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Deformation of the surface of passive medial gastrocnemius muscle (MG) was measured in vivo while performing a hysteresis test. The gastrocnemius muscle of male rats were dissected free and the distal tendon was cut. The lateral head was separated from the medial head. The muscle origins were left intact. 60-70 fluorescent, polystyrene spheres (diameter 0.7 mm) were attached to the surface of the MG. During the experiment, two-dimensional video recordings of the movements of the MG were made. The coordinates of the marker centroids were obtained by computer processing of digitized images and marker displacements as a function of time were calculated. Green-Lagrange strains in two principal directions were calculated (epsilon 1, epsilon 2) for three specimens. epsilon 1 had approximately the same direction as the muscle fibers. The longitudinal strain of the fibers (20-30%) was larger than the strain of the aponeurosis (1-5%); p0.001. No significant difference was found between the values of the transverse strains of muscle fibers and aponeurosis; the value of epsilon 2 was -6 to -9% for both tissue structures.

This study evaluates the chunking hypothesis in the context of two-element, reciprocal aiming movements. The chunking hypothesis assumes that movements toward small targets require a movement stop while movements toward large targets require a movement reversal . According to the chunking hypothesis, a movement reversal allows for a functional coupling or linkage between the forward and backward movement such that braking energy generated by antagonist activity in the forward movement is stored as elastic energy and re-utilized as acceleration energy in the backward movement (Guiard, 1993). An experiment is reported which examined movement kinematics of left-right reversal movements using four different target combinations; small-small, small-large, large-large, and large-small, for left and right targets, respectively. The results accommodated predictions derived from the chunking hypothesis. Several implications for motor control theory are discussed.