To begin, we need to be familiar with motor units. Simply put, a motor unit is the motor neuron ⚡⚡and the muscle fibres it innervates as seen in image 1 (8). Small motor units involve a small neuron innervating small muscle fibres that are suited to low intensity and long duration exercises (endurance and cardio training). Where as large motor units involve larger motor neurons innervating larger muscle fibres that are better at producing high force and high velocity movements over shorter periods of time (strength training and sprint/ jumping sports) (4).
And these motor units are always recruited in order of size. This means that smaller motor units are recruited for low intensity activities but as the stimulus increases, larger motor units are increasing recruited (4). And this is important because it means we bring a spoonful of soup to our mouth without the entire arm moving at 20m/s.
When it comes to increasing muscle fibre size (getting gainz), larger muscle fibres have a greater capacity for growth, and so we need two things; to recruit larger motor units AND a slow contraction velocity (2). This slow velocity is important because it allows more crossbridges to form (refer to previous post) which will generate more tension. And we know this because jumping/ plyometric exercises recruit larger motor units but do not increase muscle size. And this slow speed is not voluntary, you are trying to move the weight as quickly as possible but the load and/or fatigue is slowing you down.
To achieve this large motor unit recruitment at a slow velocity we can either lift heavy loads or moderate/light loads until fatigue. Let’s start with heavy loads. Lifting a 1-5RM or >85% of 1RM* typically involves full motor recruitment on all reps of a set and also involve moving with a slow speed due to the load (2). Where as when we lift moderate (8-15 reps before fatigue) or light loads (15 to 30 reps before fatigue) it is only the last ~5 reps where fatigue kicks in and slows contraction velocity which forces the larger motor units to be recruited. Prior to this, large motor recruitment is not high enough and contraction velocity is not slow enough. I say ~5 reps but this could be slightly less or slightly more depending on the individual.
So if muscle building is the goal, instead of measuring volume as total reps x sets, we should be measuring the “stimulating” number of reps where larger motor unit recruitment is high and contraction velocity is low. So this will be all of the reps when lifting up to a 5RM, but only the last 5 reps of a set when lifting moderate to lighter loads. This means when lifting loads heavier than 5RM, you will to complete extra sets to compensate for the smaller number of “stimulating reps”. But when lifting lighter loads, you will get ~5 stimulating reps per set regardless of the number of reps of weight used (as long as the set is taken to fatigue).
So what does this mean practically? It means doing 5 sets using a 3RM will give you 15 stimulating reps much the same as 15 sets using a 1RM or (however this will take much longer). It also means 3 sets of 12 and 3 sets of 25 will also give you 15 stimulating reps (~5 reps per set). The implication of this is massive for both health practitioners and gym junkies. It means that a similar amount of muscle can be built using essentially any rep range, with effects tapering off after 30 reps per set (3, 7). Images 2 and 3 (5, 1) illustrate this nicely.
So somebody who wants to increase strength with heavier loads and someone who wants to increase muscle endurance with lighter loads can achieve similar gains in muscle mass. For health professionals rehabbing injuries, it means a patient who is unable to lift heavy loads can still achieve increases in muscle mass using lighter loads, as long as they are taken to relative fatigue.
No research has been conducted on the optimal number of “stimulating” reps per muscle group per week but a recent study (6) revealed that muscle growth continued with 30 sets per week for upper body muscles and 45 sets per week for lower body muscles. This is most likely why bodybuilders primarily use moderate rep ranges- because lifting heavy loads for this number of sets taxes the sh*! out of your central nervous system and lighter loads are just tedious.
I should emphasise that your rep range should reflect the adaptations you want to make (power, strength, endurance) for your sport or your training goal but you can build muscle with any.
(1) Beardsley, C. The strength endurance continuum. Retrieved from https://www.strengthandconditioningresearch.com/perspectives/strength-endurance-continuum/
(2) Beardsley, C. 2018, July 13. What is training volume? Retrieved from https://medium.com/@SandCResearch/what-is-training-volume-286b8da6f427
(3) Lasevicius, T., Ugrinowitsch, C., Schoenfeld, B. J., Roschel, H., Tavares, L. D., De Souza, E. O., ... & Tricoli, V. (2018). Effects of different intensities of resistance training with equated volume load on muscle strength and hypertrophy. European journal of sport science, 18(6), 772-780.
(4) Milner‐Brown, H. S., Stein, R. B., & Yemm, R. (1973). The orderly recruitment of human motor units during voluntary isometric contractions. The Journal of physiology, 230(2), 359-370.
(5) Schoenfeld, B. 2018, August, 22. Effect of different intensities of training with equated volume load on muscle strength and hypertrophy. Retrieved from https://www.instagram.com/p/Bmxy853D1Ds/
(6) Schoenfeld, B. J., Ogborn, D., & Krieger, J. W. (2017). Dose-response relationship between weekly resistance training volume and increases in muscle mass: A systematic review and meta-analysis. Journal of sports sciences, 35(11), 1073-1082.
(7) Schoenfeld, B. J., Peterson, M. D., Ogborn, D., Contreras, B., & Sonmez, G. T. (2015). Effects of low-vs. high-load resistance training on muscle strength and hypertrophy in well-trained men. The Journal of Strength & Conditioning Research, 29(10), 2954-2963.
(8) What is a motor unit? (2014, February). Retrieved from https://physiopolis.wordpress.com/2014/02/24/what-is-a-motor-unit/