body composition
Body composition is the relative proportions by weight of fat and lean (muscle) tissue. Although there are sophisticated and expensive devices capable of body composition, the skinfold measurement technique–when used by a trained professional–provides the most valid and reliable means for assessing body fatness generally available.
In general, the less fat mass an athlete has, the more efficient he or she will be during his or her activity. As such, assessing body composition can be an effective tool to monitor an athlete’s readiness for endurance activities.