This Determines How Much Muscle You’ll Gain from Your Training

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00:00 What makes muscles grow?

01:02 Time under tension and repetition tempo

02:03 Hypertrophy formula – simplifications to get to the Henselmans hypertrophy model

03:17 Model vs repetition tempo

03:47 Model vs training volume and intensity

05:37 Model vs training to failure

06:51 Model vs rest intervals

09:22 Model vs training frequency

12:33 Model limitations and conclusion


The secret sauce of muscle growth is mechanical tension. Mechanical tension is by far the most important driver of muscle growth. Research is very clear on this. We can decompose mechanical tension into active and passive components, and we can also decompose it into the level of tension and the time under tension and is the integral or the area under the curve. Over time, that seems to determine how much muscle we gain from our training. The level of total tension is approximated very well by our training intensity. If you lift heavier weights, your muscles have to produce more force, which requires greater tension to generate. So the weight on the bar is directly proportional to the amount of tension in your muscles. Higher training intensities therefore mean higher levels of muscle tension.

However, the higher the training intensity, the lower the time under tension, because you cannot do as many reps with a heavier weight. time under tension, we can measure by the number of reps that you do multiplied by your repetition tempo. That is logically how long your sets take. however, repetition tempo is a little bit tricky in this regard, it may not be necessary to measure because if you lift with a slower tempo, you perform your reps at a slower speed. You can normally not do as many reps. So this typically balances out. If you lift with a faster tempo, you can do more reps but shorter, more frequent reps. But if you lift with a slower tempo, you can not do as many reps, but the reps last longer. So this kind of washes out. and there’s also an interaction effect with metabolic stress. If the set lasts longer and if you lift more explosively in turn, If the set lasts longer and if you lift more explosively in turn, that can increase the amount of force production. If you lift more explosively higher force output.

Therefore that comes from higher mechanical tension in the muscle, force and tension are inside the muscle. They are the same. It’s the tension that’s producing the force. So repetition tempo kind of washes out and is a little bit hard to measure. So repetition tempo kind of washes out and is a little bit hard to measure. So repetition tempo kind of washes out and is a little bit hard to measure. So let’s simplify the time under tension to the total amount of reps. It then logically follows that the amount of hypertrophy you’re going to get, how much muscle you’re going to build is the product of how many reps you’re doing with a given training intensity or how many reps you’re doing with a given weight.

If you’re using different weights for your sets or for different exercises, we can simplify things even further because just like for repetition tempo we can simplify things even further because just like for repetition tempo with training intensity, we know that if you lift a heavier weight, you have higher tension but less time under tension. So just like with repetition, tempo, let’s call this a wash So if the training intensity changes, we can simplify our measure of muscle growth further to the total number of sets per unit of time. of muscle growth further to the total number of sets per unit of time. Let’s call it a week for convenience sake per muscle group. So if the training intensity is the same, we can say that hypertrophy So if the training intensity is the same, we can say that hypertrophy is approximated or predicted quite well by the number of reps that you do. If the training intensity differs, we would expect hypertrophy to be predicted well simply by the number of sets per week per muscle group that you’re doing.

There, We just made the Henselmans hypertrophy model or Menno’s muscle model, if you want. Now let’s look at how this super simplified model holds up against the scientific research. Let’s start with repetition tempo. Repetition tempo I discussed in a previous video. Indeed, just as our model predicts repetition tempo seems to be mostly a wash. The tempo that you use to perform your exercises doesn’t have a lot of effect on muscle growth. As long as you at least control your weights within reason, it seems that whether you do two or eight second repetitions or even one second repetition sometimes, you can get equivalent results in terms of muscle growth. So our model performs well for repetition, tempo, for training intensity, just as our model predicts. It is indeed a wash as well. I also discussed this in a recent video on the most important things for muscle growth and strength development. Training intensity does not have any effect on muscle growth within the range of about 5 to 30 repetitions.

