Estimation of Vertical Ground Reaction Forces During Vertical Jumping in Children Using OpenCap.

Vertical ground reaction force is an important parameter for describing the developmental characteristics of young children's vertical jumping. However, its application in large-scale physical fitness monitoring and routine teaching practice is greatly limited. Previous studies have used OpenCap to estimate vertical ground reaction force during adult jumping tasks and have provided preliminary validation, but its effectiveness in young children remains unclear. To examine the correlation and agreement of vertical ground reaction force (GRF) estimated by the OpenCap markerless motion capture system during young children's vertical jumping and to explore the characteristics of vertical GRF estimated by OpenCap during the vertical jump. Kinematic and kinetic data during vertical jumping were synchronously collected from 16 young children using the OpenCap markerless motion capture system and a three-dimensional force platform, with each child completing three trials. Kinematic data were acquired using the OpenCap markerless motion capture system, and the vertical acceleration of the whole-body center of mass was calculated to estimate vertical GRF based on Newton's second law. Pearson linear correlation analysis and Bland-Altman analysis were used to examine the differences in characteristics between the estimated vertical GRF and the measured vertical GRF. The vertical GRF characteristics estimated by OpenCap showed moderate-to-high correlations with the measured values. Specifically, the time and mean impulse during the push-off phase, flight phase, and landing stabilization phase were highly correlated (r > 0.85), while the peak force and mean force during the push-off phase showed moderate-to-high correlations (r > 0.7). Bland-Altman analysis showed that the bias in time and impulse during the vertical jump was less than 15%, indicating relatively high agreement; however, the bias in peak force during the landing phase exceeded 40%, indicating weak agreement. These results suggest that the OpenCap markerless motion capture system can effectively estimate vertical GRF characteristics during young children's vertical jumping, with the best performance observed for vertical GRF variables in the push-off phase. The method used in this study may be applied to obtain vertical GRF during young children's vertical jumping in non-laboratory settings and to assist in evaluating the developmental level of young children's vertical jump performance. Nevertheless, OpenCap-derived rapid impact variables, particularly landing peak force, should be interpreted with caution.