Kinematics and Kinetics of Ski Techniques (continued):

Ski Skating Techniques

Ski reaction forces in the skating techniques are oriented approximately perpendicular to the ski surface. Because skating skis are prepared with glide wax and are without the grip waxes required for classical skiing, there is no means of using static friction to generate propulsive force. In a manner similar to speed skating, the ski is set down at an angle to the forward direction and while gliding it is placed on edge. The edged platform of the ski resists forces perpendicular to it as these simply compress the snow under the ski. Forces in other directions cannot be generated as the frictional forces are insubstantial. Figure 20 illustrates the resultant ski reaction force perpendicular to the ski surface; components can be determined if the ski positioning with respect to the snow surface (edging angle) and with respect to the forward direction are know. With resultant ski force (F_ski) normal to the ski surface, each of these angles affects the propulsive component as the sine of the angle. Thus propulsive force component from a skating ski can be calculated from:

where a is the edging angle of ski surface with respect to snow surface (0° being flat) and b is the orientation angle (0° being straight ahead). From this equation it is obvious that propulsive force increases (for a given resultant ski force) as either the orientation angle or the edging angle increases.

A common observation in skating is the relationship of ski orientation angle to skiing speed. On flat and fast terrain, the skis are angled away from the forward direction a relatively small angle while under slower conditions and on uphill terrain the ski angles increase substantially. For example, on flat terrain used during the 1992 Olympic races, ski angles were about 6 to 8 degrees (men, Smith & Heagy, 1994) and 10 to 12 degrees (women, Gregory et al., 1994). In contrast, skiers on uphill terrain of the Calgary Olympic races skated with much greater angle of skis to the forward direction (about 28-30°) (Smith et al., 1989). Mechanically, this response would be expected based on the relationship of ski angle to propulsive force component expressed in the equation above. On the flat, only air and snow drag forces resist a skier's motion requiring relatively modest propulsive forces to maintain skiing speed. On uphill terrain, gravity is an additional force against which a skier is working. This requires greater propulsive forces to maintain uphill skiing speed. These greater propulsive forces can be generated either by increasing the resultant ski reaction forces, by increasing the ski angle with respect to forward direction ( b in the equation above) or by increasing the ski edging angle on the snow surface ( a). While no force comparison of flat to uphill skiing is available, on grades of 9 and 14% ski forces have been measured (Smith, 1989). For these moderate and steep uphills, average forces were similar while ski orientation angles changed with grade. Based on this evidence, it is likely that skiers maintain similar skating force magnitudes on different terrain but generate greater propulsive force mainly through adjustment of ski orientation and edging angles.

Ski orientation angle interacts with other kinematic characteristics of a skating stroke. As ski angles increase away from the forward direction, a skier's lateral displacement during the stroke increases and displacement in the forward direction may decrease. The changing orientation angle of a ski from flat to uphill terrain also results in a modification to the effective slope up which the ski is gliding. By angling the ski laterally, a skier can increase the glide distance during a skating stroke and the glide time before the ski speed decreases substantially. Thus increased orientation angle of the ski can accomplish two things--the propulsive force component can be increased and uphill ski glide can be enhanced. These come at the expense of increased lateral motion which may be constrained by topology of the surroundings. As displacement in the forward direction during a cycle decreases with increased ski angle, a skier must increase the skating stroke rate to maintain speed but this would come at the expense of glide on each ski. At some point, stroke rate limitations combined with race course width limits restrict a skier's ability to use greater ski angles to increase propulsive force without exceeding physiological optima. These and other factors influence a skier's choice of skating technique. The main skating variations used in racing involve differences in poling timing and ski placement. These are discussed (below) within each of the primary skating techniques currently used.

[Note: the naming of skating techniques is not well standardized in English. We will use the terminology V1 and Open Field skate to refer to skating patterns where one double poling action is used per full cycle involving two skating strokes. V1 and Open Field differ in timing of poling with respect to the skating stroke. In contrast, V2 is a pattern with two poling actions per cycle; one double poling action with each skating stroke.]

V1 Skate Technique.

