Quadriceps femoris

The quadriceps femoris is a group of four muscles: the rectus femoris (the only bi-articular component, crossing both hip and knee), vastus lateralis (the largest), vastus medialis (important for patellar tracking), and vastus intermedius (the deepest). All four muscles converge on the quadriceps tendon, which crosses the knee and attaches to the tibial tuberosity via the patellar tendon, enclosing the patella within the tendon.

The quadriceps femoris is the most powerful extensor of the knee joint, generating the force needed for activities such as standing up, walking, climbing stairs, and running. The rectus femoris additionally contributes to hip flexion. The quadriceps is essential for decelerating the knee during the stance phase of walking and absorbing impact forces during landing from a jump.

A quadriceps tendon rupture typically occurs just above the patella in older adults and results in complete inability to actively extend the knee. The patient cannot perform a straight-leg raise, and a palpable defect is often felt above the kneecap. Surgical repair is almost always required to restore knee extension function, followed by prolonged rehabilitation.

The vastus medialis oblique (VMO) is the distalmost fibers of the vastus medialis muscle, which insert at an oblique angle on the medial patella. It provides a medial pull on the patella to counterbalance the lateral pull of the vastus lateralis. VMO weakness or atrophy leads to patellar maltracking—the patella shifts laterally, causing patellofemoral pain syndrome (runner's knee) and increasing the risk of patellar dislocation.

Quadriceps atrophy following knee injury or surgery is driven by arthrogenic muscle inhibition—a reflexive neural inhibition of the quadriceps triggered by pain, swelling, and joint effusion. Even small amounts of fluid in the knee joint (as little as 20-30 mL) can suppress quadriceps activation via neural pathways. This disuse atrophy can be rapid and profound, with measurable strength loss occurring within days.