Specificity refers to the type of changes the body makes in response to sports training. Very simply, what you do is what you get.
When an athlete trains, he or she repeatedly performs activities to prepare for the exact requirements of the sport. In time, the athlete's body becomes better able to meet the demands of the sport as it adapts to the training regimen.
Adaptations to training are most evident in elite athletes. For example, the effects of years of rigorous training clearly distinguish the bodies of distance runners from throwers.
For distance runners, major adaptations from the demands of sustained running include a larger, stronger heart and increased blood vessels to supply oxygen to the specific muscles involved in running. In contrast, adaptations to training for throwers include increased size and thickness of specific muscles of the body that are trained to improve power.
This principle applied to sports fitness training means that the overall energy demands of the sport determine which fitness components (e.g., strength, power, endurance) should be developed so that the requirements of the sport are matched.
For example, basketball fitness training should include some distance work with intermittent speed and agility training. In contrast, golfers would require little distance work, but train for power and flexibility.