Short Title: Interpersonal Nonverbal Movement Synchrony in Parent-Child Dyads: Analysing Play Procedures using the NICE Toolbox Long Title: How interpersonal temporal coordination manifests as movement synchrony in parent–child dyads across free-play and structured-play situations, and the extent to which the NICE Toolbox can reliably quantify genuine interactive coupling versus coincidental covariation?

Interpersonal temporal coordination refers to the alignment of bodily signals between two individuals in rhythmically matched or predictably coupled patterns during joint activity. Movement synchrony is the observable expression of interpersonal temporal coordination. It is considered a fundamental component of effective communication, emotional attunement, and shared attention. In parent–child interactions, movement synchrony provides insight into how body movement supports and reflects the quality of communication across different social contexts. To quantify these processes, the present study uses the Nonverbal Interpersonal Communication Exploration (NICE) Toolbox, which includes automated, video-based pose estimation algorithms, which analyse naturalistic interactions in both free play and structured play conditions in parent-child dyads. Sixteen parent–child dyads with children aged 4 to 10 were recruited from the university’s psychotherapeutic outpatient clinic. Each dyad completed two standardized play situations: a free play interaction and a structured play task (approximately 5–7 minutes each). In total, 32 videos were analysed using the NICE Toolbox, which extracted frame-by-frame movement data using ViTPose, a pose estimation algorithm. Subsequently, we calculated synchrony indices using windowed cross-correlation analysis. To further assess ecological validity and determine whether observed synchrony reflected genuine coordination rather than chance-level covariation, synchrony indices were compared with pseudosynchrony baselines generated through the Surrogate Synchrony (SUSY) framework. We found that observed synchrony levels were significantly higher than pseudosynchrony levels across dyads, indicating that parent–child movement coordination occurred at above-chance levels and reflected true interactive coupling. However, we were not able to find a significant difference between the play contexts. Nonetheless, the descriptives and visualization suggested that there was a slight variation depending on the play context with free play showing slightly higher variability compared to structured play. This study validates the NICE Toolbox as a reliable and scalable method for quantifying movement coordination in naturalistic settings. Furthermore, the findings highlight movement synchrony as a measurable marker of interaction quality in the context of developmental and clinical psychology research and demonstrates its potential to connect moment-to-moment coordination with broader aspects of relational functioning in naturalistic interactions. Keywords: Temporal Coordination, Movement Synchrony, Parent-Child Interaction, Body Pose Estimation, NICE Toolbox.