Time Under Tension Myth: Do Slow Reps Build Muscle?

Time under tension, or TUT for short, refers to the amount of time your muscles are working during a set. By taking longer to lift and lower the weight, you make your sets last longer. This in turn is supposed to speed up muscle growth.

In fact, most research shows that slowing down each rep won’t automatically your muscles grow faster than normal.

Your muscles can be made to grow with heavy sets lasting less than 20 seconds or lighter sets lasting 60 seconds or more.

Slower reps might make your workout feel harder, but that doesn’t mean your muscles will grow any faster.

What Is Time Under Tension?

When people talk about time under tension, or TUT for short, they’re usually referring to the amount of time your muscles are working, or under strain, during a set.

For example, let’s say you do a dumbbell curl. It takes two seconds to lift the weight and two seconds to lower it. During each rep, your biceps are under tension for a total of four seconds.

Performing a set of 10 reps at the same speed would take a total of 40 seconds. Therefore, the total length of time your muscles are under tension during that set is 40 seconds.

If you slow down each rep so that it takes six seconds, it would take 60 seconds to complete a set of 10 reps. In which case, the total TUT is now 60 seconds.

Proponents of TUT training programs also like to prescribe a specific rep tempo, where you count the number of seconds it takes to lift and lower the weight, as well as the length of time you pause at the top and bottom of the movement.

For example, a 4-1-2-1 tempo means:

• 4 seconds to lower the weight (the lengthening or eccentric phase)
• 1 second to pause at the bottom
• 2 seconds to lift the weight (the shortening or concentric phase)
• 1 second to pause at the top

By reducing your repetition speed and using a slower tempo, the muscle is under strain for a longer period of time.

Rather than power through six reps in 20 seconds, for example, you slow them down so that a set lasts somewhere between 50 and 60 seconds. This will make your muscles grow more quickly than they otherwise would do. 

That’s the idea, anyway.

In reality, a TUT training program that involves a slower repetition speed won’t automatically make your muscles grow faster than normal. In some cases, your results may end up getting worse rather than better.

Time Under Tension: Hypertrophy, Strength and Endurance

The standard advice is that to maximize strength gains, the ideal time under tension is about 20 seconds or less; to build muscle, it’s at least 40 seconds; and for muscle endurance, it’s at least 70 seconds.

In truth, your muscles can be made to grow with heavy sets lasting less than 20 seconds or lighter sets lasting 60 seconds or more, just as long as you do enough sets and push yourself hard in each one.

In one trial, researchers got a group of guys to train their legs on the leg extension machine three times a week for 10 weeks [1].

In the first group, the time under tension for each set was 30-48 seconds. Group two, on the other hand, took a longer period of time( 90- 120 seconds) to perform each set.

• Group 1 = 30-48 seconds per set
• Group 2 = 90-120 seconds per set

Which group do you think built the most muscle?

Despite the big difference in time under tension, the amount of new muscle added to both legs was almost identical.

In other words, a time under tension that was deep in the endurance range stimulated just as much muscle growth as sets that straddled the strength and hypertrophy range.

Time Under Tension Training: Bodybuilding vs Powerlifting

In a study published in the Journal of Strength and Conditioning Research, two groups of trained men took part in either a bodybuilding-type (3 sets of 10 reps) or powerlifting-type (7 sets of 3 reps) routine [2].

The time under tension per set for the bodybuilding-type routine was 30-40 seconds, while in the powerlifting-type routine it was roughly 9-12 seconds.

After two months, both groups registered an almost identical amount of muscle growth.

It was much the same story when a team of University of Central Florida researchers put a group of 33 resistance-trained men through eight weeks of resistance exercise [3].

The subjects were divided into two groups.

Group one did four sets of 10-12 reps, with each rep lasting 3-4 seconds. Group two did the same number of exercises and sets. But they used a much heavier weight that limited them to 3-5 reps.

The training program used in the study involved compound lifts like squats, deadlifts, bent over rows, as well as the bench press and overhead press.

Here’s what the time under tension looked like in both groups:

• Group 1 = 30-48 seconds per set
• Group 2 = 9-20 seconds per set

The result?

