Image by Markus Spiske from Pixabay

Becoming Superhuman

Stimulating Reps: The Ultimate Scientific Guide To Maximizing Muscle Growth

This will make you enter a new phase in your fitness life

“Don’t stop when it hurts, stop when you are done.”

Table of Content

1. The Stimulating Reps Model: Recruiting High-Threshold Motor Units
2. The Stimulating Reps Model: Force-Velocity Relationship
3. How Many Stimulating Reps Do You Need For Sufficient Hypertrophy?
4. Do You Need To Go To Task Failure?
5. Multi-Joint Exercises and Stimulating Reps: Going To Task Failure Is Not As Necessary
6. Central Nervous System Fatigue vs Peripheral Fatigue
7. Keeping Central Nervous System Fatigue Low: The Do’s and Don’ts
8. Peripheral Fatigue Is Not Your Enemy: The Benefits of Peripheral Fatigue
9. Optimal Rest Periods To Maximize Hypertrophy and Strength
10. Powerbuilding: Look Strong, Be Strong
11. Passive and Active Mechanical Tension: Stretching Your Way To Muscle Growth
12. Acknowledgements

Abstract

Want to make sure that you leave the gym knowing that your workout will cause muscle growth?

Or maybe your muscles just don’t want to grow?

The stimulating reps model may be the answer. In fact, the model even says that it is very well possible to have a workout causing no muscle growth i.e. ineffective.

Lastly, the most essential thing this model also proposes is that muscle damage does not cause hypertrophy. Instead, something else does. Read more to find out!

So first of all, what is the stimulating reps model?

The Stimulating Reps Model: Recruiting High-Threshold Motor Units

Essentially, this model says that, on average, the last 5 reps before task failure are “stimulating”. That is, they are the only reps causing sufficient skeletal muscle hypertrophy.

So why is that? First of all, we have something called Henneman’s size principle: Low-threshold motor units are recruited before high-threshold motor units.

Second, low-threshold motor units are characterized by recruiting a small amount of muscle fibers, which consists mostly of type 1 slow-twitch oxidative muscle fibers, a type of muscle fiber more effective during endurance training rather than strength and hypertrophy training.

High-threshold motor units, on the other hand, usually recruit a way bigger amount of muscle fibers that, instead, mostly consists of type 2a oxidative glycolytic and type 2x glycolytic muscle fibers which, again, are muscle fiber types that are more effective during hypertrophy and strength training.

See this for more information about the different types of muscle fibers:

Types of Muscle Fiber

Third, low-threshold motor units usually are active the whole day (e.g. when walking), which means they probably are already close to their “natural” max size i.e. not much hypertrophy can occur within those muscle fibers without any ergogenic aids.

Combining all these points, we get to see that we want to recruit high-threshold motor units as much as possible, because of their high quantity in terms of muscle fibers and their potential to still grow. So how do we do that?

Henneman’s size principle is, obviously, dependent on force i.e. bar weight x bar velocity. This means that high-threshold motor units only get recruited during high force. Again, this can be done either by a heavy bar weight, high bar velocity, or a combination of both.

We are not finished here, however. The stimulating reps model further says that high force solely due to high bar velocity is ineffective.

To understand this, we have to add another element: The force-velocity relationship.

The Stimulating Reps Model: Force-Velocity Relationship

The force-velocity relationship essentially says that, the faster the bar velocity, the lower the force. Why is that? Isn’t it the other way around i.e. F = m x a? When talking about the force-velocity relationship in muscle physiology, we actually mean something else, namely the force production of the muscles themselves:

Simply said, the force generated by a muscle is dependent on the total number of actin-myosin cross-bridges attached. Second, the attachment rate is a lot more fixed (as opposed to the detachment rate) meaning that a higher bar velocity doesn’t necessarily “speed up” the formation of actin-myosin cross-bridges.

One can, then, imagine that a higher bar velocity creates less actin-myosin cross-bridges which, in turn, results in less force generated by the muscle.

So how does the muscle, then, “compensate” its lack of force generation during high bar velocity? By recruiting high-threshold motor units i.e. more and faster muscle fibers.

Again, because of the small formation of actin-myosin cross-bridges, hypertrophy tends to be low despite recruiting high-threshold motor units.

