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 d uring isometric contractions at different muscle lengths. The orientat ion of the line of pull was always similar in our plane of analysis. I t was always aligned with the external tendon but not with the aponeur osis. 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. Functiona l implications of angular effects on muscle length changes were neglig ible in SM1. The muscle fiber was modelled as a slanted cylinder to te st the hypothesis whether angular changes of fibers with the attachmen t area during muscle shortening are related to the constancy of fiber volume. Accommodation of the change in fiber cross-sectional area whic h accompanies fiber shortening was insufficient at the bony attachment area. It is concluded that during muscle shortening fiber volume disp lacement toward the bony origin is reduced as a smaller cross-sectiona l area needs to be accommodated. Angular changes of fibers with the pr oximal and distal part of the aponeurosis were so large that they cann ot be related to the constancy of volume of a fiber with a homogeneous cross-sectional area. When angular changes of fibers are related to t heir length changes it is suggested that volume displacement and inhom ogeneous length changes along the aponeurosis can explain the experime ntal results at the aponeurosis.