Review Article

Sports Medicine 9 (5): 273-285, 1990 0112-1642/90/0005-0273/$06.50/0 © ADIS Press Limited All rights reserved. SPORT2271

Biomechanics of Crosscountry Skiing Gerald A. Smith Biomechanics Laboratory, Pennsylvania State University, University Park, Pennsylvania, USA

Contents

Summary ................. .................................................................... ............................................... 273 I. Biomechanical Analysis in Crosscountry Skiing ................ ................... ............................. 273 2. Classic Technique Kinematics .................................................................................... ......... 276 3. Classic Technique Kinetics ...................................................................................................277 4. Skating Technique Kinematics .............................................................................................279 5. Skating Technique Kinetics ..................................................................................................281 6. Conclusions ... .......................................................................................................................... 283

Summary

The mechanics of crosscountry skiing involve a complex interaction between the kinematic characteristics of the movement patterns and the kinetic relationships driving the motion. Crosscountry skiing techniques have been the subject of some biomechanical research, primarily involving the traditional diagonal stride technique. In the 1980s, skiers have developed several new approaches to moving across snow. These skating techniques rely on the generation of force from ski placements at an angle to the forward direction creating propulsive force while the ski slides forward. Biomechanical analysis of the crosscountry techniques has developed from rather simple 2-dimensional kinematic descriptions of diagonal stride to complex measurement of skating forces and 3-dimensional motion. The combined determination of forces and resultant motion has perhaps the most promise for practical application in assessments of equipment and technique. The relationship of the kinematic characteristics of skiing to the forces involved have received preliminary study but substantial gains in understanding will be necessary before approaches to optimising a skier's technique are plausible.

1. Biomechanical Analysis in Crosscountry Skiing The evolution of crosscountry skiing in the twentieth century has involved a series of dramatic changes in both equipment and technique. Traditional skiing, before the advent of ski lifts, was a mixture of what we currently describe as 'alpine'

and 'nordic' skiing. The same equipment was used both for downhill skiing as well as for ascending or touring crosscountry. Typically, rather cumbersome and heavy wooden skis, leather boots and bamboo or wooden poles were the equipment used for all terrain. The alpine and nordic branches diverged with the development of lift-serviced skiing. As alpine skiers were no longer constrained by the

274

requirements of climbing uphill, alpine ski equipment evolved into more specialised forms which emphasised greater speed and better control. Manufacturers experimented with various synthetic materials in both alpine skis and boots. However, until the early I 970s, nordic or crosscountry ski equipment had changed very little; wooden skis and leather boots were produced similar to earlier patterns. Then, several traditionally alpine ski manufacturers (initially, Kneissl and Fischer) began applying their understanding of synthetic materials to the development of crosscountry skis (Hall 1981). The plastic and fibreglass skis which resulted have been consistently faster and more durable than the traditional wooden skis. The synthetic materials have also allowed for the development of skis with mechanical characteristics such as torsional rigidity, lateral stiffness and variable longitudinal stiffness which may be adjusted to fit the requirements of different conditions and techniques. The characteristics of the new skis have stimulated experimentation with various waxing procedures and skiing techniques. The classical movement pattern on skis (fig. I) involves contralateral limbs moving synchronously: a similar pattern to human walking or running. This 'diagonal' stride movement (as it is called in skiing) depends on a stationary ski providing a platform from which a stride is made. The ski's characteristics have traditionally been controlled by the frictional properties of a thin layer of wax applied on the base of the ski. The wax 'gripped' the surface when pressed into contact with the snow, but also slid with reduced friction during the 'glide' phase of the diagonal stride. The synthetic skis developed in the later 1970s used to advantage the low frictional characteristics of polyethylene surfaces combined with relatively stiffer midsections. Such skis would

Sports Medicine 9 (5) 1990

glide primarily on the tip and tail sections with minimal pressure under the midsection. By judicious placement of the wax used for grip in the midsection of the ski, skiers were able to 'kick' the centre section of the ski into contact with the snow for traction while sliding on the low friction tip and tail section of the ski during the glide phase of the ski stride. The synthetic materials used in racing skis (primarily polyethylene base material) have allowed increased skiing velocities with longer lengths of glide (Hall 1981), stimulating racers to adopt techniques more appropriate to higher speeds. The traditional diagonal stride technique has upper speed limits due to the required ski stopping in each stride (for the 'kick' wax to grip the snow). Other techniques allow propulsive thrusts while the skis are gliding. Traditionally, skiers have used the double pole technique when speeds are too fast for the diagonal stride. This relies solely on the arms and trunk for generating force through the poles. Other techniques have been developed since 1980 to generate propulsive force through alternative ski actions. These have taken the form of various 'skating' motions in which one or both of the skis are placed at an angle to the forward motion of the skier. With the ski on edge, a reaction force normal to the ski surface includes propulsive components in the forward direction. Unlike the kick in diagonal stride, these forces can be generated while the ski is gliding. Without the ski stopping during each stride, overall velocities of the skating techniques can be considerably greater than for the diagonal stride (Endestad & Teaford 1987). The skating technique evolution began with what has been called the 'marathon' skate. This technique involves I ski in a skating motion (the ski placed at an angle to the forward direction) while

