Effect of the position of immobilization upon the tensile properties of the rat gastrocnemius muscle.

This study compared the tensile properties of the atrophied gastrocnemius unit after immobilization in a shortened position (SP) and lengthened position (LP). The left hind legs of 52 male Wistar rats were fixed with plaster casts either with the knee in flexion and the ankle in extension, or vice versa. After seven to 21 days of immobilization the load-deformation curves and the weights of both gastrocnemius muscle-tendon units from each rat were evaluated. The initial length increased about 4% in the LP immobilized muscles and decreased about 5% in the SP immobilized muscles when compared to the contralateral controls. The decrease in force to failure averaged 21% in the LP and 31% in the SP immobilized muscles after one week and continued to about 26% and 48%, respectively, in three weeks. The corresponding elongation decreased 13% and 21% in one week and 17% and 36% at the end of the third week when immobilized in the LP and SP, respectively. The decrease in elastic stiffness was similar in both groups, but the loss of energy absorption capacity before failure was more prominent in SP fixed muscles (64%) than in LP fixed muscles (49%). The loss of weight of the muscle-tendon units due to the immobilization was greater in the SP (36%) than in the LP (12%). We conclude that immobilization of the rat gastrocnemius muscle-tendon unit in an SP is followed by a more marked muscle atrophy and a greater decrease in tensile properties than immobilization in an LP.