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Knee flexion is the bending of the lower leg and foot at the knee joint, while the thigh remains stationary. The knee is the body's largest joint and is critical to efficient bipedal movements like walking, running, and jumping. The knee flexors include the hamstrings, gracilis, sartorius, gastrocnemius, plantaris, and popliteus. The hamstrings group are the main knee flexors. The quadriceps group also plays a role in controlling knee flexion.
| Characteristics | Values |
|---|---|
| Knee Flexors | Hamstrings, gracilis, sartorius, gastrocnemius, plantaris, and popliteus |
| Hamstrings | Biceps femoris, semitendinosus, semimembranosus |
| Main Knee Flexor | Gastrocnemius |
| Knee Flexion | When the lower leg and foot bend and are raised posteriorly at the knee joint, while the thigh remains fixed in a stationary position |
| Range of Motion | 115-160° |
| Antagonists | Biceps femoris, semimembranosus, semitendinosus, gastrocnemius, popliteus, gracilis, and sartorius |
| Attachments | From the anterior surface of the shaft of the femur to attach to the suprapatellar bursa |
| Function | Retracts the suprapatellar bursa and joint capsule of the knee to protect it from entrapment during extension |
| Quadriceps Group | Controls knee flexion (lengthening contractions), straightens the leg during gait and stair climbing, and influences tracking of the patella |
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What You'll Learn
- The hamstrings, gastrocnemius, plantaris, and popliteus are the main knee flexors
- The quadriceps group acts on the knee to extend it
- The popliteus helps unlock the knee from a fully extended position
- The semitendinosus and semimembranosus muscles can rotate the lower leg medially
- The gracilis muscle, which crosses the knee, is innervated by the obturator nerve
The hamstrings, gastrocnemius, plantaris, and popliteus are the main knee flexors
The knee is the body's largest joint, and knee flexion is essential for walking. The knee flexors include the hamstrings, gastrocnemius, plantaris, and popliteus. These muscles are responsible for the knee's movements and assist the knee ligaments in preventing excessive displacement in any direction.
The hamstrings are a group of three muscles at the back of the thigh: the biceps femoris, semitendinosus, and semimembranosus. They perform three common functions at the knee: flexion of the knee, internal rotation of the knee, and dynamic support of the medial collateral ligament, providing medial stability to the knee. Shortened hamstring muscles can cause back problems and imbalance in the muscle forces across the knee, placing more stress on the quadriceps muscles.
The gastrocnemius is a powerful two-headed muscle that produces large plantar flexion torques across the ankle. As it crosses the posterior aspect of the knee, it is also a knee flexor. The plantaris is a relatively small muscle that sits between the gastrocnemius and the soleus, running the entire length of the lower leg, knee to foot. It is also a flexor of the knee.
The popliteus is a flat, triangular muscle that originates from a small groove on the lateral surface of the lateral femoral condyle. It crosses the posterior knee and is a very effective internal rotator muscle, providing the torque that unlocks the knee.
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The quadriceps group acts on the knee to extend it
The knee is the body's largest joint, allowing for leg flexion and extension. Knee flexion refers to the action of bending the lower leg and foot posteriorly at the knee joint while the thigh remains stationary.
The knee flexors include the hamstrings, gracilis, sartorius, gastrocnemius, plantaris, and popliteus. The hamstrings are also important for stabilising the knee. The gastrocnemius is a powerful two-headed muscle that produces large plantar flexion torques across the ankle and, as it crosses the posterior aspect of the knee, it is also a knee flexor. The plantaris is a small muscle that sits between the gastrocnemius and the soleus and runs the length of the lower leg. The popliteus helps to unlock the knee when the knee is in full extension.
The quadriceps muscles are one of the body's largest muscle groups and are the main drivers of the knee joint. When the quadriceps muscles are tight, they prevent the full movement of the tendons supporting the knee, applying greater pressure on the kneecap. Tightness in the quadriceps can be caused by a variety of factors, including tight core muscles, which can pull the pelvis forward or backward, altering knee position and placing added stress on the quadriceps.
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The popliteus helps unlock the knee from a fully extended position
The popliteus muscle is a small but important muscle located in the posterior aspect of the knee joint. It is often referred to as the "key to unlock the knee" or the "key of the knee". This is because the popliteus muscle plays a crucial role in initiating knee flexion from a fully extended or "locked" position.
