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The knee is the largest joint in the human body and is surrounded by several muscle groups. These muscles help maintain stability and move the knee joint through motion in conjunction with ankle and hip movements. The knee joint is a hinge joint that sits between the thigh and the shin, allowing the legs to bend and straighten. The muscles that generate movement across the knee are mainly located in the thigh and can be split into anterior and posterior compartments. The knee joint is comprised of two joints: the tibiofemoral joint, which is the weight-bearing knee joint, and the patellofemoral joint, which joins the patella (kneecap) with the femur.
| Characteristics | Values |
|---|---|
| Muscles that cause movement | Sartorius, quadriceps femoris, biceps femoris, semitendinosus, semimembranosus, gracilis, popliteus |
| Joint type | Hinge joint |
| Joint composition | Bones, cartilage, muscles, ligaments, nerves |
| Joint function | Allows legs to bend and straighten |
| Joint location | Middle of the leg, where the femur meets the tibia |
| Joint stability | Provided by cruciate ligaments (anterior and posterior), medial collateral ligament, lateral collateral ligament |
| Largest joint in the body | Yes |
| Most stressed joint in the body | Arguably |
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What You'll Learn
The role of the quadriceps femoris group of muscles
The knee joint is the largest joint in the body and is surrounded by several muscle groups. The quadriceps femoris group of muscles, also known as the quadriceps, is a group of four muscles that sit on the front of the thigh. These muscles are responsible for allowing the knee to straighten, which is necessary for standing from a seated position, bringing the leg forward when walking, and kicking a ball. The two patellar tendons attach the quadriceps to the patella, which can rupture during sports.
The patella, or kneecap, acts as a fulcrum for the quadriceps femoris muscle, increasing its leverage and efficiency. This is known as the patellofemoral joint, which joins the patella with the femur. The patellofemoral joint has two key functions: increasing the leverage of the quadriceps tendon to improve muscle stability and protecting the knee joint from damage. The patella moves up and down as the knee bends and straightens, allowing for gliding motion.
The quadriceps femoris muscle also plays a role in the patellar tendon, which is one of the primary tendons within the knee. The quadriceps tendon is attached to the quadriceps muscle and helps initiate movement within the patella, the patellar tendon, and the tibia, allowing the lower leg to move forward in congruency. The contraction of the quadriceps femoris muscle is also necessary for unlocking the knee joint, allowing for flexion to occur.
Overall, the quadriceps femoris group of muscles plays a crucial role in the movement and stability of the knee joint, enabling essential functions such as walking and kicking.
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The function of the hamstrings
The hamstring muscle group is comprised of three muscles: the biceps femoris, semitendinosus, and semimembranosus. These muscles are located in the back of the thigh, starting at the pelvis and extending to the knee. The biceps femoris is the outermost muscle of the hamstring group, with the semitendinosus in the middle and the semimembranosus on the innermost side.
The hamstrings play a critical role in many human activities, from standing up from a chair to explosive actions such as sprinting and jumping. They are essential for walking, climbing stairs, and performing squats, and they are also engaged when running and doing many other leg movements. The hamstrings activate in the final 25% of the swing phase of the gait cycle, generating extension force at the hip and resisting knee extension.
The hamstrings also function as dynamic stabilizers of the knee joint, working in tandem with the anterior cruciate ligament (ACL) to resist anterior translation of the tibia during the gait cycle. This helps to protect the knee joint from damage. Additionally, the hamstrings contribute to the internal rotation of the hip and knee joints.
The hamstring muscles contain thousands of long, elastic muscle fibres that help the leg muscles contract and tighten. These fibres can be injured through muscle overload, resulting in hamstring strains that range in severity from mild overstretching to a complete tear of the muscle from a tendon or bone. To prevent hamstring injuries, it is important to exercise regularly, including stretching, strengthening, and cardio, and to always warm up before physical activity.
