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The knee joint is the largest joint in the body and is comprised of two joints: the tibiofemoral joint and the patellofemoral joint. The knee joint contains bones, cartilage, muscles, ligaments, and nerves. The muscles that cross the knee joint are mainly located in the thigh and can be split into anterior and posterior compartments. The quadriceps femoris muscle group (rectus femoris, vastus lateralis, vastus medius, and vastus intermedius) crosses the knee via the patella and acts to extend the leg. The hamstring group muscles (semitendinosus, semimembranosus, and biceps femoris) flex the knee and extend the hip. The patella is a floating bone that works as a fulcrum for the quadriceps muscle to function properly.
| Characteristics | Values |
|---|---|
| Muscles crossing the knee joint | Quadriceps femoris, Sartorius, Hamstrings (including Biceps femoris), Popliteus, Gastrocnemius, Plantaris, Articulus genu, Semitendinosus, Semimembranosus |
| Knee joint composition | Bones, cartilage, muscles, ligaments, nerves, tendons, meniscus, synovial fluid |
| Knee joint type | Hinge joint, synovial joint |
| Knee joint bones | Femur (thigh bone), Tibia (shin bone), Patella (kneecap) |
| Knee joint ligaments | Medial collateral ligament (MCL), Lateral collateral ligament (LCL), Anterior cruciate ligament (ACL), Posterior cruciate ligament (PCL) |
| Knee joint nerves | Femoral nerve, Sciatic nerve, Tibial nerve, Peroneal nerve |
| Knee joint tendons | Patellar tendon, Quadriceps tendon |
| Knee joint cartilage | Articular cartilage, Meniscus |
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What You'll Learn
Quadriceps femoris
Rectus femoris is a fusiform muscle that consists of two heads. It originates from the ilium, with one head attaching to the anterior inferior iliac spine (straight head) and the other to the supraacetabular groove (reflected head). The two heads unite to form a common muscle belly that runs down the thigh, covering the anterior aspect of this region. Rectus femoris is unique among the quadriceps femoris muscles in that it crosses both the hip and knee joints, allowing it to flex the hip and extend the knee.
Vastus lateralis, vastus medialis, and vastus intermedius, on the other hand, only cross the knee joint. They originate from the femur and attach to the patella, with their connective fibers forming the lateral and medial retinacula of the patella. These three muscles work synergistically with rectus femoris to extend the knee and stabilize the patella, protecting the knee joint from damage.
The quadriceps femoris muscle is innervated by the femoral nerve, which originates from L2, L3, and L4. It receives its blood supply primarily from the artery of the quadriceps, with contributions from the lateral circumflex femoral and superficial circumflex iliac arteries as well. The quadriceps femoris is an extraordinarily important muscle for sports and daily activities, but its crucial role in lower body movement also makes it susceptible to trauma and repetitive strain injuries.
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Hamstring group
The hamstring muscle group is a group of three muscles found in the posterior region of the thigh: semitendinosus, semimembranosus, and biceps femoris. The hamstring muscles are the primary knee flexors and are responsible for flexing the lower leg at the knee joint and extending the hip. They are also involved in explosive actions such as sprinting and jumping.
The hamstring muscles originate from the ischial tuberosity of the pelvis and run posteriorly along the length of the femur, crossing both the femoroacetabular and tibiofemoral joints. The short head of the biceps femoris is an exception as it originates from the lateral lip of the femoral linea aspera, distal to the femoroacetabular joint. This has led to some debate over whether it is a true hamstring muscle.
The semitendinosus is a largely tendinous muscle found on the medial aspect of the posterior thigh. It originates from the ischial tuberosity of the pelvis and attaches to the medial surface of the tibia. In addition to its role in knee flexion, the semitendinosus also helps to medially rotate the thigh at the hip and leg at the knee joint.
The semimembranosus muscle is flat and broad and is located deep to the semitendinosus on the medial aspect of the posterior thigh. It originates from the ischial tuberosity, attaching to the medial tibial condyle. Like the semitendinosus, the semimembranosus also contributes to medial rotation of the thigh and leg at the hip and knee joints.
The biceps femoris is the most lateral of the muscles in the posterior thigh and has two heads: a long head and a short head. It is the most frequently injured of the hamstrings, followed by the semimembranosus and semitendinosus.
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Popliteus
The popliteus muscle is a small yet significant stabilising muscle of the knee. It is located in the lower leg, behind the knee, and is the only muscle in the posterior (back) compartment of the lower leg that acts solely on the knee and not on the ankle. The popliteus muscle is responsible for "unlocking" the knee joint after extension, allowing for flexion.
