As long as you push yourself hard enough that you can't do many more reps, doing many reps with light weights works just as well as few reps with heavy weights for building muscle.
Evidence from Studies
Supporting (4)
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Neither load nor systemic hormones determine resistance training-mediated hypertrophy or strength gains in resistance-trained young men
When people train until they can't lift any more (volitional failure), it doesn't matter if they do 20+ reps with light weights or 8-12 reps with heavy weights - they get the same muscle growth. This study proved both approaches built muscle equally well.
Resistance Training Load Effects on Muscle Hypertrophy and Strength Gain: Systematic Review and Network Meta-analysis
This study only included experiments where people trained until they couldn't do any more reps. It found that whether people used light weights for many reps or heavy weights for few reps, they gained the same amount of muscle - as long as they pushed each set to failure. This supports the idea that training hard to failure matters more than how many reps you do.
The Effects of Low-Load Vs. High-Load Resistance Training on Muscle Fiber Hypertrophy: A Meta-Analysis
When both groups train until they can't do more reps, low weights work just as well as heavy weights for building muscle.
Low-Load Resistance Training to Volitional Failure Induces Muscle Hypertrophy Similar to Volume-Matched, Velocity Fatigue
Lifting light weights until you can't lift anymore builds muscle just as well as lifting with a slower speed.
Contradicting (1)
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Leaving 1-2 reps in the tank builds just as much muscle as training to complete failure.