Why fast lifts grow muscles better than slow ones
Effects of repetition duration on skeletal muscle hypertrophy in a rat model of resistance exercise.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Rats that did quick muscle contractions grew bigger muscles than rats doing slow ones, even when both did the same total work. Slow contractions didn't make muscles grow as much, maybe because they couldn't push as hard.
Surprising Findings
Acute molecular responses (protein synthesis, mTOR, ribosome biogenesis) were identical across all groups 6 hours post-exercise, yet only the short-duration group grew muscle.
Most people assume that if a workout triggers strong molecular signals right after, it must be effective—but here, all signals were the same, yet only fast reps led to growth.
Practical Takeaways
If your goal is muscle growth, consider incorporating faster, explosive reps in some sets—even if total work is matched.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Rats that did quick muscle contractions grew bigger muscles than rats doing slow ones, even when both did the same total work. Slow contractions didn't make muscles grow as much, maybe because they couldn't push as hard.
Surprising Findings
Acute molecular responses (protein synthesis, mTOR, ribosome biogenesis) were identical across all groups 6 hours post-exercise, yet only the short-duration group grew muscle.
Most people assume that if a workout triggers strong molecular signals right after, it must be effective—but here, all signals were the same, yet only fast reps led to growth.
Practical Takeaways
If your goal is muscle growth, consider incorporating faster, explosive reps in some sets—even if total work is matched.
Publication
Journal
Journal of applied physiology
Year
2025
Authors
Hikaru Kato, Takaya Kotani, Yuki Tamura, Karina Kouzaki, Kazushige Sasaki, Koichi Nakazato
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Claims (5)
Your muscles don't grow while you're lifting weights—they grow later, while you rest, because your body uses that time to repair and build new muscle tissue after the workout stresses them.
When rats lift weights with different numbers of reps, their muscles show the same early biological changes no matter if they do few or many reps—and those early changes don’t tell us which workout will make muscles grow bigger over time.
If rats lift weights slowly for longer periods, they grow less muscle than if they lift quickly or at a medium pace—even if the total effort is the same—because how fast they move matters for muscle growth.
When rats lift weights with quick, short reps, their leg muscles make more of the cellular machinery needed to grow bigger, compared to slow, long reps—and this might be why quick reps help muscles grow more.
When rats do strength exercises with very slow, long reps, they don't push as hard as when they do faster reps—even if the total effort is the same—and that might be why their muscles don't grow as much.