Dirt absorption on the ski running surface — quantification and influence on the gliding ability

We propose a thesis that minimising dirt on the running surface of skis improves the surface glide. Waxing usually improves the gliding ability of skis in the short term. But how does waxing affect pollution absorption in the long term? In this study a number of skis with a transparent base and a white background were treated by steel scraping and with different glide waxes. The gliding ability of waxed and unwaxed skis, the sliding surface whiteness and the hydrophobicity were tested and documented. Tests were performed before and after the skis had been used for different distances. It was observed that all the waxed skis (regardless of the wax used) absorbed more dirt than unwaxed, and as a result all waxed skis lose their glide ability sooner than unwaxed (freshly scraped) skis in wet snow conditions.     

Changes in the cross-country ski base properties resulting from the ski use

In this work, changes in the cross-country ski base properties resulting from stone grinding, skiing, waxing and re-stone grinding have been investigated. The surface topography, crystallinity, wettability and sliding properties of cross-country ski bases were recorded using a selection of measurement techniques: non-contact chromatic confocal microscopy, contact-type surface roughness analyser and scanning electron microscopy (topography), Raman spectroscopy (crystallinity), contact angle measurement (wettability) and a ski tester (sliding properties). The tested skis were used for 150 km and waxed 35 times. The ski base surface became smoother and the orientation and crystallisation of the base material increased. Wear decreased the hydrophobicity and increased the sliding friction. Even though re-grinding did not restore all of the ski base properties, re-grinding improved the wettability and sliding properties. The results help to optimise stone grinding interval to maintain the performance of the skis.     

Rolling resistance for treadmill roller skiing

Modern treadmills allow cross-country skiers, biathletes and ski-orienteer’s to test their physical performance under laboratory conditions using classical and freestyle techniques on roller skis. The differences in performance between tests are quite small for elite athletes, and it is therefore of great importance to control the rolling resistance of the roller skis. Otherwise different physiological tests cannot be accurately compared. This study shows that during a warm-up period of 30 min the coefficient of rolling resistance (μ _R) decreases to about 60–65% and 70–75% of its initial value for freestyle and classical roller skis, respectively. Simultaneous measurements of temperature and μ _R shows that stabilized rolling resistance corresponds to a certain running temperature for a given normal force on the roller ski. Tests were also performed on the influence on μ _R of normal force, velocity and inclination. Normal forces produced significant influence on μ _R, while different velocities and inclinations of the treadmill only resulted in small changes in μ _R.     

Friction reduction using self-waxing alpine skis

A continuously waxed ski has been developed that releases a thin film of lubricant under the base of a ski. This replicates the melt water layer observed in snow skis which is caused by frictional and solar heating. The system is particularly effective on artificial (dry) slopes where skiers slide on plastic bristles rather than snow. Speeds comparable to those achieved on snow are achieved using this system and this improves the experience for the skier. Speed enhancements on plastic slopes of up to 50?% have been achieved using solutions of polyethylene glycol in water. There is speed enhancement of approximately 9?% on artificial snow and 2?% on fresh alpine snow. The latter value is highly significant as it can be the difference between winning a medal in ski competitions and finishing outside the top ten. In addition to the quantitative data, qualitative athlete perceptions were also recorded and show that a feel like snow can be achieved on artificial surfaces. Because the lubrication system is attached to the ski, it allows personal performance enhancement irrespective of a water misting system being in operation or not. The design complies with the equipment regulations of the skiing??s international governing body so it can be used in competition.     

A new device for measuring ski running surface force and pressure profiles

The pressure/force acting between the running surface of a ski and the snow may indirectly change glide friction. Thus, measuring the pressure/force distribution may be important for a deeper understanding of glide in skiing. The present aim was to construct a device that allowed the pressure/force underneath the ski running surface (SRS) to be recorded. Pressure sensors were attached on top of a platform. Sheets of different materials were used to improve the interaction between the SRS and the sensors. Possible functions of the device are demonstrated in three applications that emphasized comparison of force distribution underneath skis selected for similarity, force distribution under both skis and a single ski as well as backward weight distribution. The results show that the device with the pressure sensors mapped pressure/force distribution underneath the skis in the applications presented, and the system can thus be a useful tool for further optimizing e.g., ski designs.     

