Differences in the coordinative structure of object manipulation between 3- and 6-year-old children: kinematic analysis with markerless pose estimation

Object manipulation is fundamental to daily activities and undergoes marked developmental changes during early childhood. This study examined differences in the coordinative structure of object manipulation during a bead-threading task in typically developing children aged 3 and 6 years. Participants were enrolled in a longitudinal follow-up study and consisted of 25 three-year-olds and 22 six-year-olds. Kinematic data were obtained from overhead video recordings and analyzed using markerless pose estimation. Joint angle time series of the upper limb and digits were extracted, and manipulation speed and coordinative structures were examined. Principal component analysis was applied to joint angle patterns, and discriminant scores were computed to quantify between-group differences in coordinative structure. Manipulation speed did not differ significantly between groups (β = -0.25, p = .070). In contrast, movement coordination differed in its underlying coordinative structure: five principal components showing relatively large standard effects accounted for 12.3% of the total variance in manipulation patterns, and discriminant scores were higher in the 6-year-old group than in the 3-year-old group. The 6-year-old group predominantly employed a precision grip characterized by thumb rotation and true pad-to-pad opposition, accompanied by stable wrist extension and smooth, arch-like flexion of the index finger. In contrast, the 3-year-old group mainly used a power-grip strategy with thumb adduction and extension of the distal joints of the index finger, accompanied by wrist flexion. These findings suggest that differences in object manipulation are not adequately captured by performance measures alone but are reflected in the underlying coordination of hand and finger movements.