The human leg is the entire lower extremity or limb of the human body, including the foot, thigh and even the hip or gluteal region; however, the precise definition in human anatomy refers only to the section of the lower limb extending from the knee to the ankle.

Legs are used for standing, walking, jumping, running, kicking, and similar activities, and constitute a significant portion of a person's mass.

In human anatomical terms, the leg is the part of the lower limb that lies between the knee and the ankle, the thigh is between the hip and knee and the term "lower limb" is used to describe the colloquial leg. This article generally follows the common usage.

The leg from the knee to the ankle is called the cnemis /ˈniːmɪs/ or crus. The calf is the back portion and the shin is the front.

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.

Animalia

Humans (known taxonomically as Homo sapiens,Latin for "wise man" or "knowing man") are the only living species in the Homo genus. Anatomically modern humans originated in Africa about 200,000 years ago, reaching full behavioral modernity around 50,000 years ago.

Humans have a highly developed brain and are capable of abstract reasoning, language, introspection, and problem solving. This mental capability, combined with an erect body carriage that frees the hands for manipulating objects, has allowed humans to make far greater use of tools than any other living species on Earth. Other higher-level thought processes of humans, such as self-awareness, rationality, and sapience, are considered to be defining features of what constitutes a "person".

Humans are uniquely adept at utilizing systems of communication for self-expression, the exchange of ideas, and organization. Humans create complex social structures composed of many cooperating and competing groups, from families and kinship networks, to nations. Social interactions between humans have established an extremely wide variety of values, social norms, and rituals, which together form the basis of human society. With individuals widespread in every continent except Antarctica, humans are a cosmopolitan species. As of November 2011^[update], the human population was estimated by the United Nations Population Division to be about 7 billion, and by the United States Census Bureau to be about 6.97 billion.

A leg is a weight bearing and locomotive structure, usually having a columnar shape. During locomotion, legs function as "extensible struts" - the combination of movements at all joints can be modeled as a single, linear element capable of changing length and rotating about an omnidirectional "hip" joint.

As an anatomical animal structure it is used for locomotion. The distal end is often modified to distribute force (such as a foot). Most animals have an even number of legs.

As a component of furniture it is used for the economy of materials needed to provide the support for the useful surface, the table top or chair seat.

Many taxa are characterized by the number of legs:

In tetrapod anatomy, leg is used to refer to the entire limb. In human medicine its precise definition refers only to the segment between the knee and the ankle. This segment is also called the shank, and the front (anterior) of the segment is called the shin or pretibia.

In bipedal tetrapods, the two lower limbs are referred to as the "legs" and the two upper limbs as "arms" or "wings" as the case may be.