When it comes to maximizing muscle growth, it’s far from uncommon to hear that time under tension is an important component.
Time under tension refers to the duration you place a muscle under strain. Typically, it’s represented as the duration of a set for an exercise.
For instance, performing 10 reps on the biceps curl with each rep consisting of a 2-second lifting phase and a 2-second lowering phase results in a total time under tension of 40 seconds.
It’s often claimed a longer time under tension maximizes hypertrophy.
It’s thought longer durations place the muscle under more strain, resulting in a more powerful stimulus for hypertrophy.
For example, numerous sources claim around 30-60 seconds of time under tension per set is optimal for building muscle.
In this article, with help from scientific research, we’ll critically assess these claims.
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Evidence Supporting Time Under Tension
Indeed, some evidence indicates a longer time under tension may enhance muscle growth, but the details are important.
Usui et al. divided 16 untrained young men into a slow or normal speed group.
Both groups trained the back squat to around 90 degrees of knee flexion for 3 sets of 10 reps with a 50% one-rep max load, three times per week for 10 weeks.
The slow speed group performed each rep with a 3-second lowering phase and a 3-second lifting phase.
The normal speed group performed each rep with a 1-second lowering phase and a 1-second lifting phase.
Thickness of the rectus femoris and vastus intermedius was measured at 30, 50, and 70% of the thigh length, while thickness of the vastus lateralis and vastus medialis was measured at 50% of the thigh length.
The normal speed group experienced virtually no growth at all measured areas, while the slow speed group experienced significant increases at 70% of the thigh length of the rectus femoris as well as 50 and 70% of the thigh length for the vastus intermedius.
Taken at face value, this study indicates slower repetition speeds result in greater muscle growth.
However, there is a noteworthy limitation with this study.
Both groups performed 3 sets of 10 reps with a 50% one-rep max load on the back squat each session.
The slow speed group would have actually been training closer to failure. Failure is defined as the point at which you cannot perform any more repetitions with a given load.
This is because, with a given load, slower rep speeds mean you perform fewer reps.
You can imagine this for yourself. When keeping load and repetition number constant, using slow rep speeds would be more fatiguing compared to faster rep speeds.
As a result, we don’t know if the greater muscle growth observed for the slow speed group was actually because of the longer time under tension or because they were training closer to failure.
As far as I’m aware, there are only two other studies in the research supporting the idea slower repetition tempos produce greater increases in measures of muscle growth.
But both of these studies share the same limitation in that the slower repetition conditions were training closer to failure.
Watanabe et al. had a group of untrained men perform 3 sets of 8 reps with a 50% one-rep max load on the knee extension and leg curl, twice per week for 12 weeks.
A slow speed group performed each exercise with a 3-second lowering and 3-second lifting phase, while a normal speed group used a 1-second lowering and 1-second lifting phase.
Increases in quadriceps and hamstring thickness were significantly greater for the slow speed group.
However, all subjects trained with 3 sets of 8 reps with a 50% one-rep max load. Consequently, the slow speed group was training closer to failure.
A second study by Watanabe et al. had untrained men train the knee extension for 3 sets of 12 reps with a 30% one-rep max load, twice per week for 12 weeks.
A slow speed group performed the exercise with a 3-second lowering and 3-second lifting phase, while a normal speed group used a 1-second lowering and 1-second lifting phase.
Increases in quadriceps cross-sectional area favored the slow speed group.
Both groups trained with 3 sets of 12 reps with a 30% one-rep max load, again meaning the slow speed group would have been training closer to failure.
Collectively, although in these three studies, the slow speed groups experienced greater muscle growth, we cannot be certain the greater growth was because they trained with a longer time under tension or because they trained closer to failure.
The only way we can know is to look at studies that not only had groups use different repetition speeds, but also had subjects perform their repetitions to failure.
Research on Time Under Tension With Failure Training
Let us evaluate a recent study by Lacerda et al., which I believe to be the best study to date exploring if a longer time under tension enhances muscle growth.
10 untrained men had one leg assigned to a 2-second condition, and their other leg assigned to a 6-second condition.
