A Novel Assessment Reveals Motor Variability as a Sensitive Marker of Neurological Development, Decline, and Plasticity

Existing motor skill assessments focus on task-based performance metrics, such as speed or achievement rate, which provide limited insight into movement quality and variability. In this study, we introduce a novel, smartphone-based approach that employs shape similarity analysis to robustly quantify motor variability, a critical yet underexplored aspect of motor control. Using acceleration data from 1675 participants aged 3-88 years as they traced circles with their hands and feet, we uncover age-dependent changes in motor variability. Motor skill matures during adolescence, stabilizes, and deteriorates from the forties, with more pronounced decline in the non-dominant hand and foot. Unlike conventional tests, our approach treats variability as an intrinsic feature of motor skill and is highly sensitive to the nuanced effects of handedness, footedness, and forced correction of hand dominance. These findings point to movement variability as a sensitive measure for tracking individual motor development, decline, and plasticity. Our automated assessment potentially offers a scalable tool for early detection of motor dysfunction and quantification of motor training during physical rehabilitation.