Many believe that eight to 12 repetitions is the golden rep range for muscle growth. Some may recommend more, and others may recommend fewer. In order to settle this dispute, we need to delve into the science behind the training principles and training variables that play into muscle growth.
Resistance training, with the goal of muscle growth, is used to initiate a hypertrophic response. This response includes the growth of myofibrillar contractile proteins as well as noncontractile elements. This is ultimately achieved through a net increase in protein synthesis. Muscle protein synthesis and proteolysis (protein degradation) are both up-regulated and down-regulated through various pathways; implemented correctly, resistance training can readily manipulate these pathways. In order to achieve this effect, an overload stimulus must be elicited — meaning a stress must be placed on the muscle over time for the muscle to adapt. There are three main factors that initiate this adaptation response: mechanical tension, metabolic stress and muscle damage. Mechanical tension refers to the amount of load, and therefore tension, placed on a muscle. It may perturb the structural integrity of the muscle cell and initiate a somatic response that ultimately leads to a net increase in muscle protein synthesis. This is likely the most important of the three, but high levels of mechanical tension alone without sufficient volume may only produce neural adaptations, as opposed to both neural and muscular adaptations. When a muscle is subjected to a certain volume of work, metabolic byproducts accumulate as a result of anaerobic glycolysis — this phenomenon is referred to as metabolic stress. This stress induces an a hormonal response, cell swelling and an increase in growth factors, which ultimately causes a net increase in muscle protein synthesis. Muscle damage generates an inflammatory response, ultimately stimulating various growth factors to promote hypertrophy; the proposed mechanism posits that nerve endings innervating damaged fibers stimulate satellite cells to induce growth. Muscle damage is the least important of the three main factors and too much of it will hinder the growth process.
In order to stress these three factors and facilitate growth, certain training variables need to be employed. The main training variable is volume, which accounts for the amount of work done. It is defined as weight multiplied by total repetitions. Volume has a dose response relationship with hypertrophy, meaning that the amount of volume determines the amount of adaptation, or, in this case, muscle growth. If more volume means more adaptation, then we would want to maximize volume, to a certain extent. Too much volume will eventually lead to either stagnation or overtraining, so volume should be kept within the athlete’s acceptable range. This range is largely determined by individual characteristics, ranging from life variables (such as sleep, stress and nutrition) to genetics and training age. Normally, the longer somebody has been training, the more volume can be utilized, as overload stimulus over time increases the total volume that can be handled at once. Periodization (the organization of training variables over the long term) may also play a role, and can be used to maximize volume over time while avoiding stagnation and overtraining. The other most important variable to consider when analyzing repetition and load schemes is intensity. Intensity quantifies the load used and is defined as a percentage of one-rep-max (1RM). This means that a higher load is a higher intensity. Inherently, a higher intensity will necessitate a lower number of possible repetitions. Since the volume formula contains both load and total repetitions, intensity has a significant effect on the volume possible to achieve.
If volume determines adaptation, then the load and repetition scheme should be chosen carefully in order to maximize this value — within reason. Lower loads tend to allow more volume per set completed, but too low a load may not be optimal for hypertrophy. Past research has indicated that loads below 65% 1RM (or around 15RM) are less effective at producing hypertrophic adaptation, as opposed to loads above this threshold. Although no consensus has been reached, recent research has shown that taking a set to muscular failure may be more important than this intensity threshold. This is partially explained by Henneman’s size principle, which states that motor units, or groups of muscle fibers, are recruited in a specific order under load. The spectrum of motor units is covered as force and fatigue requirements increase — starting with the smaller, slower-contracting fibers all the way up to the faster-contracting, larger fibers. Taking a set to muscular failure would require all motor units to be activated by the end of the bout, as fatigue facilitates the need for more motor units to be recruited in order to meet the force requirement. The question now is: What does this have to do with building muscle?
It turns out that fiber recruitment is highly correlated with muscle growth, meaning that if more muscle fibers are recruited, a higher growth stimulus is likely to occur — there is one problem, though. Taking very low loads to complete muscular failure in a normal gym setting is not very practical. The ability to place stress on the muscle itself, unlike various other factors (such as cardiovascular conditioning, grip strength or even the mental ability to continue), will be reduced as the number of required repetitions increases. What about loads above 65% 1RM? It seems as if any load within this range, except at the top end, produces similar results when volume is equated. A recent study looked into the differences between two different repetition schemes with equal volume values (weight multiplied by total repetitions). Two groups, one using three repetition sets and the other using 10 repetition sets, completed the same exercises and volume over the same period of time. The two training programs produced about the same increase in muscle cross-sectional area (a measure of hypertrophy), although there were some differences. The three-repetition group saw larger strength gains, but their training sessions took 70 minutes and they felt very fatigued by the end of the eight-week study. The 10-repetition group took 17 minutes per session and were not fatigued at all. In fact, they felt they could actually handle more volume. In order to equate volume, the 3-repetition group required seven sets per exercise, while the 10-repetition group required only three sets. Taking into account time efficiency and recovery ability, the 10-repetition protocol was likely more optimal for hypertrophy over the long run.
All things considered, as long as volume is equated, there may not actually be an optimal range of repetitions for maximum muscle growth. However, a moderate repetition range seems to be the best mix of a load high enough to allow sufficient motor unit activation but also low enough to provide sufficient volume to stimulate hypertrophy. Strength expectedly plays a role in hypertrophy as it allows more weight to be used within this moderate range, ultimately allowing more room for overload stimulus and therefore growth. Just like low-repetition strength work, high-repetition strength work focused more on fatigue resistance has its benefits as well, allowing more repetitions and therefore volume to be completed within the moderate range. Taking this analysis into account, it seems reasonable to use periodization to concentrate most of training volume within the moderate (six to 12 repetition) range, while incorporating some volume done in the lower (three to five repetition) range and some volume done in the higher (12 to 15 repetition) range.