Influence of residual force enhancement and elongation of attached cross-bridges on stretch-shortening cycle in skinned muscle fibers

Increased muscle force during stretch-shortening cycles (SSCs) has been widely examined. However, the mechanisms causing increased muscle force in SSCs remain unknown. The purpose of this study was to determine the influence of residual force enhancement and elongation of attached cross-bridges on the work enhancement in SSCs. For the Control condition, skinned rabbit soleus fibers were elongated passively from an average sarcomere length of 2.4 to 3.0 mu m, activated and then actively shortened to 2.4 mu m. For the Transition condition, fibers were elongated actively from an average sarcomere length of 2.4 to 3.0 mu m. Two seconds after the end of active lengthening, fibers were actively shortened to 2.4 mu m. In the SSC condition, fibers were lengthened actively from an average sarcomere length of 2.4 to 3.0 mu m, and then immediately shortened actively to 2.4 mu m. Increased muscle force in the SSCs was quantified by the increase in mechanical work during active shortening compared to the mechanical work measured during the purely active shortening contractions. Work enhancement was significantly greater in the SSC compared to the Transition conditions. This difference was associated with the pause given between the active lengthening and shortening phase in the Transition test, which likely resulted in a reduction of the average elongation of the attached cross-bridges caused by active stretching. Since some work enhancement was still observed in the Transition condition, another factor, for example the stretch-induced residual force enhancement, must also have contributed to the work enhancement in SSCs.