Tomas Novohradsky

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 a new operational index, the contact-integrated impulse-transfer index (ITI; the ratio of contact impulse to pre-impact foot velocity) and evaluated its reliability, agreement with an acceleration-based effective-mass estimate (M_e,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 and with 30 kg of carried equipment. Reliability was assessed using intraclass correlation coefficients; condition differences using paired tests with effect sizes and false-discovery-rate correction; method agreement using correlations, ICCs, and Bland–Altman analysis; and isokinetic strength interactions using body-mass-adjusted mixed-effects models. ITI showed poor-to-moderate single-kick reliability (ICC = 0.46–0.52) but good reliability for the five-kick mean (ICC = 0.83–0.87). It was higher with than without the carried load (26.18 ± 5.20 vs. 21.72 ± 5.05 kg; p < 0.001; d_z = 1.07), which, by definition, corresponded to a 23% greater net impulse with unchanged pre-impact foot velocity. Net impulse alone was more reliable (single-kick ICC = 0.75; five-kick mean ICC = 0.95) and showed a larger load effect (d_z = 1.39) than ITI. Contact duration increased, peak force decreased, and the force–time profile appeared more push-like under load. In a subsample with usable impact-phase marker trajectories (n = 20), ITI and M_e,acc were not significantly correlated (r = 0.06, p = 0.69) and showed negligible agreement (ICC = 0.01; Bland–Altman bias = −20.6 kg). The condition × strength interactions for concentric hip-rotator strength did not remain significant after Bonferroni correction (adjusted p = 0.067–0.096). Averaged across repeated kicks, ITI is an index of impulse transfer rather than effective (inertial) mass; front kick assessment in soldiers should include representative military equipment, and the altered kick mechanics suggest that training should include it as well.

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 (M_e,imp = J/v_pre) and evaluated its reliability, agreement with an acceleration-based estimate (M_e,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. M_e,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; d_z = 0.92), reflecting a 21% greater net impulse, while pre-impact foot velocity (v_pre) 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), M_e,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 M_e,imp (Bonferroni-adjusted p = 0.044–0.050). M_e,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.