This is called the hypertrophy zone myth. The old idea of the range being just 6 to 12 repetitions is clearly debunked by research, and research basically finds that sets within 85% to as low as 30% of your 1RM stimulate approximately equal muscle growth on a set by set basis. So just as our model predicts, training intensity doesn’t influence muscle growth, and it makes sense to simplify our measure of muscle growth to the total number of sets per week per muscle group. In the event that training intensities do differ for training volume, this model also predicts a feiture of the dose response relationship, the relationship between how much volume you do and how much muscle you gain. We know that there is a diminishing returns curve, a very strong diminishing returns effect. In fact, for some people that means that if you do one set, you get a lot of your results already, especially if you’re training to failure. If you want set, you get a lot of your results. And in the second set you get some additional results. But they are not as big as the first sets. And in the third set you get just a little bit, little bit incrementally smaller, extra effect of every additional sets that you do. This model predicts that quite well because when the training intensity is the same as it is when you’re doing multiple sets with the same weight, the number of reps per set decrease. And we said that that measure of muscle growth is the total amount of reps that you do when your training intensity is the same.

Since every set adds fewer additional repetitions, every set at a progressively smaller stimulus for muscle growth, which is exactly what we see in the research. Now let’s look at training to failure. Training to failure is also perfectly predicted by the model. I also did a recent video on this, so you can look at that on my channel training to failure is predicted extremely well simply by the total amount of repetitions. Training to failure per se does not seem to have much, if any, effect on muscle growth. Rather, it is only the effect of how many reps you’re doing that predicts muscle growth.

Multiple studies have found, for example, that if you do multiple sets further away from failure, you get similar results or equivalent results as if you do fewer sets to failure when the total amount of repetitions ends up being the same. Whereas if you are doing a fixed number of sets, train to failure only increases muscle growth when it increases the total number of reps. In a lot of studies, training to failure does not increase muscle growth when it does not increase the total amount of repetition volume because the first set is very fatiguing and therefore you get you get fewer additional repetitions in the later sets, meaning that the total set volume over time kind of equals out. And again, same total number of repetitions done with the same weights, same total amount of tension and time under tension. Therefore, same muscle growth. Our super simple model also performs very well when looking at the research on training to failure. How about rest intervals? Can our simple model also predict the effects of rest intervals on muscle growth?

In 2014, I wrote a review paper critiquing the existing stance of most researchers and the industry that short rest intervals are better for muscle growth. I showed in my research that the theory that you want short rest intervals because they promote metabolic stress and more hormonal responses is completely flawed, doesn’t work, and results are better approximated by training volume. The next year. I was part of a research group that put this idea to the test and we compared one and three minute rest intervals. we found that three minute rest intervals stimulated greater muscle hypertrophy than one minute rest intervals. And my hypothesis was that this was because of the additional volume.

Now, in that study, we could not demonstrate the link between the volume and the muscle growth very clearly. But a more recent study by Longo et al from 2022 very clearly found that the effect of rest intervals is indeed very well predicted simply by the total training volume. Longer rest periods enhance muscle growth because they increase the number of reps that you do, which increases the time under tension at a given tension. Exactly like our model says, the total amount of reps that you do with a given weight is a good measure of the total amount of time under tension, with a given tension, And therefore that stimulates more muscle growth, at least up to the point that you can recover from it, presumably. Concretely, the researchers compared the amount of muscle growth that people got from training with short rest intervals, long rest intervals, short rest intervals equated in volume to the longer rest intervals, meaning they had to do extra sets and long rest intervals equated to the short rest intervals, meaning they did fewer sets because with a longer rest interval you can do more reps per set. And here in the graph you can see the results.

The long rest interval group had the best gains together with the short rest interval group that had the volume equated to the long rest interval group. In other words, these were the two groups that did the most volume and it didn’t matter whether you got that volume from resting longer after each set, therefore doing more reps per set or whether you did more sets to get the same number of total repetitions. And you can see that the group with the short rest interval had worse results and the group with the longer rest interval also had worse results when their training volume, the number of reps was equated to that of the shorter rest interval group. In other words, your rest interval itself doesn’t seem to have a lot of effect on muscle growth. The effect is mediated entirely by the effect on training volume. We see a similar effect in the research on training frequency.

