The extensor mechanism of the knee comprises the quadriceps muscle and tendon, the patella, and the patellar tendon (also known as the infra-patellar ligament). Disruption of any of these components impedes a person’s ability to actively extend the knee or resist passive flexion. Such injuries are therefore incompatible with normal walking and standing. Failure of the quadriceps or patellar tendon is often, but not always, preceded by painful tendinopathy.
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