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How Many Reps Per Set for Muscle Growth?

I’m going to start this article by asking a hypothetical question:  If you had to pick a single repetition range to train in for growth, what would it be?  That is, imagine some very strange situation where you could only train within a certain repetition range which I’ll limit to a 3 rep spread (i.e. 9-12).  What would you choose?  Put differently, is there an optimal number of reps per set for muscle growth?

I used to ask this of friends of mine in the field and, almost with exception, the answer was pretty much the same.  This was true regardless of whether or not they had arrived at that value from experimentation and experience or just looking at the research.  So I’m going to look at a variety of different topics to show you how I got to the answer I’m going to provide.

What Stimulates Muscle Growth?

I asked a job supervisor that question once once; he was a smart-ass like me and told me “It needs lots of sunlight and water.”  Close but not quite.

The mechanism of muscle growth has been under heavy scrutiny for years and a lot of theories and ideas have come and gone in terms of both the mechanism of growth as well as what stimulates it.  Semi-amusingly, about 98% of the actual answer was known back in the 70’s.

In an exceptional paper (which I recommend the reading of to any nerds in the field) titled “Mechanism of work induced hypertrophy of skeletal muscle” a researcher named Goldspink pretty much laid it out concluding that:

It is suggested that increased tension development (either passive or active) is the critical event in initiating compensatory growth.

Basically, the development of high levels of tension within the muscle is the key/primary factor in initiating the growth process.  I’d note that there are also some elements of fatigue that may be contributing to what “turns on” the growth response.  Finally, I’d note that in order to keep stimulating growth beyond an acute training bout, there has to be an increase in muscular tension.

Basically, over time you have to add weight to the bar.  This is called progressive tension overload.

Which as another great scientist in the field (Ronnie Coleman) summed up thusly:

Everybody wants to be a bodybuilder, but nobody wants to lift no heavy-ass weights.

Or as Dante Trudell, inventor of Doggcrapp training put it so simply

Growth is stimulated by increasing strength in a moderate repetition range.

The simple fact is that, outside of heavy drug users (steroids having the capacity to stimulate growth without even training), the biggest bodybuilders are the strongest.  They grow because they provide, over time, a progressive tension overload (of course there are more variables that go into this, total workload per workout, frequency of training and diet all interact here).

But as I have been pointing out for years and years and years, if you’re not adding weight to the bar over time, you’re simply not growing. You can focus on the feel and the pump and the squeeze all you want; if you’re using the same weights 6 months from now that you’re using today, you won’t be any bigger.  You can prove this to yourself every day in the gym.

No, this doesn’t mean that you have to add weight at EVERY workout which is the fallacy of HIIT.  But over some time frame, which might be weeks or longer for advanced trainees, you have to be lifting heavier weights.

But that’s what stimulates growth in the most general sense.

Tension Overload + Some Fatigue/Volume -> Muscle Growth

With that background we move to the next topic.

How Does Muscle Generate Tension?

When you look at how muscles generate force (i.e. tension), you find that the body has essentially two methods to increase force output.  They are:

  1. Muscle fiber recruitment
  2. Rate coding

Muscle fiber recruitment is exactly what it sounds like, how many of the fibers within a muscle are actually being recruited.  Contrary to the exceptional silliness which is endlessly repeated in books and on the internet, even rank beginners can recruit 100% of the muscle fibers in their upper body.  It’s a little less, around 90% in the legs.  Rate coding refers to how quickly the body is sending electrical signals to the muscle.  As rate coding goes up, the muscle fires harder, generating more force and hence experiencing more tension.

In the muscles we’re interested in from a sports or bodybuilding standpoint, the body will generally use recruitment to increase force production up to about 80-85% of maximum force output.  In the lab, this is measured with Maximal Voluntary Isometric Contraction or MVIC, which is effectively 1 rep maximum weight.    Beyond about 80-85% of MVIC, fibers are maximally recruited and the body will use rate coding to generate further force.

For completeness, let me mention that smaller muscles in the body, the eyes and finger muscles work differently.  In general, they will use recruitment up to about 50-60% of MVIC and then rate coding handles the rest.  This allows for much finer motor control.  But it means that studies on these muscles (i.e. finger muscles) aren’t that relevant to what we do in the weight room.

Anyhow, now we have the next part of the picture, the body will recruit more fibers up to about 80-85% of maximum.  Essentially you will recruit 100% of your available muscle fibers from the first repetition of the set.

If you were to use a heavier load, 90%, you will not get any further muscle fiber recruitment.  If you use lower than 80%, you won’t get full fiber recruitment from repetition 1.  It will happen eventually, towards the end of the set approaching failure.

But only at a loading of 80-85% of maximum do you get full muscle fiber recruitment throughout the entire set.

How Many Reps Per Set for Muscle Growth?

So let’s sum up the above information.

  1. Growth is stimulated by a combination of high muscular tension and some amount of fatigue
  2. Full muscle fiber recruitment occurs at 80-85% of maximum (or near failure with lighter loads).

Because those two concepts lead us to the answer to the original question: If you had to pick a single repetition range for muscle growth to use for the rest of your career, what would it be?

The answer is 5-8 repetitions to or very near failure (1 rep in reserve maximum).

So how did I get there?

For most people 80-85% of maximum will allow about 5-8 repetitions per set to muscular failure.  This can vary slightly depending on the muscle and the movement.   Many find that they can do more repetitions in at least some leg exercises with this percentage for example.

If you were to put 90% of weight on the bar, you might only get 3 repetitions.  You wouldn’t increase fiber recruitment but you would decrease the amount of mechanical work done.  If you put 70% on the bar you might get 15 repetitions.  Taken to failure, you will actually get full recruitment near the very end.  Perhaps 3-4 of those 15 repetitions will occur under conditions of full recruitment.  Certainly you will have done more total mechanical work.  Again, all fibers will not experience high muscular tension until the end until the end.

Essentially, a near maximum set of 5-8 repetitions will allow you to perform the maximum amount of mechanical work under conditions of full fiber recruitment and muscular tension.  Heavier won’t increase recruitment but will decrease mechanical work and lighter will increase mechanical work but all fibers won’t experience it until the end.

Let me add that I am not advocating that trainees seeking maximum muscular growth only work in this range.  Rather, I am addressing a hypothetical question as a way to examine some basic physiological concepts.  Clearly other repetition range can be useful for hypertrophy  and there can be good reasons to use a variety of repetition ranges when training for maximal muscle growth.

But if you had to pick a single repetition range to use for the rest of your career for some reason, 5-8 would be the optimal number of reps per set for muscle growth.


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