Lastly, the force-velocity relationship also defines mechanical tension:

Quick recap:

1. A too low bar weight recruits low-threshold motor units, which results in small levels of hypertrophy;
2. A too low bar weight and high bar velocity does recruit high-threshold motor units, but because the formation of actin-myosin cross-bridges is very low, hypertrophy will only occur in small levels;
3. The only option that remains which also induces sufficient hypertrophy would be: (1) A sufficient loaded bar (i.e. high-threshold motor units) and (2) low bar velocity i.e. high mechanical tension;
4. Each repetition that complies to point (3) is considered to be a “stimulating repetition” or “stimulating reps”. As a side note, muscle damage does not equate to hypertrophy.

P.S. deliberately slowing down each rep to induce high mechanical tension doesn’t work. This is because the bar weight is too low and, therefore, low-threshold motor units are recruited. Every repetition with a low bar velocity due to a high bar weight, then, can only be considered to be “stimulating reps”.

Knowing our definition of stimulating reps, how many do we have to do to induce sufficient hypertrophy?

How Many Stimulating Reps Do You Need For Sufficient Hypertrophy?

A s far as I am aware, a minimum of 15 stimulating reps is needed within a single bout of exercise (let’s say within 4 hours).

So, just do 15 repetitions that all comply to point (3) right? Nope, it’s not that simple.

According to research, the maximum amount of stimulating reps within a single set is between 5 to 7 stimulating reps. For beginners, this number is closer to 5, while for elites it’s closer to 7.

This means that the last 5 to 7 repetitions within a single set can be considered stimulating repetitions.

Further, this also means that you need to do a minimum of 3 sets per muscle group consisting of more than 5 reps per set (i.e. approx. 15 stimulating reps) within a single bout of exercise, preferentially 4.

Is there a maximum limit? This seems to vary from person to person. According to this research:

" How much volume do you need to get stronger and build muscle?

The maximum sets per week to maximize hypertrophy seems to be 30 sets per muscle group for the upper body and 45 sets for the lower body.

Stimulating reps are easy to count in single-joint exercises such as bicep curls: Every repetition that happens to be slow due to a lack of force generation (rather than deliberately slowing down each rep), can be considered a stimulating rep.

Do You Need To Go To Task Failure?

Task failure simply means: The inability to continue a certain activity. In strength training, this is due to central nervous system fatigue + peripheral fatigue:

Simply said, when you are doing bicep curls and do as many reps as possible, you can be considered to have reached task-failure in the final rep.

So do we need to take each set to failure? If we take into account that the last 5 to 7 reps within a single set can be considered “stimulating reps”, then no, we don’t need to go to failure.

If you can do 10kg bicep curls for 10 reps before hitting task failure, but instead only do 9 reps, you still got 4 to 6 stimulating reps.

In fact, it has been advised to not go to task-failure each time, because it builds up long-term central nervous system fatigue, which can inhibit your other workouts in the other days of the week:

Multi-Joint Exercises and Stimulating Reps: Going To Task Failure Is Not As Necessary

For multi-joint exercises, however, calculating the amount of stimulating reps is a lot more difficult. For detailed information, I suggest listening to this podcast starting at 0:47:01:

Simply said, this has to do with the fact that there is more than one muscle group involved: The prime mover and the secondary movers.

When doing bench presses, for example, the prime mover can be considered your chest muscles. The secondary movers can be considered your triceps and shoulders, the latter mostly emphasized on the front deltoids.

In the beginning of the exercise, most of the force is generated via the prime mover i.e. chest. As one approaches task-failure, however, force is being carried over more and more to the secondary movers to compensate for the lack of force generation coming from the chest muscles (prime mover).

This means that, if you want to target your chest muscles for hypertrophy, going to task-failure is not as necessary as when targeting your biceps via single-joint exercises.

There doesn’t seem to be a clear consensus, however, how “far away” you can be from task failure to still induce at least a minimum of 1 stimulating rep to the prime mover. So it is still well advised to go close to task failure, but not as “completely” as in single-joint exercises.

For more detailed information regarding everything said in this section:

The Evidence is Lacking for "Effective Reps"

I was carelessly noting down the terms “central nervous system fatigue” and “peripheral fatigue”. What exactly do we mean by them, and how can we apply that information to adjust our exercises as optimally as possible regarding stimulating repetitions?

