PURPOSE: The objective of this study is to investigate the acute effects of static stretching (SS) on muscle hardness of the gastrocnemius medialis (MG) and gastrocnemius lateralis (LG). METHODS: Twenty young men participated in this study. MG and LG hardness was measured using shear wave ultrasound elastography before and after three bouts of 2-min SS. The measurement site of muscle hardness was at 30% of the lower leg length from the popliteal crease to the lateral malleolus. Similarly, the passive range of motion (ROM) of dorsiflexion, musculotendinous unit (MTU) stiffness determined by the slope of the portion of the passive torque-angle curve from 15 to 25 , and joint torque developed during isometric maximal voluntary plantarflexion at 0 of ankle joint angle were also measured before and after SS. RESULTS: SS increased ROM and decreased MTU stiffness significantly but did not change joint torque. The main effects of test time and muscle group on muscle hardness were significant without a significant interaction of these variables. Both differences between the relative changes in the MG and LG hardness and between the muscle hardness ratios before and after SS were not significant. A significant correlation between the muscle hardness ratios before and after SS was found. CONCLUSION: The results of this study suggest that three bouts of 2-min SS of the plantar flexors is useful for preventing muscle injury, improving muscle condition, and maintaining muscle strength, and that the acute effects of SS on the muscle hardness of MG and LG are of the same degree.
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