Only arthropods and vertebrates have the means of rapid surface locomotion. In both groups, the body is moved forward by means of the legs. Because legs provide support as well as propulsion, the sequence of movements must be adjusted to maintain the body’s centre of gravity within a zone of support; if that centre of gravity falls outside this zone, the body loses its balance and falls. It is the necessity to maintain stability that determines the functional sequence of limb movements. During propulsion – which begins with footfall and ends with liftoff – the foot and leg remain stationary as the body pivots forward over the leg. During recovery – which begins with liftoff and ends with footfall – the body remains stationary as the leg moves forward. The advance of one leg is a step; a stride is composed of as many steps as there are legs. To ensure a cycle of continuous movement, friction-reducing properties of a stable surface (ie. the ground) are needed to enable the appropriate and continuous transfer of weight from heel to toe.
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