None of the differences in body composition between the groups were statistically different. However, the researchers did find a clear trend towards greater gains in the group lifting heavier weights.

In other words, the men who trained using sets lasting 20 seconds or less were the ones that put on the most muscle.

A Slower Tempo Means Less Weight

The big downside with slowing down your reps and trying to make a set last longer is that you limit the amount of weight you can lift.

Let’s say the best you can manage on the bench press is 200 pounds for a total of 10 reps.

You do each rep in the traditional way. That is, you lower the weight under control, to your chest. Then you press it back to the start position in a fast, but controlled, fashion.

If we assume the eccentric phase of the exercise takes a couple of seconds, and another second to lift it again, each rep will last a total of three seconds. Your set of 10 reps lasts roughly 30 seconds.

What’s going to happen if you try to do each rep with a slower tempo? 

That is, you double the length of each rep so that it takes six seconds — three seconds on the way down and three seconds on the way up.

How much weight do you think you’d be able to lift, compared to what you were doing before?

There’s no way you’re going to be able to lift that same 200 pounds for 10 reps if you’re taking six rather than three seconds to do each rep.

If you want to increase the length of the set, you’ll need to drop the weight [4]. By lifting so slowly, you automatically limit the poundages you can handle on any given exercise.

Of course, you might argue that the increase in time under tension is more than enough to offset the reduction in weight. But you’d be wrong to do so.

When researchers have put slower lifting speeds to the test, the results have been less than impressive.

Slow Reps vs Fast Reps for Muscle Growth

The idea that you need to make a set last for a fixed amount of time has led some people to believe that slower lifting speeds work better for muscle growth — which they don’t.

Look at a slice of muscle tissue under a powerful microscope, and you’ll see that it’s made up of many smaller muscle fibers.

During a bout of resistance exercise, those muscle fibers are called into action to move the weight from point A to point B.

If the training you do is sufficient to stimulate those muscle fibers, your body responds in the hours and days that follow by laying down new muscle proteins, which makes those muscle fibers thicker, a process known as hypertrophy.

However, performing reps at a fixed speed of four seconds per rep versus a self-selected speed has been shown to decrease both muscle fiber activation and training volume [5].

That’s a step in the wrong direction if you want to make your muscles grow.

In fact, increasing time under tension by slowing down your reps has not been shown to deliver superior gains in muscle size.

Do Slow Reps Build Muscle?

Slow reps do build muscle. However, most research shows that they’re no more effective than faster (but still controlled) lifting speeds.

Researchers from the University of Sydney, for example, report that taking six seconds to do a dumbbell curl is no better for muscle growth than a rep lasting two seconds [6].

After six weeks of resistance exercise, there was no evidence that one rep speed was better than the other for increasing arm size. In fact, strength gains were roughly 10% greater with the faster reps.

It was much the same story when Japanese scientists compared slow and fast lifting speeds [5]. Reps lasting six seconds didn’t work any better than reps lasting three seconds for increasing whole-body muscle thickness or maximal strength.

A follow-up study, which involved the parallel squat done twice a week for six weeks, also shows no benefit to slowing down the eccentric, or lowering phase, of a rep [7].

That is, subjects taking two seconds to lift the weight and four seconds to lower it didn’t grow any faster than lifters who lowered the weight in two seconds.

What’s more, it was the faster group who gained the most strength.

They finished the study being able to squat 18 pounds (8 kilograms) more than the four-second group, despite the fact that both groups started out in roughly the same place.

A similar trial, this time using the leg extension, found that reps lasting four seconds were no more effective for building muscle than reps lasting two seconds [8].

There’s more.

Scientists from Brazil found that spending extra time on a rep by increasing the duration from two to six seconds made no difference to the speed at which muscle was gained [9].

Subjects taking part in the study trained each leg twice a week, doing 3-4 sets on the leg extension machine, with each set separated by 3 minutes of rest.

With leg extensions, you normally work both legs together. But in this study, the men trained one leg at a time, which meant that each leg could be trained using different rep speeds.

All sets were taken to failure, where the subjects were unable to complete another rep through a full range of motion.

The amount of weight lifted, number of sets, as well as the amount of rest taken between each set, was identical.

