This paper attempts to explain the complex mechanical properties of the sarcolemma of single muscle fibers. Previously published measurements on retraction zones of frog semitendinosus fibers were used as a basis for computation. A somewhat slack system of helical fibers in the sarcolemma can adequately account for the observations. It explains the shape of the tension–elongation curves of sarcolemma and its anisotropy. It permits the drastic transfiguration from normal shape to retraction zone shape in the absence of undue stresses. The angle of the helices was calculated to be 55° in the muscle fiber at rest length. This is also the angle permitting maximal volume of the muscle fiber. The Young's modulus of the fiber system was calculated to be in approximate agreement with the modulus published for collagen. With the single additional assumption that muscle fiber volume has a certain value within the range permitted by the measurements, the helical system can be shown to limit muscle fiber length between 70% and 140% of its rest length. The proposed helical fiber system is thus found to have properties that make it a likely candidate for the parallel elastic element of muscle.