The muscle fibres of the popliteus 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 the tendon of the biceps femoris. The popliteus is also attached to the lateral meniscus in the knee and draws it posteriorly during knee flexion to prevent the meniscus from being crushed between the tibia and femur as the knee flexes.
The popliteus assists in knee flexion, and its function is dependent on the position of the lower extremity, i.e. weight-bearing or non-weight-bearing. In a non-weight-bearing state, the popliteus is considered the primary internal rotator of the tibia. During the closed-chain phase of the gait cycle, when the foot is in contact with the ground, the popliteus externally rotates the femur on the tibia, "unlocking" the knee. In the open-chain phase or swing phase of the limb, the popliteus acts to internally rotate the tibia on the femur.
The popliteus muscle is supplied by the tibial nerve, from spinal nerve roots L4, L5, or S1, with approximately 2 to 3 parallel tibial nerve branches. The entry point of the nerve is the lateral distal margin of the muscle, found inferior to the fibular head, and it then splits into anterior, medial, and lateral distributions throughout the muscle.
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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. Its tendon joins with the tendons of the gracilis and semitendinosus muscles in the pes anserinus, where it inserts into the superomedial surface of the tibia. The pes anserinus is a layered supportive structure on the medial side of the knee.
The sartorius muscle can move the hip joint and the knee joint, although its actions are weak. At the hip, it can flex, weakly abduct, and laterally rotate the femur. At the knee, it can flex the leg and, when the knee is flexed, it medially rotates the leg. Sitting cross-legged demonstrates all four actions of the sartorius. The gracilis and sartorius flex and internally rotate the knee and play an important role in providing stability to the medial side of the knee.
The name "sartorius" comes from the Latin word "sartor," meaning "tailor," and it is sometimes called the tailor's muscle. This name likely refers to the cross-legged position in which tailors once sat. Similarly, in French, an older name for this muscle is "couturier" (seamstress or dressmaker), with a similar reference to "sitting as a tailor." Other hypotheses for the origin of the name include the idea that it refers to the location of the inferior portion of the muscle, the "inseam" or area of the inner thigh that tailors commonly measure when fitting trousers. Another hypothesis is that the muscle closely resembles a tailor's ribbon. Antique sewing machines required continuous crossbody pedaling, which would have resulted in tailors having particularly developed sartorius muscles.
The sartorius muscle receives 5 to 11 vessels originating from the superficial circumflex iliac, lateral femoral, deep femoral, descending geniculate, and femoral arteries. There are slight adaptive ethnic differences in the width and range of the muscle belly and tendon of the sartorius muscle. The muscle may be absent in some people.
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Gastrocnemius
The gastrocnemius is a complex muscle located in the posterior compartment of the leg, along with the soleus muscle. Together, they form half of the calf muscle. The gastrocnemius is considered a superficial muscle as it is located directly under the skin, and its shape can often be visualized through the skin. The muscle is fundamental to walking, running, jumping, and posture.
The gastrocnemius is made up of two heads: the medial head and the lateral head. The medial head 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 medial head is generally thicker and wider than the lateral head. The two heads unite to form the upper part of the tendocalcaneus, a broad tendon that inserts into the posterior surface of the calcaneus (heel bone).
The gastrocnemius plays a crucial role in plantar flexion of the foot and flexion of the leg at the knee joint. It is the main plantar flexor of the ankle joint and a secondary knee flexor. The muscle is prone to spasms, which are painful, involuntary contractions that may last several minutes. Overuse of the gastrocnemius can also lead to inflammation, and it is susceptible to injuries such as Medial Gastrocnemius Strain (MGS), commonly known as a "torn" or "strained" calf muscle.
To stretch the gastrocnemius muscle, the knee of the back leg is kept extended. The patient stands on an inclined board with feet pointing upward and heels downward, leaning forward to increase the stretch. This stretch should be approached with caution, especially when using weight-bearing exercises, as excessive stress on the arches of the feet can occur.
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Frequently asked questions
The muscles that cross the knee joint include the quadriceps femoris, the hamstrings, the sartorius, the gracilis, the plantaris, the gastrocnemius, the popliteus, the biceps femoris, the semitendinosus, and the semimembranosus.
The quadriceps femoris is a group of four muscles that extend the lower leg at the knee joint.
The hamstrings are the primary flexors of the knee and also perform hip extension.
