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Knee extension is the lowering of the leg at the knee joint from a flexed knee position, straightening the knee and entire leg. The knee extensors play an essential role in human movement. The agonist muscle that extends the knee is the quadriceps femoris muscle group, which is composed of four muscles: vastus lateralis, vastus intermedius, vastus medialis, and rectus femoris. The quadriceps muscle group is the largest muscle group in the body and forms the main bulk of the thigh. The knee joint is comprised of two joints: the tibiofemoral joint between the femur and tibia, and the patellofemoral joint, which joins the patella (kneecap) with the femur. The patella acts as an anchor for the quadriceps and patellar tendons, increasing leverage and efficiency for the quadriceps femoris muscle during knee extension.
| Characteristics | Values |
|---|---|
| Knee extension | Lowering of the leg at the knee joint from a flexed knee position, straightening the knee and entire leg |
| Knee extension range of motion | 0° to 5-10° hyperextension |
| Knee extension muscles | Quadriceps femoris (rectus femoris, vastus lateralis, vastus medius/medialis, vastus intermedius), tensor fascia latae (TFL), iliotibial tract (ITB), hamstring group (biceps femoris, semitendinosus, semimembranosus) |
| Knee extension function | Stability and protection of the patella, increased leverage for quadriceps femoris muscle, improved muscle stability |
| Knee extension assistance | Suprapatellar, pretibial, and prepatellar bursae provide cushioning and assist in movement |
| Knee extension blood supply | Lateral and inferior genicular arteries, branches of the femoral artery |
| Knee extension nerve supply | Branches of tibial, femoral, obturator, and common peroneal nerves |
| Knee extension unlocking muscles | Popliteus, gastrocnemius |
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What You'll Learn
The quadriceps femoris muscle group
The rectus femoris is the most superficial muscle of the four and covers the other three vastus muscles. It originates from the pelvis and attaches to the patella, facilitating rotation at the hip. The vastus lateralis, intermedius, and medialis originate from the femur and attach to the patella. Together, these muscles converge to form the quadriceps tendon, which attaches to the superior pole of the patella.
In addition to their role in knee extension, the quadriceps femoris muscles also have other functions. For example, the rectus femoris facilitates rotation at the hip, while the vastus muscles provide depth and support to the group. The vastus intermedius, in particular, has been found to have a companion muscle, the tensor vastus intermedius, which sits between it and the vastus lateralis. This companion muscle has been observed in several individuals, adding variation to the typical quadriceps femoris structure.
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The hamstring group muscles
The biceps femoris is similar to the biceps brachii in the upper arm and is double-headed. It originates from the pelvis and femur and attaches to the fibula. The semitendinosus and semimembranosus are synergistic muscles associated with the biceps femoris. These three muscles are sometimes referred to as the hamstring group.
The hamstring muscles work in conjunction with other muscles and ligaments to ensure knee stability. For example, the tibiofemoral joint, which is relatively weak and easily damaged, relies on the surrounding muscles and ligaments for stability. The patella (kneecap) also plays a crucial role in protecting the knee joint from damage.
The hamstrings are one of the two main muscle groups in the knee, the other being the quadriceps femoris group, which extends the knee. The quadriceps group consists of four muscles: the vastus lateralis, vastus medialis, vastus intermedius, and rectus femoris. These muscles converge to form the quadriceps tendon, which attaches to the patella.
In summary, the hamstring group muscles are vital for knee flexion, hip extension, and knee stability. They work in conjunction with other muscles and anatomical structures, such as the quadriceps and the patella, to enable smooth and stable knee movement.
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The articularis genu
During knee extension, the articularis genu plays a crucial role in pulling the suprapatellar bursa of the knee upward to prevent impingement between the femur and tibia. This action helps to stabilise the knee joint and prevent painful conditions such as suprapatellar bursitis. The muscle also assists in preventing the synovial membrane from becoming caught between the patella and femur within the knee joint.
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The vastus intermedius
Knee extension is the lowering of the leg at the knee joint from a flexed knee position, thus straightening the knee and the entire leg. The range of motion for knee extension is estimated to reach 0 degrees, but hyperextension can occur, bringing the range of motion to 5-10 degrees.
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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. The popliteus tendon passes beneath the lateral collateral ligament (LCL) and the tendon of the biceps femoris. Cadaveric dissections have also shown fibres originating from the styloid portion of the fibular head, which then runs obliquely blending with the main muscular structure.
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Frequently asked questions
The muscles that extend the knee are the quadriceps femoris muscle group, which includes the rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis.
The quadriceps femoris muscle group is responsible for extending the lower leg at the knee joint, as well as stabilizing and protecting the patella.
The hamstring group muscles, including the biceps femoris, semitendinosus, and semimembranosus, play a role in knee extension by flexing the knee and extending the hip. Additionally, the popliteus muscle helps to "unlock" the knee from a straightened or extended position.
