When people train with either 3-5 reps (heavy weights) or 20-25 reps (light weights) until they can't do more, their muscles grow about the same amount - around 7-8%.
Evidence from Studies
Supporting (2)
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Resistance Training Load Effects on Muscle Hypertrophy and Strength Gain: Systematic Review and Network Meta-analysis
This study looked at people doing resistance training until they couldn't do any more reps, comparing different rep ranges. It found that muscle growth was the same whether people did few reps (like 3-5) or many reps (like 20-25), as long as they trained to failure. This matches the claim that both rep ranges produce similar muscle thickness increases.
Muscle hypertrophy and strength gains after resistance training with different volume matched loads: a systematic review and meta-analysis.
This review of many studies found that when the total work is the same, muscle growth is similar whether you use light weights with many reps or heavy weights with few reps.
Contradicting (1)
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Neither load nor systemic hormones determine resistance training-mediated hypertrophy or strength gains in resistance-trained young men
The study tested 8-12 reps vs 20-25 reps, but the claim is about 3-5 reps vs 20-25 reps. Since the low-rep range is different, the study doesn't directly test the claim even though it found similar hypertrophy between high and low loads.