How slow should you go? A systematic review with meta-analysis of the effect of resistance training repetition tempo on muscle hypertrophy.

We systematically searched the literature for interventions that compared different resistance training tempos for eccentric (ECC) or concentric (CON) actions. We estimated pre-/post-study changes and between-condition differences in lean/muscle mass for healthy adults, restricting inclusion to studies that controlled all other training variables. Hierarchical meta-analyses were conducted within a Bayesian framework, with change estimates made within groups, and the primary emphasis placed on pairwise differences between dichotomously coded slower (repetition time: 1.7 to 4.5 s, averaging ~3.5 s) and faster (repetition time: 0.3 s to 2 s, averaging ~1 s) interventions. When combining data across all 14 included studies (4 CON and 9 ECC manipulated interventions, 1 for both in a cross-over design), meta-analyses of within-group changes showed similar pooled effects for faster (0.43 [95%CrI: 0.29 to 0.58]) and slower tempos (0.34 [95%CrI: 0.22 to 0.47)]. Meta-analyses of pairwise differences across the same studies showed central estimates that trivially favored faster over slower tempos (pooled mean = 0.09 [95%CrI: -0.04 to 0.22]), with a low probability that the between-condition effect size was at least small (p = 0.450), medium (p = 0.001), or large (p < 0.001). Subgroup analyses of pairwise differences stratified by muscle action type (ECC or CON), body region (upper- or lower-body), and training to failure (yes or no) generally produced trivial to small effects, although the certainty of estimates varied across analyses. In conclusion, resistance training tempo appears to have minimal overall effect on muscle hypertrophy, with potential differences emerging under specific conditions.