Number of repetitions and muscular hypertrophy

By Fabrizio Felici

The common advice on the ideal number of repetitions per series given to athletes is that a low number of repetitions is the best to increase muscle mass and strength while a high number of repetitions promotes muscle duration and can increase definition. Years ago studies carried out by US Navy researchers showed that a higher number of repetitions created a greater stimulus for the growth of larger muscle groups, such as the thighs and the back. But is determining the ideal number of repetitions per series so simple?

No, if we consider the overall picture which includes the variable effects of the number of repetitions on hormones and on the anabolic processes of the muscles. An example is the difference between the low number of repetitions and the moderate repetitions in the range of 8-10 repetitions per series. The execution of a low number of repetitions, less than 6, is fed by the phosphocreatine energy system. In a nutshell, this means that the use of a low number of repetitions exploits the ATP and mainly the creatine deposited in the muscle. When using the range of 8-10 repetitions, the phosphocreatine energy system does not last long enough and forces the body to turn to the next level energy source. This system is known as glycolytic because it relies on circulating blood glucose and glycogen deposited in the muscle. Also this energetic mechanism is anaerobic, since the presence of oxygen is not necessary to feed this process.

An important consideration linked to muscle growth is that the glycolytic system produces lactic acid. This is known primarily as a cause of fatigue but also promotes the release of growth hormone (GH). Furthermore, the body is able to dissociate the acid portion of lactic acid, creating lactate, which can be used by the muscles directly as an energy source.

The increase in growth hormone induced by the release of lactic acid with a moderate number of repetitions activates both systemic and local release of IGF-1 in muscle. IGF-1 activates the normally dormant immature muscle cells, called satellite cells that can then be incorporated into the trained muscle, creating an increase in muscle mass. A moderate number of repetitions accompanied by heavy loads also activates the mechano growth factor (MGF), another more powerful localized muscle growth factor than IGF-1 in promoting muscle growth.

But then if lactic acid is such a powerful stimulus for muscle growth, why not maximize the effect with an even higher number of repetitions, for example over 15 per series?

The first answer is that using so many repetitions requires the use of lighter loads. When using lighter loads, the body uses the principle of muscle recruitment. This refers to how the brain recruits the minimum number of muscle fibers to do the job. The use of lighter loads initially activates type I slow-twitch muscle fibers, which are not as sensitive to muscle growth as type II fast-twitch fibers. The other problem related to the use of light loads and a high number of repetitions is that it is possible to switch to an aerobic energy path, which tends to interfere with muscle growth, favoring processes of muscle duration, such as the increase in mitochondria cell count and the increase in oxidative enzymes. Doing a moderate number of repetitions also favors another procedure related to the anabolic effects in the muscle, involving a greater moisturizing effect in the cell. The added moisturizing effect derives from the increased blood flow in the muscle (pumping) combined with the added lactate release. These two effects produce an osmotic reaction that brings water into the muscle. The subsequent swelling of the muscle cells acts as a powerful anabolic signal by stimulating muscle protein synthesis. The moisturizing effect of the cell is more specific in type II muscle fibers because these fibers contain a water transport protein called aquaporin-4.

To sum it all up, we can say that a moderate number of repetitions in the range of 8-10 per series produces the best results for the promotion of the increase in muscle mass.

To confirm this, a study by Gerson and collaborators is very interesting, which examined the effects of three different models of series and repetitions on muscle adaptations in the vastus lateralis muscle in the front of the thigh. The study subjects were 32 men, average age 22, who performed an 8-week resistance training program. The subjects were divided into four groups:

1. Low number of repetitions, 3-5 repetitions per series, four series per exercise and 3 minutes of recovery between the series
2. Intermediate group, 3 sets of 9-11 repetitions with 2 minutes of recovery between the series
3. High number of repetitions, 2 sets of 20-28 repetitions with 1 minute of recovery between the series
4. Control group that has not carried out physical activity.

All groups, except the control group, used three leg exercises (leg press, squat and leg extension) twice a week in the first month, then three days a week in the second month. Some tests were performed before and after the 8-week study to measure maximum strength, muscle duration and other cardiorespiratory values. Muscle biopsies were also performed to evaluate the variations in fibers induced by different regimes of physical activity.

The results showed that the force increased more in group 1, the group with a lower number of repetitions, while the muscle duration increased more in the group with a high number of repetitions. The group with a low number of repetitions and the intermediate group experienced major muscle hypertrophy. In contrast, those in the group with a high number of repetitions did not experience any muscle growth.