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Updated: Jan 3, 2022

January 2022: New research has led to information in this post being updated. Please click here to read a new post on the findings.


Why is it that we do 5s, 8s, 10s, 12s, 15s or 20s? You'll find all kinds of answers from the different kinds of people you meet in the gym, but finding out the why behind it all can be a bit hard sometimes. Someone was asking me about it last night, so I decided to write a quick post to clear it all up!

We all use different rep numbers across each of our sets in every workout. Some people do the same number of reps in every single set of every single exercise, and others will carefully choose their numbers in advance and follow a plan throughout every session. But what difference does it make if we choose a specific number of reps per set?

Each group of numbers has a different purpose, and some of them overlap to a very slight extent. Depending on how many reps you perform per set, you can work your body into building strength, muscle mass or endurance.

  • 1-5 reps: Strength (the closer to one rep you use, the more likely you are to build absolute strength, and the closer to 5 you use, the more likely you are to train strength relative to your bodyweight)

  • 6-10 reps (and sometimes 12): hypertrophy - this is the process of tearing muscle fibres so that they can be rebuilt by the protein in your diet (makes 'em bigger)

  • 12-20 reps: muscular endurance - use this range to condition your muscles to contracting repeatedly for longer periods of time. Lots of lactic acid building up? Keep using this rep range and you'll notice less of it building up over time!

Knowing what each type of rep range does is all well and good, but none of this really explains why these ranges work. Why do 5 reps build strength when 8 reps do more work? Why do 8 reps build muscle when 20 reps break down more fibres? Keep reading and we'll dive a little deeper.

If you know me personally, you'll hear me talk a lot about how 1-rep maxes in training don't really serve to help with anything (that's the Mark Rippetoe influence talking). In any session, I'll work with every rep range number including 1, but I'll never max out unless I'm in a competition. I don't believe it's safe enough to justify maxing out in a gym, because you don't have the medical support or backing that you might have in a competitive environment.

This isn't to say that a couple of heavy 1-rep sets aren't going to help you progress a little in your training. I'll add more than a few sets of single reps at the top of my session to push for that little bit extra, but more often, I'll look for sets of 3-5. But why are these better than singles?

The fundamental aim of strength training sessions is to place the human body under manageable amounts of stress. Hans Selye, way back around the 1930s, published studies on General Adaptation Syndrome, a phenomenon he discovered and labelled to explain how the body responds to being placed in these situations of manageable stress.

If we can, in training, place the body under the right amount of stress, it'll realise it needs to recover in order to deal with that level of stress again in future. Most people who aren't professional strength athletes will find that their recovery takes place fully within 2 days, meaning that they'll more than likely be able to lift that little bit more every other time they enter the gym.

If you look at the diagram below, you'll see how the body responds to stress by becoming alarmed, then resists the stress placed on it by recovering and making itself more able to deal with stress. If recover is fully permitted to run its course, then the body will remain at the newly-found level of ability, visible in the resistance stage on the graph below. If recovery is not allowed to fully run its course however, the body will become overstressed, encountering too much to adapt to within a normal recovery window. This phenomenon is often referred to as "overtraining" and is visible in the exhaustion stage of the graph below.

So, when it comes time to train, why don't we just haul one rep up at around 90% effort to save time? Surely a heavy rep near the top will do as much as lighter ones at higher reps. Well, with only one rep, we'd need to come a little too close to our 1-rep max potential for our session to be deemed totally safe, so we lower the weight slightly and do a few more reps to summatively create stress events that our bodies can safely and easily recover from.

Additionally, with more reps (especially at a beginner level), we're also working to commit good movement patterns to muscle memory. This process is referred to as neuromuscular development. Once we've learned good form, we repeat it over and over to make sure our bodies can become faster and more efficient at performing it in future, ultimately unlocking a little bit more energy and focus to reach our potential with when the time comes.

If you're a beginner, you'll want to up the reps towards 5. Your body won't be used to repeating good movement patterns, and because you're quite far from your strength potential, you'll adapt to almost any stress event placed on your body, so you can afford to sacrifice some weight to work on the muscle memory side of training. As you improve in terms of good movement patterns, you can lower the rep numbers to 2-3 and focus on heavier weights to get the most out of your potential adaptations.

Mathematically, these lower rep ranges are very useful for predicting how much we could lift in a competitive environment without introducing the risk of injury present in a 1-rep max attempt. 1-rep max calculators like this one can help us quite accurately predict our potential, meaning we can aim for competitive targets without overtraining or risking injury, but the more reps we do, the harder it is to accurately make a calculation.

This lack of accuracy at higher rep ranges means that sets of 6+ generally aren't used with recorded weights; we use them to place the muscles under the kinds of stress that will force them to come back bigger as we recover, focusing less on strength gains. 6, 8, 10 and even 12 reps are less useful for building strength efficiently, but they're good for breaking down fibres so we can build them back bigger. As a result, with novice training we generally aim to reserve sets of 2-5 to big, strength-building compound lifts like the squat, bench press and deadlift, and leave the muscle-building work to less important exercises that are generally considered accessories.

Pairing strength reps in compounds with hypertrophy reps in accessories ultimately leads to more efficient strength gains with increased mass in supporting muscles, building a strong, solid and focused body that supports itself more effectively as it progresses.

Lastly, we come to those spicy sets of 12-20. The higher we go, the more painful it all seems to become, but the pain is all key to the development! In sets with smaller reps, your body uses a fuel source called ATP, but when that runs out (after just a few seconds), your muscles start respiring anaerobically, meaning that they're burning carbs and producing lactic acid. Lactic acid sits in your muscles and causes the soreness/tightness you feel when you push yourself for those longer, tougher sets. If it's not cleared away by your blood, it'll remain there and burn for up to 3 days!

If we work the muscular endurance range of repetitions in training, we'll condition our bodies to more effectively remove lactic acid, meaning that it'll take longer for the pain to set in in future. By increasing the number of reps per set, we're also getting more of those repeated patterns committed to muscle memory, allowing our bodies to learn how to perform them more effectively and more efficiently in future. If we keep performing muscular endurance sets in training, we'll find functional movements easier to perform and less exhausting to repeat!

So there we are. It was a little bit lengthy and roundabout at times, but this is why we do different ranges of repetitions in training. It's a lot to remember, but keep coming back if you forget, or even get in touch with me and we'll chat about it!

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