Acute and Chronic Effects of Stretching on Running Economy: A Systematic Review with Meta-Analysis

Background: Running economy (RE) determines the performance of endurance athletes. While stretching has been practised for decades, and is still one common integral component of warm-up routine, muscle stretching is also associated with decreased stiffness. For RE energy storage in the tendons which is accompanied with stiffness is of crucial importance. In turn, avoidance of pre-running stretching was frequently recommended. Although some studies supported this recommendation, the evidence is controversial. Nevertheless, yet, no systematic review on the effects of stretching on RE with effect size (ES) quantification was performed. Consequently, with this systematic review with meta-analysis, we aim to provide the first overview on this topic. Methods: In adherence to PRISMA 2020 guidelines, we meta-analyzed effect sizes from three databases using PICOS guidelines on stretching effects on RE in healthy participants using robust variance estimation. Heterogeneity was reduced using subgroup analyses while meta-regression evaluated whether running velocity potentially moderates results. Risk of Bias was assessed using the PEDro scale, certainty of evidence was classified via GRADE working group criteria. The study protocol was registered in Open Science Framework https://doi.org/10.17605/OSF.IO/MA8D4). Results: Overall, low certainty of evidence pooled from 15 studies with a total of 181 participants indicated that stretching did not significantly moderate RE acutely (p = 0.21-0.65), neither in general, nor were there any stretching types (dynamic, static and proprioceptive neuromuscular facilitation) that affected this result. Due to the limited number of chronic studies found in the literature, long-term stretching effects were exclusively evaluated qualitatively. Meaningful heterogeneity and reduced methodological quality (PEDro Score: 4.88, fair) contributed to certainty of evidence downgrading. Conclusions: In contrast to common beliefs that stretching decreased stiffness parameters and would therefore hamper RE, current evidence does not support any effect of stretching on RE in running athletes. However, several flaws such as no investigation of the underlying mechanisms (e.g., stiffness), small sample sizes, determining RE at different velocities, and the implementation of unreasonable stretching durations strongly biased conclusions. Especially on chronic effects there is a large demand for improved evidence, including underlying mechanisms investigation. Yet, it seems unreasonable to avoid pre-running stretching to prevent RE decreases.