How Many Reps to Build Muscle?

The optimal number of reps per set to achieve maximum muscle growth is a fiercely debated and disputed subject amongst the bodybuilding community.

If you go to any popular bodybuilding forum and ask, “What’s the best rep range for muscle growth?” you’ll have a variety of different numbers thrown at you, and you’ll end up even more confused than before you asked.

The amount of contradicting opinions on this question is staggering.

Everyone seems have their own “expert” opinion on the “perfect” rep range for muscle growth.

But what rep range should you really be training in to maximise muscle growth?

It’s a simple question, with a somewhat complex answer, that I hope to make understandable in this post.

The truth is, muscle growth can be achieved by training within more than one rep range, there’s no “magic” or “perfect” single rep for muscle growth.

There is much more overlap than people think. Increases in muscle mass are seen across of wide spectrum of repetition ranges.

With that said, there is a narrow rep range to work within most of the time to optimise for muscle growth.

Rep ranges are often categorised into 3 main “loading zones” – Low (heavy), Mid (medium), and High (light), and are pretty well accepted and validated by studies to produce the following outcomes.

Low: 1 – 5 reps are within the “Strength Range“, used primarily to increase muscular and neurological strength.
Medium: 6 – 12 reps are within the “Hypertrophy Range“, used primarily to build muscle mass.
High: 15+ reps are within the “Endurance Range“, used primarily to increase muscular endurance.

Of course, there is plenty of overlap in result outcomes across these ranges, as you can see from this illustration.

For example, you can build a lot of strength training in the mid “hypertrophy range”, and you can build a lot of muscle training in the low “strength range”. It’s just that you’ll get more of one outcome over the other, depending on which range you train within.

I’ve extensively researched this subject and from my experience and study can confirm that you should be training within the 6 – 12 rep (8- 10 being ideal) range most of the time if you want to maximize gains in muscle growth.

So why does training in this rep range produce greater gains in muscle size?

The Science Behind Rep Ranges

Let’s get one thing straight – feeling the “burn” in your muscle from very high rep training does not cause growth.

…And lifting outrageously heavy weights that induce trembling arms and bloodshot eyes is not the catalyst, either.

Growth stimuli from resistance exercise is caused by a combination of mechanical tension and fatigue induced metabolic stress, and to some degree, muscle damage.

Optimise mechanical tension and metabolic stress and you’ll maximize muscle hypertrophy

Mechanical Tension

Lifting weights creates a level of tension across your muscle fibres.

This mechanical tension regulates the size of your muscles through a process called “mechanotransduction” where the mechanical forces cause an intracellular anabolic response.

The magnitude and the duration of the load lifted creates the stimuli that brings about hypertrophic adaptations.

In other words, the type of load – or, ‘how heavy the weight is’ – is the primary driving force behind the building of muscle mass.

When your muscle adapt and grow from set / reps of resistance weight training, they aren’t responding to numbers, they’re responding to tension.

If you choose to lift a lot of heavy weight, the load is high, and the reps will be low.

If you choose to lift a very light weight, the load is low, and the reps will be high.

Now, some of you might be thinking, heavier weight lifted means more tension, and that more tension means more muscle growth.

But that’s not the case, because very heavy loads don’t allow for much time under tension, or for the accumulation of metabolic stress.

Heavy weights mean less metabolic stress, and light weights mean less mechanical tension.

Moderately heavy weight provides a good balance between the two, meaning your trained muscle will get an optimal combination of growth stimuli.

You’re not just picking 5 reps, or 8 reps, or 12 reps, you’re picking the right “load intensity” or “weight” for your goal. The number of reps you select determines the load.

To help you understand how load and reps impact growth, you need to understand something called your one rep max or 1RM.

But before we get into that, let’s talk about another contributor to growth – metabolic stress.

Metabolic Stress

Training with moderately heavy weights within the 8 – 12 rep range is where metabolic stress “kicks-in”, which is the exercise induced accumulation of metabolites within the muscle.

Minimal metabolic stress is seen in training routines involving very heavy loads.

