Whether you lift light weights or heavy weights until you can’t do another rep, your muscle fibers don’t seem to grow significantly more with one than the other — but we can’t be sure because the results are too uncertain.

Scientific Claim

When resistance training is performed to muscular failure, low-load (≤60% 1RM) and high-load (>60% 1RM) training show no statistically significant difference in hypertrophy of type I or type II muscle fibers in the quadriceps of young adults, though the wide prediction intervals (–0.71 to 1.28 for type I; –0.28 to 0.88 for type II) indicate the true effect could range from substantial benefit of one load type to the other.

Original Statement

“In this meta-analysis, there were no significant differences between low-load and high-load resistance training on hypertrophy of type I or type II muscle fibers. The 95% confidence and prediction intervals were very wide, suggesting that the true effect in the population and the effect reported in a future study conducted on this topic could be in different directions and anywhere from trivial to very large.”

From study:The Effects of Low-Load Vs. High-Load Resistance Training on Muscle Fiber Hypertrophy: A Meta-Analysis

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 is a meta-analysis of non-RCTs with unknown randomization status, so causal language is inappropriate. The authors correctly avoid claiming equivalence and instead report non-significance with wide intervals, aligning with association-level inference.

More Accurate Statement

“When resistance training is performed to muscular failure, low-load (≤60% 1RM) and high-load (>60% 1RM) training are associated with similar type I and type II muscle fiber hypertrophy in the quadriceps of young adults, but current evidence is insufficient to determine whether one load condition is superior due to wide prediction intervals.”

Evidence from Studies

Supporting (1)

The Effects of Low-Load Vs. High-Load Resistance Training on Muscle Fiber Hypertrophy: A Meta-Analysis

Systematic Review With Meta-Analysis

Human

2020 Aug

This study found that lifting light weights and heavy weights to exhaustion both build muscle in the thighs about the same, but the results are so mixed that either could be better—we just don’t know for sure yet.

Contradicting (0)

No contradicting evidence found

Source Study

The Effects of Low-Load Vs. High-Load Resistance Training on Muscle Fiber Hypertrophy: A Meta-Analysis

Score: 39

DOI: 10.2478/hukin-2020-0013

Similar Assertions

Lifting light or heavy weights until failure doesn’t clearly make your fast-twitch muscle fibers bigger in your thighs — the data is too mixed to say one is better.

94%

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.

91%

We just don’t know yet if light or heavy weights make muscles grow better — the results could swing either way in future studies, so we need more research.

85%

We can’t say whether light or heavy weights are better for growing muscle fibers in the thighs — we just don’t have enough good data yet, and we need more studies with more people.

80%

Resistance training to muscular failure induces equivalent hypertrophy in both slow-twitch (type I) and fast-twitch (type II) muscle fibers, regardless of load or repetition range.

79%