Dynamic and static stretching on hamstring flexibility and stiffness: A systematic review and meta-analysis

Introduction: Hamstring injuries are one of the most common types of damage in sports. Insuffi-cient flexibility and high stiffness are important reasons for it. Stretching is often used in warm-up activities before exercises to increase flexibility, among which dynamic stretching (DS) and static stretching (SS) are the most widely used. The effects of these two stretching techniques on the flexibility or stiffness of the hamstring still need to be clarified. Objective: This study aimed to compare the short-term, medium-term, and long-term effects of DS and SS on improving hamstring flexibility and stiffness via a meta-analysis of RCTs.Methods: RCTs were identified from PubMed, Cochrane Library, Web of Science, and PEDro from inception to July 28, 2022. The methodological quality was evaluated using the PEDro scale. The mean difference and 95% confidence interval of the outcome variables before and after stretching were calculated and the extracted data were quantitatively processed using a random or fixed effects model.Results: A total of 27 RCTs and 606 participants were included. In terms of improving the ROM of the hamstring, there was no significant difference in the acute (MD,-0.70, 95% CI,-1.54 to 0.14; Z = 1.63, P > 0.05) and sub-acute effects (MD, 1.71, 95% CI,-2.80 to 6.22; Z = 0.74, P > 0.05) between a single bout of SS and DS, while the acute (MD,-5.13, 95% CI,-7.65 to-2.61; Z = 3.99, P < 0.05) and sub-acute effects (MD,-5.30, 95% CI,-6.33 to-4.27; Z = 10.04, P < 0.05) of multiple bouts of SS was superior to DS; There was no significant difference in the medium-term effect between the two stretching techniques (MD, 3.48, 95% CI,-2.57 to 9.53; Z = 1.13, P > 0.05), but the long-term effect of SS was better than DS (MD, -10.40, 95% CI,-10.97 to-9.83; Z = 35.57, P < 0.05). Regarding the length of the hamstring, the acute (MD,-0.41, 95% CI,-1.09 to 0.26; Z = 1.20, P > 0.05) and sub-acute effects (MD,-0.73, 95% CI,-1.69 to 0.22; Z = 1.51, P > 0.05) of a single bout of DS and SS were similar. Two studies have compared the effects on hamstring stiffness, with one showing similar effects, and the other showed that DS was superior to SS. One study showed no difference in the magnitude of change in improving passive torque. No studies explored the effect of DS and SS on hamstring myofascial length. Only one study demonstrated no significant difference in hamstring thickness.Conclusions: A single bout of DS and SS have similar short-term effects in improving hamstring ROM and length, while multiple bouts of SS can significantly improve hamstring ROM compared to DS. DS and SS showed similar effects on hamstring myofascial length.