Developmental joint control in two-foot vertical jumps of 3-5-year-old children

This study used continuous age (months) to quantify phase-specific joint quasi-stiffness, variability (CV) of angles and moments, and instantaneous joint power of the hip, knee, and ankle during two-foot countermovement jumps in children aged 3-5 years, aiming to identify sensitive windows for early motor screening or intervention.

Ninety-one healthy preschoolers performed two-foot countermovement jumps. Kinematics (100 Hz) and ground reaction forces (1000 Hz) were recorded. Joint moments and power were computed via inverse dynamics, with power calculated as joint moment × angular velocity and normalized to body weight. For each phase and joint, quadratic regression was fitted; linear regression was used if the quadratic term was non-significant. Assumptions were checked and p values near 0.05 were interpreted conservatively.

Squat-phase quasi-stiffness showed no age association; knee and ankle angle CV increased with age, hip moment CV decreased, and ankle moment CV was non-significant. During propulsion, ankle quasi-stiffness increased slightly with age, angle CV showed minor age effects with quadratic trends for knee and ankle, and moment CV remained stable. Power timing was consistent across ages; 4-year-olds had the highest knee peak power and variability, while 5-year-olds showed smoother and higher propulsion output.

Development is joint- and phase-specific, with stable squat stiffness but increasing distal variability and modest ankle stiffness increase during propulsion. A transient peak in power variability at age four may mark a sensitive window for targeted screening or early intervention, with ankle propulsion mechanics as the most sensitive marker.