Just as diagonal stride is used under conditions where there are relatively large resistive forces (uphill, headwind, or very slow snow), V1 skating technique is used similarly. Rather like a low gear, V1 skating involves slightly higher frequency than does V2 or Open Field when used at similar speeds. As can be seen in Figure 21, an assymmetical application of poling is involved with both poles operating nearly together along with one side's skating stroke but not the other skate. Pole positioning is also different on each side. These side to side differences have been named in a variety of ways by coaches. Perhaps most common phrasing is to call the side for which skating and poling are together the "strong" side while the side where skating occurs without poling is the "weak" side. In this phrasing, one would refer to the strong side pole or the weak side ski, for example. These terms are not to imply that the poling or skating forces on the weak side are necessarily less than the strong side but simply designate a synchronization of propulsion from poling and skating. In Figure 21, the skier is poling while skating on the left side, thus the left side is the "strong" side and right side is the "weak" side. In the figure, typical pole positioning can be observed. The strong side pole is planted in a nearly vertical position and poling continues with the pole oriented close to the forward direction. The weak side pole is inclined further from vertical at pole plant and is oriented in the direction of the strong side ski.

As a technique well suited for uphill skating, the ski positions in the V1 can be adjusted from rather narrow to quite a wide "V" between skis as required to suit hill steepness. Wide "V" placement positions the skis somewhat across an uphill and allows them to glide further than if placed straight uphill. Each ski must be edged during its skating stroke to generate propulsive force components and this is accomplished as a skier smoothly pushes from one ski onto the other with a complete weight shift. The assymmetrical pole positionings are partly a result of the typically wide "V" ski positions on uphills. The weak side pole must be outside of the weak side ski and not interfere with its glide. As the skier is stepping forward from that ski, this necessitates a more lateral positioning of the pole plant. In addition, the orientation of the weak side pole toward the strong side ski (rather than forward) serves to enhance glide in that direction and to facilitate weight transfer from the weak side to the strong side ski. Timing of the V1 skate nearly synchronizes poling with stepping onto the strong side ski. Poling ends as the skier skates from the strong side and steps onto the weak side ski. These temporal relationships are illustrated in the phase plot of Figure 22. As with many ski techniques, phase proportions of the V1 skate are adjusted as terrain and snow conditions require.

Skating forces applied to the skis are primarily perpendicular to the ski surface. In V1 skating, these resultant forces are modest when compared to running or jumping. Resultant skating forces reach peak values of about 1.5 to 2 times body weight. Force patterns on the weak side ski are similar to the double peaked ground reaction forces of walking and similar to those observed in other skating strokes. On the strong side, skiers may often step onto the ski more smoothly because of concurrent poling. For many skiers, the smoothly increasing force to a peak is observed. This pattern is probably advantageous in reducing ski drag force and allows the ski to more smoothly plane over snow, enhancing glide.

Poling forces in V1 skating are about half of a skier's body weight, at maximum. When the resultant forces from skis and poles are resolved into their component directions, the relative effectiveness of these propulsive sources can be analyzed. Skating ski propulsive force depends on the ski's orientation with respect to forward and on its edging angle. Due to the multiplication of the sine function of each of these angles (Figure 20), propulsive force is a relatively small fraction of the resultant skating force. In contrast, propulsive poling force depends mainly on the angle of pole inclination. While not as effectively angled as is seen in the classic double pole technique, V1 poling does develop substantial propulsive force. When analyzed over a full V1 skating cycle, average propulsive force from each pole is about 10% of body weight while each ski develops about 6% of body weight (Smith, 1989). These proportions of propulsive force suggest that V1 skating on uphill terrain is driven in large part by poling forces from arm and trunk musculature. On flat terrain, it is likely that somewhat less reliance on poling force may typically be used in skating. Nevertheless, it is clear that high level skiers require the ability to generate substantial poling force.

While V1 skating involves using both poles almost synchronously, the movement patterns during poling are somewhat different than observed with classic double poling. Trunk flexion during V1 poling is less than 25 degrees or about half that observed in double poling. Body center of mass vertical motion, about 15 cm, is similarly much less than in double poling. Unlike other skating techniques, the trunk flexion and center of mass drop is in part done prior to poling. This timing results in the lowest center of mass position near the end of poling and near the end of strong side skate. The upward reaction forces associated with this downward then upward motion of the trunk serve to enhance poling and skating forces when poles and skis are most effectively oriented for propulsive force. In contrast, the V2 and Open Field skating techniques described below use other timing strategies to enhance propulsive force.

