This is an excerpt from Science and Development of Muscle Hypertrophy-2nd Edition by Brad Schoenfeld.
While resistance training volume is generally thought of in terms of sets, repetitions, and total work, a concept called time under tension (TUT) also can be considered a relevant variable. TUT can be operationally defined as the total amount of time that a muscle, or group of muscles, endures mechanical stress during resistance exercise. Anecdotally, some fitness professionals have put forth the claim that sets should have a TUT of 40 to 60 seconds to optimally build muscle.
Research into the role of TUT in muscle development is limited. In one of the few studies that attempted to directly investigate the topic, Burd and colleagues (32) carried out an acute, within-subject design in which subjects performed a leg extension exercise at 30% of 1RM with a slow tempo (6-0-6) with one leg and trained the other leg at the same intensity of load with a fast tempo (1-0-1). Three sets were performed for each condition with a 2-minute rest interval between sets, resulting in a 6-fold greater TUT in the slow-tempo condition. Post-exercise muscle biopsies showed significantly greater increases in myofibrillar protein synthesis and intracellular anabolic signaling favoring the slow-tempo condition; differences primarily manifested 24 to 30 hours after the training bout. While on the surface these findings seemingly support the importance of TUT as a driver of hypertrophy, conclusions were confounded by the fact that people in the slow-tempo condition performed all sets to volitional failure while the number of repetitions performed for the fast-tempo condition were matched to that of the slow-tempo condition. Thus, rather than providing insights into the hypertrophic effects of TUT, the results reinforce the importance of challenging the muscles with a high level of effort for muscle building.
Studies comparing superslow training to traditional training whereby both conditions are performed to volitional fatigue have not shown a benefit from higher TUTs; in fact, evidence indicates training in a traditional fashion produces superior hypertrophy despite a substantially lower TUT (221). A caveat to these findings is that the higher TUT in the superslow condition was at the expense of a much lower intensity of load. How these variables interact with one another to affect muscle growth is not clear.
Despite the paucity of objective evidence, a logical case can be made that TUT does play a role in hypertrophy. However, it appears the effects are more related to the time a muscle is worked over the duration of a training session than to the TUT for a given set. In support of this hypothesis, my lab showed that performing a powerlifting-style workout consisting of 7 sets of 3RM produced increases in muscle growth similar to those of a bodybuilding-style workout consisting of 3 sets of 10RM (208). Although TUT in the powerlifting-style sets was markedly lower than in the bodybuilding-style sets (~9 seconds vs. ~30 seconds, respectively), the total TUT for the training session was approximately equal because of the greater number of sets performed for the powerlifting-style condition. These findings are in contrast to a follow-up study showing that when the total number of sets were equated, a bodybuilding-style workout (10RM) elicited greater hypertrophic adaptations compared to a powerlifting-style workout (3RM) (213). Here, the TUT was markedly greater both during each set as well as over the course of the training session.
It also can be hypothesized that not all repetitions equally contribute to hypertrophy. For example, the initial repetitions in a set of 25RM are relatively easy to execute; only when fatigue begins to manifest does the set become challenging. In contrast, the initial repetitions during a 6RM set are substantially more challenging to complete from the outset, and conceivably would promote greater anabolic stimulation. A case therefore can be made that the TUT in the 6RM protocol would have greater hypertrophic relevance than that of the higher-repetition set. Hence, to some extent TUT should be considered in the context of the repetition range in which a set is performed and the corresponding duration of repetitions that are challenging to complete.
Another inherent issue with TUT is that it considers the duration of the repetitions as a whole and thus neglects to take into account the individual portion of the actions. For example, a set carried out at a 4-0-1 tempo (4-second concentric actions, 1-second eccentric actions) would have the same TUT as a set carried out at a 1-0-4 tempo (1-second concentric actions, 4-second eccentric actions), provided the number of repetitions is equated between sets. This has potentially important implications given the research that shows differential intracellular signaling and hypertrophy responses (72) between concentric and eccentric actions.
All things considered, evidence indicates that TUT plays a role in muscle hypertrophy. However, its implications must be considered in the context of the resistance training variables comprising a given routine (i.e., repetition range, tempo of eccentric versus concentric actions). Within limits, it appears that the total TUT accumulated for a muscle group in a given session, or perhaps over time (e.g., weekly), has the most relevance from a muscle growth standpoint. A rationale for speculation exists whereby a longer TUT (>60 seconds per set) may be beneficial for targeting hypertrophy of Type I muscle fibers; this hypothesis warrants further exploration.