Fig. 1. The diagonal stride is the primary technique used in classic crosscountry skiing. The skis are constrained to move in the forward direction by machine set tracks in the snow surface.

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Strong side skate

Weak side skate Cycle length

Fig. 2. The VI skating cycle includes a double poling motion associated with skating on one side (strong side) but no poling with the skating of the other side (weak side).

the other ski glides straight forward in the set tracks. The skating motion is accompanied by simultaneous poling thrusts with both poles. Initially, this technique was used only on relatively flat sections of the course as an alternative to the double pole and the double pole with stride techniques. Uphills were skied using traditional diagonal stride methods requiring 'kick' wax on the ski. As skiers developed proficiency and strength in skating, it became apparent that uphills could also be skated (Tabor 1985). This eliminated the need for kick wax on the skis, thus reducing the frictional drag characteristics and increasing the overall skiing speed. The American skier, Bill Koch, was the first Nordic World Cup racer to apply the marathon skate technique to the hilly World Cup races (Borowski 1986). His winning of the World Cup series in 1982 provided dramatic evidence of the increased velocity obtainable by a strong skate technique over traditional crosscountry methods. Other skating techniques were subsequently found to be advantageous on hilly terrain (Borowski 1986). During the glide phase of the marathon skate there is no propulsive force. Skating uphill, the glide speed decreases rapidly demanding a high cycle rate to maintain momentum. The alternate stride skate (or 'V I') alleviates this problem. In VI skating (fig. 2), what was the glide ski of the marathon skate is placed at an angle to the hill and used for a skating stroke but is not accompanied by poling. This 'weak side' skate provides increased glide length on the ski and less decre-

ment of the ski's glide speed because of the angle to the hill while providing an additional propulsive phase to the skating cycle. The 'strong side' skating action of the V I (where poling accompanies skating) involves some modification from the marathon skate as well (fig. 3). Other skating techniques have been developed for flat terrain and steep uphills: the every stride skate (or 'V2') and the diagonal skate. These differ in the timing of the poling actions while maintaining similar ski positioning to the VI skate. However, the VI has become the dominant skating technique currently employed in racing (Woodward 1988). The technique is somewhat adaptable for various terrain and speed conditions through modification of ski and pole angulation, timing of the poling thrusts and overall tempo (Borowski 1987). The skating techniques have proven to be substantially faster than traditional methods used in racing. Street et al. (1986) estimated skating to be as much as 23% faster than the traditional techniques. This inequality of techniques resulted in the International Ski Federation (FIS) designation of half of the World Cup competitions to be 'classic' events (with skating restricted) and half to be 'free technique' events allowing skating. This approach has been reasonably well accepted by the skiing community and will be maintained in the foreseeable future. Despite the environmental difficulties typically encountered in data collection, crosscountry skiing has been the subject of extensive biomechanical re-

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2. Classic Technique Kinematics

Strong side pole

Weak side pole

Strong side ski

Weak side ski

Fig. 3. The V I skating motion is asymmetrical. The side where poling is synchronised with the skating motion is the 'strong side' while the 'weak side' skate is not combined with poling.

search. However, the majority of that research has dealt with the classic technique of diagonal stride. The recently developed skating techniques have generated a great deal of interest in the mechanics of the skating motion, such as the apparent speed advantage skating has over classic techniques. In addition, the continuing evolution of skating has stimulated discussion and speculation concerning the kinematics observed and has demanded explanation in terms of the relevant kinetic variables. Biomechanical studies of human locomotion have contributed to the general understanding of the kinematic and kinetic characteristics typical of various movement patterns. While the kinematic description of how a human body moves in space is often of interest in comparing individuals or techniques, the kinetic evaluation of movement in terms of force and energy is a more basic approach to understanding the reasons behind an observed pattern. The review of biomechanical research in crosscountry skiing which follows has been organised in terms of kinematic and kinetic as well as classic and skating categories.

