Maximus Gage
Knee flexion is the bending of the lower leg and foot at the knee joint, while the thigh remains stationary. The knee joint is composed of the distal femur, proximal tibia, and patella. The patella is the attachment point for the quadriceps femoris muscle, which extends the leg. The hamstring group of muscles, including the semitendinosus, semimembranosus, and biceps femoris, flex the knee and extend the hip. The popliteus muscle, located behind the knee, is responsible for unlocking the knee joint after extension, allowing flexion to occur. Other muscles that contribute to knee flexion include the sartorius, gracilis, gastrocnemius, plantaris, and rectus femoris.
| Characteristics | Values |
|---|---|
| Knee flexion range of motion | 115-160° |
| Knee flexion muscles | Sartorius, gracilis, biceps femoris (long, short), semitendinosus, semimembranosus, gastrocnemius, plantaris, popliteus |
| Knee extension muscles | Quadriceps, tensor fascia latae (TFL), iliotibial tract (ITB) |
| Knee flexor | Popliteus |
| Knee extensor | Rectus femoris |
| Knee joint | Distal femur, proximal tibia, patella |
| Knee flexor muscle group | Hamstrings (semitendinosus, semimembranosus, biceps femoris) |
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What You'll Learn
The role of the rectus femoris muscle
The rectus femoris is one of the four muscles that make up the quadriceps femoris group, the others being the vastus medialis, vastus intermedius, and vastus lateralis. It is the most anterior of the quadriceps muscles and is situated in the middle of the front of the thigh. The rectus femoris is a fusiform-shaped muscle with superficial bipenniform fibres and deep, vertically oriented fibres. It is the only muscle in the quadriceps group that crosses the hip.
The rectus femoris is a two-way acting muscle as it crosses over the hip and knee joints. It functions to extend the knee and assists the iliopsoas in flexing the hip. It is also a hip flexor and is referred to as the "kicking muscle" due to its involvement in activities involving forceful knee extension. The muscle is highly contracted in a state of active insufficiency, which occurs when full extension of the knee limits its ability to flex the hip and vice versa.
The rectus femoris is innervated by the femoral nerve, which originates from lumbar nerve roots 2, 3, and 4. Blood is supplied to the muscle via the descending branch of the lateral circumflex femoral (LCF) artery. The muscle ends in a broad and thick aponeurosis that occupies the lower two-thirds of its posterior surface, gradually narrowing into a flattened tendon that is inserted into the base of the patella.
The rectus femoris is involved in gait, particularly during the toe-off phase, and it helps to stabilize the knee in extension. The most common trigger point in the rectus femoris lies near the pelvic attachment and can refer pain in and around the patella and deep into the knee joint. This can be a source of knee pain that is easily overlooked as the target zone is located a significant distance from the trigger point.
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The quadriceps femoris muscle group
The quadriceps femoris is essential for daily activities such as climbing stairs, getting up from a chair, and walking, especially downhill. It also plays a significant role in sports like basketball, cycling, running, and any activity that involves jumping or kicking. Knee extension strength is a major factor in performance on static and dynamic balance tests.
The rectus femoris muscle is important in regulating knee flexion. Its overactivity can cause reduced knee flexion during the swing phase of gait, known as stiff-knee gait. Removal of the rectus femoris actuator from a model resulted in hyperflexion of the knee, while increased excitation input to the actuator reduced knee flexion.
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The hamstring group muscles
Knee flexion is when the lower leg and foot are bent and raised posteriorly at the knee joint, while the thigh remains stationary. The sartorius, gracilis, biceps femoris, semitendinosus, semimembranosus, gastrocnemius, plantaris, and popliteus are the most significant contributors to knee flexion.
The semitendinosus and semimembranosus muscles are synergistic muscles associated with the biceps femoris. They are inserted into the medial tibial condyle and can rotate the lower leg medially. Shortened hamstring muscles at the back of the thigh can result in a stationary hip, causing back problems in athletes and other patients.
The popliteus muscle, located in the lower leg, is responsible for unlocking the knee joint after extension, allowing for flexion to occur. It is a flat, triangular muscle that originates from a small groove on the lateral surface of the lateral femoral condyle. As it travels distally, its flat body obliquely crosses the posterior knee to the medial side, where it inserts into the posterior surface of the proximal tibial shaft. The popliteus muscle is a one-joint knee flexor and also serves as a medial rotator of the tibia and femur.
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The popliteus muscle
During the open-chain (swing) phase of the gait cycle (when the foot is above the ground and the tibia is free to move), the popliteus muscle medially rotates the tibia on the femur. This action provides stability to the tibia during knee flexion. The popliteus muscle is also the main stabiliser of the posterior knee region, connecting to the posterior joint capsule and popliteofibular ligament.
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Knee flexion range of motion
Knee flexion is when the lower leg and foot are bent and raised posteriorly at the knee joint, while the thigh remains stationary. The sartorius, gracilis, biceps femoris, semitendinosus, semimembranosus, gastrocnemius, plantaris, and popliteus are the most significant contributors to knee flexion. The knee joint's range of motion (ROM) is an important clinical parameter used in knee assessment. The range of motion is determined by the amount of fluid in the knee, which can cause pain and limit mobility. This is especially important following surgery, as fluid buildup can cause scar tissue to form thick fibres of collagen, limiting the range of motion for life.
The popliteus muscle, located in the lower leg, is responsible for "unlocking" the knee joint after extension, allowing for flexion. The biceps femoris, semitendinosus, and semimembranosus muscles are also involved in flexion, while the quadriceps femoris group of muscles is responsible for extension. The patella is the attachment point for the quadriceps femoris muscle, providing increased leverage and protecting the knee joint.
The hamstring group of muscles, including the semitendinosus, semimembranosus, and biceps femoris, flex the knee and extend the hip. These muscles originate from the pelvis and femur and attach to the fibula. The gastrocnemius is a powerful two-headed muscle that produces large plantar flexion torques across the ankle and also acts as a knee flexor. The plantaris is a small muscle that sits between the gastrocnemius and soleus and is susceptible to ACL injuries.
The knee's range of motion can be measured with a goniometer, which assesses joint angles. However, it has limitations in analysing the ROM during dynamic activities such as walking or squatting. For such analyses, motion capture devices are used to evaluate a patient's gait and joint conditions. Gaining a good range of motion early on, particularly within the first few weeks after surgery, is crucial to rapid recovery and improving mobility.
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Frequently asked questions
The sartorius, gracilis, biceps femoris, semitendinosus, semimembranosus, gastrocnemius, plantaris and popliteus are the most significant contributors to knee flexion.
The popliteus muscle is a one-joint knee flexor. It is a flat, triangular muscle that originates from a small groove on the lateral surface of the lateral femoral condyle. It flexes the knee and rotates it medially, serving as a medial rotator of the tibia and the femur.
The gastrocnemius is a powerful two-headed muscle known for its ability to produce large plantar flexion torques across the ankle. It is also a knee flexor.
