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What is time under tension?

This is an excerpt from M.A.X. Muscle Plan 2.0-2nd Edition, The by Brad Schoenfeld.

Time Under Tension

Some fitness pros have put forth the hypothesis that an optimal time under tension (TUT) exists for hypertrophy training. Simply stated, TUT is the duration of a set; the longer the set, the higher the TUT. Although TUT recommendations differ somewhat between proponents of the theory, most peg the hypertrophic sweet spot at approximately 30-60 seconds. While the concept of TUT may sound good in theory, the question is, does it have research-based support?

My doctoral dissertation work provides some intriguing insights into the topic. The study investigated muscular adaptations between a bodybuilding-style routine (three sets of 10 reps per exercise) versus a powerlifting-style routine (seven sets of three reps per exercise); the additional sets performed by the powerlifting group equated the total work performed between groups (18). The results: After eight weeks of training, both groups showed similar increases in hypertrophy. Now here’s the interesting thing pertaining to TUT. The duration of the sets for the powerlifting group was much shorter compared to the bodybuilding group, which seemingly refutes the concept of an optimal TUT. However, because the powerlifting group performed more than twice as many total sets as the bodybuilding group, the total TUT per exercise was similar over the course of the study. This suggests that the TUT for a given muscle should be viewed in the context of a training session (or perhaps across weekly sessions to account for training frequency) as opposed to the duration of a set. In support of this theory, we carried out a follow-up study that again compared muscle growth in powerlifting- versus bodybuilding-type training; however, as opposed to my dissertation study, both groups performed three sets per exercise. Thus, TUT for each session was substantially higher in the bodybuilding group. At the conclusion of the eight-week training period, results showed that the bodybuilding group achieved greater increases in muscle size compared to the powerlifting group. Although causality cannot be drawn from correlation, the findings of these two studies when taken in combination seem to suggest that per-session TUT is more relevant to hypertrophy than a per-set basis.

Note that training tempo impacts TUT. If you lift slower at a given rep range, then TUT necessarily increases during the set. If an optimal per-set TUT does in fact exist, we’d expect that slower concentric tempos promote greater hypertrophy than faster tempos. However, there is no evidence this is the case when training is carried out with equal levels of effort (19). In fact, performing concentric reps very slowly seems to have somewhat of a detrimental effect on growth (see the Superslow Training sidebar in chapter 2). Moreover, TUT doesn’t distinguish between durations of the concentric and eccentric actions. As discussed in chapter 2, evidence shows that the two actions may benefit from different training tempos. Thus, it would seem misguided to calculate TUT as a single value without considering the individual components that compose a repetition.

Bottom line: The concept of an optimal TUT for muscle building is overly simplistic and lacks supporting objective evidence. It’s better to focus on total training volume (as discussed in chapter 2); if you do that, achieving a sufficient TUT ultimately will take care of itself.