Effect of acute static stretching on the passive mechanical properties of the Achilles tendon: a systematic review and meta-analysis

Background/Aims Little is known about the effectiveness of static stretching on the tendinous structures. The aim of this study was to verify the effect of acute static stretching on the passive mechanical properties of the Achilles tendon. Methods Databases were searched with the descriptors ‘muscle stretching exercises,’ ‘adult’ and ‘Achilles tendon’ with a sample consisting of physically active young adults. Two blind reviewers performed the review. Results A total of four studies were selected. After the meta-analysis, no significant difference was observed for range of motion (−0.73; 95% confidence interval −2.18–0.71; I 2 =0%), muscle-tendon unit (−0.02; 95% confidence interval −0.23–0.19; I 2 = 83%), muscle (−0.43; 95% confidence interval −0.92–0.06; I 2 = 0%) and tendon stiffness (3.70; 95% confidence interval 1.57–5,83; I 2 =0%; P=0.0007) and passive torque (−1.35; 95% confidence interval −4.75–2.05; I 2 = 0%). Maximum voluntary isometric contraction showed an alteration with a longer stretching time (1215 seconds). Conclusions Passive static stretching (60–1215 seconds) does not significantly alter the analysed variables.