An Impulse-Based Effective Mass Estimate Is Sensitive to a Carried-Load Condition During Front Kicks in Military Cadets

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Abstract
Effective mass reflects the body mass mechanically coupled during impact. Acceleration-based effective-mass estimates require noise-sensitive differentiation of segment kinematics. This study introduced an impulse-based estimate (Me,imp = J/vpre) and evaluated its reliability, agreement with an acceleration-based estimate (Me,acc), and sensitivity to a carried-load condition. Forty male military cadets performed five maximal front kicks onto a force plate synchronized with three-dimensional motion capture under two fixed-order conditions: barefoot without load (NL) and with 30 kg of carried equipment (WL). Reliability was assessed using intraclass correlation coefficients; condition differences using paired tests with effect sizes and FDR correction; method agreement using correlations, ICCs, and Bland–Altman analysis; and isokinetic strength interactions using body-mass-adjusted mixed-effects models. Me,imp showed poor-to-moderate single-kick reliability but good reliability for the five-kick mean (ICC = 0.84–0.87). It was higher in WL than NL (25.95 ± 5.44 vs. 21.89 ± 5.14 kg; p < 0.001; dz = 0.92), reflecting a 21% greater net impulse, while pre-impact foot velocity (vpre) remained unchanged. Contact duration increased and the force–time profile became more push-like under WL. In a subsample with usable impact-phase marker trajectories (n = 20), Me,imp and Me,acc were not significantly correlated (r = 0.10, p = 0.54) and showed negligible agreement (ICC = 0.02; Bland–Altman bias = −21.5 kg). Stronger concentric hip rotators were associated with a smaller WL–NL increase in Me,imp (Bonferroni-adjusted p = 0.044–0.050). Me,imp should be interpreted as a contact-integrated index of impulse-transfer capacity, reliable when averaged across repeated kicks, rather than as true inertial mass or an interchangeable substitute for Me,acc.

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