A leg is a weight bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modelled 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.

Terminology

  • Uniped: 1 leg, such as clams
  • Biped: 2 legs, such as humans and birds
  • Triped: 3 legs, which doesn't occur naturally in healthy animals
  • Quadruped: 4 legs, such as dogs and horses.

Many taxa are characterised by the number of legs:

  • Tetrapods have four legs.
  • Arthropoda: 4, 6 (Insecta), 8, 12, or 14 legs. Some arthropods have more than a dozen legs; a few species possess over 100. Despite what their names might suggest, centipedes ("hundred feet") might have fewer than 20 or more than 300 legs, and millipedes ("thousand feet") have fewer than 1,000 legs, but up to 750.

Components

A leg is a structure of gross anatomy, meaning that it is large enough to be seen unaided. The components depend on the animal. In humans and additional mammals, a leg includes the bones, muscles, tendons, ligaments, blood vessels, nerves, and skin. In insects, the leg includes most of these things, except that insects have an exoskeleton that replaces the function of both the bones and the skin.

Sometimes the end of the leg, or foot, is considered part of the leg; additional times it is considered separate. Similarly, the hip joint or additional place where the leg attaches to the main body might be considered separate or part of the leg.

Tetrapod legs

The leg of a wooly mammoth (reconstruction).

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

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

Arthropod leg

Robotic leg

A robotic leg is moved by an actuator, which is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into a few kind of motion.