Open Field Skate Technique.

At first glance, the Open Field skate may be mistaken with the V1 technique described above. It involves poling along with one side's skating stroke but not the other side, as does V1. But in contrast, Open Field skating is used primarily under fast conditions and is rarely used on slopes more than modestly uphill. The difference is in glide under these conditions.

Figure 24 shows the movement pattern involved in Open Field skating. Double poling in the illustration is occurring as the skier is skating from the left ski. This association would make left the skier's strong side in this case. Poling and the strong side skating stroke end as the skier steps onto the other ski (weak side). The combined poling and skating impulse briefly accelerates the skier onto the weak side ski. Under fast conditions considerable glide is obtained on this ski before skating back onto the strong side ski. Subsequent poling occurs after additional gliding on the strong side ski. Timing of these poling and skating phases is subtly different than in V1. Under fast conditions an Open Field skating cycle may be 1.5 seconds or more in duration. In contrast, V1 is usually shorter, 1.2 to 1.3 seconds. The additional time is spent gliding on either ski. In particular, considerable glide is obtained on the strong side ski before pole plant, poling phase and a propulsive skating stroke. In V1, pole plant and strong side skating occur almost synchronously.

With more time, the double poling motion of Open Field skating can involve a larger range of trunk flexion and greater vertical center of mass motion. This increases pole inclination to a more effective positioning for generating propulsive force. On flatter terrain typical of the Open Field technique, ski placements are much closer to the forward direction (less than 10 degrees). This allows both poles to be oriented nearly forward--also increasing the effectiveness of Open Field poling over that which is possible with the V1 skate. Unfortunately, systematic measurements of pole and ski forces in the Open Field (and V2) techniques have not been carried out. Thus the proportions of propulsive force due to poling and skating are unknown.

V2 Skate Technique.

Unlike V1 or Open Field skating, the V2 technique involves symmetric poling-skating movement patterns on each side. Used primarily under fast conditions on flats or to maintain momentum over rolling terrain, V2 has become widely used in recent years of racing. Timing of the double poling action is similar to that of the open field skate with respect to the strong side skate stroke. In V2, poling occurs near the latter part of each skate stroke as the skier is about to step from the skating ski to gliding on the other. Figure 26 illustrates this pattern for half of a V2 cycle. Second half of the cycle repeats with a similar poling pattern while skating on the other side. A visual comparision of the poling phases of V2 and Open Field skating (Figures 26 and 24) confirms the similarity of the two techniques.

While poling-skating patterns are similar in the two techniques, timing is dramatically different. Overall cycle times for V2 range from 1.5 to 2 seconds. Thus each of the poling actions must be completed in less than 1 second--much less time than is available with the other skating techniques. This requires considerable acceleration of the arms with each poling motion. As V2 frequency increases (when sprinting, for example), range of motion at the shoulder and elbow in late poling may be reduced to decrease poling time and quicken recovery to the next poling action. An additional consequence of the brief time available in poling is the modest trunk flexion that occurs. There simply isn't time to move the trunk down and recover in position and in time for the next pole plant.

Forces in V2 skating follow a pattern shown in Figure 28. The two peaks in each skating stroke correspond with initial ski set down and with a vigorous propulsive thrust at the end of the stroke which helps transfer a skier's weight to the other ski. As ski angles in V2 skating are generally quite small (skis aligned close to the forward direction), the propulsive forces that can be generated during each skate stroke cannot be very large. However, when used on flat terrain and with fast snow, rather modest propulsive force is sufficient to maintain speed. Unfortunately, no systematic study of V2 skating forces has been undertaken currently and we know relatively little about how propulsion is distributed between poling and skating forces.

Other Skating Techniques.