Electrical and mechanical response of skeletal muscle to electrical stimulation after acute passive stretching in humans: A combined electromyographic and mechanomyographic approach

Abstract

Flexural and torsional rigidity are important properties of skis. However, the flexural and torsional rigidity that lead to optimal performance remain to be established. In the present study, four pairs of slalom skis that differed in flexural and torsional rigidity were tested by advanced and expert skiers. Using a 10-item questionnaire, different aspects of the skis' performance were rated on a 9-point scale. For each pair of skis, physical measurements were compared with the ratings of the two groups of skiers. Correlations (Spearman) were then determined between (i) different mechanical properties of the skis (static and dynamic), (ii) subjective assessments of the participants, and (iii) properties of the skis and the participants' assessments. The latter showed that expert skiers rate the aspects of the skis more accurately than advanced skiers. Most importantly, expert skiers are particularly sensitive to torsion of the skis. These results suggest that such highly rated elements should be addressed in future ski designs.     

An experimental study to compare the grip of classical style roller skis with on-snow skiing

Cross-country skiers use roller skis for their snow-free training with the aim of imitating skiing on snow. Also, exercise laboratories evaluate the biomechanics and physiology of cross-country skiing using roller skis on a treadmill. The roller skis on the market that are constructed for use in the classical style are equipped with a front and a back wheel, one of which has a ratchet to enable it to grip the surface when diagonal striding and kick double poling (static friction). The aim of this study was to investigate static friction coefficients (μ_S) of ratcheted wheel roller skis, and compare the results to the μ_S reported from skiing on snow with grip-waxed cross-country skis. Also, a new type of roller ski with a camber and adjustable grip function was evaluated. The results showed that ratcheted wheel roller skis, on a treadmill rubber mat and on dry and wet asphalt surfaces, reached μ_S values that were five to eight times greater than the values reported from on-snow skiing with grip-waxed cross-country skis. For the roller skis with a camber and adjustable grip function, the μ_s could be varied from no grip at all up to the level of the tested ratcheted wheel roller skis.     

What static and dynamic properties should slalom skis possess? Judgements by advanced and expert skiers

Flexural and torsional rigidity are important properties of skis. However, the flexural and torsional rigidity that lead to optimal performance remain to be established. In the present study, four pairs of slalom skis that differed in flexural and torsional rigidity were tested by advanced and expert skiers. Using a 10-item questionnaire, different aspects of the skis' performance were rated on a 9-point scale. For each pair of skis, physical measurements were compared with the ratings of the two groups of skiers. Correlations (Spearman) were then determined between (i) different mechanical properties of the skis (static and dynamic), (ii) subjective assessments of the participants, and (iii) properties of the skis and the participants' assessments. The latter showed that expert skiers rate the aspects of the skis more accurately than advanced skiers. Most importantly, expert skiers are particularly sensitive to torsion of the skis. These results suggest that such highly rated elements should be addressed in future ski designs.     

Comparisons of the ski turn techniques of experienced and intermediate skiers

We compared selected kinematic variables for four different ski turn techniques performed by five experienced and five intermediate male skiers. The four ski turn techniques were the upstem turn, the downstem turn, the parallel turn and the parallel step turn. Each turn was divided into the initiation phase and the first and second steering phases. Most of the statistically significant differences (P < 0.05) between the two groups were found for the initiation phases of the four turns. Both the hip axis-hand axis angle and the edging angle of the uphill ski were significantly different between the two groups for the upstem turn at the beginning of the initiation phase. For the downstem turn, significant differences between the groups were found at the start of the initiation phase for the hip axis-hand axis angle, the shoulder axis-fall line angle, and the edging angle of the uphill ski. The standard deviation of the distance between the tips of the two skis over the second steering phase also differed significantly between the two groups. For the parallel step turn, significant differences were found at the start of the initiation phase for the edging angle of the downhill ski and the downhill ski to movement direction angle. Significant differences were also found for the edging angle of the downhill ski in the middle of the second steering phase and the shoulder axis to movement direction angle at the end of this phase. For the initiation phase of the parallel turn, significant differences were found for the timing of setting the ski pole, the uphill knee angle at the start of this phase and the range of the knee angle of the uphill leg from the start to the end of this phase. For this turn, significant differences between the two groups were also found for the edging angle of the downhill ski in the middle of the second steering phase and the shoulder axis to movement direction angle at the end of this phase. One of the reasons it was possible to identify a few significant differences only for the turns analysed, was the variability within the intermediate group: for most of the variables analysed, the standard deviation was much higher for the intermediate than for the experienced group.     

