Logan Steele
The knee is the body's largest joint, connecting the thigh to the lower leg and allowing for a wide range of movements. The knee joint is formed by the femur (thighbone), tibia (shinbone), and patella (kneecap). Several muscles, tendons, and ligaments work together to enable knee movement, with the hamstrings being the primary muscle group responsible for knee flexion or bending. The hamstrings attach at the base of the femur and the top of the tibia, controlling knee bending and providing stability during extension. Other muscles, such as the biceps femoris, semitendinosus, and semimembranosus, also contribute to knee flexion and extension, while the quadriceps femoris group of muscles are key extensors, stabilizing the patella. The knee's complex anatomy and range of motion make it susceptible to injuries, highlighting the importance of understanding the muscles involved in knee bending and their functions.
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What You'll Learn
Hamstrings
The hamstrings are skeletal muscles located at the back of the thigh. They are made up of three muscles: the biceps femoris, semitendinosus, and semimembranosus. These muscles are responsible for flexing or bending the knee and are involved in many daily activities such as walking, running, jumping, and climbing stairs.
The hamstrings attach at the base of the femur (thighbone) and the top of the tibia (shinbone). They cross and act upon two joints – the hip and the knee – and are therefore termed biarticular muscles. When the knee is bent, the hamstrings contract, and when the knee is extended, they lengthen.
The biceps femoris is the most commonly injured hamstring muscle, followed by the semitendinosus. A "pulled hamstring" or strain is a common hamstring injury, which can range from mild to severe. Mild hamstring strains may cause muscle pain or swelling, but the leg is usually still usable. However, a complete tear can result in severe pain and swelling, and the individual may be unable to use their leg.
Hamstring injuries are very common, especially in athletes who run and sprint. To avoid hamstring injuries, it is important to stretch, warm up, and not push through pain in the hip, knee, and leg.
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Quadriceps femoris
The quadriceps femoris muscle, commonly known as the quad muscle, is the strongest muscle in the human body. It is located in the anterior compartment of the thigh, along with the sartorius muscle. The quadriceps femoris is composed of four muscles that make up the front of the thigh: three deep-lying vastus muscles (lateralis, intermedius, and medialis) and the rectus femoris, which covers them. The rectus femoris can be further broken down into two heads that originate from two sites on the ilium: the anterior inferior iliac spine (straight head) and the supraacetabular groove (reflected head).
The quadriceps femoris is the most voluminous muscle in the human body and is essential for daily activities such as climbing stairs or getting up from a chair. It is also crucial for walking, running, jumping, and squatting. The muscle allows for the extension of the lower leg at the knee joint and flexion of the thigh at the hip joint. The rectus femoris can flex the hip, while its synergistic action with the vastus lateralis, vastus medialis, and vastus intermedius extends the knee. The vastus medialis plays a crucial role in stabilising the patella and the knee joint during gait.
The quadriceps femoris muscles are a common target for manual therapy and physical therapy for repetitive strain injuries, such as those sustained from skiing. They are also the most common site of myositis ossificans. In strength training, the quadriceps are trained through various leg exercises, including squats, leg presses, and leg extensions.
The patella (kneecap) is the attachment point for the quadriceps femoris muscle and is connected to the tibia by a ligament. The four heads of the quadriceps femoris are directed downward to form a single tendon, the quadriceps muscle tendon, which fits onto the patella. Some connective fibres from the vastus lateralis and vastus medialis attach themselves to the margins of the patella, while the connective fibres of the vastus intermedius insert at the base of the patella.
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Popliteus
The popliteus muscle is located behind the knee joint and is responsible for "unlocking" the knee joint after extension. It does so by rotating the femur on the tibia, allowing for the lower leg to be flexed. The popliteus muscle is also involved in both the closed-chain phase and open-chain phase of the gait cycle. During the closed-chain phase, when the foot is in contact with the ground, the muscle externally rotates the femur on the tibia. In the open-chain phase, or swing phase, when the limb is not in contact with the ground, the popliteus acts to internally rotate the tibia on the femur.
The popliteus muscle is a small, thin, flat, triangular-shaped muscle in the lower leg. It forms the floor of the popliteal fossa and is located in the deep posterior compartment of the leg. It is considered a major stabilising muscle of the knee and 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 is also connected 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 muscle fibres of the popliteus originate from the lateral condyle of the femur and insert on the posterior surface of the tibia, above the soleal line. The popliteus tendon runs beneath the lateral collateral ligament and the tendon of the biceps femoris. The popliteus is supplied by the tibial nerve, with nerve roots from L4, L5, and S1. There are approximately 2 to 3 parallel tibial nerve branches.
Injury to the popliteus muscle can cause posterolateral rotatory instability of the knee. This condition can be treated with arthroscopic popliteus sling reconstruction using the popliteus portal.
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Biceps femoris
The biceps femoris is a muscle of the thigh located to the posterior, or back. It is a long muscle that is part of the hamstrings muscle group. The hamstrings attach at the base of the femur and the top of the tibia. 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 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. The short head arises from the lateral lip of the linea aspera, between the adductor magnus and vastus lateralis, extending up almost as high as the insertion of the gluteus maximus.
The biceps femoris muscle runs from the ischial tuberosity to the proximal part of the fibula, crossing both the hip and knee joints. It is similar to the biceps brachii in the upper arm and is double-headed. The muscle's main action is the flexing of the lower leg at the knee. The long head flexes the knee, extends the hip, and laterally rotates the lower leg when the knee is slightly flexed. The short head flexes the knee and laterally rotates the lower leg when the knee is slightly flexed. Both heads provide rotary stability by preventing forward dislocation of the tibia on the femur during flexion.
The biceps femoris is supplied by the terminal branches of the sciatic nerve. The long head receives innervation from its tibial division, while the short head is innervated by the common fibular division. The majority of the blood supply for the biceps femoris comes from branches of the deep femoral artery, such as the perforating arteries and medial circumflex femoral artery.
Avulsion of the biceps femoris tendon is common in sports that require explosive bending of the knee, such as sprinting. This can lead to a rare condition called Snapping Biceps Femoris Tendon, which causes pain on the lateral side of the knee and a painful snap during active and passive knee flexion.
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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 passes behind the medial condyle of the femur and ends in a tendon, which joins the tendons of the gracilis and semitendinosus muscles in the pes anserinus. The pes anserinus is a layered supportive structure on the medial side of the knee, where the tendons attach onto the anteromedial proximal tibia.
The sartorius muscle can move both the hip and knee joints, but its actions are weak, making it a synergist muscle. 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 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. Antique sewing machines required continuous cross-body pedalling, which would have given tailors particularly developed sartorius muscles due to the combination of lateral rotation and flexion of the hip and flexion of the knee.
The sartorius muscle may be absent in some people. It receives 5 to 11 vessels originating from the superficial circumflex iliac, lateral femoral, deep femoral, descending geniculate, and femoral arteries. The muscle may be innervated by the femoral nerve and may originate from the outer end of the inguinal ligament, the notch of the ilium, the ilio-pectineal line, or the pubis.
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Frequently asked questions
The hamstrings are the muscles that control flexion or bending of the knee. The biceps femoris, semitendinosus, and semimembranosus muscles are also responsible for flexing the lower leg at the knee.
The knee joint is formed by the femur (thighbone), the tibia (shinbone), and the patella (kneecap).
The knee helps support your weight and lets your legs bend and move. Almost any movement that uses your legs relies on your knees, such as walking, running, and jumping.
The muscles that extend the knee are called extensors. The quadriceps femoris is a group of four muscles that extend the lower leg at the knee joint.
