Can You Isolate VMO During Exercise?

Pittsburgh Pirates designated hitter Andrew McCutchen (22) grabs his knee after falling to the ground to avoid an inside pitch. Mandatory Credit: Charles LeClaire-USA TODAY Sports

The vastus medialis obliquus (VMO) muscle, a part of the quadriceps muscle group, plays a crucial role in stabilizing the patella and ensuring proper knee function. The VMO is often a focal point in rehabilitation exercises to improve knee stability and address patellofemoral pain syndrome (PFPS). An imbalance between the VMO and the vastus lateralis (VL) contributes to the development of patellofemoral pain syndrome, and VMO atrophy or inhibition contributes to patellar instability⁽¹⁾.  A prevalent belief in sports science and rehabilitation is that specific exercises, particularly knee extensions, can target and isolate the VMO⁽¹⁾. This narrative permeates common society, and it isn’t uncommon to hear athletes say, “I need to strengthen this muscle on the inside of my knee.”

Researchers use electromyography (EMG) to measure the activation levels of the VMO and VL muscles during exercises. They represent these measurements as a ratio (VMO/VL) to assess VMO activity, with a ratio greater than 1 indicating preferential VMO activation. Clinicians may alter hip, knee, ankle, or forefoot positions or co-contract lower extremity muscles to enhance VMO activity. However, it may not be possible to selectively strengthen the VMO^(2,3,4).

Understanding the relationship between the vastus medialis weakness and patellar tracking is useful in diagnosing, treating, and preventing patellar subluxation and dislocation. Furthermore, the VMO is the first muscle to atrophy following knee injury⁽⁵⁾.

VMO Anatomy and Function

The VMO is the most distal and medial portion of the vastus medialis muscle, part of the quadriceps femoris group (see figure 1). Its oblique fibers are oriented at an angle to the femur, contributing to its unique role in knee mechanics. Based on a muscle fiber orientation, VMO is not primarily involved in knee joint extension and indirectly contributes to the knee extension torque⁽⁶⁾.

The anatomical structure of the quadriceps muscle group further complicates the idea of isolating the VMO. The quadriceps muscles share a common neural innervation and tendon, the quadriceps tendon, which inserts into the patella. This shared neural and tendon structure means that any contraction of the quadriceps muscles will inherently involve some degree of coordinated activity across all components. Vastus medialis obliquus might contribute to generating the knee joint torque consistently through the range of motion. Therefore, facilitating the neural drive for VMO is important during rehabilitation⁽⁶⁾.