What’s the difference between a set of 5 reps and 15 reps?

The answer: not much.

Historically, the strength & conditioning world has abided by the idea that low reps (~1-5) builds strength, moderate reps (6-12) builds muscle size, and high reps (~15+) builds muscular endurance.

However, this idea was largely developed solely based on the fact that it “makes sense” at face value.

More recent research on the subject paints a very different picture, though.

Studies now show that your proximity to failure is the ONLY thing that matters for all three of these goals: strength, size, and muscular endurance. [1]

• Keep in mind “muscular endurance” is different than normal, cardiovascular endurance.

Proximity to failure just refers to how close you got to being unable to performing any further reps.

In other words, it’s how hard you try in any given set.

So, imagine we’re comparing two different sets.

In one set, you did 15 reps and could have cranked out 17 at max effort.

In another set you did 6 reps and could have cranked done 8 at max effort.

Therefore, your proximity to failure in each set was 2 reps.

So, these two sets would provide the exact same stimulus for building size and strength.

So, how close to failure do you need to get for a set to be stimulating enough?

The best evidence suggests getting within 3 reps shy of failure is likely to be the sweet spot. [2]

On lower volume programs (lower # of sets), it might be wise to get closer to failure in your sets & vice versa.

With all that said, there are still some valid reasons why you might opt for lower or higher reps in your sets.

For instance, with bicep curls, going too heavy (~4-6 reps) might require too much stabilization effort elsewhere in the body to stimulate your biceps sufficiently.

Conversely, opting for extra high reps (~12-15+ or so) on squats might leave you fatigued before the musculature of your legs is sufficiently stimulated.

So, the take-home is this:

The best repetition range is dependent on the movement you’re training, and your preference, more than the specific adaptation you’re after.

When deciding on what weight and/or reps to use on a movement, go with what feels best for you in that movement above all else; and then focus on giving a high effort in the set.