A comparison of vertical stiffness values calculated from different measures of center of mass displacement in single-leg hopping

This study assessed the agreement between Kvert calculated from 4 different methods of estimating vertical displacement of the center of mass (COM) during single-leg hopping. Healthy participants (N = 38) completed a 10-s single-leg hopping effort on a force plate, with 3D motion of the lower limb, pelvis, and trunk captured. Derived variables were calculated for a total of 753 hop cycles using 4 methods, including: double integration of the vertical ground reaction force, law of falling bodies, a marker cluster on the sacrum, and a segmental analysis method. Bland-Altman plots demonstrated that Kvert calculated using segmental analysis and double integration methods have a relatively small bias (0.93 kN.m-1) and 95% limits of agreement (-1.89 to 3.75 kN.m-1). In contrast, a greater bias was revealed between sacral marker cluster and segmental analysis (-2.32 kN.m-1), sacral marker cluster and double integration (-3.25 kN.m-1), and the law of falling bodies compared with all methods (17.26-20.52 kN.m-1). These findings suggest the segmental analysis and double integration methods can be used interchangeably for the calculation of Kvert during single-leg hopping. The authors propose the segmental analysis method to be considered the gold standard for the calculation of Kvert during single-leg, on-the-spot hopping.