Acute and Long-term Effects of Nordic Hamstring Exercise Training on Hamstring Muscle Stiffness: A Systematic Review and Meta-analysis

Context: The existing literature provides inconsistent information regarding the effects of Nordic hamstring exercise (NHE) on hamstring muscle stiffness after both acute and chronic training periods. Objective: To systematically review the acute and long-term training effects of NHE on hamstring muscle stiffness in healthy people and athletes. Data Sources: A comprehensive literature search was conducted using the Embase, Medline, SPORTDiscus, Cochrane CENTRAL, Scopus, and Web of Science databases. Study Selection: A total of 459 unique records were screened, and 17 articles were selected for full-text evaluation based on the following inclusion criteria: implementation of NHE, reporting of pre- and postintervention hamstring stiffness outcomes in either acute or long-term training contexts, and inclusion of healthy people and athletes. The methodological quality of the included studies was assessed using a modified version of Downs and Black’s quality assessment tool. Study Design: Systematic review and meta-analysis. Level of Evidence: Level 2. Data Extraction: Training dosage and hamstring stiffness were extracted for the acute and long-term effects of NHE. Results: A total of 12 studies were included in the final analysis, comprising 6 on acute effects and 6 on long-term training effects. Of the 12 studies, 10 were rated as high quality and 2 as low quality. The evidence indicated that NHE acutely increases hamstring stiffness after training. However, no significant changes in hamstring stiffness were observed between the intervention and control groups after 6 to 10 weeks of training, with a mean difference of −2.25 (95% CI, −5.13 to 0.63) and P = 0.13 ( I 2 = 80%). Conclusion: A single NHE session produces a transient increase in the hamstring muscle stiffness. However, longitudinal training programs incorporating NHE do not result in significant stiffness adaptations. Consequently, while a single NHE session may be beneficial for short-term performance enhancement or neuromuscular priming, its efficacy in inducing long-term changes in hamstring stiffness appears limited.