Whether you lift light weights or heavy weights until you can't do another rep, your slow-twitch muscle fibers in your thighs grow about the same amount — but we can't be sure because the results are too uncertain.
Scientific Claim
Low-load (≤60% 1RM) and high-load (>60% 1RM) resistance training performed to muscular failure show no statistically significant difference in type I muscle fiber hypertrophy in the quadriceps of young, untrained individuals, with a standardized mean difference of 0.28 (95% CI: -0.27 to 0.82), suggesting the true effect could range from modest atrophy to substantial hypertrophy under either condition.
Original Statement
“In the meta-analysis for the effects of low-load vs. high-load resistance training on type I muscle fiber hypertrophy, there was no significant difference between the training conditions (standardized mean difference: 0.28; 95% confidence interval: –0.27, 0.82; p = 0.316; I² = 18%; 95% prediction interval: –0.71, 1.28).”
Evidence Quality Assessment
Claim Status
appropriately stated
Study Design Support
Design supports claim
Appropriate Language Strength
association
Can only show association/correlation
Assessment Explanation
The study design (meta-analysis of heterogeneous studies with unknown randomization status) cannot establish causation. The language 'no significant difference' and inclusion of confidence intervals appropriately reflect uncertainty.
Evidence from Studies
Supporting (1)
The Effects of Low-Load Vs. High-Load Resistance Training on Muscle Fiber Hypertrophy: A Meta-Analysis
This study found that lifting light weights and lifting heavy weights, both until you can’t do another rep, lead to about the same amount of growth in slow-twitch muscle fibers — so neither is clearly better.