Human leg

The human leg is the lower limb, encompassing the thigh, knee, lower leg, ankle, and foot, and sometimes the hip or buttock area. Each leg contains thirty bones, with the main ones being the femur (thigh bone), tibia (shinbone), and fibula. The section from hip to knee is the thigh, while the part from knee to ankle is the lower leg, also called the shank or crus, which includes the shin at the front and the calf at the back. Legs are essential for standing, various movements, and activities like dancing, and they make up a large portion of a person's body weight. Over time, evolution shaped the human leg specifically for efficient walking on two legs. Although other primates can stand upright briefly, it costs them much more energy. Human legs are notably long and powerful: leg length relative to trunk length is 171% in humans, compared to 128% in chimpanzees and 111% in orangutans. Female legs typically have greater hip anteversion and tibiofemoral angles, while male legs tend to have longer femurs and tibias.

In anatomy, the term "leg" is often restricted to the lower leg between the knee and ankle. The thigh sits between the hip and knee, and the whole structure is called the lower limb or lower extremity. Bipedalism also shifted the body's center of gravity, reorganized internal organs, and changed the trunk's form and biomechanics. The human spine's double S-curve acts as a shock absorber, transferring weight from the trunk to the feet. Many leg muscles, especially the gluteals, knee extensors, and calf muscles, are adapted for upright walking.

The leg's major bones—femur, tibia, and fibula—are all long bones. The patella, or kneecap, is the body's largest sesamoid bone, located in front of the knee. Most leg bones have palpable bumps and edges that serve as anatomical landmarks, such as the anterior superior iliac spine, greater trochanter, medial condyle of the tibia, and medial malleolus. Exceptions include the hip joint and the femoral neck and shaft. The large joints of the lower limb usually align along a straight mechanical axis, the Mikulicz line, which runs from the hip joint (femoral head) through the knee (intercondylar eminence of the tibia) to the center of the ankle. In the tibia, the mechanical and anatomical axes match, but in the femur they diverge by about 6°, creating a femorotibial angle of roughly 174° in a normally aligned leg. A leg is considered straight when, with feet together, both the medial malleoli and medial condyles touch. If the knee joint center is lateral to the mechanical axis (with more than 3 cm between the ankles), it is genu varum; if medial (with more than 5 cm between the knees), it is genu valgum. These conditions cause uneven joint loads and stretch the thigh's adductors or abductors. The angle between the femoral neck and shaft (collodiaphysial angle) changes with age: about 150° at birth, decreasing to 126–128° in adults, and 120° in old age. Abnormal angles lead to coxa vara (small angle) or coxa valga (large angle), often combined with genu varum or genu valgum. A line through the femoral neck and one through the femoral condyles form a torsion angle, which converts hip flexion into rotary movements of the femoral head. Increased torsion turns the limb inward, decreased torsion turns it outward, both limiting mobility.

Hip muscles can be classified by location or innervation, by insertion points (posterior in two layers and anterior), or by function (extensors, flexors, adductors, abductors). Some hip muscles also act on the knee or vertebral joints. Because many have broad origins and insertions, they often participate in several different movements. In the hip joint, lateral and medial rotation occur.