Quickest descent line during alpine ski racing

Time differences between medalists at Olympic or World Cup alpine ski races are often less than 0.01 s. One factor that could affect these small differences is the line taken between the numerous gates passed through while speeding down the ski slope. The determination of the ‘quickest line’ is therefore critical to winning races. In this study the quickest lines are calculated by direct optimal control theory which converts an optimal control problem into a parameter optimization problem that is solved using a nonlinear programming method. Specifically, the problem is described in terms of an objective function in which state and control variables are implicitly involved. The objective function is the time between the starting point and finishing gate, while state variables are positions of the ski-skier systems on a ski slope, rotational angles of skis, velocities, and rotational velocity at a discrete time, i.e., a node. The control variable at each node is the skier-controlled edging angle between the ski sole and snow surface. Equations of motion of the ski-skier system on a ski slope are numerically satisfied at the midpoint between neighbouring nodes, and the original problem is converted into a nonlinear programming problem with equality and inequality constraints. The problem is solved by the sequential quadratic programming method in which numerical calculations are carried out using the MATLAB Optimization Toolbox. Numerical calculations are presented to determine the quickest lines of an uphill and a downhill ski turn with a starting point, first gate, and second gate (finish line) having been successfully carried out. The quickest line through four gates could not be calculated due to numerical difficulty. Instead, the descent line was respectively calculated for an uphill and downhill turn and simply added, giving a resultant time that represents an upper bound.     

高山滑雪初级教学应注意的问题

高校开展高山滑雪教学对培养大学生滑雪的兴趣、爱好,以及锻炼行的养成,达到娱乐与健身的目的具有重要作用。由于高校缺乏统一的高山滑雪教材,对开设的滑雪初级教学中存在的一些问题未能寞现有效的共性认识,严重制约了滑雪运动在高校的普及与发展。提出在高山滑雪初级教学中应注意使大学生了解高山滑雪板的结构与性能,学会选用不同类型的高山滑雪板;选择合适的高山滑雪鞋与固定器,固定器的强度要求指标控制在4～5kg为好;高山滑雪服与滑雪杖的选用时注重性价比等问题,探索提高冬季高山滑雪教学质量之策。     

High-resolution friction measurements of cross-country ski bases on snow

Participants in the sport of snow skiing devote considerable effort to reduce sliding friction of the ski. A large industry is devoted to providing products and ski preparation methods with claims of improved ski performance, sometimes at considerable expense. Despite this attention to the topic, there are very little data available to skiers that quantify the effect these products and methods have on reducing ski friction. Determining the coefficient of friction of a ski on snow presents a significant technical challenge. Our approach has been to develop a tribometer incorporating a series of proximity sensors to test commercially available ski equipment and waxes on natural snow. We developed a test protocol that significantly reduces the experimental noise associated with variable environmental conditions enabling us to differentiate the difference in friction between two pairs of skis with a resolution of 0.001. A large body of test data was acquired over a wide range of environmental conditions to quantify the effect of ski wax and base texturing treatments in terms of coefficient of friction in a way that is of practical use to skiers. An exercise physiology power model was used to estimate the ski race time difference that could be expected from changing the coefficient of friction.     

