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The knee is the body's largest joint, and several muscles are involved in its flexion and extension. The primary knee flexors are the hamstring muscles, which span the posterior thigh. The biceps femoris, semitendinosus, and semimembranosus muscles are responsible for flexing the lower leg at the knee. The popliteus muscle, located in the lower leg, unlocks the knee joint after extension, allowing for flexion. Other muscles involved in knee flexion include the sartorius, gracilis, gastrocnemius, plantaris, and quadriceps femoris. These muscles collectively contribute to the movement and stability of the knee joint.
| Characteristics | Values |
|---|---|
| Number of muscles that flex the knee | 7 |
| Hamstring muscles | Semitendinosus, Semimembranosus, Biceps femoris |
| Other muscles | Sartorius, Gracilis, Gastrocnemius, Popliteus |
| Muscles that unlock the knee | Popliteus |
| Muscle that rotates the lower leg | Biceps femoris |
| Muscle that extends the leg | Quadriceps femoris |
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Hamstring muscles
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. They are responsible for flexing the knee and extending the hip.
The biceps femoris muscle is on the outside of the back of the thigh. It can flex the knee, extend the thigh at the hip, and rotate the lower leg from side to side when the knee is bent. The semitendinosus muscle is located between the semimembranosus and biceps femoris, and it shares the same function as the latter. The semimembranosus muscle, on the other hand, is found on the innermost side of the back of the thigh. While it flexes the knee joint and extends the thigh at the hip, it also rotates the hip and lower leg.
The hamstring muscles are skeletal muscles, which means that they are under voluntary control. They are essential for various leg movements, such as walking, climbing stairs, running, and squatting. They also play a critical role in more explosive actions, such as sprinting and jumping.
In summary, the hamstring muscle group, consisting of the biceps femoris, semitendinosus, and semimembranosus, is responsible for flexing the knee and extending the hip. These muscles enable various leg movements and are crucial for maintaining dynamic stability in the knee joint. However, they are also prone to injuries, particularly among athletes, and require proper care to prevent and manage strains effectively.
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Gastrocnemius
The gastrocnemius is a powerful two-headed muscle located in the posterior compartment of the leg, along with the soleus muscle. Together, these muscles form the calf muscle. The gastrocnemius is considered a superficial muscle as it is located directly under the skin, and its shape is often visible through the skin.
The gastrocnemius is a complex muscle that plays a fundamental role in walking, running, jumping, and posture. It affects the entire lower limb and the movement of the hip and lumbar area. The muscle is prone to spasms, which are painful, involuntary contractions that may last several minutes. A severe ankle dorsiflexion force may result in a medial gastrocnemius strain (MGS) injury, commonly referred to as a "torn" or "strained" calf muscle.
The medial head of the gastrocnemius originates from the posterior surface of the medial condyle of the femur, while the lateral head originates from the lateral surfaces of the epicondyle. The two heads unite to form the upper part of the tendocalcaneus, which then narrows until it reaches the calcaneus, where it expands for its insertion. The gastrocnemius, along with the soleus, is the main plantar flexor of the ankle joint and also functions as a secondary knee flexor.
To stretch the gastrocnemius muscle, the knee of the back leg is kept extended, and the patient stands on an inclined board with feet pointing upward and heels downward. This stretch can be increased by the patient leaning forward. The gastrocnemius may contain up to four trigger points, with two medial trigger points found in the medial head of the muscle near the knee.
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Popliteus muscle
The popliteus muscle is a small yet significant stabilising muscle of the knee. It is a thin, flat, triangular-shaped muscle in the lower leg, forming the floor of the popliteal fossa and constituting a part of the posterolateral corner of the knee. It is the only muscle in the posterior compartment of the lower leg that acts solely on the knee joint and not on the ankle.
The popliteus muscle is a capsular structure, although extra-articular, and separates the lateral meniscus from the lateral collateral ligament. The muscle fibres originate from the lateral condyle of the femur and insert on the posterior surface of the tibia, superior to the soleal line. The popliteus tendon runs beneath the lateral collateral ligament and tendon of the biceps femoris.
The popliteus muscle is often referred to as the "key" to unlocking the knee. It is responsible for initiating knee flexion by laterally rotating the femur on the tibia. This unlocks the knee joint after full extension, allowing for flexion. In the closed chain phase of the gait cycle, when the foot is in contact with the ground, the popliteus muscle externally rotates the femur on the tibia. In the open-chain phase, or swing phase, of the gait cycle, the popliteus muscle acts to internally rotate the tibia on the femur.
