Knee Flexors

Original Editor - Lucinda hampton

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Introduction[edit | edit source]

Knee Flexion

The knee flexors include the set of hamstrings, gracilis, sartorius, gastrocnemius, plantaris, and popliteus. Most of these knee flexors also internally or externally rotate the knee[1]. The hamstring muscles are the primary knee flexors. They play a key role in everyday movements, eg running, walking.

Bending of the knee is known as flexion. The opposite movement is extension. Flexion and extension are controlled by opposing muscle groups.

Hamstrings[edit | edit source]

Hamstrings Group

The semitendinosus, semimembranosus, and biceps femoris (long and short heads) make up the hamstring group.

Spanning the posterior thigh, the hamstring muscles are the primary knee flexors. The hamstrings also perform hip extension, excepting the short head biceps femoris.

As the three of the four hamstrings cross the hip, as well as the knee, the position of the hip (and knee) can significantly affect the functional length of these muscles. As such, both the extensibility and the maximal force generated by the hamstrings are highly dependent on the position of the hip.

Gracilis and Sartorius[edit | edit source]

10,11,12. Tendons of sartorias, gracilis, and semi-tendinosus

The gracilis and sartorius flex and internally rotate the knee and play an important role in providing stability to the medial side of the knee. They also have roles at the hip. These muscles attach proximally at the hip.

Distally these muscles course posterior to the medial-lateral axis of rotation of the knee, and with semitendinosus form a collective insertion on the proximal-medial tibia, commonly known as the pes anserinus. These three muscles all perform three common functions at the knee: Flexion of the knee; Internal rotation of the knee; Dynamic support of the medial collateral ligament, providing medial stability to the knee

Gastrocnemius and Plantaris[edit | edit source]

The gastrocnemius produces a large plantar flexion torques across the ankle, and as the muscle crosses the posterior aspect of the knee, it is also a knee flexor. The relatively small plantaris, which also crosses the posterior knee, so is a flexor of the knee.[1]

Popliteus[edit | edit source]


The popliteus helps unlock the knee when the knee is in full extension. The hamstrings don't have much leverage from that position, and the small popliteus is crucial here. The popliteus externally rotates femur on tibia, the locked ligaments loosen, and hamstrings can then flex free[2]

See Also[edit | edit source]

Goniometry: Knee Flexion

Flexion Deformity of the Knee

Manual Muscle Testing: Knee Flexion

References[edit | edit source]

  1. 1.0 1.1 Mansfield PJ, Neumann DA. Essentials of kinesiology for the physical therapist assistant e-book. Elsevier Health Sciences; 2018 Oct 23. Available: (accessed 16.2.2022)
  2. Radiopedia Knee Available: (accessed 16.2.2022)