Several of the earliest biomechanical studies of crosscountry skiing originated from the Swiss Federal Institute of Technology, Zurich, Switzerland, and from the Biomechanics Laboratory at the University of Illinois at Urbana-Champaign. The studies utilised high speed films of world class skiers during races. Waser (1976) analysed the First International 15km race in Davos, Switzerland, in December 1974. Films of the men's 15km race at the 26th International Ski Competition in Le Brassus, Switzerland (January 1977), were analysed by other Swiss researchers. Soliman (1977) analysed diagonal stride technique on flat terrain, Haberli (1977) analysed diagonal stride on uphills, and Andres (1977) analysed the double pole with stride techniques on flat terrain. Each of the studies involved selected kinematic parameters which were analysed with respect to race performance. A consistent finding from the diagonal stride studies was the relationship of stride length with performance: the best skiers exhibited longer length strides on all terrain. On the other hand, Andres' (1977) analysis of double pole with stride technique suggested that stride rate was positively related to performance: faster skiers were double poling at higher tempo than slower skiers. The University of Illinois studies were based on films from the Gitchi Gammi Games (Telemark, Wisconsin, December 1977) and from the Lake Placid Pre-Olympic Games (1979). Dillman (1979) corroborated the relationship of stride length and performance observed previously. Martin (1979) analysed an uphill section of the men's 15km race in the Pre-Olympics while India (1979) analysed a flat section of the women's IOkm race. Ofthe various kinematic and temporal variables which were analysed in each case, stride length was emphasised as perhaps the most significant factor distinguishing the best of the elite skiers. A similar study by Marino et al. (1980) analysed diagonal stride characteristics of 9 competitors in the women's IOkm race of the North American Championships in 1979. Stride length was emphasised as being strongly related to velocity as well

Biomechanics of Crosscountry Skiing

as to race performance. Marino et al. (1980) also subdivided the stride into several phases. They found glide phase distances to be a particularly important component of the total stride length. Komi and colleagues' (1982) study of male World Championship racers (from Lahti, Finland, 1978) included kinematic analysis of centre of mass (CM) velocity variations throughout a diagonal stride as well as hip joint and knee joint angular responses. The 5 elite skiers exhibited quite individual patterns of velocity variation in a complete stride. Angular patterns were similar between skiers but included some individual characteristics as well. Gagnon (1980, 1981) systematically analysed the relationship of diagonal stride velocity to stride kinematics. Three controlled speeds ranging from 4.5 to 6.8 m/sec were observed on flat terrain. Stride rate and length, centre of mass motion and several angular measures were determined. Skiers were found to control velocity by increasing stride rate (decreasing stride time) while maintaining relatively constant stride lengths. This should not be confused with the findings of previous studies detailed above which derived from race observations. Faster skiers generally ski with longer stride lengths than slower skiers, but the mechanism used by fast or slow skiers in adjusting velocity is adjustment of stride rate.

3. Classic Technique Kinetics Kinetic studies of crosscountry skiing have taken 2 directions: determination of mechanical energy relationships exhibited by skiers, and measurement of forces applied to skis and poles. A series of papers dealing with mechanical energy levels of the diagonal stride have been based in part on high speed films from the men's 15km World Championship race in Lahti, Finland, 1978. Two sites were observed: nearly flat terrain (1.6 degrees) and a moderate uphill (9 degrees). Norman et al. (1985) compared the body segment energy utilisation exhibited by the elite skiers of the World Championship race on flat terrain with the patterns exhibited by 'recreational' cross country skiers on flat terrain. Norman and Komi (1987) studied II elite

