The VMO is the obliquely oriented distal fibres of the vastus medialis, distinguished from the more proximal longitudinal fibres by their greater angle (55 to 70 degrees from vertical versus 25 to 40 degrees for the longitudinal fibres). These oblique fibres are the critical medial patellar stabiliser and are the focus of patellofemoral pain and patellar instability rehabilitation. The VMO is the first quadriceps muscle to atrophy after knee injury or surgery.
| Origin | Medial supracondylar line of the femur and the adductor magnus tendon (the oblique portion) |
|---|---|
| Insertion | Medial border of the patella (the more oblique distal fibres) |
| Nerve Supply | Femoral nerve (L2, L3, L4) |
| Blood Supply | Femoral artery |
| Actions | Extension of the knee; Medial patellar stabilisation — the only muscle producing a medial pull on the patella; Last 15 to 30 degrees of knee extension — particularly active in terminal extension |
|---|
The VMO's medial pull on the patella counterbalances the lateral pull of the vastus lateralis, maintaining central patellar tracking in the trochlear groove during knee extension. Loss of this balance allows lateral patellar tilt and excessive lateral facet pressure that drives patellofemoral pain.
VMO bulk is assessed by comparing the teardrop-shaped muscle mass above the medial patellar border between knees. Atrophy is visible as flattening of this area and measurable by circumferential thigh girth. Selective VMO activation during terminal knee extension exercises, step-downs, and Spanish squats is the cornerstone of patellofemoral pain and patellar instability rehabilitation.
The VMO is the distinctly visible teardrop-shaped muscle mass on the medial lower thigh just proximal and medial to the patella, palpable and visible during the last 30 degrees of knee extension and during the drop catch of a squat.
Selective VMO inhibition and atrophy in knee pain producing lateral patellar tilt and increased patellofemoral pain, managed with targeted VMO strengthening in terminal knee extension.
