Dominic Stone
The knee joint is a highly functional system that allows for movements like flexion and extension, as well as medial and lateral rotation. The knee is the largest joint in the body and is arguably the most stressed, translating the actions of the flexor and extensor muscles. The knee joint is formed by articulations between the patella, femur, and tibia. The patella acts like a pulley for the quadriceps femoris, increasing its mechanical advantage and providing greater angular force. The knee can be locked and unlocked through the screw-home mechanism, which involves the rotation of the tibia and femur. The popliteus muscle is a crucial knee flexor and plays a role in unlocking the knee by rotating the femur on the tibia. Other muscles that contribute to knee rotation include the gracilis, sartorius, biceps femoris, and gastrocnemius.
| Characteristics | Values |
|---|---|
| Muscles that rotate the knee | gracilis, sartorius, biceps femoris, semitendinosus, semimembranosus, gastrocnemius, popliteus |
| Main contributors to medial rotation | gracilis, sartorius, popliteus |
| Main contributors to lateral rotation | biceps femoris |
| Muscles that flex the knee | gracilis, sartorius, semitendinosus, semimembranosus, biceps femoris, popliteus, gastrocnemius |
| Muscles that extend the knee | quadriceps femoris |
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What You'll Learn
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 inferiorly and mediolaterally towards the tibia, passing beneath the lateral collateral ligament and the tendon of the biceps femoris. The popliteus tendon pierces the joint capsule but does not enter the synovium.
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The gracilis and sartorius muscles
The sartorius muscle is the longest muscle in the human body. It is a long, thin, superficial muscle that runs down the length of the thigh in the anterior compartment. It can flex, weakly abduct, and laterally rotate the femur at the hip. At the knee, it can flex the leg, and when the knee is flexed, it medially rotates the leg. The sartorius muscle is also sometimes called the tailor's muscle, likely because tailors would sit in a cross-legged position, demonstrating all four actions of the muscle.
The popliteus muscle is another important muscle for knee rotation. It helps unlock the knee when it is in full extension and externally rotates the femur on the tibia. This movement loosens the locked ligaments and allows the hamstrings to flex freely.
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The hamstrings
The biceps femoris is located on the outside of the back of the thigh. Its functions include flexing the knee, extending the thigh at the hip, and rotating the lower leg from side to side when the knee is bent. The semitendinosus is the longest muscle in the hamstring group, and it works together with the long head of the biceps femoris to resist knee extension. The semimembranosus is located on the innermost side of the back of the thigh, and it shares the same functions as the semitendinosus.
The hamstring muscles are susceptible to injuries, especially in athletes who engage in running and sprinting. A "pulled hamstring" is a common injury that occurs when the muscle fibres are overstretched but not torn. More severe hamstring strains include partial tears and complete tears, where the muscle tissue detaches from the tendon or bone. To prevent hamstring injuries, it is important to stretch, warm up, and avoid pushing through pain in the hip, knee, and leg.
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The biceps femoris
The two heads of the biceps femoris perform knee flexion. The long head is a weaker knee flexor when the hip is extended (because of active insufficiency). For the same reason, the long head is a weaker hip extender when the knee is flexed. When the knee is semi-flexed, the biceps femoris, in consequence of its oblique direction, rotates the leg slightly outward. The long head also gives posterior stability to the pelvis.
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The semimembranosus and semitendinosus muscles
To strengthen the semimembranosus and semitendinosus muscles, individuals can perform exercises such as walking, climbing stairs, and doing squats. It is important to properly warm up and stretch before engaging in physical activities to reduce the risk of injury. If pain persists in the back of the thigh, it is recommended to seek medical attention.
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Frequently asked questions
The knee joint is a complex system that allows for flexion and extension, as well as medial and lateral rotation. The popliteus muscle is a key player in rotating the knee, unlocking it by causing lateral rotation of the femur on the tibia.
The gracilis, sartorius, and popliteus muscles are the main contributors to medial rotation.
The biceps femoris is responsible for lateral rotation.
Yes, the hamstrings, including the semitendinosus and semimembranosus, can also rotate the lower leg medially.
Understanding the mechanics of the knee joint is crucial for diagnosing and treating injuries. For example, assessing medial and lateral rotation can help determine damage to the respective medial and lateral ligaments.
