What is the SAID principle?

The principle of specificity is commonly referred to as the SAID principle, which stands for "Specific Adaptation to Imposed Demands." The SAID principle states that the body will adapt to the specific demands placed upon it. In other words, you get what you train for. For example, an individual wanting to improve specific sport skills, the training movements and exercises chosen should resemble the movement patterns encountered in the individual's particular sport or activity, as closely as possible. Another example of the SAID principle would be an individual wanting to recover from a lower leg injury. This particular individual should select initial exercises that focus on improving strength and stability, along with muscular endurance.

The SAID principle stands for the "specific adaptation to imposed demands" and means that essentially, the body will specifically adapt to the types of demands placed on it.  For example, in individuals that sit for extended periods, their body will start to change and structurally adapt to the demands of sitting.  Conversely, this principle can be applied to exercise as well.  When individuals perform resistance training exercises repeatedly with light weights for many repetitions, the body will adapt by developing higher levels of muscular endurance. However, that same individual would probably not be very good at lifting maximal weights for low repetitions because they have not trained for it.