For optimal muscle building (hypertrophy), most sets will fall somewhere between five and 20 reps per set as long as they are performed close to failure. Failure is defined as the point where you can’t perform another additional rep with good technique.
Generally speaking, lower reps lead to greater strength, whereas higher reps encourage muscle growth. Below five reps will lead to good strength increases but very little muscle growth (and we see this in powerlifters and Olympic weightlifters who return to Bodyscan with new personal bests but little, if any, increase in muscle mass).
Above 20 reps, the muscle adapts and becomes better at endurance and contracting at low levels for long periods. Consider a marathon runner who is, in effect, doing thousands of reps – endurance athletes like these have very low muscle mass.
You may have heard of slow-twitch and fast-twitch muscle fibres. There may be some debate about whether there are just two types of fibre but it is accepted that slow-twitch fibres are built for endurance (the runner), while fast-twitch fibres are optimised for strength and power.
Slow-twitch endurance fibres are smaller than fast-twitch, which is why your training regime should focus on maximising stimulus on the bigger, fast-twitch type; slow-twitch fibres contribute less to muscle hypertrophy, which is why endurance athletes are skinny.
The ratio of fast-twitch to slow-twitch muscle fibres we each have varies based on our genetics and our usual activity but it is not set in stone. How you train (your weights and reps) will encourage the increase and development of one or the other type of muscle fibre. So a runner can become a boxer (and vice versa) by changing the way they train and increasing the number of slow- or fast-twitch fibres in their muscles.
The way the body responds to different weights and rep ranges is why doing 10 reps of 20kg will not have the same effect as 200 reps of 1kg.
As for where in the 5-20 rep range you should aim for, different muscle groups and individuals respond differently, so I advocate some variety. You may feel different muscles really “working” at different ranges and giving you the best ‘pump’. Also, generally speaking, big compound movements (like squat and deadlift) lend themselves to lower ranges, while isolation exercises that target small muscles (like lateral raises and bicep curls) respond better to higher reps.
Injury prevention is very important in a successful weight training regime and so performing all your exercises with lower reps (below 8) would increase your risk. We see that muscle mass is more adaptive and faster to strengthen than connective tissue (tendons and ligaments). Adding in some higher rep work (15-20 reps per set) will allow lower loads to be lifted, less stress on the connective tissue leading to a lower injury risk whilst still maintaining the benefits for muscle growth on the muscle.
Muscle growth is an adaptive response and therefore in my opinion, it makes sense to have a variety of rep targets within the 5-20 rep range across the week of training. Just doing 10 reps for every set is likely to yield less of an adaptive response from the body as the body is receiving the same stimulus and it is the adaptation which causes muscles to grow.
In summary, keeping all sets between 5-20 reps and varying the rep target across your weekly training regime will see you yielding good results.
However, with all the above said, what matters more is the quality of your exercise. If your form is bad (and you are using momentum or gravity to move the weight, or not working the muscles the exercise was designed to stimulate) then you are wasting your time and the rep range becomes irrelevant!
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