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 (I2 = 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.