Non-local acute stretching effects on strength performance in healthy young adults

Background Static stretching (SS) can impair performance and increase range of motion of a nonexercised or non-stretched muscle respectively. An underdeveloped research area is the effect of unilateral stretching on non-local force output. Objective The objective of this review was to describe the effects of unilateral SS on contralateral, non-stretched, muscle force and identify gaps in the literature. Methods A systematic literature search following Preferred Reporting Items for Systematic Review and Meta-Analyses Protocols guidelines was performed according to prescribed inclusion and exclusion criteria. Weighted means and ranges highlighted the non-local force output response to unilateral stretching. The Physiotherapy Evidence Database scale was used to assess study risk of bias and methodological quality. Results Unilateral stretching protocols from six studies involved 6.3±2 repetitions of 36.3±7.4 seconds with 19.3±5.7 seconds recovery between stretches. The mean stretchinduced force deficits exhibited small magnitude effect sizes for both the stretched (-6.7±7.1%, d=-0.35: 0.01 to -1.8) and contralateral, non-stretched, muscles (-4.0±4.9%, d=-0.22: 0.08 to -1.1). Control measures exhibited trivial deficits. Conclusion The limited literature examining non-local effects of prolonged SS revealed that both the stretched and contralateral, non-stretched, limbs of young adults demonstrate small magnitude force deficits. However, the frequency of studies with these effects were similar with three measures demonstrating deficits, and four measures showing trivial changes. These results highlight the possible global (non-local) effects of prolonged SS. Further research should investigate effects of lower intensity stretching, upper versus lower body stretching, different age groups, incorporate full warm-ups, and identify predominant mechanisms among others.