When it comes to resistance training, the number of repetitions you can perform in a given time frame is an important consideration for muscle growth and strength development. Traditionally, you might perform enough reps of an exercise so that you work for 40 seconds or so without stopping. That’s about the time it takes to complete 10 to 12 reps, says Brad Schoenfeld, a leading expert in the field of muscle hypertrophy.
Time Under Tension and Muscle Growth
The concept of time under tension (TUT) has gained significant attention in the fitness community. TUT refers to the total amount of time a muscle or group of muscles experiences mechanical stress during a resistance exercise. Some fitness professionals have suggested that sets should have a TUT of 40 to 60 seconds to optimize muscle building.
However, research on the direct relationship between TUT and muscle hypertrophy is limited. A study by Burd et al. (2010) found that a slow tempo (6-0-6) with a 30% 1RM load resulted in greater increases in myofibrillar protein synthesis and intracellular anabolic signaling compared to a fast tempo (1-0-1) at the same intensity. But the study was confounded by the fact that the slow-tempo condition was performed to volitional failure, while the fast-tempo condition matched the number of repetitions.
Repetition Duration and Muscle Hypertrophy
A systematic review and meta-analysis by Schoenfeld et al. (2015) found that a wide range of repetition durations (0.5 to 8 seconds) resulted in similar increases in muscle hypertrophy when the sets were performed to volitional failure. This suggests that the total time under tension may be more important than the duration of individual repetitions.
Another study by Wilk et al. (2018) demonstrated that movement tempo in strength training impacts training volume, both in terms of repetitions and total time under tension. Slower tempos, particularly in the eccentric phase, resulted in a higher time under tension per set.
While the optimal TUT for muscle growth is still being researched, it appears that a wide range of repetition durations can be effective, as long as the sets are taken close to failure. The total time under tension accumulated over a training session or week may be more relevant than the TUT of a single set.