Movement Direction Is the Primary Determinant of Force and Impulse in the Knife-Hand Strike (Sonkal Taerigi) in ITF Taekwon-Do

Background: The effectiveness of striking techniques in combat sports depends not only on peak force but also on how force is applied over time. The knife-hand strike (sonkal taerigi) in ITF taekwon-do can be executed in inward and outward directions; however, biomechanical differences between these variants and the role of limb laterality remain unclear. This study aimed to evaluate the effects of movement direction and limb side on selected kinetic variables. Methods: Fifteen experienced male taekwon-do practitioners (black belts, ≥10 years of training) performed knife-hand strikes using both hands (right and left) and two movement directions (inward and outward) on a ground reaction force platform. Three trials were recorded for each condition. The analyzed variables included peak resultant force (F), relative force (Fr), contact time (t), and impulse (J). Paired t-tests or Wilcoxon signed-rank tests were applied depending on data distribution, and effect sizes were calculated. Results: Inward strikes produced significantly higher resultant force (F), relative force (Fr), impulse (J), and slightly longer contact time (t) compared to outward strikes (all p ≤ 0.001), with large to very large effect sizes. The effect of limb side was limited and statistically significant only for impulse (p = 0.031), indicating generally high bilateral symmetry. Differences in contact time, although significant, were of negligible practical magnitude. Conclusions: Movement direction is the primary determinant of biomechanical effectiveness in the sonkal taerigi technique. Inward strikes provide more favorable mechanical conditions for force and impulse generation, whereas the influence of limb laterality is minimal. Impulse appears to be a sensitive and functionally relevant indicator of striking performance and may be particularly useful for performance assessment and training monitoring.