What we are going to be talking about is the manipulatable elements of fat and muscle, rather than genetically determined height and proportions. Of course, propensity to build muscle and fat storage is to a degree genetically determined. But they can be heavily manipulated by training, nutrition and recovery.
First up, spoiler alert: we can’t truly say what is OPTIMAL for performance. Because we can’t test it. We can say that individuals with ‘x’ body composition tend to be most successful in a certain sport, and that ‘y’ body composition confers certain physical properties that are expected to drive performance. But we cannot categorically state the optimal body composition.
In fact, it is likely that a single optimal body composition does not exist. Because body composition has to be considered relative to an individual’s proportions (e.g. a taller F1 driver typically needs to be proportionally lighter than their shorter counterpart in order for optimal running of the car), as well as other factors that impact performance and may have an indirect impact on body composition. For example, energy availability to support immune function and reduce injury risk (more on this later).
Whilst ‘optimal’ may not exist, body composition does interplay with performance. Therefore it is worthwhile considering how … because optimising performance is what the competitive athlete must do to succeed!
First up, muscle. Strength and power depends on muscle. So any sport that depends on strength and power depends on well-developed muscle, at least in the muscle groups relevant for the sport! Take powerlifting, Olympic lifting, rugby, strongman etc. It is worth noting that muscle strength, whilst related to muscle size is not equivalent to muscle size. Strength and hypertrophy (muscle mass) can be uncoupled. So in sports where strength:bodyweight must be optimised, it is beneficial to maximise strength gains without necessarily adding maximal amounts of muscle mass. Take gymnastics, for example. An athlete must be strong to move themselves in space, but as light as possible with this strength to enable them to do more complex moves, more efficiently, and with as little stress and fatigue exerted on the body as posisble.
Outside of strength based sports, muscle is still vital. A runner must have strong leg and butt muscles to exert force against the ground to propel themselves forwards as fast and efficiently as possible. In such sports there is going to come a point where further addition of muscle mass – even though it is in functional muscle – becomes detrimental. A sprinter with the quads of a bodybuilder is going to have trouble running with mechanics efficient enough to move fast and / or for long!
Now, body fat. Sports that require athletes to move their bodyweight in space typically benefit from reduced body fat. It is ‘deadweight’ that the body must move, without getting any performance return. Take running, for example. Around 80% of energy expended in running is said to be moving bodyweight. So you can see how adding significant fat mass could significantly increase the muscle and energetic fatigue a runner experiences, causing them to run slower and / or for shorter durations. It must also be noted that ‘deadweight’ doesn’t just refer to fat mass. It also refers to non functional muscle mass. To again use a runner as an example … an jacked bodybuilder-esque upper body is deadweight!
Excess body fat can also limit range of motion essential to certain sports. Excess fat deposition in the lower half of the body may prevent an individual getting full depth in a squat. In contrast, in the upper body it may assist in a bench press because it reduces the depth of movement required to complete the lift! In addition, in endurance swimmers the buoyancy provided by body fat can help preserve energy to complete the swim in the fastest possible time.
High bodyweight from fat or muscle can help either apply load to move things, e.g. weight or humans, or to avoid being moved by load, e.g. other humans. Take a rugby forward for example. They want to stay on their feet when hurtled at by another heavy player, and withstand and apply force in the front of a scrum. This is most easily achieved by being heavy themselves. You are not likely to find a 65kg forward!!!
However, there are always trade offs. Take the rugby forwards for example. Yes they need to apply and withstand force, but they still need to survive an 80 minute match where they are required to repeatedly move up and down the pitch. Moving their weight to do this is energy intensive. So, they want to be as big as they need to be and no bigger! Plus with a high aerobic capacity to enable them to move as fast as they need to move. And with as much functional muscle as possible to support rather than be a deadweight in moving.
Trade offs become even more complex is multidisciplinary sports like CrossFit. Athletes need to move their body in space in gymnastics and bodyweight movements such as burpees. So we think ‘light’. But they also need to move as heavy load as possible. So we think ‘muscle’ and ‘weight’, and sometimes in muscle groups that are not so ‘functional’ in the bodyweight movements, e.g. you don’t need big quads to do push ups! Ultimately such athletes aim to maximise their strength : bodyweight ratio, and develop large energetic systems in order to effectively move a relatively high muscle weight repeatedly in space.
Weight class sports throw in another complication. Weight class sports were developed because in strength and combat based sports, there is an inherent advantage of size. Load lifts load. Weight exerts force. So, weight classes were developed in order to remove the inherent advantage of being bigger. However, athletes attempt to regain this advantage by competing in a weight class lower than their true walkaround weight, through manipulation of body water, carbohydrate and fibre stores as well as rapid caloric restriction. Essentially so they are ‘bigger’ than their opponent. Nowadays, particularly in combat sports, most athletes do this and so it is less about gaining an advantage and more about not being at a disadvantage. Weight class sports are typically strength and power sports. So muscle is an advantage. But an athlete must avoid adding so much muscle that they can no longer make their weight class (albeit I would argue that often these individuals may then be better placed in a higher weight class, if their skill and performance has increased with the increase in muscle mass). They can of course lose it through weight loss, but it is not clear what the advantage might have been to working on gaining it in the first place in that instance.
Finally, we must also consider the impact of maintaining a certain body composition on other factors impacting performance. Most importantly, health. Talking physical health first. In sports where athletes compete at a light weight to optimise performance, e.g. endurance runners, this weight may not be optimal for health. As it may mean the athlete’s caloric intake is too low to have the energy availability to support the immune system, reproductive system, and so on. Over the long term this can lead to a deterioration in physical health. This has an indirect impact on performance as energy for training, recovery and adaptation is compromised. And a direct impact as the athlete is more susceptible to injury. Gymnastics, combat sports, physique sports can all encounter similar situations. In addition, chronic restriction of food intake increases the risk of disordered eating and eating disorders, as well as reducing social pleasures associated with food. In such sports, periodisation of nutrition and body composition may effectively balance health and performance needs over the long term. This involves periods of higher high quality calorie intake and increased bodyweight out of competition, and managed periods of lower energy intake to optimise body composition for competition.
In summary, body composition impacts performance in multiple ways in different sports. As ever, there are trade offs to be made. And, as ever, the precise trade offs to be made and to what extent are going to depend on the individual athlete.
Some articles of interest
Posthumus et al. (2020). Physical and Fitness Characteristics of Elite Professional Rugby Union Players. Sports, 8, 85, doi:10.3390/sports806008
Reale et al. (2020). Body composition of elite Olympic combat sport athletes. European Journal of Sport Science, 20 (2), 147-156
Stellingwerff, T., Morton, J.P., and Burke, L.M. (2019). A Framework for Periodised Nutrition for Athletics. International Journal of Sport Nutrition and Exercise Metabolism, 29 (20), 141 – 151