According to the BBC, specificity is a principle in sport training that states that training a certain body part or component of fitness yields benefits that are specific and related to it. It implies that one needs to incorporate a certain exercise or skill into a training program if one hopes to become better at it. For example, someone who wishes to become a faster runner should spend time running.
The principle of specificity also applies to individual aspects of a given sport. For example, if a long-distance runner is hoping to run a good time on a course with a lot of hills, he should spent a lot of time running on hills to work on that aspect of his physical fitness; he is sure to be unprepared if he trains for the race on a treadmill. A powerlifter who hopes to improve his one repetition maximum on the squat should spend significant time working in lower repetition ranges.
On the other hand, the principle of specificity should not be taken to mean that athletes should only practice the exact activities for which they are training. Often, practicing the same activity as one plans to compete at every training session is a recipe for injury. For example, new marathoners seldom run the full 26.3-mile marathon distance before the day of the race.
Instead, they usually build up to a lesser distance slowly and include some short interval training, then "taper" to lighter mileages in the weeks leading up to the race. Running the full marathon distance for new marathoners is often enough of an effort to require weeks of recovery. When designing a training plan, it is vital to strike a balance between hard, competition-level efforts and easier sessions to avoid injury and steadily improve fitness.