Mechanical and geometrical properties of the rat semimembranosus lateralis muscle during isometric contractions.

The orientation of the line of pull and geometrical characteristics of rat semimembranosus lateralis muscle (SM1) were measured in a plane during isometric contractions at different muscle lengths. The orientation of the line of pull was always similar in our plane of analysis. It was always aligned with the external tendon but not with the aponeurosis. Muscle and fiber length changes were similar. Aponeurosis length changes were very small. Angular changes of aponeurosis with respect to the line of pull were larger than those of muscle fibers. Functional implications of angular effects on muscle length changes were negligible in SM1. The muscle fiber was modelled as a slanted cylinder to test the hypothesis whether angular changes of fibers with the attachment area during muscle shortening are related to the constancy of fiber volume. Accommodation of the change in fiber cross-sectional area which accompanies fiber shortening was insufficient at the bony attachment area. It is concluded that during muscle shortening fiber volume displacement toward the bony origin is reduced as a smaller cross-sectional area needs to be accommodated. Angular changes of fibers with the proximal and distal part of the aponeurosis were so large that they cannot be related to the constancy of volume of a fiber with a homogeneous cross-sectional area. When angular changes of fibers are related to their length changes it is suggested that volume displacement and inhomogeneous length changes along the aponeurosis can explain the experimental results at the aponeurosis.