Chronic Effects of Static Stretching Exercises on Muscle Strength and Power in Healthy Individuals Across the Lifespan: A Systematic Review with Multi-level Meta-analysis

Abstract

Background: The current literature on the chronic effects of static stretching (SS) exercises on muscle strength and power is unclear and controversial.

Objective: We aimed to examine the chronic effects of SS exercises on muscle strength and power as well as flexibility in healthy individuals across the lifespan.

Design: Systematic review with meta-analysis of (randomized) controlled trials.

Data sources: A systematic literature search was conducted in the databases PubMed, Web of Science, Cochrane Library, and SPORTDiscus up to May 2022.

Eligibility criteria for selecting studies: We included studies that investigated the chronic effects of SS exercises on at least one muscle strength and power outcome compared to an active/passive control group or the contralateral leg (i.e., using between- or within-study designs, respectively) in healthy individuals, irrespective of age, sex, and training status.

Results: The main findings of 41 studies indicated trivial-to-small positive effects of chronic SS exercises on muscle strength (standardized mean difference [SMD] = 0.21, [95% confidence interval 0.10-0.32], p = 0.001) and power (SMD = 0.19, 95% confidence interval 0.12-0.26], p < 0.001). For flexibility, moderate-to-large increases were observed (SMD = 0.96, [95% confidence interval 0.70-1.22], p < 0.001). Subgroup analyses, taking the participants' training status into account, revealed a larger muscle strength improvement for sedentary (SMD = 0.58, p < 0.001) compared with recreationally active participants (SMD = 0.16, p = 0.029). Additionally, larger flexibility gains were observed following passive (SMD = 0.97, p < 0.001) compared with active SS exercises (SMD = 0.59, p = 0.001). The chronic effects of SS on muscle strength were moderated by the proportion of female individuals in the sample (β = 0.004, p = 0.042), with higher proportions experiencing larger gains. Other moderating variables included mean age (β = 0.011, p < 0.001), with older individuals showing larger muscle strength gains, and the number of repetitions per stretching exercise and session (β = 0.023, p = 0.004 and β = 0.013, p = 0.008, respectively), with more repetitions associated with larger muscle strength improvements. Muscle power was also moderated by mean age (β = 0.006, p = 0.007) with larger gains in older individuals. The meta-regression analysis indicated larger flexibility gains with more repetitions per session (β = 0.094, p = 0.016), more time under stretching per session (β = 0.090, p = 0.026), and more total time under stretching (β = 0.078, p = 0.034).

Conclusions: The main findings indicated that chronic SS exercises have the potential to improve muscle strength and power. Such improvements appear to benefit sedentary more than recreationally active participants. Likewise, chronic SS exercises result in a marked enhancement in flexibility with larger effects of passive, as compared with active, SS. The results of the meta-regression analysis for muscle strength indicated larger benefits of chronic SS exercises in samples with higher proportions of female, older participants, and a higher number of repetitions per stretching exercise and session. For muscle power, results suggested larger gains for older participants. Regarding flexibility, findings indicated larger benefits following a higher number of repetitions per exercise and a longer time under stretching per session as well as a longer total time under stretching.

Fig. 1

Flow chart illustrating the different stages of search and study selection. SS static stretching

Fig. 2

Ordered caterpillar plot with prediction intervals of all effects for muscle strength, muscle power, and flexibility. Positive values indicate chronic static stretching exercise-related improvements in muscle strength, muscle power, and flexibility. The highlighted space between the two vertical lines indicates the 95% prediction interval

Fig. 3

Contour-enhanced funnel plot for all effects to visualize publication bias

Fig. 4

Graphical display of study heterogeneity plot for all effects

Fig. 5

Meta-analytic regression plot of Physiotherapy Evidence Database (PEDro) scale scores (%) for muscle strength, muscle power, and flexibility. Black outlined subgroup shows meta-regression with clear effect

Fig. 6

Subgroup plots of effects of chronic static stretching exercises in categorical subgroups on muscle strength, muscle power, and flexibility including prediction intervals. Black outlined plots show subgroups with clear effects and gray outlined plots show subgroups with unclear but significant effects

Fig. 7

Meta-analytic plots of effects of chronic static stretching exercises in continuous subgroups on muscle strength, muscle power, and flexibility including prediction intervals. Black outlined subgroups show meta-regressions with clear effects