Background and Study Aim: In the early literature on karate, it is empirically suggested that biomechanical variable such as executing time and impact force are affected by the stance. The aim of the research is to verify the hypothesis: there is no difference in the maximum impact force between natural (shizen-tai) and front (zenkutsu-dachi) stances; the maximum impact force may differ between the two stances because of the weight distribution on the lower limbs, and a greater engagement of body parts (effective mass) and joints. Material and Methods: The impact force of a straight punch was acquired for eight highly trained back belt karate practitioners (age: 47.5 ±10.13 years; height: 1.76 ±0.03 m; mass: 86.08 ±17.43 kg; expertise: 32.13 ±8.87 years) using two load cells (maximum capacity: 2000 N) fixed between two parallel boards with the same dimensions of a target. The subjects executed a straight punch while adopting two stances: natural and front stance targeting the force measurement device at full power. The effect of natural and front stances on the impact force (peak) was analyzed using the nonparametric Wilcoxon rank test. Additionally, the nonparametric bootstrap resampling technique was employed to investigate the robustness of the results. Results: The impact force (peak) of the straight punch is average 2260.79 ±538.44 N and average 2645.59 ±538.44 N for the natural and front stance, respectively. A straight punch in front stance presents statistically significant higher impact force (peak) than the same technique while adopting a natural stance. Conclusions: In the front stance (zenkutsu-dachi), karate athletes are able to engage more body parts and consequently increase effective mass during impact, which transfers greater momentum, and consequently generates a high impact force. These results were corroborated by the nonparametric bootstrap paired t-statistic which rendered similar conclusions.
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