The function of the popliteus muscle depends on the position of the lower extremity, i.e., weight-bearing or non-weight-bearing. In weight-bearing states, the knee 'locks' into full extension as the femur medially rotates on the tibia. To 'unlock' the knee, the popliteus muscle contracts, causing flexion and lateral rotation of the femur on the tibia. In non-weight-bearing states, the popliteus muscle acts as the primary internal rotator of the tibia.
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Sartorius
The sartorius muscle is the longest muscle in the human body. It is a thin, long, superficial muscle that runs down the length of the thigh in the anterior compartment. The sartorius muscle originates from the pelvis and attaches to the tibia. It runs obliquely across the upper and anterior part of the thigh in an inferomedial direction. It passes behind the medial condyle of the femur to end in a tendon. This tendon curves anteriorly to join the tendons of the gracilis and semitendinosus muscles in the pes anserinus, where it inserts into the superomedial surface of the tibia.
The sartorius muscle is innervated by the femoral nerve. It may originate from the outer end of the inguinal ligament, the notch of the ilium, the ilio-pectineal line, or the pubis. The muscle may be split into two parts, and one part may be inserted into the fascia lata, the femur, the ligament of the patella, or the tendon of the semitendinosus. The tendon of insertion may end in the fascia lata, the capsule of the knee joint, or the fascia of the leg. It is important to note that the sartorius muscle may be absent in some people.
At the hip, the sartorius muscle can flex, weakly abduct, and laterally rotate the femur. At the knee, it can flex the leg; when the knee is flexed, the sartorius medially rotates the leg. Sitting cross-legged demonstrates all four actions of the sartorius. The sartorius muscle plays an important role in stabilising the pelvis, especially in women. This is due to the constrictive effect that the muscles on both sides of the body have on the pubic symphysis.
The pes anserinus (Latin for "Goose's Foot") refers to the conjoined tendons of the gracilis, semitendinosus, and sartorius muscles. It lies superficially to the tibial insertion of the medial collateral ligament of the knee. One of the conditions that can disrupt the use of the sartorius is pes anserine bursitis, an inflammatory condition of the medial portion of the knee. This condition usually occurs in athletes from overuse and is characterised by pain, swelling, and tenderness.
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Biceps femoris
The biceps femoris is a muscle of the thigh located to the posterior, or back. It is the most lateral hamstring muscle and is located in the posterior thigh. The biceps femoris is a long muscle that runs from the ischial tuberosity to the proximal part of the fibula, crossing the hip and knee joints.
The biceps femoris has two heads, the long head and the short head, which have different origins but share a common insertion. The long head is considered part of the hamstring muscle group, while the short head is sometimes excluded from this characterisation as it only causes knee flexion and is activated by a separate nerve (the peroneal, as opposed to the tibial branch of the sciatic nerve). The long head arises from the lower and inner impression on the posterior part of the tuberosity of the ischium, which is a common tendon origin with the semitendinosus muscle and the sacrotuberous ligament. The short head arises from the lateral lip of the linea aspera, between the adductor magnus and vastus lateralis muscles.
Both heads of the biceps femoris perform knee flexion. The long head also extends the hip, while the short head does not. When the knee is slightly flexed, the biceps femoris rotates the lower leg laterally. The biceps femoris also provides rotary stability to the knee by preventing forward dislocation of the tibia on the femur during flexion.
Avulsion of the biceps femoris tendon is common in sports that require explosive bending of the knee, such as sprinting. Snapping biceps femoris tendon is an unusual condition that can cause pain on the lateral side of the knee and a painful snap during active and passive knee flexion. This condition may be due to overuse, prolonged sporting activity, or congenital factors.
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Frequently asked questions
There are seven muscles that flex the knee: semitendinosus, semimembranosus, biceps femoris, sartorius, gracilis, popliteus, and gastrocnemius.
The semitendinosus and semimembranosus are synergistic muscles that originate from the pelvis and attach to the tibia. They are responsible for flexing the lower leg at the knee joint.
The biceps femoris is a double-headed muscle that originates from the pelvis and attaches to the fibula. It can rotate the lower leg so that the foot points laterally.
The gracilis is a thigh adductor that crosses the knee and flexes and medially rotates the leg.
The popliteus is a flat, triangular muscle that unlocks and weakly flexes the knee. It is a medial rotator of the tibia and the femur.