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The patellofemoral joint
Patellofemoral joint pain (PFP) is a common issue for athletes, with females reporting more pain. Biomechanical dysfunction is a significant contributing factor to PFP. The patellofemoral joint is stabilised by the patellar retinaculum, particularly its medial and lateral components. The medial patellofemoral ligament (MPFL) is a crucial dynamic and static stabiliser, minimising lateral translation of the patella. The lateral patellofemoral ligament (LPFL) is another important stabiliser, preventing medial subluxation or dislocation of the patella.
The quadriceps muscle is the largest and strongest extensor muscle, consisting of the rectus femoris and the vastus group. The vastus medialis obliquus and vastus lateralis keep the kneecap in the femoral groove during knee straightening. The vastus group produces 80% of the knee extension torque, while the rectus femoris produces 20%. The vastus medialis has two fibre directions: the vastus medialis longus and the vastus medialis obliquus, which attach at different angles to the quadriceps tendon.
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The knee flexors
The gracilis, sartorius, and plantaris muscles are also knee flexors. The gracilis and sartorius flex and internally rotate the knee, providing stability to the medial side of the knee. The plantaris muscle crosses the posterior knee and is a flexor of the knee. The gastrocnemius is a powerful two-headed muscle that produces large plantar flexion torques across the ankle. As the gastrocnemius crosses the posterior aspect of the knee, it is also a knee flexor.
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The role of the popliteus muscle
The popliteus muscle is a small, thin, flat, triangular-shaped muscle located in the lower leg, behind the knee joint. It is a deep muscle, forming the floor of the popliteal fossa, and is the only muscle in the posterior compartment of the lower leg that acts solely on the knee and not on the ankle.
The popliteus muscle has two main functions: it unlocks the fully extended knee joint, and it stabilises the knee during flexion. When the knee is fully extended, the femur rotates slightly on the tibia to lock the joint into place, allowing for efficient load-bearing. The popliteus muscle unlocks the knee by laterally rotating the femur on the tibia, allowing flexion to occur. This action is essential for walking, standing up, and sitting down.
During the closed-chain phase of the gait cycle, when the foot is in contact with the ground, the tibia is laterally rotated on the femur, locking the knee joint. The popliteus muscle then acts to unlock the knee, allowing for flexion. During the open-chain phase, when the foot is above the ground and the tibia is free to move, the popliteus muscle medially rotates the tibia on the femur, providing stability to the tibia during knee flexion.
The popliteus muscle is also involved in preventing injuries to the knee. It inhibits excessive tibial rotation and prevents significant anterior translation of the knee. However, activities such as sprinting or running downhill can overcome the popliteus muscle, leading to posterolateral knee pain and instability. Therefore, it is important to avoid or modify such activities to prevent injury.
The popliteus muscle receives its arterial blood supply from branches of the popliteal artery, including the inferior medial and lateral genicular arteries. It is supplied by the tibial nerve, with spinal nerve roots ranging from L4 to S1. The popliteus tendon connects to the medial aspect of the head of the fibula via the popliteofibular ligament and passes over the posterior joint capsule of the knee joint.
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Frequently asked questions
The knee is a complex joint that is surrounded by several muscle groups. The muscles that generate movement across the knee are mainly located in the thigh and can be split into anterior and posterior compartments. The four quadricep muscles attach at the base of the femur, just above the knee joint. The hamstrings attach at the base of the femur and the top of the tibia, controlling flexion or bending of the knee. The biceps femoris, semitendinosus, and semimembranosus muscles are responsible for flexing of the lower leg at the knee.
The patella is a floating bone that acts as a fulcrum for the quadriceps muscle. The patellofemoral joint allows the patella to move up and down, and the knee bends and straightens. The patella also increases the leverage of the quadriceps tendon to improve muscle stability and protect the knee joint from damage.
Extensors are muscles that let you extend your knee out. The extensors that control your knee include the biceps femoris. Flexors pull your knee in, including the articularis genus, rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis. The gracilis and sartorius flex and internally rotate the knee, providing stability to the medial side of the knee.
