Movement Science: Occupational Acquisition of a Milestone—From Sitting to Quadruped

Abstract

When infants are seated on the floor and are presented with objects at increasing lateral distance, a critical point is reached at which they must assume a quadrupedal position to obtain the object. This study was designed to examine the developmental course of this transition from sitting to quadrupedal position within the context of purposeful action. It asks how infants adjust their actions in response to objects placed at a small distance (5 cm) beyond their previously measured maximum reach. It also examines how they adjust their actions when the reach distance approaches the biomechanical limits of the sitting posture.Five infants were videotaped weekly from the onset of independent sitting (19–26 weeks) to when they would assume quadruped position in a lateral reaching task. Toys were presented to alternate sides at increasing distances to determine the maximum reachable distance each week. Trials were administered at 5 cm beyond the maximum distance and two closer distances. The infants' ability to reach improved linearly with experience. Each infant assumed quadrupedal position in 8–10 weeks, reaching to distances of 128–164% of arm length. Each infant also rotated his/her posture to assume a quadruped position with minimum asymmetry. Lateral asymmetries in use were significant for 3 of the 5 subjects.Stability in sitting posture is necessary before the transition to the quadruped stage is elicited in this task. The data suggest that infants perceive the need to change their posture concurrently with the ability to assume quadrupedal position.