Kataura, S, Suzuki, S, Matsuo, S, Hatano, G, Iwata, M, Yokoi, K, Tsuchida, W, Banno, Y, and Asai, Y. Acute effects of the different intensity of static stretching on flexibility and isometric muscle force. J Strength Cond Res 31(12): 3403-3410, 2017-In various fields, static stretching is commonly performed to improve flexibility, whereas the acute effects of different stretch intensities are unclear. Therefore, we investigated the acute effects of different stretch intensities on flexibility and muscle force. Eighteen healthy participants (9 men and 9 women) performed 180-second static stretches of the right hamstrings at 80, 100, and 120% of maximum tolerable intensity without stretching pain, in random order. The following outcomes were assessed as markers of lower limb function and flexibility: static passive torque (SPT), range of motion (ROM), passive joint (muscle-tendon) stiffness, passive torque (PT) at onset of pain, and isometric muscle force. Static passive torque was significantly decreased after all stretching intensities (p ≤ 0.05). Compared with before stretching at 100 and 120% intensities, ROM and PT were significantly increased after stretching (p ≤ 0.05), and passive stiffness (p = 0.05) and isometric muscle force (p ≤ 0.05) were significantly decreased. In addition, ROM was significantly greater after stretching at 100 and 120% than at 80%, and passive stiffness was significantly lower after 120% than after 80% (p ≤ 0.05). However, all measurements except SPT were unchanged after 80% intensity. There was a weak positive correlation between the intensities of stretching and the relative change for SPT (p ≤ 0.05), a moderate positive correlation with ROM (p ≤ 0.05), and a moderate positive correlation with passive stiffness (p ≤ 0.05). These results indicate that static stretching at greater intensity is more effective for increasing ROM and decreasing passive muscle-tendon stiffness.

The effect of static stretching on passive stiffness of the hamstrings was not maintained as long as the changes in ROM, stretch tolerance, and isometric muscle force. Therefore, frequent stretching is necessary to improve the viscoelasticity of the muscle-tendon unit. Muscle force was decreased for 30 minutes after stretching; this should be considered prior to activities requiring maximal muscle strength.

Although static stretching is commonly performed to improve flexibility, it has been reported to have detrimental effects on muscle force and performance. Previous studies have reported that low-intensity and short-duration isometric contraction after static stretching inhibits stretch-induced force loss. However, the detailed effects of low-intensity short-duration isometric contraction after static stretching on flexibility, such as passive torque and stiffness, are currently unclear. Therefore, the current study sought to reveal the effects of low-intensity and short-duration isometric contraction after static stretching on flexibility and muscle force. Sixteen healthy participants (eight men, eight women) performed 300-s static stretching (SS), 300-s static stretching followed by 6-s 30% maximum isometric voluntary contraction (MIVC) (SS-30% MIVC), and 6-s 30% MIVC (30% MIVC) of the right hamstring on three separate days, in random order. Range of motion (ROM) and passive stiffness during knee extension, peak passive torque, and MIVC torque were obtained before and after exercise. ROM and peak passive torque were significantly increased after all exercises, whereas passive stiffness and MIVC torque were significantly decreased after SS only. Moreover, ROM after SS and SS-30% MIVC were significantly greater than that of 30% MIVC, and peak passive torque after SS-30% MIVC was significantly greater than that of SS, while passive stiffness after SS was significantly lower than that of 30% MIVC. These results indicate that low-intensity and shortduration isometric contraction after static stretching is effective for inhibiting stretch-induced force loss while also inhibiting the effects of static stretching on passive stiffness.