After 14 weeks of training, ultrasound scans showed no significant difference in muscle growth between the two legs.

Irrespective of rep speed, rectus femoris and vastus lateralis – two of the muscles that make up the quads – grew at much the same rate.

One of the main limitations of this study is the fact it was done on untrained beginners, who tend to grow no matter what they do.

Would trained participants respond in the same way?

A follow-up study, published in the Journal of Strength and Conditioning Research, done with trained versus untrained subjects, also shows no advantage to slower rep speeds [10].

This time, a group of men who’d been lifting weights for at least three years, and were able to squat an average of double their bodyweight, took part in the study.

The study was set up in much the same way. 

Participants trained one leg with a fast rep speed, taking 1 second to lift and lower the weight. The other leg was trained with a slower tempo, 1 second to lift the weight and 3 seconds to lower it.

The men did sets of 8-10 reps in weeks 1–4, with a fourth set added in weeks 5–8. 

A 2-minute and 3-minute rest interval was allowed between sets and each leg, respectively.

To ensure that differences in training volume didn’t skew the results, volume load (sets x reps x weight) was equated between legs.

After eight weeks of training, there was no significant difference in muscle growth whether the reps were done using a fast or slow speed.

Much like the findings in untrained subjects, slower reps had no benefits for hypertrophy.

In short, as long as you’re lifting and lowering the weight under control, rather than letting the weight control you, there’s very little point in trying to make a set last for a specific amount of time.

Is Time Under Tension a Myth?

Time under tension isn’t a myth in the sense that putting muscle fibers under mechanical tension is one of the factors required to stimulate hypertrophy.

What is a myth is the idea that the length of time a muscle is under tension during a set is of critical importance when it comes to building muscle.

You also need to consider 1) the degree of tension and 2) the total amount of tension a muscle is exposed to during a workout.

From Dr Brad Schoenfeld, author of Science and Development of Muscle Hypertrophy:

Mechanical tension is directly related to the magnitude of load or weight you’re lifting. If you perform a rep at your 1 rep max (RM), it will necessarily create more mechanical tension than a rep performed at 50% 1RM. Thus, sets of long durations will necessarily involve lower levels of tension than those of shorter durations, assuming training is carried out near or to momentary muscular failure.”

Dr Brad Schoenfeld

In other words, the muscle-building stimulus generated by a given bout of resistance exercise depends not just on the length of each set, but also:

• The amount of weight you lift during those sets
• Intensity of effort, or how close each set is taken to momentary muscular failure

You will no doubt come across many people saying that time under tension is “important” when it comes to muscle growth, which of course it is.

However, saying that TUT is “important” doesn’t mean anything, because your muscles are under tension for a given amount of time during ANY type of resistance exercise.

And changing the speed at which you perform each repetition is far from being the only way to alter TUT. You can also:

• Do more sets of a given exercise.
• Do more exercises per muscle group.
• Increase the number of reps.
• Train a muscle group more frequently.

Making a set last longer by lifting a lighter weight and slowing down each rep is a long way from being the only way, or even the most effective way, to increase time under tension.

Time Under Tension vs Reps

Is it better to count reps or time under tension? In most cases, you’re far better off counting reps. There’s very little benefit in counting the number of seconds it takes to complete each rep or calculating time under tension for each set.

The length of time a muscle is under tension for during a set is a lot less important than the total amount of tension it’s exposed to during a workout, the degree of tension, as well as how often that tension is applied over the course of a week.

None of this means that strength training programs using lighter weights and higher reps won’t make your muscles bigger.

In fact, training with lighter weights and higher reps is a highly effective way to stimulate growth in both the fast twitch and slow twitch muscle fibers, just as long as you train with a sufficient intensity of effort (i.e. you make sure your muscles are in a high state of fatigue by the end of a set).

Final Thoughts

As long as your resistance training program is set up properly, and you’re using a lifting speed that’s appropriate for the exercise you’re doing and the weight you’re lifting, time under tension isn’t something you need to spend any time measuring or even thinking about.

A lot of people go to the gym, train hard, and get bigger and stronger without paying the slightest bit of attention to time under tension. I’d highly recommend that you do the same.