Long‐term static stretching can decrease muscle stiffness: A systematic review and meta‐analysis

Stretch training increases the range of motion of a joint. However, to date, the mechanisms behind such a stretching effect are not well understood. An earlier meta‐analysis on several studies reported no changes in the passive properties of a muscle (i.e., muscle stiffness) following long‐term stretch training with various types of stretching (static, dynamic, and proprioceptive neuromuscular stretching). However, in recent years, an increasing number of papers have reported the effects of long‐term static stretching on muscle stiffness. The purpose of the present study was to examine the long‐term (≥2 weeks) effect of static stretching training on muscle stiffness. PubMed, Web of Science, and EBSCO published before December 28, 2022, were searched and 10 papers met the inclusion criteria for meta‐analysis. By applying a mixed‐effect model, subgroup analyses, which included comparisons of sex (male vs. mixed sex) and type of muscle stiffness assessment (calculated from the muscle‐tendon junction vs. shear modulus), were performed. Furthermore, a meta‐regression was conducted to examine the effect of total stretching duration on muscle stiffness. The result of the meta‐analysis showed a moderate decrease in muscle stiffness after 3–12 weeks of static stretch training compared to a control condition (effect size = −0.749, p < 0.001, I 2 = 56.245). Subgroup analyses revealed no significant differences between sex ( p = 0.131) and type of muscle stiffness assessment ( p = 0.813). Moreover, there was no significant relationship between total stretching duration and muscle stiffness ( p = 0.881).