In sports, body composition refers to the overall weight and size of an athlete and the proportion of that weight made up of muscle, fat or bone. Body composition is important in nearly every sport, but different sports have vastly different ideal body compositions. Even if athletes are skilled and in good physical shape, improper body composition can drastically lower their effectiveness at a given sport.
Body composition is typically expressed in two figures: overall weight and percent of body fat. This gives a much better picture of an athlete's body type than just measuring overall weight. After all, there is a drastic difference between two men who weigh 200 pounds if one is at 30 percent body fat and the other is 15 percent body fat.
Body fat is most commonly measured by using a skinfold caliper at certain areas of the body; the measurements are entered into a calculator to estimate body fat. While this form of measurement is fairly precise if it is done the same way every time, making it useful to measure overall trends in body fat percentage, it is not necessarily accurate. When more accuracy is desired, hydrostatic weighing is usually employed. Because fat is less dense than muscle, the weight of a body in water and outside of water is compared to reach an accurate body fat reading.
In most sports, especially those that involve endurance, it is advantageous to have a low body fat percentage. This is because body fat weighs down an athlete, and unlike muscle, it provides no utility. However, in some contact sports, the extra weight from fat confers an advantage. For example, athletes who play positions on the offensive line in football typically have somewhat high body fat percentages. The extra weight makes them more difficult to push around, so it is easier for them to block members of the opposing defense.