You curl the dumbbells up, then stretch out the return, counting seconds because more time under tension seems like it should mean more growth. But making each repetition slower has not shown a consistent muscle-building advantage. Controlled movement and the slowest possible movement are different goals.
A 2025 review of 14 studies found minimal overall growth differences when researchers changed the lifting or lowering phase. That leaves room for useful tempo work without making a particular cadence mandatory.
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Slower wasn’t a reliable growth upgrade
The review included 278 healthy adults, mostly young and untrained. It compared changes in muscle size after programs manipulating the speed of either the lifting or lowering phase.
Overall, the pooled difference between faster and slower conditions was small. That does not prove every tempo works identically in every situation.
Tempo instructions and monitoring varied. Repetitions also naturally slow near failure, complicating comparisons. The review did not establish one ideal count for your dumbbell curl.
Use a curl to check control
In its seated dumbbell-curl instructions, the American Council on Exercise keeps the head, shoulders and butt against a backrest, with feet on the floor. The basic version raises the weights without arching the back, keeps the wrists neutral, and gently lowers the dumbbells toward the starting position.
Those are observable movement checks. In that setup, does your back stay against the support, and can you return the weights deliberately? A longer countdown does not, by itself, tell you whether those parts of the exercise stayed consistent.
Also separate a phase from a whole repetition. As a hypothetical example, two seconds lowering plus one second lifting makes three seconds before adding any pauses. “Three seconds per rep” and “three seconds lowering” describe different instructions, rather than interchangeable targets.
If a coach gives you a count or pause while teaching a movement, ask what it is meant to change. A technique drill can serve a purpose without needing to be sold as the best muscle-growth strategy.
Match the bigger decisions to your goal
The ACSM’s 2026 resistance-training guidance emphasizes individual goals and regular participation. It distinguishes heavier loading for strength, higher weekly training volume for muscle growth, and an emphasis on fast lifting intent for power. Those goals are related, but they do not make the same demands.
That is a reason to ask what your curl is doing in the program before changing its cadence. Are you practicing the movement, completing planned muscle-building work, or following a specific coaching instruction? Choose the task before treating extra seconds as an improvement.
If your current dumbbells have become manageable but the next pair is too large a jump, our guide to adding reps before increasing dumbbell weight explains another progression option within a chosen rep range. Tempo, repetitions and load are separate adjustments; you do not need to change all three together.