Background: Static stretching (SS) is routinely used in sports and clinical settings to increase joint range of motion (ROM). However, the mechanisms underlying improvements in ROM remain unclear. Objective: We aimed to determine the effects of a single session (acute) and multiple sessions (chronic) of SS on stretch tolerance, passive stiffness and fascicle length, and whether such effects are moderated by specific training parameters and participant characteristics. A secondary aim was to explore the mechanisms associated with improved ROM. Methods: Seven databases (CINAHL Complete, Cochrane CENTRAL, Embase, Emcare, MEDLINE, Scopus and SPORTDiscus) were systematically searched up to 6 June, 2024. Randomised and non-randomised controlled trials investigating the effects of acute (single session) or chronic (two or more sessions) SS on muscle-tendon unit structure (fascicle length), mechanical properties (stiffness) or stretch tolerance (maximum tolerable passive resistive torque) compared to non-stretching passive controls (adults aged = 18 years) were included. The effects of SS were examined using a multi-level meta-analysis, with associations between changes in maximum tolerable passive resistive torque, stiffness and fascicle length with improvements in ROM examined using multivariate meta-regression. Results: Data from 65 studies representing 1542 adults (71% male; mean ± standard deviation age = 26.1 ± 11 years) were included. We found a small decrease in overall stiffness following both acute (Hedges` g = 0.42, 95% confidence interval [CI] 0.21, 0.63, p < 0.001) and chronic SS (Hedges` g = 0.37, 95% confidence interval 0.18, 0.56, p < 0.001), and a moderate increase in maximum tolerable passive resistive torque following chronic SS (Hedges` g = 0.74, 95% CI 0.38, 1.10, p < 0.001). Neither acute nor chronic SS had a significant effect on fascicle length. For acute SS, greater reductions in overall stiffness were found with moderate (p < 0.002) and high SS intensities (p = 0.02) compared with low-intensity SS, and in individuals with normal flexibility compared with those with poor flexibility at baseline (p < 0.001). Conversely, the effects of chronic SS on overall stiffness and maximum tolerable passive resistive torque were not moderated by stretching intensity, intervention length, baseline flexibility or sex (p > 0.05). Last, improved ROM following chronic SS was significantly associated with both decreased overall stiffness (g = 0.59, 95% CI 0.08, 1.10, p = 0.03) and increased maximum tolerable passive resistive torque (g = 0.74, 95% CI 0.41, 1.09, p < 0.001). Conclusions: While both acute and chronic SS reduced overall stiffness, stretch tolerance only increased following chronic SS. Neither acute nor chronic SS altered fascicle length. The effect of acute SS on reduced overall stiffness was greater when stretching at a moderate or higher intensity and in those with normal flexibility. Increased ROM was significantly associated with decreased overall stiffness and increased stretch tolerance following chronic SS. Understanding the mechanisms underlying SS will assist coaches and clinicians in deciding whether and when to prescribe SS to their athletes and patients. Key Points 1. Evidence from randomised and non-randomised controlled trials indicates that static stretching reduces overall stiffness following a single session (acute) and multiple sessions (chronic), whereas increased stretch tolerance is only observed following multiple sessions. Static stretching alone does not appear to increase fascicle length. 2. Specifically, acute reductions in overall stiffness occur only when stretching at moderate or high intensities, and in those with `normal` flexibility. 3. Conversely, the magnitude of reduced overall stiffness and increased stretch tolerance following chronic static stretching was not influenced by stretching intensity, duration of stretching intervention, sex or baseline level of flexibility. 4. Increased range of motion was significantly associated with both reduced overall stiffness and increased stretch tolerance.
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