The TUT (time under tension) method refers to how long the muscle is under tension, i.e. contracts while resisting weight during each set.
In other words, it’s a way of calculating the total amount of work you place on a muscle. Since the key factors in muscle growth are the duration of the stimulus and the amount of tension, it’s generally accepted that maximizing muscle growth requires training with sets that last in the range of 30 to 60 seconds.
Muscles put under longer bouts of strain will experience a more extensive breakdown, resulting in increased growth. That being said, the method does not differentiate among concentric, isometric and eccentric contractions, which are all included in a traditional set.
A recent study done by researchers from Brazil and the U.S. conducted a study in which 22 male subjects (who had previous training experience) performed two different training protocols on the Smith machine. While both protocols included three sets, three minutes of rest and one-rep maximum, the slow protocol had six reps with a six-second rep duration and the fast protocol consisted of 12 reps with a three-second rep duration.
Although the time under tension was the same for both protocols, the results showed that muscle activation and blood lactate concentrations were higher in the subjects performing the fast protocol. This means that in order to adequately activate the muscle, the duration of your reps shouldn’t be lower than three seconds.
Furthermore, other researches have shown that the optimal number of sets for muscle growth is in the range of 8 to 12, while the best way to gain muscle strength is to perform 1-6 reps per set.