Dynamic force responses of skeletal muscle during stretch-shortening cycles

Muscle damage due to stretch-shortening cycles (i.e., cyclic eccentric/concentric muscle actions) is one of the major concerns in sports and occupational related activities. Mechanical responses of whole muscle have been associated with damage in neural motor units, in connective tissues, and the force generation mechanism. The objective of this study was to introduce a new method to quantify the real-time changes in skeletal muscle forces of rats during injurious stretch-shortening cycles. Male Sprague Dawley rats (n=24) were selected for use in this study. The dorsi flexor muscle group was exposed to either 150 stretch-shortening cycles (n=12) or 15 isometric contractions (n=12) in vivo using a dynamometer and electrical stimulation. Muscle damage after exposure to stretch-shortening cycles was verified by the non-recoverable force deficit at 48 h and the presence of myofiber necrosis. Variations of the dynamic forces during stretch-shortening cycles were analyzed by decomposing the dynamic force signature into peak force (F-peak), minimum force (F-min), average force (F-mean), and cyclic force (F-a). After the 15th set of stretch-shortening cycles, the decrease in the stretch-shortening parameters, F-peak, F-min, F-mean, and F-a, was 50% (P<0.0001), 26% (P=0.0055), 68% (P<0.0001), and 50% (P<0.0001), respectively. Our results showed that both isometric contractions and stretch-shortening cycles induce a reduction in the isometric force. However, the force reduction induced by isometric contractions fully recovered after a break of 48 h while that induced by stretch-shortening cycles did not. Histopathologic assessment of the tibialis anterior exposed to stretch-shortening cycles showed significant myofiber degeneration and necrosis with associated inflammation, while muscles exposed to isometric contractions showed no myofiber degeneration and necrosis, and limited inflammation. Our results suggest that muscle damage can be identified by the non-recoverable isometric force decrement and also by the variations in the dynamic force signature during stretch-shortening cycles.