When you lift weights until you can't anymore, you build similar muscle size no matter how heavy the weights are, but lifting heavier weights makes you stronger and faster at moving them than lighter weights.
Evidence from Studies
Supporting (3)
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Resistance Training Load Effects on Muscle Hypertrophy and Strength Gain: Systematic Review and Network Meta-analysis
The study shows that lifting weights until you can't do more leads to similar muscle growth no matter the weight, but heavier weights build more strength, which matches the claim.
Impact of high versus low fixed loads and non-linear training loads on muscle hypertrophy, strength and force development
This study shows that lifting heavy or light weights to failure makes muscles grow the same, but only heavy weights make you stronger and faster at producing force.
Impact of high versus low fixed loads and non-linear training loads on muscle hypertrophy, strength and force development
This study shows that lifting heavy weights to failure makes you stronger and faster, while lifting lighter weights to failure only builds similar muscle size, just like the claim says.
Contradicting (2)
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Low-Load Resistance Training to Volitional Failure Induces Muscle Hypertrophy Similar to Volume-Matched, Velocity Fatigue
The study didn't test the same thing as the claim—it compared low-load failure training to high-load non-failure training, not failure training at high vs. low loads, so it doesn't fully support or contradict the claim.
Comparing the effects of low and high load resistance exercise to failure on adaptive responses to resistance exercise in young women
The study found that when women trained until they couldn't do any more reps, both heavy and light weights made muscles bigger and stronger similarly, but the claim said heavy weights would make them much stronger—so the study shows the claim isn't right.
Gold Standard Evidence Needed
According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.