Lifting light weights or heavy weights makes your muscles grow the same size, as long as you push until you can’t do another rep.
Evidence from Studies
Supporting (3)
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Resistance training load does not determine resistance training-induced hypertrophy across upper and lower limbs in healthy young males.
When people lift weights until they can’t do another rep, whether they use heavy or light weights, their muscles grow about the same — the key is pushing until exhaustion, not how heavy the weight is.
Even with very light weights, if you lift until you can’t do another rep, your muscles still grow just as much as with heavy weights—this study proves it.
Divergent Strength Gains but Similar Hypertrophy After Low-Load and High-Load Resistance Exercise Training in Trained Individuals: Many Roads Lead to Rome.
This study found that lifting light weights until you can’t do another rep builds muscle just as well as lifting heavy weights until failure — so you don’t need to lift super heavy to get bigger muscles.
Contradicting (2)
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Muscle Failure Promotes Greater Muscle Hypertrophy in Low-Load but Not in High-Load Resistance Training
Lifting light weights only builds muscle if you push to exhaustion, but lifting heavy weights builds muscle even if you don’t go all the way to failure — so failure doesn’t help equally at all weights.
Changes in muscular strength following nine weeks of high- or low-load resistance training.
Even though both groups lifted until they couldn’t do another rep, the heavy lifters got much stronger than the light lifters, suggesting that lifting heavier weights still gives better muscle growth, even when going all the way to failure.