Central Nervous System Fatigue vs Peripheral Fatigue

Central nervous system fatigue, or central fatigue, simply means “mental fatigue”. However, one doesn’t necessarily have or can be aware of this central fatigue for it to impair exercise and, thus, reduce the amount of stimulating reps per set.

Central fatigue can occur due to all kinds of things, but the main ones are: Intra-set central fatigue and post-workout central fatigue.

Intra-set central fatigue can further occur via two main ways: High rep range or heavy weight.

It has been proposed that high rep range mostly causes the motor cortex itself to fatigue while heavy weight causes the motor neurons, instead, to fatigue. Out of these two, however, the most detrimental seems to be high rep range, which we will get to later on.

Second, post-workout central fatigue simply happens due to muscular damage from the previous workout. The more muscle damage that workout has caused, the more post-workout central fatigue.

Lastly, we have peripheral fatigue, which is fatigue within the muscles themselves. Peripheral fatigue is mostly caused due to accumulation of metabolic products such as adenosine diphosphate (broken down from adenosine triphosphate).

So how does knowing the definitions of central and peripheral fatigue improve our workout i.e. stimulating reps? Simply said, central fatigue seems to lessen the maximum stimulating reps per set, while peripheral fatigue doesn’t seem to.

Remember, stimulating reps occur within the muscles not in the brain. This, then, is why central fatigue reduces the maximum stimulating reps per set while peripheral fatigue doesn’t. If central fatigue is high enough, the maximum stimulating reps per set might even be zero.

So how can we keep central fatigue as low as possible to ensure an effective workout?

Keeping Central Nervous System Fatigue Low: The Do’s and Don’ts

Again, central fatigue can occur both due to high rep range, heavy weight, or muscular damage. Let’s take a look between high rep range and low weight vs low rep range and heavy weight, which one is better in keeping central fatigue as low as possible?

Simply said, low rep range and heavy weight seems to be the best option if you want to keep central fatigue as low as possible. Why? Because the maximum amount of stimulating reps per set is around 5 to 7, so why would you do 30 reps anyways?

Second, central fatigue does not seem to increase at a much faster tempo when increasing the weight and lowering the reps per set. This means that doing 20 bicep curls with 5 kilo produces way more central fatigue than doing 10 bicep curls with 10 kilo i.e. the ratio is about the same.

Again, the maximum stimulating reps per set is around 5 to 7, so the optimal rep range would be 8 to 10 for hypertrophy. I personally would advise 10 reps per set, because a lower rep range would require a much higher weight, which could wear out the tendons a lot quicker requiring a higher frequency of “deload weeks”. This, of course, depends on the number of sets and workouts per week.

So we talked about the negative side-effects of central fatigue. How about peripheral fatigue?

Peripheral Fatigue Is Not Your Enemy: The Benefits of Peripheral Fatigue

Again, peripheral fatigue is the fatiguing of the muscles themselves either due to accumulation of metabolic products (e.g. ADP) or due to muscular damage from the previous workouts (this can accumulate too, which can result in overtraining).

Only one of them is truly beneficial, namely peripheral fatigue due to accumulation of metabolic products.

Why is peripheral fatigue due to muscular damage not beneficial? Because it has been proposed that muscles first try to repair themselves before undergoing hypertrophy. Again, logically, this is why you train different body parts per day of the week.

Peripheral fatigue due to muscular damage, unlike central fatigue, can be felt i.e. a burning sensation or pain.

So let’s go back to the beneficial version: Peripheral fatigue due to accumulation of metabolic products. How can this benefit us in relation to stimulating reps?

Imagine the beginning of an exercise, you are doing bicep curls for 10 reps while feeling fresh i.e. no central or peripheral fatigue. As you progress from rep to rep and get closer to task failure (the 10th rep), peripheral fatigue increases more and more.

In the first few reps, the recruitment of high-threshold motor units isn’t that necessary. As peripheral fatigue increases, however, the recruitment of high-threshold motor units becomes more and more necessary to compensate for the lack of force production coming from the lower-threshold motor units.

Again, high-threshold motor units contain the fastest and highest quantity muscle fibers maximizing hypertrophy.

Lastly, bar velocity is also very high in the beginning of the exercise, and if we take the concept of the force-velocity relationship again, we can see how the force production is rather low i.e. low mechanical tension per muscle fiber and, therefore, low hypertrophy.

As peripheral fatigue increases, however, bar velocity slows down increasing the mechanical tension per muscle fiber as one approaches task failure i.e. the 10th rep.

