The vastus medialis muscle is a part of the quadriceps muscle group, located on the front of the thigh. It is the most medial, or inner, of the quadriceps muscles. It extends the entire length of the thigh. The portion of the muscle that is just above the knee is sometimes referred to as the vastus medialis obliquus, or VMO. This muscle is used to extend the leg at the knee and to stabilize the patella, which is also known as the kneecap.

The muscle is a common center of weakness because it will not become fully strengthened unless the leg is regularly extended fully. The vastus medialis can be strengthened by performing exercises such as knee extensions, leg presses, and squats. The muscle is more fully activated when the knee is at a greater angle, especially when the leg is completely extended. Therefore, quadriceps exercises that involve a full range of motion are best for strengthening this muscle.