| Research purpose: the purpose of this study is to determine the influence of the height of center of gravity on the speed of the upper board,the speed of the take-off and the economy of the take-off of the long jump athletes before the take-off,so as to determine whether it is necessary to lower the center of gravity before the take-off of the long jump,and to provide a basis for future research and training.Research methods: The subjects are 7 long jump students,all of whom were level 2.The subjects were ranked,and the test was performed in a given order,with each person performing six trials.The experiment was divided into two experiments with the same test method.The first test: the subjects performed the trial jump according to their own running rhythm and gave full play to the running speed.The second experiment: the subjects are required to consciously lower the height of the center of gravity in the last few steps of the run-up to take off.In the middle of the two experiments,a training course is arranged three days apart,two times in total,each training time is about one hour.The subjects are mainly trained to complete the take-off of the center of gravity in the last steps of the run-up.Two constant speed cameras were used to film the subjects’ four steps after the run-up and the complete technical movements of the take-off.Both cameras are positioned on the right side of the auxiliary runway,one camera is positioned in the right front of the approach direction,and the other camera is positioned in the right rear of the approach direction.The included Angle of the main optical axis of the two cameras is 90°.The shooting frequency is 60 frames /s and the shutter speed is 1/1000 s.Before the test,the large-range space calibration method was adopted for calibration.In addition,three geodetic coordinates were placed in the take-off area to establish the geodetic coordinate system.The geodetic coordinate system,Y-axis,pointed to the right at the take-off front,X-axis parallel to the auxiliary runway,z-axis perpendicular to the ground,and the origin was set to the left of the take-off front.This study analyzed at least 5 frames from 5～6 frames before the take-off from the four steps of the run-up to the highest point of the body’s center of gravity.The dalian sharp motion analysis system was applied to obtain the coordinates of 21 analytic points of the subject’s body,and the two cameras were synchronized according to several key characteristic moments.The 3-d coordinates of the analytic points in the geodetic coordinates were calculated by DLT method,and the coordinate data were smoothed by Butterworth low-pass filtering method with a truncated frequency of 10 Hz.According to the smooth three-dimensional coordinates,the coordinates of the human body’s center of gravity,the Angle of take-off,the take-off speed and other indicators are further calculated.The paired T test was used to analyze the relationship between the height of center of gravity and the take-off speed before the two jumps.Unitary regression analysis was used to determine the relationship between horizontal velocity loss and vertical velocity increment during take-off support phase.Statistical analysis of all data was completed using SPSS23.0 and EXCEL,and the significance level was defined as p<0.05.The test results were expressed as Mean±SD.Results: in the height of the center of gravity,the height of the center of gravity of the low center of gravity was significantly lower than that of the high center of gravity of the low center of gravity by four steps(p=0.010),three steps(p=0.008),two steps(p=0.010)and one step(p=0.012).In step size,there was no significant difference in step size between high and low gravity center(p=0.199),three steps(p=0.300),two steps(p=0.467)and one step(p=0.173).In terms of the landing distance,the landing distance of the lower center of gravity was significantly higher than that of the higher center of gravity by four steps(p=0.012)and one step(p=0.018),while the landing distance of the lower center of gravity by three steps(p=0.283)and two steps(p=0.056)had no significant difference.In the horizontal speed of the approach,the horizontal speed of the approach with high center of gravity and low center of gravity was reversed by four steps(p=0.291),three steps(p=0.238),and two steps(p=0.218),but the horizontal speed of the approach with low center of gravity was significantly higher than that of the approach with high center of gravity(p=0.025).At the moment of take-off,there was no significant difference in the take-off distance between the high center of gravity and the low center of gravity(p=0.245),the height of the take-off center of gravity(p=0.265),the take-off speed(p= 0.228),the horizontal take-off speed(p=0.224),and the vertical take-off speed(p=0.316).During the take-off support phase,the horizontal velocity loss of high and low center of gravity(p=0.108),the vertical velocity increment(p=0.433),and the vertical velocity increment and horizontal velocity loss ratio(p=0.089)had no significant difference.There was no significant difference in horizontal velocity loss,vertical velocity increment,vertical velocity increment and horizontal velocity loss ratio between high center of gravity and low center of gravity during the take-off support phase.The relationship between the horizontal velocity loss and the vertical velocity increment is significantly affected by the velocity conversion coefficients of the high and low center of gravity during the take-off support phase.The smaller the value of velocity conversion coefficient,the smaller the horizontal velocity loss caused by each unit vertical velocity increment.The larger the value of velocity conversion coefficient,the greater the horizontal velocity loss caused by each unit vertical velocity increment.Conclusions: 1.A gradual lowering of the body’s center of mass in the last two steps of the run with a low center of gravity will help maintain the horizontal approach speed.2.The height of the center of gravity before takeoff did not affect the horizontal and vertical velocity of the subjects in this study,and the horizontal velocity loss during takeoff was linearly correlated with the vertical velocity increment.3.There is a linear correlation between horizontal velocity loss and vertical velocity increment during take-off.4.The height of the center of gravity affects the value of the velocity conversion coefficient of the take-off during the upper plate.5.The low center of gravity upper plate is conducive to improving the economy of take-off and reducing the horizontal velocity loss caused by the increment of vertical velocity per unit. |
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