A client mentioned recently that one of her goals for the year was to add lean muscle, but she was not sure how to program for it. The question underneath is one almost everyone asks: how many reps to build muscle? The gym-floor answer is “8 to 12”, and it is not wrong, but it is not the whole story either. Muscle grows across a much wider range than that once you understand what actually stimulates it. Here is the science, explained simply, and what it means for your training at Hammer’s Gym in Nunawading.
First, motor units
A motor unit is a motor neuron and the muscle fibres it controls. Small motor units involve a small neuron controlling small, fatigue-resistant fibres suited to low-intensity, long-duration work such as endurance and cardio. Large motor units involve bigger neurons controlling larger fibres that produce high force at high speed for short periods, the fibres used in strength training, sprinting and jumping.
Motor units are recruited in order of size. Low-intensity tasks use the small ones; as the demand rises, progressively larger units join in. That is why you can lift a spoon to your mouth without your whole arm firing at full power.
What makes muscle grow
The large muscle fibres have the greatest capacity for growth, so to build muscle you need two things at once: recruitment of the large motor units, and a slow contraction speed, because slow movement gives the muscle time to develop high tension. We know slow speed matters because explosive plyometric exercises recruit large motor units yet do not build much size. Importantly, the slowness is not voluntary: you are trying to move the weight fast, but the load or your fatigue is slowing you down.
Two ways to get there
Heavy loads. Lifting in the 1 to 5 rep range, above roughly 85 per cent of your max, recruits essentially all your motor units on every rep, and the bar moves slowly simply because it is heavy. Every rep of the set is a stimulating rep.
Moderate or light loads to fatigue. With moderate weights (8 to 15 reps to fatigue) or light weights (15 to 30 reps to fatigue), the early reps are fast and do not recruit the largest units. Only the final five or so reps, when fatigue slows the movement and forces full recruitment, are truly stimulating. The exact number varies a little between people, but about five is a good rule.
Count stimulating reps, not total reps
If muscle is the goal, measuring volume as sets times reps is misleading. What matters is the number of stimulating reps, where recruitment is high and speed is low. Every rep counts when lifting at a five-rep max or heavier; only the last five count when lifting lighter to fatigue. So five sets at a three-rep max gives about 15 stimulating reps. Three sets of 12 to fatigue also gives about 15. So does three sets of 25 to fatigue. The implication is significant: a similar amount of muscle can be built with almost any rep range, with the effect tapering off beyond about 30 reps per set, provided you take the set close enough to fatigue.
What this means in practice
- Pick the rep range that suits your goal and your joints. Someone chasing strength can build muscle with heavy triples; someone protecting sore knees can build it with sets of 20. Both work. See types of weight training explained.
- Take sets close to fatigue when lifting moderate or light loads, or the stimulating reps never arrive. Stopping at 12 when you could do 20 does little.
- Heavier loads need more sets to accumulate the same stimulating reps.
- Moderate reps are the practical sweet spot for most people, which is why bodybuilders live there: heavy sets tax the nervous system and very light sets are tedious.
- Progress the load over time. Whatever the range, you must do a little more as you adapt; see progressive overload explained simply.
How much volume per week?
Research has not settled the optimal number of stimulating reps per muscle per week, but growth has been shown to continue up to around 30 hard sets a week for upper-body muscles and 45 for lower-body in trained lifters. Most people do well between 10 and 20 hard sets per muscle group per week, spread over two or three sessions. Our balanced gym program guide shows how to arrange it.
Match the range to your goal, but know you can grow with any
Your rep range should reflect the adaptation you want most: power, strength or endurance for your sport or your training. But if muscle is on the list, you can build it in any of them as long as the effort is there. Questions about your program, or a niggle getting in the way? Our personal trainers and in-house osteopath at Hammer’s Gym are happy to help; ask at reception about a free assessment. New to Hammer’s? Claim a free one-day trial in Nunawading, minutes from Mitcham, Blackburn, Box Hill and Doncaster.