The gluteus medius is one of the muscles that make up the gluteus muscle group. This muscle is small compared to the gluteus maximus, but helps perform the key function of hip abduction, or moving the thigh away from the midline of the body.
This movement is key during exercise, and can act as a protective mechanism for the knee. For example, if the hip is not properly abducted when landing from a jump, the knee joint will attempt to attenuate these forces. The knee joint and surrounding musculature is not as strong as the hip joint structure, and is more likely to fail in this compromising position.
The picture on left demonstrates an athlete not using his glut med, with the knee falling in towards the body, into a compromising position. The picture on the right demonstrates proper form. The athlete is activating his glut med, which pushes the knee away from the midline, and also takes the ankle out of valgus (falling towards the midline).
The action of the knee adducting during exercise is one of the main causes for ACL, MCL and meniscus injury. The use and activation of the glut med is under utilized by coaches and training staff, when teaching proper mechanics to athletes.