The quadricep's vastus medialis, often called the vastus internus, is located just above the knee on the medial side of the leg. During a leg extension movement it is active whether you are at 10 degrees, 30 degrees, 60 degrees or 90 degrees of motion. But in order to maximize it's develop make sure the athlete extends his or her legs completely and pauses at the top of the movement.
Pause at the top of the movement to Get the Medialis Strong.