Remember, peripheral fatigue does not decrease the amount of stimulating reps per set, unlike central fatigue. In this example, then, we got all our 5 to 7 stimulating reps that we could get within this single set and, hence, maximizing hypertrophy.

If peripheral fatigue is beneficial, should we do short rest periods to keep peripheral fatigue high and also decrease the reps needed in the next sets to approach task failure, while keeping the reps above 7 to get all the stimulating reps? In terms of maximizing hypertrophy, it may not be that simple in the long-term.

Optimal Rest Periods To Maximize Hypertrophy and Strength

According to many studies, the optimal rest period appears to be between 2 to 3 minutes between each set of the same muscle group. So why is that?

Their reasoning, as far as I have read, is because of the accumulation of central nervous system fatigue. Remember how this is the “bad” type of fatigue, decreasing the maximum amount of stimulating reps you can perform per set:

Second, it has been shown by research that central fatigue tends to decay exponentially, rather than linearly. One can imagine, then, if most of its decay happens at the last minute, the difference between 1 minute and 2 minutes can be pretty significant.

If you were to do 10 reps of bicep curls with only 1 minute of rest in between, the high presence of central fatigue after the first set may only enable you to gain 3 stimulating reps every set thereafter, instead of 5.

Another reason why longer rest periods is more beneficial, is because of its greater strength gains (rather than just muscle size). Longer rest periods allow you to use heavier weights, which makes you stronger more quickly, and the greater increases of strength from workout to workout may enable you to gain muscle size quicker, in the long-term.

One can imagine that, if you were to become twice as strong as you currently are and also bump up your weights respectively despite your muscle size being the same as they were before, hypertrophy gains would come a lot more easily.

Lastly, the main difference between hypertrophy and strength (even though hypertrophy increases strength), is that hypertrophy is simply the increase of muscle size while strength is more due to stronger tendon strength and, therefore, less tendon compliance:

Now, if you want to maximize hypertrophy of every single muscle group via very specific exercises, this may require you 2 hours of gym time every day just because of waiting 3 minutes between each set. The alternative to that, however, is powerbuilding.

Powerbuilding: Look Strong, Be Strong

One way to not be in the gym for 2 hours every day, is to simply alternate between more hypertrophy focused days or weeks (“building”) and strength (“power”): Powerbuilding.

Powerbuilding is nothing more than a variant of periodization and, so far, data says that periodization may bump up your strength gains by a few percentages (which is a lot in the long-term):

Periodization: What the Data Say * Stronger by Science

The mesocycle between hypertrophy and strength can be as long as you desire, you can do 3 days of strength and then 3 days of hypertrophy, or 1 week of strength and then 1 week of hypertrophy. Remember, no program is perfect, the perfect program is whatever you can consistently follow.

As a side note, according to Dr. Layne Norton, starting your program with strength before hypertrophy is more effective than the other way around:

The Ultimate Layne Norton PHAT Workout For Size And Strength

On the strength days, you perform a low amount of exercises with heavy weights and long rest periods (2 to 3 minutes). On the hypertrophy days, on the other hand, you perform many exercises with moderate weights and short rest periods (<1 minute).

But you just said that 2 to 3 minutes is superior for both hypertrophy and strength gains!

Yes, if we truly want to maximize everything we can. But in all honesty, do you want to stay in the gym for 2 hours or more every day on the hypertrophy days?

To top this article, there is more than one way to induce hypertrophy than just moving weights: stretching.

Passive and Active Mechanical Tension: Stretching Your Way To Muscle Growth

Stretching your muscles before a workout and after the workout (or even between sets) may promote extra gains in hypertrophy:

Why You Should STRETCH Between Every Set From Now On (New Science Explained) on YouTube

Remember, high mechanical tension simply means high force per muscle fiber, which also can be done by stretching these muscle fibers.

So how long should you stretch? As far as I have read, the general guideline is to stretch for around 30 seconds per muscle group.

Acknowledgements

I want to thank Chris Beardsley for introducing me to the concept of mechanical tension and stimulating reps. I also highly recommend reading his articles:

Chris Beardsley - Medium

I want to thank Stronger by Science for introducing me to the concept of periodization and its many benefits.

And for those interested, I also get most of my information from Jeff Nippard and Jeff Cavaliere on YouTube.