With regard to body composition, it is necessary, before proceeding further, to dwell on establishing a priori what the optimal body composition should be. An extremely simplified model of the human body sees it as the sum of four components: water, fat, non-lipid cellular solids (i.e., non-fat cellular solids), and non-lipid extracellular solids (Wang et al. 1992; Heymsfield, Waki, 1991; Wang et al. 2011; Heymsfield et al. 2015; Prado et al. 2025). For our purposes, however, it’s useful to consider a non-exhaustive reclassification of these (with some categories overlapping). However, here, it’s useful to consider a reclassification of these that, while not exhaustive and taxonomically suboptimal (some categories overlap), will allow us to focus on what’s truly important for our purposes. The classes are: fat, muscle mass, and body water. Each of these can be modified in quantity (and sometimes in quality) through training and nutrition. As far as cell mass is concerned, intervention is focused more specifically on skeletal muscle mass and, to a much lesser extent, that of the heart muscle (the direct influence of nutrition on the latter is negligible). It is therefore necessary to establish what the optimal distribution of each of these three elements should be, depending on the athlete’s specific performance requirements.
The mass of water
As far as water is concerned, as long as it is retained in physiological amounts (around 60 per cent (Lewis, Braunstein, 2024; Périard et al. 2021; O’Brien et al. 1999; Heymsfield et al. 2018; Sawka et al. 2015)) it makes no positive difference to athletic performance. Individuals with a water mass above or below the values considered physiological will certainly have health problems (Dmitrieva et al. 2024; Edmonds et al. 2021; Li et al. 2023; El-Sharkawy et al. 2021). The only goal one should set with regard to water mass is to keep it within a physiological range.
Fat mass
As far as fat mass is concerned, on the other hand, most athletes need to reduce it as much as possible, but there are exceptions to this rule.
Inert ballast
More specifically, excess fat constitutes an inert ballast that slows down movement and generates stress on the joints that have to support it. If, then, the athlete in question practises a sport in which weight categories are involved, he or she will certainly have an interest in ensuring that the maximum amount of body weight is such as to guarantee performance, so he or she will have reason to desire a body composition with a relative scarcity of fat in favour of tissue that is more functional for the production of performance (muscle mass).
Source of stability for the body
Sometimes, however, even fat can be a valuable performance aid. A fat physique, in fact, because it is heavier, creates more friction with the ground, stabilising the position. For example, a discus thrower or a hammerer might benefit from a body composition with a fair amount of fat mass. Furthermore, as body weight increases, the difficulty of maintaining ground support decreases. This can be very useful for a wrestler (of course there are pros and cons to consider). Finally, fat tends to favour buoyancy of the body. An endurance swimmer will benefit, therefore, from retaining a certain amount of fat mass, which, rather than hindering him, will help him save energy (Shaw, Mujica, 2018; Baldassarre et al. 2017).
Source of energy
Fat is also an abundant source of energy. Usually, even people with extremely low fat mass percentages possess sufficient fat tissue to produce plenty of energy for basic needs (Hargrove et al. 2008; Dulloo, Jacquet, 1999; Hall, 2008). Some studies show, however, that a fat mass percentage of less than 8 per cent is correlated with a higher incidence of injuries as well as relatively low performance, but this is probably not due to a lack of fat mass per se, but rather to the side effects of a dietary regimen that reduces fat mass to this extent (Mountjoy et al. 2014; Mountjoy et al. 2018). Such a regime, in fact, could easily lead to problems with recovery from training fatigue.
Muscle mass
As is well known, the amount of muscle mass is one of the elements that generate strength, so all athletes with an interest in developing this motor capacity should desire a relatively muscular physique. Others, however, with different needs, may need to limit it.
