Effects of eccentric contraction on force enhancement in rat fast‐twitch muscle

The aim of this study was to elucidate the effects of eccentric contraction (ECC) on force enhancement in rat fast‐twitch skeletal muscle. Gastrocnemius (GAS) muscles were subjected to 200 ECCs in situ by electrical stimulation. Immediately before and after the stimulation, isometric torque produced by ankle flexion was measured at an ankle angle of 90°. After the second torque measurement, the superficial regions of the muscles were dissected and subjected to biochemical and skinned fiber analysis. ECC did not induce changes in the amount of degraded titin. After ECC, isometric torques in the GAS muscles were markedly reduced, especially at low stimulation frequency. ECC increased passive torque in whole muscle and passive force in skinned fibers. Passive force enhancement and the ratio of passive force to the maximal Ca(2+)‐activated force, but not residual force enhancement, were augmented in the skinned fibers subjected to ECC. An ECC‐induced increase in titin‐based stiffness may contribute to the increased PFE. These results suggest that skeletal muscle is endowed with a force potentiation system that can attenuate ECC‐induced force reductions.