Moderate evidence

Rep speeds anywhere from about half a second to eight seconds per rep produce similar muscle growth.

In practice

Control the weight rather than counting seconds. Prescribed tempos aren't doing much beyond keeping you honest about not throwing the weight around.

What it doesn't say

Very slow reps — beyond roughly ten seconds — do appear to be worse for growth.

Source

• Schoenfeld BJ, Ogborn DI, Krieger JW (2015). Effect of repetition duration during resistance training on muscle hypertrophy: a systematic review and meta-analysis. Sports Medicine, 45(4):577–585. doi.org/10.1007/s40279-015-0304-0