If you logically think about it, this makes sense because training frequency and rest intervals are essentially the same thing. We call it a rest interval. When it’s 5 minutes, but when it’s five days, we start talking about training frequency. In effect, though, it just means that you’re training a muscle, there’s a stimulus on contractile tissue, and there’s a certain amount of time that passes and then you do it again. and at some point we start distinguishing between a rest interval and a long rest interval, which we call training frequency. So most of the research on training frequency finds that if you equate total volume, training frequency does not have an effect on muscle growth. You can train a muscle twice a week, four times a week, once a week, every single day, even twice a day, although there’s not that much research on twice a day. And you get equivalent muscle growth if the total training volume is the same between conditions. However, when you allow people to increase their training volume with their training frequency, there is a chance for greater gains with higher training frequencies.

This is illustrated well by a recent study by Neves et al from 2022, they compared three conditions, one group did nine sets once per week, one group did three sets three times per week, and the other condition did the three sets three times per week equating the volume to the group doing nine sets once per week. Now what happened is that doing nine sets once per week had led to the same gains in muscle cross sectional area – muscle growth. When the total repetition volume or the total work volume I think they used in the study – it leads to the same thing. If you’re using the same weight – was equated. In other words, it didn’t matter. Whether you train a muscle once or three times a week if the total amount of repetitions that you’re doing remains the same. However, if you spread out to volume over more sessions, it does not normally keep your number of reps the same.

That’s something academics do in their research studies because it equates for the effect of training volume and we want to isolate variables. But in practice, in the gym when we’re training, we are not in a volume equated condition. So what happens and in this study, this also happened. When you let people do three sets three times per week instead of nine sets in one day you get more reps when you do it in three sessions than you do it in one session because you are more fresh every time you are training. So in this case, instead of about 2% increase in quadriceps cross-sectional area from the group training once per week or three times per week with the same volume, the group that was training three times per week now left to their own devices, meaning they increased their total training volume, they got approximately double the muscle growth, 4% increase in muscle quadriceps cross-sectional area. So this, just like the rest interval research shows that train frequency per se does not seem to have a big effect on muscle growth.

But if you are increasing your training volume, whether it’s repetitions or number of sets by increasing your training frequency, then it does increase muscle growth. And needless to say, research also supports that. If you increase your training volume like you literally just do, you go from one session per week and you do sessions per week. That also increases muscle growth because you’re literally just doing a lot more volume in terms of sets and the repetitions. So again, our super simple model performs quite well, explaining the effect of training frequency on muscle growth. Now, the Henselmans hypertrophy model, if you will, is definitely not perfect. It will break down when you start looking at very complicated niche scenarios, drop sets, some kinds of advanced training techniques.

The model does not perfectly explain these results, but I think that this model is currently the best we have and it has a lot of advantages. For one, it’s extremely simple. We just predict muscle growth by the amount of repetitions that you do for that muscle. If the weight is the same and if the weight is not the same, we just look at the total number of sets that you do for that muscle group per week. it’s super simple. It’s quantifiable, it’s concrete, you can literally measure it and it fits with a lot of the scientific research on a lot of training variables. It predicts all of them very well. So in that sense, I think it is a model that is very practical to use and it is also theoretically completely substantiated because the model just simplifies mechanical tension to operational, concrete training variables. So it just equates the amount of tension with the training intensity, and then it equates the time under tension with the number of reps that you do. Therefore, the model simply says that the total amount of muscle that you’re going to gain for any muscle group is predicted by the total number of repetitions per week that you’re doing for the muscle group.

If we’re holding the training intensity, which is the tension the same and if the tension differs, then we’re simplifying it even further to the total number of sets per week, per muscle group. It’s a very simple model, has theoretical support. It fits quite well with the research and I think this may help you cut through the noise and also predict in your own training program if you change some things. Will this increase muscle growth or not? Is this really important?

So, I hope this helps you in your own training.

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About the author

Menno Henselmans

Formerly a business consultant, I've traded my company car to follow my passion in strength training. I'm now an online physique coach, scientist and international public speaker with the mission to help serious trainees master their physique.

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