While V1, V2 and Open Field techniques are the primary choices of skating skiers, several alternatives are used under certain conditions. Variations of ski technique are in some ways analogous to gearing of a bicycle or car; low gears are used up steep hills, high gears on downhills. V1 skating is generally of higher frequency (like a lower gear) than either Open Field or V2 techniques, but occasionally very steep slopes are encountered which require an even lower gear. Diagonal skate, sometimes called flying herringbone, combines contralateral arm and leg actions. Single poling is synchronized with a skating stroke of the opposite side and the direction of poling is oriented nearly parallel with the skating ski. This enhances glide during each skating stroke and allows progression up very steep slopes.

"High gear" skating must sometimes exceed speeds at which V2 is effective. As speed increases the time available for poling decreases because the stationary planted pole basket passes out of reach behind a skier in a time period too short for generating force. But free skating without poling remains possible under high speed conditions. Skating is an unusual mode of locomotion which does not require the ski (or skate) to be stationary. Despite gliding forward, when placed on edged the ski can be used as a platform from which to skate. Provided a ski is oriented at an angle to the forward direction, skating from the edged ski will create propulsive force even at high speed. Free skating in this manner is used under very fast conditions and generally involves ski placements at somewhat wider angles than would be seen for V2 skating in the same terrain.

Finally, marathon skate technique is sometimes used when a classic track is available along with a skating lane on flat terrain. With one ski gliding straight ahead in set tracks, the other ski is placed at an angle to it out of the tracks and with a skating stroke used to propel the skier. Double poling is combined with the skating stroke into a propulsive phase followed by an extended glide on the track ski. Marathon skating can be relatively fast and is a relaxing alternative to other skating strokes. It was the first skating technique used for extended periods of skiing during the transition period of skating technique development of the early 1980's.

What makes skating fast? Within several years of exploration of skating, the marathon skate, then V1, V2 and Open Field skating became distinct movement patterns. As early explorers of skating won more and more races in the 1980's, it was clear that skating techniques are usually faster than classic techniques. Comparisons of race times over comparable distances, terrain and conditions suggest that skating can be as much as 20 to 25% faster than classic skiing. What factors contribute to this advantage?

Ski motion during skating differs from ski motion during diagonal stride or kick double pole in two respects: skating skis glide throughout a stroke while kicking a classic ski requires it to be momentarily stopped. Second, skating requires a ski to be placed on edge forcing the ski to bite into the snow while classic skis run flat on the surface. These differences provide both advantages and disadvantages for skiing speed.

By performance, it is clear that continuous gliding during skating provides greater advantage than is lost due to increased drag force due to ski edging. Propulsive force in skating is generated by reaction forces perpendicular to a ski's surface. Being perpendicular to the ski surface, reaction forces during skating have no component in the direction of the ski and do not slow it down. This allows displacement of the ski during propulsion unlike the stationary requirements of kicking a classic ski. Coaching suggestions to skaters often include the comment "push through the heel" to minimize any tendency to step forward off of the ski rather than laterally. Such inappropriate stepping forward introduces a reaction force which slows a ski's glide and interferes with skating performance. Optimal skating technique keeps reaction forces perpendicular to the ski which allows glide to continue during a skating stroke.

Ski edging is has long been thought to affect glide characteristics of both alpine and nordic skis. Ski edging angle is a measure of a ski's flatness to the snow surface. It is thought to influence glide by affecting ski penetration into surface snow layers which may increase snow drag force while providing a firm platform from which skating forces are generated. Conventional wisdom from ski coaches suggests that a "flat ski" will glide faster than an edged ski. In skating, glide directly affects cycle length. As faster skiers tend to ski with greater cycle lengths it is a common connection to relate ski edging to glide and to performance. While it is reasonable to expect snow drag forces to be greater on an edged ski than on one that is flat (due to deeper penetration and increased plowing), this has not been demonstrated and the magnitude of the increased drag is unknown. The typical description of fast skating techniques like the V2 and the Open Field includes a long glide phase on each ski prior to pushing off with a vigorous knee extension. The implication of some coaching suggestions is that a relatively static flat ski position be maintained during the early parts of each skating stroke where the ski is mainly gliding.

