A Contact-Integrated Impulse-Transfer Index for Front Kicks Under a Carried-Load Condition in Military Cadets

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 (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 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; dz = 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 (dz = 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 Me,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.