跳台滑雪中运动员采用什么姿态飞得既稳且远1)
WHAT POSE DOES THE ATHLETE USE TO FLY STEADILY AND FAR IN SKI JUMPING1)
通讯作者: 2)马赟,副研究员,研究方向为生物力学与仿生。E-mail:yun_ma@bit.edu.cn;3)霍波,教授,研究方向为运动生物力学、骨力学、细胞力学、呼吸力学。E-mail:huobo@bit.edu.cn
责任编辑: 胡漫 王永会
收稿日期: 2022-02-22
基金资助: |
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Received: 2022-02-22
作者简介 About authors
针对冰雪运动项目跳台滑雪,本文阐述了跳台滑雪运动员在比赛过程中助滑、起跳、飞行及着陆阶段的动作技术特点及其所蕴含的力学原理,设计制作了实验道具并开展了演示实验,说明了起跳阶段适当蹬地与飞行阶段维持特定姿态是跳台滑雪运动员取得优异成绩的关键因素。本文还探讨了相关演示实验在基础力学教学中的应用前景。
关键词:
It explains the technical characteristics and mechanical principles in the ski jumping sport during the four phases, i.e., approach, takeoff, flight and landing. Relevant experiments have been carried out to investigate the dynamics. It is found that the leg thrust against the ground during takeoff phase and maintaining specific body posture during flight phase are the key factors to ensure the athletes to achieve satisfactory scores during the game. This paper also discusses the possible applications of the relevant experiments for teaching theoretical mechanics.
Keywords:
本文引用格式
蒋量, 陈雪, 高先智, 黄毅, 马赟, 霍波, 吴霞, 王宁.
JIANG Liang, CHEN Xue, GAO Xianzhi, Huang Yi, MA Yun, HUO Bo, WU Xia, WANG Ning.
跳台滑雪是一项充满挑战与观赏性的项目,起源于挪威。在1924年法国夏慕尼举办的第一届冬奥会上,跳台滑雪就被列为了正式项目。北京2022年冬奥会上,跳台滑雪分项设3个男子项目、1个女子项目和1个混合项目,共产生5枚金牌。我国跳台滑雪项目开展较晚,但近年来发展迅速。中国跳台滑雪队取得1男2女共3个北京冬奥会参赛资格,获取参赛资格的人数取得了历史突破。
国家跳台滑雪中心("雪如意")是中国首座跳台滑雪场馆,为2022年北京冬奥会的比赛场馆。国家跳台滑雪中心的滑道分为HS140大跳台和HS106标准跳台,HS140大跳台长度为110 m,落差为135 m,HS106标准跳台长度为106 m,落差为115 m。运动员从如此高的跳台上跳下来,是如何保证着陆平稳而不受伤的呢?在不受伤的前提下,运动员又如何飞得更远呢?跳台滑雪按技术动作可分为助滑、起跳、飞行和着陆四个阶段,跳台滑雪运动员在比赛中需要无助力条件下呈半蹲式姿态沿助滑道向下加速,在起跳台蹬地展开,在空中两腿和雪板呈"V"字型进行滑翔,最后以弓箭步姿态完成着陆[1]。对此,本文将从跳台滑雪的四个阶段解读跳台滑雪运动员飞得又稳又远的力学原理。
1 助滑阶段
