A form of locomotion in which the torso is parallel to the ground and the four limbs are used for propulsion. At the start of crawling, weight is equally distributed across all four limbs in a posture such that the arms are extended downward from the shoulder at shoulder width, palms on the floor with the fingers pointing forward, thighs are vertical and hip-width apart, and the knees are on the floor with lower legs and feet pointing backward. When crawling forward, the arms and thighs move parallel to the body midline, the legs flex alternately, and the arm and leg on the opposite side of the body move simultaneously. Most infants, however, hardly ever comply with all details of this two-part description as there is considerable individual variation in both the starting posture and the subsequent pattern of interlimb coordination. As a motor milestone, crawling is typically preceded by creeping, during which both arms are extended and both legs flex symmetrically, initially with the stomach on the supporting surface. Some infants, however, start crawling without ever having had experience with creeping (and a few go from sitting to upright locomotion, without ever creeping or crawling).
See Coordination, Cruising, Gross motor abilities, Locomotion, Motor milestone, Rhythmical stereotypies