The purpose of this study was to investigate the possible mechanisms underlying residual force enhancement in skeletal muscle Force enhancement is defined as an increase in the steady-state force following active stretch compared to the purely isometric force at the same length. Whole cat soleus muscle was chosen as the preparation for this study. Three novel results were found: (1) contrary to literature, force enhancement did not increase with stretch magnitude, but reached a plateau for moderate stretches at long muscle lengths; (2) force enhancement was associated with an increase in passive forces at all muscle lengths; and (3) force enhancement was linearly related to increases in stiffness. These results suggest that residual force enhancement in the cat soleus is caused by a passive component, possibly a change in "free" length of the molecular spring titin, and an active component associated with an increase in the proportion of attached cross-bridges.