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The muscles surrounding the knee function to both move and stabilize the joint. The two main muscle groups are the quadriceps on the anterior side of the knee and femur, and the hamstrings on the posterior side.

The four muscles of the quadriceps: vastus lateralus, vastus medialus, vastus intermedius and rectus femoris function to extend the knee. The muscles join together to form the common quadriceps tendon. Tendons are part of the muscle, and attach muscle to bone. Within the quadriceps tendon is the patella (knee cap.) The patella is a sesamoid bone, which provides increased leverage to the quadriceps muscle to improve its efficiency.

The three posterior hamstring muscles: biceps femoris, semitendinosis, and semimembranosis function to decelerate, stabilize and bend the knee joint, and attach to the posterior part of the tibia and fibula.

There are two other important muscles of the knee complex. The gastrocnemius pushes the foot down (plantar flexes) and helps bend the knee. The popliteus helps unlock the knee from a straightened or extended position. Two adductor muscles, the adductor magnus and gracilis, cross the knee joint and help rotate the leg and can be a source of inflammation.

Common conditions

  • patella tendinitis

  • ruptured quadriceps tendon

  • quadriceps strain

  • hamstring strain

  • torn hamstring

  • muscle strain

  • gastrocnemius tear

  • gastrocnemius strain

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