跳台滑雪运动员在助滑阶段的主要任务是维持滑行的动态平衡和加速稳定性,利用助滑道获得最大的起跳初速度。在助滑阶段,运动员保持下蹲姿态(图1),这里面主要涉及到两个力学原理:一是下蹲姿态可降低人体重心,提高滑行稳定性,防止摔倒;二是运动员沿着角度约为35$^\circ$的斜坡滑道下滑,在重力作用下,沿助滑道斜坡获得越来越大的助滑速度。
图1
需要注意的是,在助滑阶段速度增大的同时所受风阻也越来越大,这就类似于骑自行车时速度越快所感受到的阻力越大,或者是坐过山车时会感受到远大于在地面行走时的空气阻力。另外,由于空气阻力与受风面积成正比,助滑阶段运动员会将身体前倾,与滑雪板保持平行,呈现符合空气动力学要求的流线型,减少空气阻力。
助滑阶段是运动员积累动能并获得最大助滑速度的过程。助滑阶段结束后,运动员还会进一步蹬地起跳以进一步增加自身速度。运动员具体是如何做到的呢?接下来就需要对起跳阶段进行解读。
2 起跳阶段
起跳是跳台滑雪整个技术动作的关键,起跳动作的好坏决定了运动员的成绩[2]。跳台滑雪运动员离开跳台瞬间的速度可达90~120 km/h (即25~33 m/s),相当于高速公路上高速行驶的汽车车速。当运动员以25 m/s以上的速度下滑至台端的起跳板(它与水平方向相比向下倾斜9$^\circ$~11$^\circ$)时,运动员会向上奋力一跳,身体抛向空中。运动员起跳时两腿快速下蹬,具体来讲是运动员顺着助滑道快速滑行,当雪板尖到达台端附近时立即起跳,躯干向前伸展。考虑到助滑阶段的高速度,掌握起跳的最佳时机是衡量运动员技术水平高低的一个重要标准。为了更好地理解起跳蹬地力对飞行距离的影响,本文设计以下演示实验。
在本演示实验中,将人体半蹲且下肢蹬伸的过程简化成被钢球与跳台的碰撞,跳台尺寸统一为100 mm,$\times$,100 mm,$\times$,10 mm,使用不同材质的方板(橡胶、椴木、大理石和铁)以模拟不同的蹬伸力量对跳跃距离的影响(图2)。将小球从同一高度滚动至跳台,由于材质的不同,钢球在跳台上碰撞后的离台速度会存在差异,从而产生不同的飞行距离。通过本演示实验,将跳台材料从橡胶、椴木、大理石一直变到铁,其对应的弹性模量由小到大,钢球受到的反弹力越来越大,飞行距离也随之变长。可见,跳台滑雪运动员在起跳时需要充分发力,双腿的蹬踏力越大,飞行距离也会越远。
图2
3 飞行阶段
在起跳完成后,运动员进入空中开始滑翔,运动员通常将身体尽可能前倾,下肢微曲,双手伸展并置于身体两侧,滑板张开成V型,如同滑翔的老鹰,御风而行,紧盯远处的猎物,这样的姿态称为"V"型姿态。在飞行过程中,运动员将受到自身重力及气动力的作用。气动力可以分解为沿运动员速度反方向的阻力$F_{\rm d}$和垂直于速度方向的升力$F_{\rm l}$,其大小可通过以下公式计算得到
其中,$\rho$为空气密度,与跳台所处的海拔高度有关;$v$为运动员的速度;$A$为运动员的迎风面积;$C_{\rm l}$为升力系数;$C_{\rm d}$为阻力系数。升力系数、阻力系数与运动员的飞行姿态和滑雪服材质有关。有研究表明,当将雪板侧滑角从0$^\circ$增加至20$^\circ$时,升力系数可增大2倍以上[3]。在飞行阶段,运动员需要找到一个合适的姿态使其受到的升力较大且阻力较小,从而飞得更远。在常见的V型飞行姿态下,运动员与雪板的整体姿态与飞机机翼类似,近似上曲下平,通过这种飞行姿态可将升力与阻力的比值提升至较高值,从而获得最长的飞行时间,产生最大的飞行距离。
本文通过以下演示实验探讨空中的技术动作对空气阻力的影响(图3):将一个人体模型固定在小型风扇前,在保持其他条件不变的情况下,将人体模型雪板的夹角从0$^\circ$变为45$^\circ$。实验结果表明,随着雪板的张开,弹簧测力计的示数逐渐减小,即升力逐渐增大,这表明运动员的飞行距离也会逐渐增大。本演示实验验证了雪板"V"型飞行姿态相较于平行姿态能产生更高的升力。
图3
4 着陆阶段
在完成优美的飞行后,运动员需要完成最后也是最惊险的一个阶段,即着陆阶段,那么运动员是通过什么技术动作实现安全稳定地着陆于着陆坡的呢?