Both legs trained the unilateral knee extension for 3-4 sets to failure with a 50-60% one-rep max load, 2-3 times per week for 14 weeks.
The leg assigned to the 2-second condition performed the exercise with a 1-second lifting phase and a 1-second lowering phase.
The 6-second condition leg performed the exercise with a 3-second lifting phase and a 3-second lowering phase.
I should emphasize again that both conditions involved performing repetitions to failure with their respective repetition speed.
On average, the 6-second condition involved a time under tension of between 37 to 52 seconds, whereas the 2-second condition involved a time under tension between 25 and 38 seconds, so some of their sets were performed under the 30-second threshold which numerous sources recommend.
However, both conditions produced similar increases in rectus femoris and vastus lateralis cross-sectional area.
Therefore, when repetitions are performed to failure, a longer time under tension does not appear to enhance muscle growth.
Another recent study by Chavez et al. supports these findings.
Untrained men performed the unilateral knee extension for 3 sets to failure with a 70% one-rep max, twice per week for 8 weeks.
Some subjects trained one leg with a 2-second lifting and 2-second lowering phase, called the 4-second condition, and their other leg with a self-selected repetition speed. On average, subjects performed a 0.9-second lifting and 0.9-second lowering phase with this leg.
Therefore, time under tension was greater with the 4-second condition leg.
Again, I should emphasize both conditions involved performing repetitions to failure.
However, increases in vastus lateralis cross-sectional area were similar between both conditions.
Now, these are not the only two studies assessing the impacts of different time under tension durations on direct measures of muscle growth when subjects carry out their repetitions to failure.
A meta-analysis by Schoenfeld et al. sums up the other studies.
For those unaware, meta-analyses are studies that combine the results of numerous individual studies looking at the same topic.
Combining the results of 4 studies, increases in measures of muscle growth were similar between fast and medium repetition speeds. In this analysis, fast rep speeds were classified at repetitions that lasted between 0.5 to 4 seconds, whereas medium rep speeds were repetitions that lasted between 4 and 8 seconds.
Therefore, this analysis indicates that provided repetitions are performed to failure, there is no difference in muscle growth between performing reps with a 0.5-second duration or 8-second duration.
Super Slow Rep Speeds Are Inferior?
Interestingly, there is a study not included in the Schoenfeld et al. analysis suggesting that super slow rep speeds may actually be inferior for building muscle.
Schuenke et al. had untrained women perform the leg press, squat, and knee extension for 3 sets, 2-3 times per week for 6 weeks.
Some subjects performed each exercise to failure with a 1-2 second lifting phase and a 1-2 second lowering phase using an 80-85% one-rep max load. This was the traditional strength group.
Other subjects performed each exercise to failure with a 10-second lifting phase and a 4-second lowering phase using a 40-60% one-rep max load. This was the super slow group.
Increases in vastus lateralis slow-twitch and fast-twitch fiber cross-sectional area were significantly greater for the traditional strength group compared to the super slow group.
Now, this is only one study, so I believe we cannot be truly confident repetition tempos longer than 10 seconds are suboptimal. More research is needed.
Why Slow Reps and Fast Reps Are Likely Similarly Effective For Hypertrophy
So, when overviewing the current evidence, it appears repetition durations lasting from 0.5 seconds up to 8 seconds produce comparable muscle growth, opposing the notion a longer time under tension enhances growth.
Now, all of these studies were conducted on untrained subjects. That includes the 4 studies in the Schoenfeld et al. meta-analysis.
One might argue that in trained individuals, a longer time under tension would enhance muscle growth.
To my knowledge, there aren’t any papers with trained subjects in this area unfortunately.
However, physiologically, I believe it seems unlikely a longer time under tension would enhance muscle growth, provided repetitions are performed to or very close to failure.
I say this because, irrespective of repetition speed, mechanical tension would likely be adequately high provided repetitions are carried out near failure.
Mechanical tension is currently the best-categorized mechanism of muscle growth.
Mechanical tension is equal to the force produced by a muscle.
One key component of how much force a muscle produces is the number of muscle fibers recruited.
