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How much time under tension is needed for muscle growth?

This is an excerpt from Science and Development of Muscle Hypertrophy 3rd Edition by Brad Schoenfeld.

PRACTICAL APPLICATIONS
Is There an Ideal Time-Under-Tension to Maximize Muscle Growth?

While training volume is typically quantified through sets, repetitions, and total workload, the concept of TUT represents another potentially relevant variable. TUT can be operationally defined as the total time a muscle or a muscle group endures mechanical stress during resistance exercise. Some practitioners have proposed, albeit without robust empirical support, that an optimal hypertrophic stimulus occurs when sets last between 40 and 60 seconds.

Empirical research directly investigating TUT remains limited. In a notable study on the topic, Burd and colleagues (36) examined the acute effects of repetition tempo using a within-subject design. Participants performed unilateral leg extensions at 30% of the 1RM with a slow tempo (6-0-6) on one leg and a fast tempo (1-0-1) on the other. Each condition involved 3 sets separated by 2-minute rest intervals, resulting in approximately 6 times greater TUT in the slow-tempo condition. Post-exercise biopsies revealed higher rates of myofibrillar protein synthesis and greater activation of anabolic signaling pathways in the slow-tempo leg, with differences most pronounced 24 to 30 hours post-­exercise. However, interpretation of these findings is limited by the study’s design—slow-tempo sets were performed to volitional failure, whereas the fast-tempo condition was repetition matched and thus sets were terminated far from failure, thereby confounding the role of TUT with effort. Thus, these results underscore the importance of training with high levels of effort rather than isolating TUT as a causal factor in muscle growth.

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 (227). Importantly, the extended TUT in superslow protocols is accompanied by markedly lower loading intensities, complicating the interpretation of the independent effect of TUT. The precise interplay between tempo, load, and hypertrophic response remains unclear.

Although direct evidence is scarce, it is plausible that TUT contributes to hypertrophy through cumulative mechanical, and possibly metabolic, stress. However, its relevance may be more pronounced when considering total session or weekly TUT rather than the duration of a single set. Supporting this notion, my lab demonstrated comparable hypertrophy between a high-load, low-repetition (7 × 3RM) protocol and a moderate-load, moderate-repetition (3 × 10RM) protocol when total session TUT was similar (215). These findings contrast with a follow-up study from my lab showing that, when the total number of sets was equated, a bodybuilding-style workout (10RM) elicited greater hypertrophic adaptations compared to a powerlifting-style workout (3RM) (219). Here, the TUT was markedly greater for the bodybuilding-style workout both during each set and over the course of the training session.

It is also reasonable to infer that not all repetitions within a set contribute equally to muscle growth. For example, in a 25RM set, early repetitions impose minimal neuromuscular challenge; only when fatigue begins to manifest does the set become demanding. In contrast, the initial repetitions in a 6RM set are mechanically demanding and may promote a more potent hypertrophic stimulus. Thus, the effective TUT—periods during which muscle fibers are highly recruited and closer to failure—may be more relevant than total TUT per se. That said, there may be fiber type–specific relevance to different TUT durations. As mentioned earlier in the chapter, some evidence suggests that higher-repetition sets (and thus a greater intraset TUT) may target Type I fibers, resulting in their preferential hypertrophy, perhaps at the expense of complete stimulation of Type II fibers. Research on the topic remains equivocal and requires further study.

In summary, TUT appears to influence hypertrophic outcomes, although its effects are interdependent with other training variables such as load intensity, repetition range, and tempo. The cumulative TUT accrued across a session or training week may be a more meaningful determinant of muscle growth than TUT per set. While it is conceivable that higher per-set TUT (>45 seconds) may preferentially stimulate Type I fibers, empirical confirmation is lacking, and further research is needed to establish definitive guidelines.

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