运动员着陆时,两脚成弓箭步并前后分开,身体重量分别落于两脚,雪板后端略领先于板尖着陆,两腿屈膝作缓冲,两臂左右平伸,以维持身体平衡(图4)。为什么要采用弓箭步呢?这是因为要充分利用动量定理。动量定理即为物体动量在运动阶段开始与结束时的变化量等于该物体在整个运动阶段所受的冲量$F\Delta t$,公式表达式为$F\Delta t=mv_{2}-mv_{1}$。具体来讲,运动员在着陆过程中,质量不变,速度最终变为零,动量的改变量为定值,为了尽可能减小身体受到的冲击力(即坡面支撑力),需要通过屈膝来延长作用时间,以此实现安全着陆。
图4
5 结论与展望
本文分析并解释了跳台滑雪中助滑、起跳、飞行和着陆阶段中的相关力学问题。跳台滑雪是一项极具挑战的比赛项目,蕴含着许多技术细节,在如此长的飞行距离和高度落差中,考验的是运动员的勇气和技巧,只有通过对各个阶段运动技术细节的完美掌控,运动员才能飞得更远。
此外,本文为解释跳台滑雪起跳和飞行阶段力学问题所设计的演示实验,可进一步开发并应用于理论力学、空气动力学等课程教学的演示实验,用直观的方法引导学生思考相关的力学问题和原理,进而掌握相关力学知识。
参考文献
Biomechanics research in ski jumping, 1991—2006
,In this paper, I review biomechanics research in ski jumping with a specific focus on publications presented between 1991 and 2006 on performance enhancement, limiting factors of the take-off, specific training and conditioning, aerodynamics, and safety. The first section presents a brief description of ski jumping phases (in-run, take-off, early flight, stable flight, and landing) regarding the biomechanical and functional fundamentals. The most important and frequently used biomechanical methods in ski jumping (kinematics, ground reaction force analyses, muscle activation patterns, aerodynamics) are summarized in the second section. The third section focuses on ski jumping articles and research findings published after the establishment of the V-technique in 1991, as the introduction of this technique has had a major influence on performance enhancement, ski jumping regulations, and the construction of hill profiles. The final section proposes topics for future research in the biomechanics of ski jumping, including: take-off and early flight and the relative roles of vertical velocity and forward somersaulting angular momentum; optimal jumping patterns utilizing the capabilities of individual athletes; development of kinematic and kinetic feedback systems for hill jumps; comparisons of simulated and hill jumps; effect of equipment modifications on performance and safety enhancement.
Dependence of ski jump length on the skier's body pose at the beginning of take-off
,A kinematical model of the ski jumper's body pose at the beginning of take-off was proposed. A method of measuring skier's body coordinates based on the results of video recordings and office information technologies was created. Kinematical parameters of the skier's body pose at the beginning of take-off were determined using sport competition results of 33 ski jumpers. Five parameters of the pose which show statistically significant correlation (р < 0.03) with the jump length were determined. A part of variation of the model parameters in the total variation of the jump length is almost equal to 53%, and relative correlation is strong and significant (p < 0.005). Recommendations regarding optimization of the body pose at the beginning of take-off were formulated.
Ski jumping takeoff in a wind tunnel with skis
,The effect of skis on the force-time characteristics of the simulated ski jumping takeoff was examined in a wind tunnel. Takeoff forces were recorded with a force plate installed under the tunnel floor. Signals from the front and rear parts of the force plate were collected separately to examine the anteroposterior balance of the jumpers during the takeoff. Two ski jumpers performed simulated takeoffs, first without skis in nonwind conditions and in various wind conditions. Thereafter, the same experiments were repeated with skis. The jumpers were able to perform very natural takeoff actions (similar to the actual takeoff) with skis in wind tunnel. According to the subjective feeling of the jumpers, the simulated ski jumping takeoff with skis was even easier to perform than the earlier trials without skis. Skis did not much influence the force levels produced during the takeoff but they still changed the force distribution under the feet. Contribution of the forces produced under the rear part of the feet was emphasized probably because the strong dorsiflexion is needed for lifting the skis to the proper flight position. The results presented in this experiment emphasize that research on ski jumping takeoff can be advanced by using wind tunnels.
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