Dominic Stone
Knee flexion is a critical movement in the human body, allowing for bending at the knee joint. This movement plays a significant role in various activities, such as walking, running, and sitting. The primary muscles involved in knee flexion are the hamstrings, along with several other key muscles. The hamstring group consists of three main muscles: semitendinosus, semimembranosus, and biceps femoris. These muscles span the posterior thigh and are essential for bending the knee. In addition to the hamstrings, other muscles that contribute to knee flexion include the gracilis, sartorius, popliteus, and gastrocnemius.
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What You'll Learn
The hamstrings
In summary, the hamstrings are a group of three muscles that play a crucial role in flexing the knee and enabling various leg movements. They are essential for knee flexion and stabilisation, but they are also susceptible to injury, especially in athletes. It is important to maintain strong and flexible hamstrings for overall knee health and to prevent injuries through proper stretching and warming up.
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The gracilis and sartorius
The sartorius, on the other hand, is the longest muscle in the human body. It runs diagonally across the thigh, aiding in knee flexion and hip rotation. The sartorius, along with the gracilis, provides dynamic support to the medial collateral ligament, which is crucial for maintaining overall knee stability during activities such as running and walking.
Both the gracilis and sartorius muscles work together to flex and internally rotate the knee. They also play a significant role in providing stability to the medial side of the knee. These muscles attach proximally at the hip and distally course posterior to the medial-lateral axis of rotation of the knee. Along with the semitendinosus, they form a collective insertion on the proximal-medial tibia, commonly known as the pes anserinus.
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The popliteus
The muscle fibres of the popliteus originate from the lateral condyle of the femur and the posterior horn of the lateral meniscus, via a strong tendon called the popliteus tendon.
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The biceps femoris
Injuries to the biceps femoris tendon are common in sports that require explosive bending of the knee, such as sprinting and soccer. Avulsion of the tendon, or subluxation and dislocation, can lead to a condition known as Snapping Biceps Femoris Tendon, causing pain on the lateral side of the knee and a painful snap during active and passive knee flexion.
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The semitendinosus and semimembranosus
The semimembranosus is a relatively large muscle that originates from a small facet on the rough superolateral surface of the ischial tuberosity. The semimembranosus tendon appears at the mid-thigh level and continues caudally toward the point of insertion at the medial condyle of the tibia. It is a hamstring muscle that acts on the hip and knee joints. It is located on the posteromedial side of the thigh deep to semitendinosus and medial to the biceps femoris. Its origin is the ischial tuberosity on the inferior pelvis, and it has a complex distal insertion connected to the popliteus muscle, the medial meniscus, the medial collateral ligament, and the tibia.
The semitendinosus muscle is a member of the posterior compartment of the thigh, which also includes the biceps femoris and the semimembranosus muscles. These three muscles together are collectively referred to as the hamstring muscle complex, which serves as hip extensors and knee flexors that are integral to gait and running. The semitendinosus, just medial to the biceps femoris, arises from three locations: the medial border of the ischial tuberosity, the medial border of the proximal tendon of the long head of the biceps femoris, and the proximal aponeurosis. Distal to the ischial tuberosity, the semitendinosus becomes fusiform until it reaches below the middle of the thigh.
The semimembranosus and semitendinosus muscles work together to enable movements vital for activities like running, walking, and jumping. The hamstrings initiate knee flexion when activated, pulling the heel toward the glutes. During running and walking, they help control the extension of the leg and provide stability when the knee is flexed. In addition to knee flexion, the hamstring group plays a significant role in hip extension, especially the semitendinosus and semimembranosus.
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Frequently asked questions
There are several muscles that contribute to knee flexion. The hamstrings are the primary knee flexors and consist of three muscles: semitendinosus, semimembranosus, and biceps femoris.
In addition to the hamstrings, the gracilis, sartorius, gastrocnemius, and popliteus muscles are also involved in knee flexion.
The gracilis and sartorius muscles are essential for knee flexion and internal rotation, providing dynamic support to the medial collateral ligament for medial knee stability.
The gastrocnemius contributes to knee flexion and helps prevent knee joint hyperextension, acting as a dynamic stabiliser.
The popliteus muscle is a one-joint knee flexor that rotates the knee medially, initiating the unlocking of the knee joint after full extension.
