tension length. Such extreme shortening affects both the morphology and physiology of the muscle; the morphological change seems to be limited to the myofilaments. The physiological effects in frog sartorius muscle include an increase in resting oxygen consumption and changes in the relation between fiber length and isometric tension similar to those found in isolated muscle fibers
