<P> Evolution has provided the human body with two distinct features: the specialization of the upper limb for visually guided manipulation and the lower limb's development into a mechanism specifically adapted for efficient bipedal gait . While the capacity to walk upright is not unique to humans, other primates can only achieve this for short periods and at a great expenditure of energy . The human adaption to bipedalism is not limited to the leg, however, but has also affected the location of the body's center of gravity, the reorganisation of internal organs, and the form and biomechanism of the trunk . In humans, the double S - shaped vertebral column acts as a shock - absorber which shifts the weight from the trunk over the load - bearing surface of the feet . The human legs are exceptionally long and powerful as a result of their exclusive specialization to support and locomotion--in orangutans the leg length is 111% of the trunk; in chimpanzees 128%, and in humans 171% . Many of the leg's muscles are also adapted to bipedalism, most substantially the gluteal muscles, the extensors of the knee joint, and the calf muscles . </P> <P> The major bones of the leg are the femur (thigh bone), tibia (shin bone), and adjacent fibula, and these are all long bones . The patella (kneecap) is the sesamoid bone in front of the knee . Most of the leg skeleton has bony prominences and margins that can be palpated and some serve as anatomical landmarks that define the extent of the leg . These landmarks are the anterior superior iliac spine, the greater trochanter, the superior margin of the medial condyle of tibia, and the medial malleolus . Notable exceptions to palpation are the hip joint, and the neck and body, or shaft of the femur . </P> <P> Usually, the large joints of the lower limb are aligned in a straight line, which represents the mechanical longitudinal axis of the leg, the Mikulicz line . This line stretches from the hip joint (or more precisely the head of the femur), through the knee joint (the intercondylar eminence of the tibia), and down to the center of the ankle (the ankle mortise, the fork - like grip between the medial and lateral malleoli). In the tibial shaft, the mechanical and anatomical axes coincide, but in the femoral shaft they diverge 6 °, resulting in the femorotibial angle of 174 ° in a leg with normal axial alignment . A leg is considered straight when, with the feet brought together, both the medial malleoli of the ankle and the medial condyles of the knee are touching . Divergence from the normal femorotibial angle is called genu varum if the center of the knee joint is lateral to the mechanical axis (intermalleolar distance exceeds 3 cm), and genu valgum if it is medial to the mechanical axis (intercondylar distance exceeds 5 cm). These conditions impose unbalanced loads on the joints and stretching of either the thigh's adductors and abductors . The angle of inclination formed between the neck and shaft of the femur, (collodiaphysial angle), varies with age--about 150 ° in the newborn, it gradually decreases to 126 - 128 ° in adults, to reach 120 ° in old age . Pathological changes in this angle results in abnormal posture of the leg: A small angle produces coxa vara and a large angle in coxa valga; the latter is usually combined with genu varum and coxa vara leads genu valgum . Additionally, a line drawn through the femoral neck superimposed on a line drawn through the femoral condyles forms an angle, the torsion angle, which makes it possible for flexion movements of the hip joint to be transposed into rotary movements of the femoral head . Abnormally increased torsion angles results in a limb turned inward and a decreased angle in a limb turned outward; both cases resulting in a reduced range of a persons mobility . </P> <Table> Function of hip muscles <Tr> <Th> Movement </Th> <Th> Muscles (In order of importance) </Th> </Tr> <Tr> <Td> Lateral rotation </Td> <Td> <P> Sartorius Gluteus maximus Quadratus femoris Obturator internus Gluteus medius and minimus Iliopsoas (with psoas major ♣) Obturator externus All functional adductors except gracilis * and pectineus Piriformis </P> </Td> </Tr> <Tr> <Td> Medial rotation </Td> <Td> <P> Gluteus medius and minimus (anterior fibers) Tensor fasciae latae * Adductor magnus (long medial fibers) Pectineus (with leg abducted) </P> </Td> </Tr> <Tr> <Td> Extension </Td> <Td> <P> Gluteus maximus Gluteus medius and minimus (dorsal fibers) Adductor magnus Piriformis Semimembranosus * Semitendinousus * Biceps femoris * (long head) </P> </Td> </Tr> <Tr> <Td> Flexion </Td> <Td> <P> Iliopsoas (with psoas major ♣) Tensor fasciae latae * Pectineus Adductor longus Adductor brevis Gracilis * Rectus femoris * Sartorius * </P> </Td> </Tr> <Tr> <Td> Abduction </Td> <Td> <P> Gluteus medius Tensor fasciae latae * Gluteus maximus (fibers to fascia lata) Gluteus minimus Piriformis Obturator internus </P> </Td> </Tr> <Tr> <Td> Adduction </Td> <Td> <P> Adductor magnus (with adductor minimus) Adductor longus Adductor brevis Gluteus maximus (fibers to gluteal tuberosity) Gracilis Pectineus Quadratus femoris Obturator externus Semitendinosus * </P> </Td> </Tr> <Tr> <Th> Notes </Th> <Th> ♣ Also act on vertebral joints . * Also act on knee joint . </Th> </Tr> </Table>

What is the joint at the top of the leg called