Biomechanical analysis of different starting strategies utilized during cross-country skiing starts

The present study was designed to analyse and compare the kinetics and kinematics associated with three different starting strategies during classic cross-country ski racing. Inside a ski tunnel, 12 elite male skiers performed three sets of three 38?m starts. Each set included one start using: double poling only (DP), diagonal stride only (DIA) and freely chosen (FREE) (i.e. where subjects used the strategy or combination of strategies they felt was fastest) in random order. The first 18?m was performed on a series of force plates that measured horizontal and vertical forces followed by 20?m of a standard snow track. Additionally, cycle characteristics and joint angles were measured. DIA and FREE were faster over 38?m than DP (P?.01). Net horizontal impulse (taking into account both positive and negative impulses) 5–10?m after the start was lower during DP than during DIA and FREE (both P?<?.05). All subjects skied faster when using only DIA for the entire 38?m. Furthermore, the sum duration and frequency of propulsive contacts over the first 18?m was less in DP than DIA and FREE (P?相似文献   

Finite element simulation of the ski–snow interaction of an alpine ski in a carved turn

Skiing manufacturers depend on the development of new skis on trial and error cycles and extensive product testing. Simulation tools, such as the finite element method, might be able to reduce the number of required testing cycles. However, computer programs simulating a ski in the situation of a turn so far lack realistic ski–snow interaction models. The aim of this study was to (a) implement a finite element simulation of a ski in a carved turn with an experimentally validated ski–snow interaction model, and (b) comparison of the simulation results with instantaneous turn radii determined for an actual carved turn. A quasi-static approach was chosen in which the ski–snow interaction was implemented as a boundary condition on the running surface of the ski. A stepwise linear function was used to characterise the snow pressure resisting the penetration of the ski. In a carved turn the rear section of the ski interacts with the groove that forms in the snow. Two effects were incorporated in the simulation to model this situation: (a) the plasticity of the snow deformation, (b) the influence of the ski’s side-cut on the formation and shape of this groove. The simulation results agreed well with experiments characterising snow penetration. Implementation of the groove in the ski–snow interaction model allowed calculation of the instantaneous turn radii measured in actual turns, but also caused significant numerical instability. The simulation contributes to the understanding of the mechanical aspects of the ski–snow interaction in carved turns and can be used to evaluate new ski designs.     

A ski robot system for qualitative modelling of the carved turn

A robot that simulates a number of human leg joint motions during carved turns has been developed. Each leg had six degrees of freedom like those of human athletes. An on-board computer controlled the sequence of joint angles in an open-loop mode during skiing on an artificial grass slope. The relations among joint motions, reacting forces and turn trajectory were investigated by programming various motions of the robot. At first, the effect of basic joint motions, such as abduction–adduction and flexion–extension of the hip, knee and ankle joints were investigated. Then the sequence of a top athlete’s joint motions, measured in a separate study, was applied to investigate its effect on the ski turn. The human-inspired programme produced a more even force balance between the skis and also a higher-quality turn. The requirements for a successful physical model of a human skier are discussed.     

Influence of ski materials on muscle activity

The purpose of this study was to examine whether differences in construction between the compact ski, the racing ski and the soft ski influence the behavioural and electromuscular responses of the user. Eight qualified male ski instructors performed two 'shuss' trials and three different basic turns. Six muscles (M. biceps femoris, M. gastrocnemius lateralis and medialis, M. rectus femoris, M. vastus lateralis and M. tibialis anterior) were studied, using a conventional but portable electromyographic (EMG) registration with telemetric synchronization, active electrodes and a six-channel portable data recorder. Muscle contractions were continuously registered and visualized in raw EMG form and linear envelopes. The differences between the mean rectified EMG data of dynamic contractions while skiing and the mean rectified EMG data of the maximal voluntary contraction were used in the primary analysis of data, from which the participation levels of the muscles investigated could be calculated for each type of ski. Based on this comparison, differences between the effects on muscle activity of the three types of skis were unimportant. In a second phase, the normalized linear envelopes of all subjects were graphically superimposed and averaged. This was performed for each muscle, for each movement, for each leg and for each ski tested. The EMG data were considered in combination with anthropometric values, with snow characteristics and with the velocity of skiing. This study showed systematic differences between the use of the racing, soft and compact ski. On average the soft ski showed the lowest muscle activity patterns and thus the most economical muscular efforts for all muscles investigated and within all movements.     