In a very general sense, muscles consist of slow-twitch fibers and fast-twitch fibers.
Slow-twitch fibers are highly resistant to fatigue but produce low amounts of power, whereas fast-twitch fibers are less fatigue resistant but produce high amounts of power.
Generally, when force requirements are low, slow-twitch muscle fibers are recruited, but as force requirements increase, faster-twitch muscle fibers are recruited.
High levels of muscle fiber recruitment, so high levels of slow-twitch and fast-twitch fiber recruitment, is one important component for inducing muscle growth.
Regardless of time under tension, performing repetitions to or very close to failure likely results in sufficiently high levels of muscle fiber recruitment.
A study by Morton et al. demonstrates this.
A group of men with at least 2 years of training experience performed the leg extension for 3 sets with 4 different conditions.
One condition involved performing repetitions to failure with an 80% one-rep load using a 1-second lifting, 1-second pause, and 1-second lowering phase. On average, the time under tension for this condition was 26 seconds per set.
The second condition involved performing repetitions to failure with an 80% one-rep max load too, but using a 3-second lifting, 1-second pause, and 3-second lowering phase. On average, the time under tension for this condition was 31 seconds per set.
The third condition involved performing repetitions to failure with a 30% one-rep max load using a 1-second lifting, 1-second pause, and 1-second lowering phase. On average, the time under tension for this condition was 54 seconds per set.
The fourth condition involved performing repetitions to failure with a 30% one-rep max load using a 3-second lifting, 1-second pause, and 3-second lowering phase. On average, the time under tension for this condition was 76 seconds per set.
The researchers found that all four conditions resulted in similar recruitment of slow-twitch and fast-twitch fibers from the vastus lateralis. Note, recruitment was measured via glycogen depletion of the fibers.
Put differently, regardless of load or repetition speed, overall muscle fiber recruitment was likely similar.
This supports the idea that provided repetitions are carried out near to failure, mechanical tension is likely sufficiently high regardless of the repetition tempo used.
Supporting Evidence From Rep Range Research
The research on rep ranges provides further evidence that a longer time under tension does enhance muscle growth.
This includes research done on trained subjects.
For instance, Morton et al. found that in men with at least 2 years of training experience, there was no difference in vastus lateralis fast-twitch and slow-twitch fiber growth after training the leg press and knee extension with either 8-12 reps to failure or 20-25 reps to failure.
Of course, performing 20-25 reps means you experience a longer time under tension versus performing 8-12 reps, yet this did not matter.
Minimal Time Under Threshold?
Finally, I should mention that theoretically, there may be a minimal time under tension threshold per set required for optimal hypertrophy.
For instance, we’ve detailed in a previous article that one-rep max training does not produce muscle growth.
Therefore, accumulating some reps and consequently some time under tension is likely needed to elicit optimal adaptations.
As we’ve mentioned already, reps between 5 and 35 are likely similarly effective for muscle growth, when reps are carried out to or near failure. Therefore, the time under tension accumulated with 5 normal speed reps may be the minimal time under the tension threshold per set.
Although, as we’ve discussed in a previous article, reps as low as 3 could build muscle effectively, so the threshold could be even lower.
Ultimately, it would be interesting to see more research evaluating the minimal time under tension threshold for muscle growth.
In conclusion, I believe there is sufficient evidence to suggest that a longer time under tension does not enhance muscle growth. Furthermore, the numerous recommendations that sets should last from 30-60 seconds are not substantiated.
However, this should not be misinterpreted to say that using a longer time under tension is useless.
Slowing your repetition speeds could be a way of applying progressive overload. For instance, in one session you may perform 3 sets of 8 reps with 20kg on an exercise using a 2-second lifting and 2-second lowering phase. Next session, you may use the same load, reps, and sets but use a 3-second lifting and 3-second lowering phase.
Additionally, using a variety of repetition speeds in your training program could potentially be one effective way of ensuring more balanced growth across a muscle, as there is some evidence that different repetition tempos produce different regional muscle growth. We’ve covered the research on this in a previous article.