gait is a general term covering a series of modes of forward progression. In humans, it encompasses the only two familiar types: walking and running. However, other bipeds like kangaroos have radically different mechanisms of gait, and quadrupeds such as the horse have a much fuller repertoire of gaits including walking, trotting, cantering, and galloping, that involve quite different patterns of movement. Human walking and running varies only in speed.

The general principle is that increases in speed are associated with lifting each foot earlier in the stride cycle and placing it so that stride length is increased.

The choice of gait is largely determined by the energy cost of progression. In each case the most economical gait is selected, although the mechanism by which the central nervous system makes the selection remains unknown.

The power requirement for gait increases linearly with speed in each mode of gait. However, the efficiency with which this power is converted into forward motion varies. In general, each limb is used like a pendulum, swinging forward passively at low speeds and accelerated forward at higher speeds of progression. The increased flexion of the limb at the knee as humans change from walking to running can be regarded as a attempt to shorten the length of the pendulum, so allowing faster swings forward. This pendulum action is supplemented by storage of energy in the elastic components of the limb, with its subsequent release in the next phase of movement. This is obvious in the ‘spring in the step’ of youngsters, that allows smooth storage and release of energy during gait. It also allows a low energy cost of movement. (This reaches its most sophisticated form in the bouncing gait of the kangaroo and, as a consequence, kangaroos have the most energy-efficient gait in the animal kingdom.)

The management of energy transfer between its kinetic and potential forms requires complex control within the central nervous system. The limbs have to be folded to allow swing phases and stiffened to allow stance in precisely co-ordinated ways. In addition, muscle contraction can be used to stiffen the natural springiness of tendons, so allowing more energy storage.

Many neurological conditions, such as Parkinson's disease and stroke, affect the timings of the muscle activity. This severely affects the efficiency of movement and imposes a large additional demand for muscle activity, which is obvious in slowed gait and reduced endurance.

R. H. Baxendale

See also walking.