Influence of ski materials on muscle activity

The purpose of this study was to examine whether differences in construction between the compact ski, the racing ski and the soft ski influence the behavioural and electromuscular responses of the user. Eight qualified male ski instructors performed two ‘shuss’ trials and three different basic turns. Six muscles (M. biceps femoris, M. gastrocnemius lateralis and medialis, M. rectus femoris, M. vastus lateralis and M. tibialis anterior) were studied, using a conventional but portable electromyographic (EMG) registration with telemetric synchronization, active electrodes and a six‐channel portable data recorder. Muscle contractions were continuously registered and visualized in raw EMG form and linear envelopes. The differences between the mean rectified EMG data of dynamic contractions while skiing and the mean rectified EMG data of the maximal voluntary contraction were used in the primary analysis of data, from which the participation levels of the muscles investigated could be calculated for each type of ski. Based on this comparison, differences between the effects on muscle activity of the three types of skis were unimportant. In a second phase, the normalized linear envelopes of all subjects were graphically superimposed and averaged. This was performed for each muscle, for each movement, for each leg and for each ski tested. The EMG data were considered in combination with anthropometric values, with snow characteristics and with the velocity of skiing. This study showed systematic differences between the use of the racing, soft and compact ski. On average the soft ski showed the lowest muscle activity patterns and thus the most economical muscular efforts for all muscles investigated and within all movements.     

Influence of mass on the speed of wheelchair racing

Reducing the mass of a racing wheelchair improves the winning time even for relatively small mass reductions over short distances (100 m). In this study, the improvement of speed and winning time on mass reduction was modelled mathematically, the speed profiles of 100 m wheelchair races as well as the rolling resistance and drag area of the athlete–wheelchair system was determined experimentally, and the improvement of the winning time through mass reduction was calculated from the mathematical model and experimental data. For winning times of 30 s, the relative improvement of the winning time is at least equal to the relative reduction of mass. For winning times of 15 s, the relative improvement of the winning time is at least half the relative reduction of mass. A reduction by 1 kg results in an overall improvement of the winning time of about 1–2.3% for winning times of 15–30 s (100-m race). In absolute terms, a mass saving of 1 and 5 kg on a 60-kg system would reduce a 15-s winning time for the 100 m by 0.132 and 0.66 s, respectively, and a 30-s winning time by 0.63 and 2.86 s, respectively. The mass of a wheelchair is the most important parameter for improving the winning time. The influence of the mass on the winning time is 4 or 1.5 times greater than the influence of the rolling friction coefficient, and 4 or 5.5 times greater than the influence of the drag area in a 15 or 30-s race, respectively. These principles should be considered when designing a racing wheelchair.     

环境风对跳台滑雪空中飞行气动特性的影响

目的:探讨环境风对跳台滑雪空中飞行气动特性的影响。方法:通过计算流体力学(computational fluid dynamics,CFD)方法数值模拟预测了不同环境风下跳台滑雪空中飞行空气动力学特性,并探究了水平方向环境风、竖直方向环境风以及侧向环境风对气动特性的影响。将跳台滑雪运动员与滑雪板看成一个多体系统,建立在空中飞行某一种普遍姿态下此多体系统的精细化三维几何模型与网格模型,采用部分时均(partially averaged Navier-Stokes,PANS)湍流模型进行数值模拟,提取多体系统的受力及力矩情况,直观地显示多体系统周围的流场信息。数值预测涉及的水平方向风风速包括-4 m/s、-2.5 m/s、-1 m/s、0 m/s、1 m/s、2.5 m/s、4 m/s等工况;竖直方向风风速包括-8 m/s、-4 m/s、-2.5 m/s、-1 m/s、0 m/s、1 m/s、2.5 m/s、4 m/s、8 m/s等工况;侧向风风速包括1.5 m/s、3.0 m/s、4.5 m/s、7.5 m/s、10.5 m/s、13.5 m/s等工况。结果:1)水平方向环境风下多体系统...