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The knee is a modified hinge joint, and the act of bending the knee is known as knee flexion. Knee flexion is essential for walking and involves the lower leg and foot bending and being raised posteriorly at the knee joint while the thigh remains stationary. The sartorius, gracilis, biceps femoris, semitendinosus, semimembranosus, gastrocnemius, plantaris, and popliteus muscles are the most significant contributors to knee flexion.
| Characteristics | Values |
|---|---|
| Number of muscles that flex the knee | 7 |
| Names of muscles that flex the knee | Semitendinosus, Semimembranosus, Biceps femoris, Sartorius, Gracilis, Popliteus, Gastrocnemius |
| Muscles that medially (internally) rotate the tibia | Popliteus, Gracilis, Semimembranosus, Semitendinosus |
| Muscles that laterally rotate the tibia | Biceps femoris |
| Muscles that flex and internally rotate the knee | Sartorius, Gracilis |
| Muscles that provide dynamic support to the medial collateral ligament | Sartorius, Gracilis, Semitendinosus |
| Muscles that cross both the ankle and knee joint | Gastrocnemius, Plantaris |
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What You'll Learn
The role of the gracilis and sartorius muscles
The gracilis and sartorius muscles are essential knee flexors that play a significant role in knee stabilization and function. The gracilis muscle, located on the inner side of the thigh, is a thin, strap-like muscle that helps flex the knee and contributes to internal rotation. It also serves to adduct the thigh, providing tibial medial rotation.
The sartorius muscle, on the other hand, is the longest muscle in the human body. It runs diagonally across the thigh, aiding in knee flexion and hip flexion, external rotation, and abduction. The contraction of the sartorius muscle allows for inward or medial rotation of the tibia against the femur, enabling individuals to sit in a cross-legged position. Additionally, the sartorius muscle is believed to be important for climbing.
Both the gracilis and sartorius muscles work together to flex and internally rotate the knee while providing dynamic support to the medial collateral ligament, which is crucial for maintaining the stability of the inner knee. This support is especially important during activities such as running and walking. While these muscles are considered minor contributors to knee flexion, they facilitate coordinated actions of the knee and hip, enhancing overall stability.
The understanding of the roles of the gracilis and sartorius muscles is particularly relevant in rehabilitation settings. Their functions become vital when recovering from injuries, as they assist in regaining the range of motion and strength without overloading the primary flexors, specifically after a hamstring injury. This knowledge is essential for athletes aiming to optimize performance and individuals seeking to maintain joint health.
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Semitendinosus, semimembranosus and biceps femoris
The knee joint is a very complicated hinge joint formed by the distal femur, proximal tibia, and patella. The muscles that bend the knee are called knee flexors. The knee flexors include the gracilis, sartorius, and the hamstring muscle group.
The hamstring muscle group consists of three muscles: the semitendinosus, semimembranosus, and biceps femoris. These muscles are also called true hamstrings as they cross both the hip and knee joints and are, therefore, involved in knee flexion and hip extension. They contract when the knee is bent and lengthen when the knee is extended.
The semitendinosus and semimembranosus muscles are inserted into the medial tibial condyle and can rotate the lower leg medially. The distance between the origin and insertion of these hip extensors and knee flexors varies depending on the angle of the hip and knee joint. Shortened hamstring muscles at the back of the thigh can result in a stationary hip and lead to back problems in athletes and other patients.
The biceps femoris is the most frequently injured of the hamstrings, followed by the semitendinosus. The short head of the biceps femoris is an exception to the rule that hamstring muscles originate from the pelvis and run along the femur as it originates from the lateral lip of the femoral linea aspera. For this reason, some argue that the short head of the biceps femoris is not a true hamstring muscle.
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The popliteus muscle
The muscle fibres originate from the lateral condyle of the femur and the posterior horn of the lateral meniscus, via a strong tendon called the popliteus tendon. It then runs diagonally across the posterior upper tibia and a portion of the joint capsule, to lie as the deepest muscle of the posterior knee region. The popliteus tendon passes beneath the lateral collateral ligament (LCL) and the tendon of the biceps femoris.
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The gastrocnemius muscle
The gastrocnemius and the soleus come together above the heel and attach to the Achilles tendon. The two muscles form the triceps surae, or the "three-headed muscle of the calf". The plantaris muscle and a portion of its tendon run between the gastrocnemius and soleus. The gastrocnemius is the larger of the two muscles and forms the bulk of the calf muscle. It has two heads that start on the inside and outside of the thighbone (femur) and go down the back of the leg.
To stretch the gastrocnemius muscle, the knee of the back leg is kept extended, and the patient must be standing on an inclined board with feet pointing upward and heels downward. The stretch is greater if the patient leans forward. The gastrocnemius may contain up to four trigger points, with two medial trigger points found in the medial head of the muscle.
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Knee flexors and gait modifications
The knee flexors are able to overpower the quadriceps, which may lead to gait modifications such as anterior trunk bending to compensate for weak quadriceps. Gait modifications are a nonsurgical approach to reducing the external knee adduction torque in patients with knee osteoarthritis. Gait retraining has been shown to reduce the adduction torque in patients with knee osteoarthritis.
The knee flexors include the gracilis and sartorius muscles, which flex and internally rotate the knee, and the semitendinosus, which also has a role in rotating the lower leg medially. The distance between the origin and insertion of these hip extensors and knee flexors varies, depending on the angle of the hip and knee joint. Shortened hamstring muscles can cause back problems as they result in a stationary hip, which can be compensated for by bending forward at the lumbar spine.
Gait modification is a useful approach to treating knee osteoarthritis, with the ultimate goal of reducing the peak knee adduction moment to reduce peak medial contact force during stance. Variable stiffness shoes, lateral wedge insoles, and valgus knee braces have produced the largest reductions in peak medial contact force. However, valgus knee braces have caused significant discomfort for patients.
There are challenges with gait modifications, such as the difficulty of getting patients to achieve and retain the new gait pattern. Verbal instructions alone are often insufficient, and the decrease in knee pain may not outweigh the discomfort caused by the assistive device. Furthermore, a decrease in peak knee adduction moment does not guarantee a decrease in peak medial contact force.
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Frequently asked questions
There are seven muscles that flex the knee: semitendinosus, semimembranosus, biceps femoris, sartorius, gracilis, popliteus, and gastrocnemius.
The semitendinosus and semimembranosus muscles are inserted into the medial tibial condyle and can rotate the lower leg medially. They are also known as the hamstrings.
The biceps femoris is a lateral rotator of the tibia. It can rotate the lower leg so that the foot points laterally.
The gracilis, sartorius, and semitendinosus muscles form a collective insertion on the proximal-medial tibia, commonly known as the pes anserinus. 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.
