Postural practices in infancy: How skill status and environment shape early motor development

The development of infant posture is shaped by a complex convergence of intrinsic abilities and environmental contexts. This longitudinal study examined how infants' postural engagement evolved across the first year of life, focusing on the roles of motor skill acquisition and movement context alongside age. Using naturalistic home video data, we tracked the frequency, duration, and diversity of postures across over 180 observations. Our findings reveal that postural transitions and engagement patterns shift dynamically with skill status and movement contexts (restrained vs. unrestrained), rather than following a linear age-based trajectory. Movement context effects were posture-specific: supine posture declined with age only in the unrestrained context; belly-on-the-floor posture declined with age overall; sitting flipped with skill - pre-sitters sat more in the restrained context, whereas sitters sat more in the unrestrained context; crawling showed no main effect of age but differed by crawling status, with an age × crawling status interaction; and upright increased modestly with age and showed the largest unrestrained context advantage, especially among cruisers. These results highlight the importance of considering how multiple interacting factors, not just chronological age, contribute to early postural development, extending skill-based accounts of motor development by demonstrating that movement context interacts with skill status in posture-specific ways.