Hip joint position and architecture of rat semimembranosus muscle: implications for length-force characteristics.

Alteration of hip joint angle, while keeping the knee position constant, changes the orientation of the biarticular rat semimembranosus lateralis muscle with respect to the bony origin at the pelvis. Geometrical and length-force characteristics of the muscle were determined in two experimental groups, each with a different orientation: angles between the muscle and line of origin were set at 68.1 +/- 5.3 degrees (condition 68) and 121.9 +/- 9.1 degrees (condition 122). For muscles of condition 68 and condition 122, the number of sarcomeres in series was similar in both proximal fibers (11,436 +/- 356 and 11,555 +/- 175, respectively) and distal fibers (12,585 +/- 844 and 12,787 +/- 288, respectively). At similar distal fiber lengths, the sarcomere length in proximal fibers was higher in condition 122 than in condition 68, indicating a change in heterogeneity of fiber sarcomere lengths in the muscle. Hence, length-force relationships of proximal and distal sarcomeres were more scattered with respect to muscle length in condition 122 than in condition 68. Range of distal fiber length between optimum and zero muscle force was increased by 2.7 mm (13%) in condition 122. These data indicate that biarticular muscles with bony origins can exhibit different length-force characteristics at similar distal fiber lengths. During hip joint movements and fiber shortening, muscle fibers of such biarticular muscles use shifting fiber length-force relationships with respect to muscle length.