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Extensor mechanism

The extensor mechanism consists of the anatomical muscle/tendon chain of structures which work together to straighten the knee.

Chief of these is the quadriceps muscle group (made up of the vastus medialis, vastus intermedius, vastus lateralis and rectus femoris muscles). The four heads of the quadiceps join together to form a common tendon (quadriceps tendon above the patella and patellar tendon below the patella). Within this tendon is housed the patella bone (kneecap).

This chain of structures reaches from hip bone to tibia bone and contraction of the muscle/tendon unit straightens (extends) the knee.

The patella plays a key role in this process by altering the 'lever arm' and thus the mechanical forces involved.