A Comparison of Vertical Stiffness Values Calculated from Different Measures of Center of Mass Displacement in Single-Leg Hopping

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Kurt L. Mudie Western Sydney University

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Amitabh Gupta Western Sydney University

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Simon Green Western Sydney University

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Hiroaki Hobara National Institute of Advanced Industrial Science and Technology

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Peter J. Clothier Western Sydney University

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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.

Mudie, Gupta, Green, and Clothier are with the School of Science and Health, Western Sydney University, Sydney, NSW, Australia. Hobara is with the Human Informatics Research Institute, National Institute of Advanced Industrial Science and Technology, Tokyo, Japan.

Address author correspondence to Kurt L. Mudie at kurt.mudie@vu.edu.au.
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