Many physical ailments lead to inhibition (your ability to elicit a muscle contraction has been reduced) of particular muscles. For example, knee pain often leads to the inhibition of the vastus medialis muscle of the quadriceps—an important muscle for proper patellar (knee cap) tracking and general knee stability.
Target exercises may be used to retrain your nervous system on appropriately contracting a target muscle. This is often achieved by specific movement patterns and goading—or calling attention to the muscle by tapping or scraping it while you attempt to initiate a contraction.