The purpose of this study was to choose between two popular models of skeletal muscle; the parallel elastic component is parallel with both the contractile element and the series elastic component (model A), or is parallel with only the contractile element (model B). Passive and total forces were obtained at a variety of muscle lengths for the medial gastrocnemius muscle in anesthetized rats. Passive force was measured prior to the contraction (passive A), or was estimated for the fascicle length (measured with sonomicrometry) at which peak total force occurred (passive B). Active force was peak total force minus passive force (A or B) at each fascicle or muscle length. Optimal length, that fascicle length at which active force is maximized, was 13.1±1.2 mm with passive A , and 14.0±1.1 mm with passive B (p<0.01). Furthermore, the relationship between double-pulse contraction force and length was broader when calculated with passive B than with passive A. When the muscle was held at a long length, passive force decreased due to stress-relaxation, with no change in fascicle length at the peak of the contraction and a smaller corresponding decrease in peak total force. There is no explanation for the apparent increase in active force that would be obtained when subtracting passive A from the peak total force. Therefore, to calculate active force, it is appropriate to subtract passive force measured at the fascicle length corresponding to the length at which peak total force occurs, not passive force measured at the length at which the contraction begins.