What the research actually says

Findings from published meta-analyses on training. Each one shows the numbers, the sentence in the paper they came from, who was studied, and — the part most summaries leave out — what the study does not support.

We publish the limits because a finding without its caveats is how good research turns into bad advice. How we check these numbers.

• Meta-regression · 67 total studies

  How weekly set volume relates to muscle growth

  Across 67 studies, the posterior probability that the marginal slope exceeded zero — that gains rise as weekly set volume rises — was 100% for both muscle size and strength.

  Joshua C Pelland (2026) · Sports Medicine (Auckland, N.Z.)

• Meta-regression

  How close to failure does a set need to be?

  Across the studies pooled in this analysis, how close a set was estimated to be taken to failure showed a negligible relationship with strength gain, while muscle growth increased as sets were terminated closer to failure.

  Zac P Robinson (2024) · Sports Medicine

• Meta-regression · 67 total studies

  Training frequency per muscle group, at equal volume

  Across 67 studies of 2058 participants, this multi-level meta-regression found that gains in muscle size and strength increase as weekly set volume increases (posterior probability that the marginal slope exceeded zero: 100% for both outcomes), though both best-fit models suggest diminishing returns, with those for strength considerably more pronounced.

  Pelland JC (2026) · Sports medicine (Auckland, N.Z.)

• Meta-analysis · Twenty-eight studies

  Heavy versus light loads for muscle growth and strength

  Across 28 studies that included only training in which sets were performed to volitional failure, low, moderate and high loads produced no detectable difference in muscle hypertrophy, while strength gains were greater with high-load and moderate-load training than with low-load training.

  Lopez P (2021) · Medicine and Science in Sports and Exercise

• Meta-analysis · 9 studies, 19 measurements

  Rest between sets and muscle growth

  Across 9 randomized-design studies contributing 19 measurements of lean/muscle mass in healthy adults, the controlled head-to-head comparisons of short versus longer inter-set rest all had credible intervals spanning zero: central estimates tended to favor longer rest at the arm and thigh, while the whole-body estimate was closer to zero and marginally favored shorter rest.

  Singer A (2024) · Frontiers in sports and active living

• Meta-analysis · Twelve studies

  Training at long versus short muscle lengths

  Twelve studies in young adults found that resistance training at longer versus shorter mean muscle lengths produced trivial differences in regional muscle growth at the proximal, mid-belly and distal sites, and the percentage of posterior distributions falling within regions of practical equivalence was high across all sites.

  Dorian Varovic (2025) · International Journal of Sports Medicine

• Meta-analysis · Eleven studies

  Does the order of exercises in a session matter?

  A systematic review and meta-analysis of eleven training studies compared performing multi-joint exercises first with performing single-joint exercises first.

  Nunes JP (2021) · European journal of sport science

• Meta-analysis · 43 studies

  Does adding cardio reduce muscle growth or strength?

  Across 43 studies, adding aerobic training on top of an identical strength training programme did not compromise muscle hypertrophy or maximal strength development.

  Schumann M (2022) · Sports Medicine (Auckland, N.Z.)

• Meta-analysis · 83 studies with more than 400 effect sizes

  What static stretching costs before you train

  Across 83 studies pooled in a multilevel meta-analysis of controlled pre-post trials, an acute bout of static stretching produced a small reduction in maximal strength (ES = -0.21) versus passive controls, rising to a high-magnitude reduction (ES = -0.84) for stretching durations of 60 s or more per bout; the authors' conclusion ties this confirmation of earlier findings to strength testing of isolated muscles such as leg extensions or calf raises.

  Warneke K (2024) · Journal of sport and health science

• Meta-analysis · Six studies, with eight groups, were included in the review.

  What happens to muscle when training stops

  In older adults, muscle size increased significantly over 9-24 weeks of resistance training (Cohen's d 0.99; 95% CI 0.63, 1.36).

  Grgic J (2022) · International Journal of Environmental Research and Public Health

• Meta-analysis · Thirty-five studies met the inclusion criteria.

  Periodised versus non-periodised training, at equal volume

  When volume was equated between conditions, periodised resistance training showed a small advantage over non-periodised training for 1RM strength (ES 0.31, 95% CI [0.04, 0.57], P = 0.02), while muscle hypertrophy did not differ between the two (ES 0.13, 95% CI [-0.10, 0.36], P = 0.27).

  Lukas Moesgaard (2022) · Sports Medicine (Auckland, N.Z.)

• Meta-analysis · Nineteen articles (42 outcomes)

  Blood flow restriction training versus heavy lifting

  Pooled across all studies, high-load resistance training produced slightly greater strength gains than low-load blood-flow-restriction training, but the gap was small.

  Chang H (2023) · Frontiers in physiology

• Meta-regression · 227 outcome measures derived from 69 publications

  Sleep loss and exercise performance

  Across 227 outcome measures from 69 publications, sleeping 6 hours or less in any 24-hour period, compared with a normal-sleep control of more than 6 hours in any 24-hour period, was associated with a mean change in exercise performance of - 7.56% (95% CI - 11.9 to - 3.13), with near-total heterogeneity (I2 = 98.1%).

  Craven J (2022) · Sports medicine (Auckland, N.Z.)

• Meta-analysis · Forty-nine studies

  Eccentric versus concentric loading for muscle growth

  In a 2026 Sports Medicine systematic review and meta-analysis of 49 studies and 773 participants, the authors' estimated effects suggest that accentuated eccentric loading — loading the eccentric phase more heavily than the concentric phase — showed greater acute responses than constant-load resistance training in four outcomes: blood lactate concentration immediately post-intervention, growth hormone concentration immediately post-intervention, muscle electrical activity during the eccentric phase (SMD = 0.37; p = 0.01), and rating of perceived exertion immediately post-intervention (SMD = 1.72; p = 0.01).

  Xing Zhang (2026) · Sports Medicine