Quadriceps tendon rupture is caused by eccentric contraction, where the quadriceps tendon lengthens while its associated quadriceps muscle is contracting, or shortening. This is a higher force exerted on the tendon than a normal physiological load generated by concentric contraction, where the tendon and the muscle both shorten. A typical mechanism of injury occurs when the knee is flexed and the foot / ankle is planted and typically stuck, while a person falls backward and contracts their quadriceps muscle to counteract the fall. The knee thereby is forced into flexion while the quadriceps attempts to straighten the knee, and the extensor mechanism fails.

Quadriceps tendon rupture usually occurs in patients over 40 years of age. It often affects the nondominant lower extremity. It occurs when the tendon fails at its insertion on the proximal patella.

Risk factors include diabetes, renal failure, connective tissue disease, and rheumatoid arthritis / autoimmune disease; pre-existing tendonitis / disease is usually present when a rupture occurs. Medication risk factors include anabolic steroids and corticosteroids (systemic or injection into the tendon), aromatase inhibitors, statins, and fluoroquinolones.

Quadriceps tendon rupture is typically, but not always, an isolated injury. Bilateral extensor mechanism injuries can occur. Ipsilateral knee ligamentous injury occurs about 9%-10% of time in association with quadriceps tendon rupture.