Without trying to generalise and stereotype a group of individuals, one of the main reasons for attending a gym and performing resistance training is to bring about hypertrophic changes (i.e. size/growth) in a given muscle region.
So how do you achieve this? Many factors influence the development of muscular mass. These variables include but are by no means limited to frequency, intensity, time spent and type of training undertaken. A further factor which has yet to be considered independently but that falls into one of the larger categories above is repetitions. Repetitions performed during a set should be relatively straight forward to comprehend but often causes unnecessary confusion (Schoenfeld, 2000).
What repetitions do you select for maximal hypertrophic effect? Is your decision based upon science and knowledge or do you tend to guess and alternate the load chosen dependant on your mood and how you are feeling at that time? The purpose of this article is to discuss the optimal repetitions required for maximal muscular hypertrophy and provide the scientific reasoning behind these numbers. This will allow you the opportunity to make an informed decision next time you hit the gym, based upon research.
Most individuals are aware that performing 15+ repetitions within any given set is aimed more towards muscular endurance than hypertrophic changes. Furthermore, dropping the repetitions too low and performing less than 5 per set is more likely to develop pure strength and power than anything else. With repetitions too high and too low not bringing about the desired muscular changes, it is likely that somewhere in between the two will bring about the desired effect.
This isn’t too far from the truth, with 8-10 repetitions often advocated as the ideal range to be working in to bring about these desirable hypertrophic changes (Schoenfeld, 2000).
So what is it about this mid range training that stimulates optimal growth levels within your given muscle regions?
Firstly, training within the 8-10 range has been demonstrated to stimulate and recruit the maximum number of muscular fibres. Muscle fibre recruitment is relatively straight forward and works in an orderly fashion to bring about maximal recruitment. Smaller motor units are initially recruited and this progresses onto the larger ones until all motor units are recruited.
The mid range training is perfect for this recruitment. When enhancing the weight and performing less than 5 repetitions, the smaller motor units are often skipped in favour of the larger motor units straight away. This prevents full recruitment of all motor units and as a result limits the potential of muscular growth (Zehr et al, 1994).
Secondly, previous research has highlighted that the release and secretion of anabolic hormones is at its greatest following a bout of mid range repetition exercise (Kraemer et al, 1993). These endogenous, anabolic hormones are released after the target muscle has been subjected to stress and assists in initiating the growth process; the greater the level of this hormone, the greater the potential for hypertrophic development.
The release of this desirable hormone is often linked to the levels of lactic acid within the body, with lactic acid build up facilitating its release. Lactic acid is a by product of glycolysis, the energy process that predominantly powers mid range repetition exercise. If you were to perform greater repetitions, for example 15, an alternative energy system is primarily utilised and the lactic acid response is limited, therefore limiting this hormonal release (Roemmich et al, 1997).
Thirdly, and following on from the second point, lactic acid also causes a significant impact upon the release of human growth hormone. As the name aptly suggests, this hormone is a powerful stimulator of muscular hypertrophy whilst at the same time mediating the release of insulin like growth factor. These are both potent, anabolic hormones. Kraemer et al. (1990) supported these findings and demonstrated a heightened secretion of growth hormone following a 10 repetition set compared to that of lower repetitions.
Finally, the time a muscle spends under tension is directly linked with muscular damage. Although muscular damage might sound negative, it is this micro fibre damage that stimulates growth and is imperative for muscular hypertrophy (Evans et al, 1991).
The easiest way to understand this phenomenon is by considering the two key factors which bring about this muscular damage; time and load. When performing less than 5 repetitions the load is enhanced but the time spent performing is decreased. Conversely, when aiming for 15 repetitions the opposite occurs; the load is decreased but the time spent performing is increased.
Somewhere in the middle lays the happy medium, that being 8-10 repetitions. During this, both the time and load are considered optimal for causing muscle damage.
So there you have the justification behind utilising the 8-10 repetition range for maximal muscular hypertrophy. In summary, during this mid range of repetitions you will experience maximal motor unit recruitment, heightened release of various growth hormones through lactic acid production and the creation of an environment where muscular development will ensue following prior micro fibre damage. All these factors will ensure bigger, fuller target muscles.
Remember, if you are looking to bring about hypertrophic changes then as well as sticking to the 8-10 repetition range you have to ensure that all other factors are in place too. Your muscles will only continue to develop if they are constantly overloaded and tested.
Your load selected should also be suitable so that you are reaching fatigue between 8-10 repetitions; there’s no point performing 8-10 repetitions utilising a load that you would normally perform 15 with. It might sound obvious but I’ve seen it with my own eyes in gyms nationally!
The research contained within this article clearly supports the use of 8-10 repetitions per set for maximal hypertrophic development. Does this mean that you should never train any other repetitions? Absolutely not. Everything has its place within your training; higher repetitions can be used to develop muscular endurance and your lactic threshold, whilst lower repetitions will develop pure strength and may assist you break through any training plateau.
Whatever you do, remember to train smart and train with a purpose.