Atypical Interpersonal Motor Synchrony and Intrapersonal Movement Variability in Autism: Pose Estimation Analysis of Free-play

Socio-motor behaviours, like interpersonal motor synchrony (IMS), are widely thought to be atypical in autism, yet they remain poorly defined, mechanistically unclear, and difficult to detect by eye. Studies exploring IMS have predominantly required participants to be stationary, making them ill-suited to observing the diverse behaviours and phenotypes associated with autism, especially in young children and naturalistic settings. We analysed 60 autistic and 27 typically developing young children aged 1 - 4 years during free-play segments of ADOS-2 diagnostic sessions. To measure IMS between child-assessor dyads, we applied 2-D pose estimation technology, OpenPose, and dynamic time warping (DTW). Our results show that child-assessor dyads with autistic children exhibited lower IMS than controls did in both trunk regions and head regions. Autistic children also demonstrated higher levels of intrapersonal movement variability (IMV). IMS and IMV were negatively correlated, which is a novel observation. Both lower IMS and higher IMV in autistic children were associated with higher levels of autistic traits, lower cognitive functioning skills, and lower adaptive functioning skills. Our findings highlight the potential of pose estimation and DTW to reliably detect atypical IMS and IMV in autistic children. Combined, these methods show promise as early behavioural markers, bringing us closer to non-invasive, digital tools for precise autism phenotyping in naturalistic settings.