Influence of stress relaxation and load during static stretching on the range of motion and muscle–tendon passive stiffness

Passive torque during static stretching was decreased. This phenomenon occurs due to stress relaxation. However, no studies have investigated the relationship stress relaxation and changes in range of motion (ROM) or muscle–tendon unit stiffness. Moreover, no study calculated the total volume during static stretching as total work and investigated the relationship between total work and ROM changes or muscle–tendon stiffness. This study aimed to investigate the relationship between stress relaxation, total work from measuring passive torque normalized by muscle thickness and ROM, and muscle–tendon stiffness. A total of 63 healthy students voluntarily participated in this study. Static stretching was performed for the plantar flexor muscles at 120 s. Outcomes were assessed on ROM, passive torque at dorsiflexion ROM, and muscle–tendon stiffness. Stress relaxation was defined as the change of torque from the start to end of static stretching. Total work was defined as the sum of the torque from the start to end of static stretching. Stress relaxation was found to be significantly correlated with muscle–tendon stiffness whether normalized by muscle thickness or not (r = 0.603, r  = 0.599). Furthermore, total work was significantly correlated with muscle–tendon stiffness, regardless of normalization (r = − 0.276, r = − 0.327). However, the relationship among stress relaxation, total work and ROM, and passive torque at dorsiflexion ROM was not significant. Stress relaxation and total work are associated with muscle–tendon stiffness changes after stretching, but not associated with ROM changes.