Absolute force and relative force
The most typical, as well as the best known, method of measuring an athlete’s strength is to subject him to a weightlifting test. The more weight he shows he can lift according to the technical parameters established beforehand, the more his strength will be assessed. Well, in this regard, it is possible to classify strength into two subgroups: absolute strength is given, precisely, by the weight, usually expressed in kilos or pounds, that the athlete lifts regardless of other factors; relative strength, on the other hand, is given by the ratio between weight lifted and body weight. Now, it is clear that most athletes interested in power and speed, being oriented to move their body before anything else, will have to engage in cultivating relative strength first, but there are exceptions to this rule. In the section on fat mass, above, we saw that having a relatively high body weight can sometimes translate into advantages for athletic performance. Athletes whose specific performance would benefit from greater body stability should therefore seek not so much relative as absolute strength.
The influence of muscle mass on relative strength
We have already made it clear that relatively large muscle mass generally corresponds to relatively high muscle strength. However, this does not always apply to relative strength. There are several reasons for this. Firstly, muscle tissue is made up of myofibrils, which are actively involved in force production, but also of other components that are not functional for this purpose. More specifically, an increase in muscle mass may imply not only an increase in myofibrils in series, but also an increase in the volume of sarcoplasm (a substance with a predominantly colloidal structure within which the myofibrils are immersed), which does not assist force performance. It is therefore possible to distinguish two different types of hypertrophy (increase in cell volume) and two different types of hyperplasia (increase in the number of cells) of muscles: a myofibrillar or functional hypertrophy/hyperplasia, which corresponds to an increase in strength, and a sarcoplasmic or dysfunctional hypertrophy/hyperplasia, which does not imply an improvement in the aforementioned motor capacity. It is clear that the latter, by causing an increase in weight without this being balanced by an increase in muscular strength, will damage the relative expression of the latter rather than improve it (Roberts et al. 2020; Van Every et al. 2026; Travis et al. 2020) However, it is worth specifying that this latter type of muscle anabolism may also have a positive influence on relative strength, although this is manifested indirectly and to an extent which does not guarantee a performance advantage for athletes interested in remaining light. Sarcoplasmic anabolism, in fact, implies greater hydration of muscle tissue (Roberts et al. 2020), which, as a result, tends to recover more quickly from training fatigue (Harris et al. 2019) and be less prone to injury (Convertino et al. 1996; Ozkan, Ibrahim, 2010). Through training, it is possible to steer the muscle growth process in a functional direction rather than one that is only partially dysfunctional, so much so that, usually, more muscular athletes, although generally stronger than athletes with less muscle mass in absolute terms, show less relative strength. Another reason why an increase in muscle mass may not correspond to an increase in relative strength is that a muscle, even if it is rich in myofibrils, may increase in weight more than it is able to compensate for this increase with an improvement in strength (Atkins, 2004; Latella et al. 2022). The relationship between muscle mass and relative strength, however, is not always linear. In fact, the greatest relative strength is sometimes observed not in extremely light athletes, but in those in low to medium weight categories.
The influence of muscle mass on absolute strength
As for absolute strength, however, this usually tends to improve as muscle mass increases. Athletes interested in absolute strength, therefore, should attempt, through appropriate training and nutrition strategies, to increase muscle (myofibrillar) mass.
Superfluous muscle mass
For other power and speed athletes, on the other hand, even a large increase in relative strength may not be functional for the specific performance, so they should not even be interested in the myofibrillar increase. This is due to the fact that strength exerts a rather limited positive influence for some of these types of performance. More specifically, these are those performances in which the preservation of the motion of the body or a body segment is required against very low resistances or those consisting only of the weight of the moving body part. The greater the resistance offered by the moving body, the greater the force required to conserve its speed. For example, having to throw a heavy object as far as possible or as fast as possible, the greater the positive influence of force will be the heavier the object. With regard to acceleration, on the other hand, force has a greater influence, but still limited to the weight of the object to be accelerated. Many sprinters choose not to try to develop the squat force beyond a load twice that of the body, because they consider it unnecessary.
Counterproductive muscle mass
Finally, there are some athletes whose specific performance does not require great strength. On the other hand, they have an interest in keeping themselves light in order to be facilitated in conserving energy during tests in which they are required to move for long distances. These athletes may even be interested in a decrease in muscle mass. The most typical example is the marathon runner.