Metabolic stress also causes muscle cell swelling and increased muscle fibre recruitment – factors that contribute to growth promotion of the whole muscle.

Cell swelling is a phenomenon that increases intracellular hydration and has been shown to increase protein synthesis and reduce muscle degradation, which is really the key behind building muscle.

Muscle Fibres and Reps

Skeletal muscles are made up of a mixture of two main types of muscle fibres.

Type 1, known as “slow twitch” fibres, respond better to light weights / high reps. They don’t fatigue easily and are not very powerful.
Type 2, known as “fast twitch” fibres, respond better to heavy weights / low reps. They fatigue easily and are the most powerful.

Your muscles are made up of around 50% type 1 and 50% type 2 fibres, give or take. The lower body tends to have slightly more type 1 fibres compared to the upper body.

It would make sense to assume that lighter, high rep sets would favour growth of type 1 fibres, and that heavier, low rep sets would favour growth of type 2 fibres. There is some scientific evidence out there to support this theory.

However, training with moderately heavy loads within the 8 – 12 rep range taken close to failure will recruit ALL muscle fibres, fairly equality – Type 1, Type 2a and Type 2b – so there’s no need for fibre specific training.

This rep range gives the best combination of muscle fibre recruitment, metabolic stress, mechanical tension, and time under tension for the goal of muscular hypertrophy.

Understanding Your One Rep Max to Build Muscle

To build muscle, you need to maximize muscle protein synthesis and inhibit protein breakdown.

Maximum protein synthesis has been shown to occur between 70-85% of your one rep max (1RM).

(1RM – One repetition maximum, is the maximum amount of weight a person is able to lift for a single repetition*)

Don’t know your 1RM? No problem.

The chart below will help you find a good estimate, but before you get to that – here’s an interesting study I want to share with you.

A study in 2009 conducted by Kumar et al. (Journal of Applied Physiology) measured the fluctuations in muscle protein synthesis after weight training.

They found that the anabolic response (muscle building) increased by:

30% after training with weights that were 20% of 1RM*
40% after training with weights that were 40% of 1RM
100% after training with weights that were 60% of 1RM
130% after training with weights that were 75% of 1RM
100% after training with weights that were 90% of 1RM

As we can see from this information, the peak of muscle growth occurs when training with a weight that is around 75% of our 1RM.

The reason that the muscle building response is lower at 90% of 1RM is because the weight is just too heavy.

This weight causes the nervous system to fatigue first, not the muscle fibres.

This means that the muscles are not receiving enough time under tension to trigger the adaptive growth response, or any metabolic stress.

So how many reps is 75% of your 1 RM?

Take a look at the chart below to see how 1RM percentages relate to repetitions:

100% of 1RM = 1 rep
95% of 1RM = 2 reps
93% of 1RM = 3 reps
90% of 1RM = 4 reps
87% of 1RM = 5 reps
85% of 1RM = 6 reps
83% of 1RM = 7 reps
80% of 1RM = 8 reps
77% of 1RM = 9 reps
75% of 1RM = 10 reps
67% of 1RM = 12 reps
65% of 1RM = 15 reps

As you can see, 75% of your one rep max works out at around 10 reps. So, training anywhere between 8 – 12 reps to concentric failure is the optimum rep range for muscle growth.

Final Recommendations

The bottom line is that most of your training should be done within 8 – 10 reps if your goal is to maximize muscle growth.

Around 85% of your training should be within this moderate intensity loading zone.

The other 15% can be made up of any other rep range.

For example, you might want to switch to heavy, low rep work for a week or two, a couple of times a year.

Or you may want to switch to some lighter, high rep work for a couple of weeks.

There may be some benefit to utilizing rep ranges across the entire spectrum, such as increasing strength, or endurance, but most of your training should be within the 8 – 10 “hypertrophy zone”.

This will optimise your sets for growth by providing optimal levels of metabolic stress and mechanical tension, while also allowing you to get the best quality work in by being able to exercise muscle groups with the greatest range of motion while maintaining good form throughout each set.

Last updated: 01/12/2017

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