T Nation

Effects of Rep Tempo/Time Under Tension for Muscle?

Hii, im searching info about tempo, TUT and effects in muscle, if only uses for mTOR activation, for lactate or muscle damage.

I just wrote a new “Question of Strength” column that explain all of that in great details.

Here is the unedited version:


Q. Does Time Under Tension matter for hypertrophy or is mostly bro-science?

A. Time Under Tension refers to the duration, in seconds, of a set. For example, if you perform 10 reps with a 3 seconds eccentric/lowering phase and a 1 second concentric/lifting phase the TUT will be roughly 40 seconds.

Now, does it matter for muscle growth?

Yes and no.

I would say that it is not a primary factor involved in growth, and that the set duration itself isn’t really important, but the physiological response to a certain duration, might play a certain role in muscle growth.

Let me be clear: the TUT itself and the physiological response (e.g. lactate and growth factors accumulation during a set lasting 40-60 seconds) are not what I would call a growth stimulus. BUT they can increase the adaptation from the stimulus.

So, what is the main stimulus for growth?

If we were to use an equation it would look something like this:

(Muscle fiber recruitment + Mechanical loading of these fibers) X Number of reps = Hypertrophy stimulus

The number of muscle fibers recruited is dependent on the amount of force required relative to your maximum force production potential DURING THAT REPETITION.

You can increase the amount of force required relative to your maximum by:

  1. Using heavier weights (heavier weights = greater percentage of your maximum)

  2. Creating fatigue (by doing « higher » reps or using supersets) which decreases your relative maximum from rep to rep (because the muscle is tired).

  3. Accelerating as much as possible when lifting the weight (F= m x a). But we will see why this is good for strength and power, but not optimal for hypertrophy.

Mechanical loading refers to imposing a mechanical stress to the muscle fibers. Simply put, you must create as much muscle tension as possible the stretch the muscle fibers while tension is still there.

Tension is related to the amount of force the muscle needs to produce. The more force is required, the higher the tension will be.

Let’s briefly look at how muscles contract. Each muscle fiber has actin and myosin filaments. They connect with each other when the myosin heads hook themselves to the actin. This is called a cross-bridge.

Then, the myosin heads pull the actin, shortening the muscle.

It looks like this:

Now understand that the more force/tension you need to produce, the more bridges you will form.

During the eccentric phase of an exercise, when the muscles fibers are lengthening (being stretched), if the bridges remain connected, that’s when you create muscle damage and stimulate mTOR activation. The two most important hypertrophy stimuli.

The bridges can only shorten the muscle fiber, they cannot lengthen it.

If the bridges remain connected, and muscle tension is high, and you perform an eccentric action the load is lengthening the fibers while the bridges are trying to shorten them. This is mechanical stress and it is the main trigger for growth.

That’s why if you have zero control of the weight during the eccentric (very low tension), it is much less effective for growth because when you don’t actively resist the weight, muscle tension goes down, the number of bridges goes down, and the potential for muscle damage and mTOR activation is lower.

To maximise the mechanical stress you must:

  1. Create as many actin-myosin cross-bridges as possible (which is how muscle contracts; the more force you need to produce, the more of those bridges you create).

  2. Keep tension fairly constant during the repetition. That’s why too much acceleration can decrease mechanical loading because when you produce too much momentum, you need less force from the muscle to lift the weight, so you decrease the number of bridges formed.

  3. Lengthen/stretch the fibers while the tension remains high. Less tension means less actin-myosin cross-bridges which means less hypertrophy stimulation (that’s why it is more effective to do the eccentric under control, not fast). The stretching of the fibers while there are producing tension is responsible for the two main hypertrophy stimuli: muscle damage and mTOR activation.

Recruit as many fibers as you can, create a lot of bridges in those fibers, lengthen the fibers while as many bridges as possible are still formed. And do that over several reps. That’s it!


That’s why the number of repetitions is more important than the Time Under Tension.

With every rep you do, you have a new bout of mechanical stress via a new loaded lengthening of the muscle fibers.

Take two hypothetical scenarios to illustrate my point.

A) 10 reps with 225lbs and a 2010 tempo (TUT of 30 seconds)

B) 5 reps with 225lbs and a 5010 tempo (TUT 30 seconds)

Even if the load and TUT are the same; scenario A is more effective.

The number of times you stretch a muscle fiber in a set has an important impact on muscle damage and mTOR activation. See it this way: every time you get to stretch a fiber under load, you trigger growth.

Now if you add a third scenario where you are doing 10 reps with a 5010 tempo (so a TUT of 60 seconds) would it be more effective? Nope! Because to do that you would have to use significantly less weight. Probably something like 185-195lbs instead of 225lbs. The lower weight you lead to less muscle tension and each rep would be less effective.

CAVEAT: In that example, the longer time under tension will have some benefits that can increase muscle growth though. When you reach the 40-60 seconds range, if the intensity is high enough so that you reach failure in that time zone, you will be producing a lot of lactate and growth factors. Which can help with the adaptation to training.

If we look at the three scenarios the order of effectiveness would thus be:

10 reps at 225lbs with a 2010 tempo
10 reps at 195lbs with a 5010 tempo
5 reps at 225lbs with a 5010 tempo

And if we added a fourth scenario, 10 reps at 235lbs with a 10X0 tempo, it would likely be the least effective scenario out of the four.

By completely decreasing tension during the eccentric and by lifting explosively, you minimize mTOR and even muscle damage. This would be great for strength and power, but not great for hypertrophy.

The new order would be:

10 reps at 225lbs with a 2010 tempo
10 reps at 195lbs with a 5010 tempo
5 reps at 225lbs with a 2010 tempo
10 reps at 235lbs with a 10X0 tempo


TUT will never be the main growth stimulus. All it does is lead to certain physiological responses, like lactate and growth factor production, which can play a small role in hypertrophy.

Local growth factors can help stimulate protein synthesis slightly (which speeds up muscle tissue repair and building) while lactate can increase follistatin levels which can inhibtit myostatin a bit.

Lower myostatin leads to the possibility to build more muscle. But don’t jump on the lactate bandwagon just yet, it likely will not make a huge difference.

My recommendation is that on most exercises, for most people, trying to get gradually stronger for sets of 6 to 10 reps (some will say up to 12) while controlling the eccentric to maintain tension is the best way to stimulate growth.

I do like to shoot for longer TUT on exercises that won’t cause much muscle damage (isolation work, especially if you the eccentric is not loaded for the whole range like lateral raises, barbell curls, etc.) and with people who have a harder time repairing muscle damage (older individuals or people with a very high stress level.

For these cases shooting for a TUT of 40-60 seconds with a moderate weight ca be beneficial.

By the way, those of you who now my work, know that I use a lot of slow eccentric work. I’m not being contradictory: I use slow eccentrics for other reasons than stimulating maximum growth, like improving motor learning, strengthening tendons and becoming stronger eccentrically.


Thanks!!! Awesome the way that you are implicate with the knowledge.

Thank you so much for this detailed explanation, coach!