INTRODUCTION: The main purposes of take-off phase are to raise the centre of mass and to gain a forward-rotating angular momentum (Schwameder 2008). This study focused on the former, and aimed to analyze the age and gender differences by the take-off force. METHOD: The take-off forces exerted by Japanese ski jumpers were obtained from the four summer competitions at the Sapporo Okurayama jumping hill (HS=134m) from 2008 to 2010. The forces perpendicular to the take-off table (normal force) were sampled at 480Hz during the last 10m on the jumping platform. The force data of 375 jumps of 76 jumpers (57 males, 19 Females) were analyzed. The data were classified in four groups by the age and gender; Male Jr., Male Sr., Female Jr. and Female Sr.. The jumper under 20 years old belongs to Jr. group. It was assumed that the total weight of the equipment was 10.5kg for males and 10.0kg for females, respectively. The net force exerted by jumper (Fz) was calculated by subtracting the gravity and centrifugal force acted on a Jumper. And then, an extension impulse (F2t), take-off duration (T1) and displacement of center of gravity (dCOG) were calculated. One-way ANOVA was used to compare means among groups. Differences with P<.05 was considered significant. Post hoc analyses were performed with the Sheffe's test. RESULTS: Mean values of each item are shown in Table 1. Fzt of Male Sr. was the largest in all groups (P<.05). By the comparison between the genders, F2t of Female group was about 60% of that of Male group. T1 of Male Jr. was significantly big in comparison with other three groups. On the other hand, T1 of Female Sr. was smaller than those of Male groups. In dCOG, significant difference was recognized between genders, and the value of Female group was approximately 56% of that of Male group. DISCUSSION: Big gender gap was identified in Fzt and dCOG. These results suggest that it is inferior in lower limbs extension muscular strength of Female group in comparison with Male group. Female groups probably shifted to an initial flight before extending enough. T1 represents a start timing of take-off motion and the duration of concentric contraction of the extensor in lower limbs. The reason why the start timing of Male Jr. was earlier might be relatively small weight and large extension impulse (Fzt). CONCLUSION: This study revealed that the age and gender differences in take-off phase. The extension muscular strength of Female group is remarkably small in comparison with Male group. The Sr. groups showed extension impulse bigger than the Jr. groups.
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