When the foot is in contact with the ground, the popliteus muscle laterally rotates the femur on the tibia, which unlocks the knee joint. This action releases the "screw-home mechanism" that occurs during full knee extension, facilitating smoother knee flexion. For example, when transitioning from a standing position to a partial squat, the popliteus muscle slightly rotates the femur externally, unlocking the knee. This function of the popliteus muscle is essential for walking, standing up, and sitting down.
The popliteus muscle is also involved in medial rotation. When the leg is not in contact with the ground and the knee is flexed, the popliteus muscle medially rotates the tibia on the femur. This medial rotation helps to stabilise the knee joint during weight-bearing activities.
In addition to its role in unlocking the knee, the popliteus muscle contributes to the stability and functioning of the knee joint. It is the main stabiliser of the posterior knee region and helps control the internal and external rotation of the tibia during walking and running.
The popliteus muscle is unique in its inverted origin and insertion. Typically, the origin is the more stable and fixed attachment point, while the insertion is the movable attachment point that pulls on a bone to create movement. However, in the case of the popliteus muscle, this relationship is inverted, with the tendinous attachment originating from the proximal bone and the fleshy attachment representing the muscle's insertion.
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The semitendinosus and semimembranosus muscles can rotate the lower leg medially
The knee flexors include the set of hamstrings, which are made up of the semitendinosus, semimembranosus, and biceps femoris. The semitendinosus and semimembranosus muscles are inserted into the medial tibial condyle and can rotate the lower leg medially.
The semitendinosus is a superficial muscle of the posterior thigh, bordered by the long head of the biceps femoris laterally and the semimembranosus medially. The semimembranosus is also a superficial muscle of the posterior thigh, with the semitendinosus bordered anteromedially by the posteromedial intermuscular septum. The semitendinosus can be palpated by locating the space between the two large bands that comprise the hamstring tendons just superior to the posterior knee. As you move your fingers up the "valley" created by the division in the hamstrings, you can feel the musculotendinous junction invest into the long cylindrical muscle bellies. The muscle just medial to this valley is the semitendinosus.
The semitendinosus and semimembranosus muscles originate from the tuberosity of the ischium by a tendon common with the long head of the biceps femoris. Their insertion is on the proximal part of the medial surface of the tibia and deep crural fascia. The tendon of the semitendinosus contributes the distal fibres of the pes anserinus tendon, which is comprised of the distal tendons of the semitendinosus, gracilis, and sartorius. The semimembranosus tendon is separated from the medial tibial plateau, medial head of the gastrocnemius, semitendinosus, and medial cruciate ligament by a U-shaped bursa.
The semitendinosus and semimembranosus muscles play a role in knee internal rotation, along with the gracilis, popliteus, and medial gastrocnemius. They also aid in knee flexion and dynamic support of the medial collateral ligament, providing medial stability to the knee.
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The gracilis muscle, which crosses the knee, is innervated by the obturator nerve
The gracilis muscle is a long, slender muscle located in the medial (adductor) compartment of the thigh. It is the most superficial muscle on the medial side of the thigh and is thin and flattened, broad above, and narrow and tapering below. It is also the weakest member of the adductor muscle group, which includes the adductor longus, adductor brevis, adductor magnus, and pectineus muscles.
The gracilis muscle is the only hip adductor that crosses and acts on two joints: the hip and the knee. It extends from the coxal bone to the tibia, allowing for thigh adduction and flexion, as well as leg flexion and medial (internal) rotation. These actions are important for balancing the trunk during activities such as walking.
The gracilis muscle is innervated by the obturator nerve, specifically the anterior branch of the obturator nerve (L2-L4). The obturator nerve is a branch of the lumbar plexus, arising from the L2-L3 spinal nerves. The gracilis muscle receives its vascular supply primarily from the 'artery to the adductors', a branch of the deep femoral artery.
The gracilis muscle plays a crucial role in knee flexion, assisting the hamstring muscles. This function is particularly evident during the initial swing phase of walking or during activities such as boat rowing. Additionally, the gracilis tendon is commonly used in reconstructive surgery, especially for repairing torn tendons and ligaments in the knee.
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Frequently asked questions
Knee flexors are the set of hamstrings, including the semitendinosus, semimembranosus, and biceps femoris.
The gracilis, sartorius, gastrocnemius, plantaris, and popliteus muscles also contribute to knee flexion.
The gastrocnemius is a powerful two-headed muscle that produces large plantar flexion torques across the ankle. It is also a knee flexor.
