The human leg is the lower limb of the human body, including the thigh, knee, lower leg, ankle, foot, or sometimes even the hip or buttock region. There are thirty bones in each leg. The major bones are the femur (thigh bone), tibia (shinbone), and adjacent fibula.
The part of the leg between the hip and the knee is called the thigh. The part between the knee and the ankle is called the lower leg, shank, crus, or in anatomy simply the leg. The lower leg includes the shin in the front and the calf at the back.
Legs are used for standing, many forms of human movement, recreation such as dancing, and constitute a significant portion of a person's mass. Evolution has led to the human leg'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. In humans, female legs generally have greater hip anteversion and tibiofemoral angles, while male legs have longer femur and tibial lengths.
Structure
In human anatomy, the lower leg or crus (or shank) is the part of the lower limb that lies between the knee and the ankle. In the lower leg, the calf is the back portion, and the tibia or shinbone together with the smaller fibula make up the shin, the front of the lower leg. Anatomists restrict the term leg to this use, rather than to the entire lower limb. The thigh is between the hip and knee and makes up the rest of the lower limb. The term lower limb or lower extremity is commonly used to describe all of the leg.
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 an 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 has also affected the location of the body's center of gravity, the reorganization of internal organs, and the form and biomechanism of the trunk. In humans, the double S-shaped vertebral column acts as a great 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 for 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.Thieme Atlas of Anatomy (2006), p. 360
Bones
The major bones of the leg are the femur (thigh bone), tibia (shin bone), and adjacent fibula, which are all long bones. The patella (kneecap) is a sesamoid bone (the largest in the body) 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.Thieme Atlas of Anatomy (2006), p. 361 Notable exceptions to palpation are the hip joint, and the neck and body, or shaft of the femur.
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.Thieme Atlas of Anatomy (2006), p. 362
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 result in abnormal posture of the leg: a small angle produces coxa vara and a large angle 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 result in a limb turned inward and a decreased angle in a limb turned outward; both cases resulting in a reduced range of a person's mobility.Platzer (2004), p. 196
Muscles
Hip
+ Function of hip musclesPlatzer (2004), pp. 244–47 Movement Muscles(in order of importance)Lateralrotation•Sartorius•Gluteus maximus•Quadratus femoris•Obturator internus•Gluteus medius and Gluteus minimus•Iliopsoas(with psoas major♣)•Obturator externus•All functional adductorsexcept gracilis* and pectineus•PiriformisMedialrotation•Gluteus medius andminimus (anterior fibers)•Tensor fasciae latae*•Adductor magnus(long medial fibers)•Pectineus (with leg abducted) Extension•Gluteus maximus•Gluteus medius andminimus (dorsal fibers)•Adductor magnus•Piriformis•Semimembranosus*•Semitendinosus*•Biceps femoris*(long head)Flexion•Iliopsoas(with psoas major♣)•Tensor fasciae latae*•Pectineus•Adductor longus•Adductor brevis•Gracilis*•Rectus femoris*•Sartorius* Abduction•Gluteus medius•Tensor fasciae latae*•Gluteus maximus(fibers to fascia lata)•Gluteus minimus•Piriformis•Obturator internusAdduction•Adductor magnus(with adductor minimus)•Adductor longus•Adductor brevis•Gluteus maximus (fibersto gluteal tuberosity)•Gracilis•Pectineus•Quadratus femoris•Obturator externus•Semitendinosus* Notes ♣ Also act on vertebral joints. * Also act on knee joint.
There are several ways of classifying the muscles of the hip: By location or innervation (ventral and dorsal divisions of the plexus layer); By development on the basis of their points of insertion (a posterior group in two layers and an anterior group); and By function (i.e. extensors, flexors, adductors, and abductors).Platzer, (2004), p. 232
Some hip muscles also act either on the knee joint or on vertebral joints. Additionally, because the areas of origin and insertion of many of these muscles are very extensive, these muscles are often involved in several very different movements. In the hip joint, lateral and medial rotation occur along the axis of the limb; extension (also called dorsiflexion or retroversion) and flexion (anteflexion or anteversion) occur along a transverse axis; and abduction and adduction occur about a sagittal axis.
The anterior dorsal hip muscles are the iliopsoas, a group of two or three muscles with a shared insertion on the lesser trochanter of the femur. The psoas major originates from the last vertebra and along the lumbar spine to stretch down into the pelvis. The iliacus originates on the iliac fossa on the interior side of the pelvis. The two muscles unite to form the iliopsoas muscle, which is inserted on the lesser trochanter of the femur. The psoas minor, only present in about 50 per cent of subjects, originates above the psoas major to stretch obliquely down to its insertion on the interior side of the major muscle.Platzer (2004), p. 234
The posterior dorsal hip muscles are inserted on or directly below the greater trochanter of the femur. The tensor fasciae latae, stretching from the anterior superior iliac spine down into the iliotibial tract, presses the head of the femur into the acetabulum but also flexes, rotates medially, and abducts to hip joint. The piriformis originates on the anterior pelvic surface of the sacrum, passes through the greater sciatic foramen, and inserts on the posterior aspect of the tip of the greater trochanter. In a standing posture it is a lateral rotator, but it also assists extending the thigh. The gluteus maximus has its origin between (and around) the iliac crest and the coccyx, from where one part radiates into the iliotibial tract and the other stretches down to the gluteal tuberosity under the greater trochanter. The gluteus maximus is primarily an extensor and lateral rotator of the hip joint, and it comes into action when climbing stairs or rising from a sitting to a standing posture. Furthermore, the part inserted into the fascia latae abducts and the part inserted into the gluteal tuberosity adducts the hip. The two deep glutei muscles, the gluteus medius and minimus, originate on the lateral side of the pelvis. The medius muscle is shaped like a cap. Its anterior fibers act as a medial rotator and flexor; the posterior fibers as a lateral rotator and extensor; and the entire muscle abducts the hip. The minimus has similar functions and both muscles are inserted onto the greater trochanter.Platzer (2004), p. 236
The ventral hip muscles function as lateral rotators and play an important role in the control of the body's balance. Because they are stronger than the medial rotators, in the normal position of the leg, the apex of the foot is pointing outward to achieve better support. The obturator internus originates on the pelvis on the obturator foramen and its membrane, passes through the lesser sciatic foramen, and is inserted on the trochanteric fossa of the femur. "Bent" over the lesser sciatic notch, which acts as a fulcrum, the muscle forms the strongest lateral rotators of the hip together with the gluteus maximus and quadratus femoris. When sitting with the knees flexed it acts as an abductor. The obturator externus has a parallel course with its origin located on the posterior border of the obturator foramen. It is covered by several muscles and acts as a lateral rotator and a weak adductor. The inferior and superior gemelli muscles represent marginal heads of the obturator internus and assist this muscle. These three muscles form a three-headed muscle (tricipital) known as the triceps coxae. The quadratus femoris originates at the ischial tuberosity and is inserted onto the intertrochanteric crest between the trochanters. This flattened muscle act as a strong lateral rotator and adductor of the thigh.Platzer (2004), p. 238