However, this static flat ski emphasis is not typical of elite skiers. Figure 29 illustrates mean ski edging angles during fast skating on flat terrain during the men's 50 km race of the 1992 Olympics. None of the 17 elite skiers analyzed in that study exhibited a ski edging phase where a flat ski was statically maintained. Most skiers set the ski down on the snow initially with it being flat to the surface and all moved away from the initial positioning immediately. Static posturing to enhance ski glide has not been observed for elite skiers and it is likely to be a disadvantageous skating technique. This observation must not be misunderstood to mean that ski edging and a flat ski are unimportant characteristics for ski glide. Note in Figure 29 that despite smoothly increasing edging angles on the strong side skate over the last 30% of the cycle, the ski is only 10 degrees from flat. This modest amount of edging may have little effect on ski glide while allowing a skier to dynamically stroke from side to side. It is only later in each skating stroke when ski reaction forces are largest that the skis are substantially edged. Some skiers are observed to set the ski down on the lateral edge (negative ski angle), rotate through flat and onto the medial ski edge during the glide phases on each side. This technique may be advantageous on flat, fast terrain as it may prolong the time where the ski is within a few degrees of flat while promoting a continuous dynamic movement toward the next skating stroke.

While ski glide is particularly essential in skating, it is also a crucial component of fast classical skiing. Following sections will address equipment and technique characteristics which affect glide and overall skiing performance.

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Figure Captions for this section:

Figure 20. Ski reaction force in skating is aimed approximately perpendicular to the ski surface. To determine the components of the resultant force, ski edging angle (a) and ski orientation angle (b) must be measured. Using the resultant force and sine functions of these angles, propulsive force can be calculated.

Figure 21. V1 skating technique involves a double poling motion synchronized with a skating stroke on one side (referred to as the strong side) while the arms are in recovery swinging forward during the skating stroke on the other side (weak side). The skier in the figure is poling during the first half of the cycle shown (positions 1 to 4) while skating on the left (strong) side. Last half of the cycle involves skating on the right (weak) side.

Figure 22. V1 skate phase diagram. The bars represent points in a full cycle when poles or skis are in contact with the snow. The pattern shows poling along with left side skating in the first half of the cycle. Skating on the right ski continues through the last half and slight overlaps with the next cycle.

Figure 23. V1 skate reaction forces. The graph begins with time = 0 at initial pole plant of the weak side pole. Strong side skating begins approximately synchronized with the strong side pole plant. Reaction forces in skating are relatively smooth and reach magnitudes of about two body weights.

Figure 24. Open Field skating involves a double poling action synchronized with the end of a skating stroke on one side (strong side) while the arms swing forward in recovery during the second skating stroke (weak side). Considerable glide occurs with both skating strokes which increases overall cycle length and decreases cycle frequency for the Open Field technique.

Figure 25. Open Field skate phase diagram. The bars represent points in a full cycle when poles or skis are in contact with the snow. The pattern shows poling along with left side skating in the first quarter of the cycle. Skating on the right ski continues through the middle half. Skating on the strong side (left) begins well before pole plant. Note this timing difference compared with V1 skating. Considerable glide occurs on the strong side ski prior to poling.

Figure 26. V2 skating involves a double poling action with each skating stroke. This requires rapid return of the arms to a forward position ready for the next pole plant. Overall frequency of V2 is low and stride length relatively large compared to V1 and Open Field skating. This diagram shows only the first half of a V2 cycle.

Figure 27. V2 skate phase diagram. The bars represent points in a full cycle when poles or skis are in contact with the snow. The pattern shows poling along with left side skating in the first quarter of the cycle. Poling with right side skating occur in the last half of the cycle. Note the extended glide on each ski prior to poling. (Data for V2 phases of this diagram from Bilodeau et al., 199X).

Figure 28. V2 skate reaction forces. The graph begins with time = 0 at initial pole plant. The second poling action begins at the mid-point of the cycle.

Figure 29. Ski edging angle during Open Field skating. Heavy solid lines are mean edging angles during each skating stroke when the ski was in contact with the snow. Curves above and below the mean values are ± 1 standard deviation based on the 17 skiers of this study from the men's 50 km race at the 1992 winter Olympic Games, Albertville, France. Note the skiers set down the ski in a nearly flat position (0 degrees) and then smoothly increased edging angle until the end of the skate stroke. Skiers did not maintain a flat ski to enhance glide but smoothly changed edging angle. Reproduced with permission (Smith et al., 1994).

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