If you lift and lower weights quickly or slowly, as long as you do both phases the same way, your muscles grow about the same amount—speed doesn’t make a big difference.
Evidence from Studies
Supporting (2)
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Comparison of the Effects of Eccentric, Concentric, and Eccentric-Concentric Isotonic Resistance Training at Two Velocities on Strength and Muscle Hypertrophy.
The study had people lift weights fast or slow, both on the way up and down, and found that no matter which speed they used, their muscles grew about the same amount.
How Slow Should You Go? A Systematic Review With Meta-Analysis of the Effect of Resistance Training Repetition Tempo on Muscle Hypertrophy
The study found that lifting weights quickly or slowly doesn’t make a meaningful difference in muscle growth — so whether you go fast-fast or slow-slow, your muscles grow about the same.
Contradicting (0)
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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.