Development of prone extension postures in healthy infants.

The purposes of this study were to 1) investigate the sequence of development of head, upper extremity, and lower extremity extension in the prone extension posture in healthy infants; 2) identify the variations in the sequence; and 3) compare these postures within the sequence with the development of prone-on-elbows and prone-on-hands postures. Twenty healthy, full-term infants were observed longitudinally from 8 to 28 weeks of age. Two of the infants could not complete the study, and two infants never used the prone extension posture. The data from 16 of the infants, therefore, were analyzed. The results indicate that the sequence of development of the prone extension posture consists of head extension, followed by lower extremity extension, and then upper extremity extension. The results also indicate that upper extremity extension in the prone extension posture does not appear to be a prerequisite for the prone-on-elbows or prone-on-hands postures. These findings suggest that infants may use different strategies for developing motor control in the head and lower extremities than in the upper extremities. This information adds to our knowledge of the normal developmental process and may have significance in the planning of physical therapy programs.