Abstract
Muscular hypertrophy depends on metabolic exhaustion as well as mechanical load on the muscle. Mechanical tension seems to be the crucial factor to stimulate protein synthesis. The present meta-analysis was conducted to determine whether stretching can generate adequate mechanical tension to induce muscle hypertrophy. We used PubMed, Web of Science, and Scopus to search for literature examining the effects of long-term stretching on muscle mass, muscle cross-sectional area, fiber cross-sectional area, and fiber number. Since there was no sufficient number of studies investigating long-lasting stretching in humans, we only included original animal studies in the current meta-analysis. Precisely, we identified 16 studies meeting the inclusion criteria (e. g. stretching of at least 15 min per day). The 16 studies yielded 39 data points for muscle mass, 11 data points for muscle cross-sectional area, 20 data points for fiber cross-sectional area, and 10 data points for fiber number. Across all designs and categories, statistically significant increases were found for muscle mass ( d = 8.51; 95% CI 7.11–9.91), muscle cross-sectional area ( d = 7.91; 95% CI 5.75–10.08), fiber cross-sectional area ( d = 5.81; 95% CI 4.32–7.31), and fiber number ( d = 4.62; 95% CI 2.54–6.71). The findings show an (almost) continuous positive effect of long-term stretching on the listed parameters, so that it can be assumed that stretch training with adequate intensity and duration leads to hypertrophy and hyperplasia, at least in animal studies. A general transferability to humans—certainly with limited effectiveness—can be hypothesized but requires further research and training studies.