This short article is aimed at anyone and everyone, therefore I am going to refrain from delving deep into physiology and describing how muscles grow and keep things simple.

Now, how can I claim that the number of repetitions to build muscle is irrelevant? It seems to be common knowledge that 6-12 reps is optimal for muscle growth, and anything higher is endurance training and anything lower improves strength. This may be somewhat true for strength gains but in general this concept falls into the category of bro science.

This has been backed by many studies including those from Schoenfeld et al, Chestnut et al, and Campos et al (et al = and others) who all compared 3-5 reps to around 10 reps, which found similar muscle growth between groups whilst the lower rep groups gained more strength. The other end of the spectrum is also backed by science with studies from Mitchell et al, Morton et al and Burd et al all showing that very high rep sets (30% 1RM, about 30 reps) are just as effective as moderate rep sets (80%, 1RM, about 10 reps) when taken to failure.

To understand why, is to understand muscle growth, how muscles are recruited and fatigue. I am going to talk in layman’s terms:

  • Mechanical tension on individual muscle fibres causes muscles to grow.
  • The size principle states that as the set gets harder more muscle must be recruited.
  • The force velocity relationship shows the link between slower contractile velocities and high force, ergo you’re not going to lift your one rep max quickly.

Putting these three things together gives us our why. Tension is not defined by load, so if the weight is heavy and 5 reps can be performed then rep speed will start slow and finish slow, while recruitment will start high and finish high. If the weight is light and 30 reps can be performed, the rep speed will start fast and finish slow while recruitment will start low and finish high. The key here is that both sets will FINISH slow and therefore recruitment will be high. This was looked at in a study by Izquierdo et el which showed that the last few reps in any set to failure regardless of load was incredibly similar.

So, where does this leave us?

It’s all well and good knowing the theory, but we have to be practical and apply this to our training. We know that any set taken to failure between 5-30 reps will give us similar levels of muscle growth. Sets in the upper range of 25-30 may not be practical due to lower levels of recruitment at the start and therefore really having to push to failure. If you’ve ever completed a true 30 rep max you will understand the deep psychological trauma that accompanies it. For this reason, paired with the fact that central nervous system fatigue is worse with higher rep sets (another common misconception is that heavier weights cause more CNS fatigue), the upper limit I’ll use myself and with my clients is 20 reps.

We also have to look at the nature of individual exercises. For a large, multi-joint, axially loaded (top down) exercise like a barbell squat, it’s best to stick to lower rep ranges. For a small, single-joint, non-axially loaded exercise, like a leg extension, it’s best to stick to moderate-high rep ranges.

To conclude, most of your exercises should be between 5-20 reps per set with lower ranges best suited to large multi joint exercises and higher ranges best suited to smaller single joint exercises.