Kinematic modulation across the ontogenetic transition to active social engagement

Spontaneous general movements provide a natural neuromotor baseline for quantifying context-related kinematic modulation as infants transition toward active social engagement. In this longitudinal prospective study, we analyzed 47 infants recorded at 4, 8, 12, and 16 weeks post-term age using 3D markerless motion tracking. Kinematics were compared across three contexts: beginning with the child alone (solitary, baseline), followed by the caregiver's approach, and then the subsequent social context. Motion space, posture, and movement speed were quantified to index age-specific changes in body control and orientation across contexts. Across development, motion space expanded from a predominantly horizontal distribution at 4 weeks to greater vertical and midline occupancy by 16 weeks. Context-related postural modulation emerged with age, shifting from initially minimal differences to sustained head and upper-body orientation toward the caregiver by 12 weeks. Remarkably, speed showed a nonlinear trajectory: infants exhibited a brief reduction in movement speed around caregiver arrival at 4 weeks, a more sustained reduction at 8 weeks, and a reversal to post-arrival acceleration beginning at 12 weeks. These findings highlight continuous ontogenetic adaptation from the outset, in which context-sensitive motor modulation is tuned across development to the infant's current repertoire and environmental demands, in service of increasingly active social engagement. Our study offers a new framework for detecting potential divergence that may lie less in specific early social behaviors than in cross-domain coordination and context-sensitive modulation.