Effect of repeated bout effect induced by eccentric exercises on joint stiffness endurance in males

The purpose of this study was to investigate the effect of the repeated bout effect induced by eccentric exercises on joint stiffness endurance. Twelve males performed fatigue tasks (5 sets of 50 hopping) three times: first fatigue task on one side (FFT), second fatigue task on the same side 2 weeks after FFT (SFT), and opposite fatigue task on the contralateral side (OFT). Ankle joint stiffness and electromyographic activities of the plantar flexor and tibial anterior muscles during drop jump using only the ankle joint were measured before and after the fatigue task. Active muscle stiffness was calculated according to changes in estimated muscle force and fascicle length during fast stretching after submaximal isometric contractions. After FFT and OFT, joint stiffness and active muscle stiffness significantly decreased, whereas the electromyographic activities of the plantar flexor and tibial anterior muscles during pre-landing, eccentric, and concentric phases did not change. After SFT, joint stiffness decreased slightly, whereas active muscle stiffness did not. Furthermore, the relative changes in joint stiffness and active muscle stiffness were significantly lower in SFT than in FFT and OFT. These results suggest that joint stiffness and active muscle stiffness endurance are significantly enhanced 2 weeks after repeated eccentric exercises produced muscle damage.