Purpose: Passive muscle stiffness and muscle architecture at a given joint angle, as well as slack angle of the muscle have been shown to change after an acute bout of stretching. However, it remains unclear whether passive muscle stiffness at a given fascicle length is reduced after stretching. We aimed to elucidate the acute effect of static stretching on the passive fascicle stiffness using ultrasound shear wave elastography. Methods: Shear modulus, fascicle length, and slack angle of the medial gastrocnemius (MG) as well as passive plantar flexion torque during passive dorsiflexion were measured before and after a 5-min static stretching in 14 healthy males. Results: After stretching, passive torques were significantly reduced at >50% of range of motion (ROM). Shear modulus at a given fascicle length was significantly reduced at >80% of the change in fascicle length during passive dorsiflexion. Slack angle of MG was observed at the middle part of ROM and significantly shifted toward more dorsiflexed position after stretching. Conclusion: The present study showed the significant effectiveness of static stretching on the passive fascicle stiffness. Furthermore, the present results suggest that both the shift in slack angle and the reduction in passive fascicle stiffness contribute to produce the change in passive torque-joint angle relationship during passive dorsiflexion. Notably, the contribution of the reduced passive fascicle stiffness to the decrease in passive torque is substantial over the latter part of ROM.
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