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skiers on both of the filming sites in the World Championship race while Norman and Ounpuu (1987) compared a selected set of 5 of the best with 5 of the slowest skiers on the flat terrain at Lahti and also 5 best with 5 slowest skiers from an uphill section (10 degrees) of a recent World Cup race (men's 15km, Labrador City, 1985). In each of these studies relatively similar methods were employed. The digitised records from the high speed films were analysed in terms of body segmental energy levels using methods detailed by Winter (1979a) and Pierrynowski et al. (1980, 1981). The comparison of recreational with elite skiers (Norman et al. 1985) found that larger within segment potential and kinetic energy exchanges during swing and pushing phases of the diagonal stride occurred with the elite skiers. Further, these best skiers demonstrated consistently longer glide and stride length, probably due to higher leg swing and greater use of gravitation force as a supplement to muscle force in the leg swing. Norman and Komi's (1987) comparison of mechanical energetics on 2 slopes involved calculation of mechanical work rate (work output divided by period of time), mechanical energy transfers and mechanical task cost (MTC, work to move Ikg 1m). The MTC was described as the 'best overall relative indicator of mechanical effort, and thus technique efficiency of individual skiers .. .' Based on the MTC, terrain was found to significantly affect a skier's energy cost. The uphill slope involved 2.2 times the cost of the flatter terrain (1.8 versus 4.0 J/kg/m). Norman and Ounpuu (1987) sought to differentiate the faster from the slower skiers on flat and uphill terrain on the basis of selected kinematic and kinetic variables. However, no differences were found between the groups of skiers on the flat terrain where similar kinematic patterns and segmental energy relationships were observed across the range of performances. On the uphill terrain, characteristic kinematic and kinetic differences were found between groups. Hip range of motion as well as several arm angular position and velocity variables differentiated the faster and slower groups. Kinetically, larger energy transfers (both within and

278

between segments} and lower MTC were observed for the faster skiers. Norman et al. (1989) used similar MTC methods combined with estimates of oxygen uptake in an analysis of selected male skiers in the Calgary Olympic 30km race on an 11.8 degree uphill. The assumption of complete energy exchanges between body segments combined with muscle efficiency estimates for positive and negative work of 25 and 120%, respectively, resulted in conservative estimates of oxygen cost. They found mechanical power outputs and estimated oxygen uptakes to be substantially greater for the faster skiers than for the slower skiers of the sample. The methodology developed for estimating metabolic cost from biomechanical measures holds considerable promise as it provides a noninvasive technique for metabolic measurement. However, validation of the technique for crosscountry skiing (or other movement patterns) has not been completed to date. The measurement of forces applied to skis and to poles is an important element in understanding ski locomotion patterns. Two instrumentation approaches have been reported in the literature. Komi (1987) described a long force plate array set under the snow surface and an alternative approach using a small force plate mounted on a ski. He described several advantages and disadvantages of each approach. Adapting standard laboratory force plates to a winter outdoor environment requires careful mounting of adjacent plates to physically separate both plates and surrounding snow and ice. Calibration of the force plates after mounting can be accomplished in the normal direction to the plate surface but is not readily accomplished in lateral directions. However, once a set of force plates is in place beneath a ski track, it is relatively simple to measure repeated trials of many skiers. Such measurements are limited to the length of the force plate array used (Komi reported a 6m length). Using an array offorce plates is generally necessary to obtain independent force measures from each ski and pole. Komi (1987) also described a small, portable force plate system to be attached to a ski between the binding and the upper surface. Such a device

Sports Medicine 9 (5) 1990

allows for measuring forces from numerous consecutive strides. The small force plate described by Komi used strain gauges to measure forces normal to the ski surface as well as front-back forces. Separate plates were made for the forefoot-binding and for the heel. The ski poles were also instrumented with strain gauges oriented to measure compression in the axial direction. Calibration of the plate was problematic, with frequent recalibration necessary for the front-back force measurement. Various means of recording data from such a system are possible. Komi's various studies have used telemetry methods for transmitting data to stationary receivers and recording devices while others have used portable acquisition systems employing cassette tape recorders to store the data (i.e. Pierce et al. 1987). Ekstrom was perhaps the first to use instruments to measure forces involved in crosscountry skiing. He used a portable telemetry-force plate system similar to the approach described above but with 5 load cells instead of strain gauges. This apparatus, described in several of Ekstrom's papers (1981, 1983, 1985), was constructed with 3 cells beneath a plate for normal forces and 2 cells bracketed vertically to respond to front-back forces. Unfortunately, other than representative force tracings for typical diagonal strides, little analysis was presented in any of the papers. Dal Monte et al. (1983) developed a similar force measurement system for analysis of the diagonal stride. Their methodology was to incorporate high speed filming simultaneous with force measurement. The force curves illustrated were of a single subject's diagonal pattern over 2 strides, but were not resolved into component forces. The analysis did not describe individual differences in force application observed between skiers or under varying conditions. Pierce et al. (1983, 1987) described a portable ski force plate system developed for analysis of diagonal stride, double pole, double pole with kick and skate stride (of unidentified technique). Forces were reported in resultant form only; propulsive and normal components of the applied forces were not detailed. Peak forces on the skis were as great

Biomechanics of Crosscountry Skiing

as 164% of bodyweight, while poling force maxima were about 10 to 17% of bodyweight. The force plate array system described by Komi (1987) in his review paper of force measurement techniques in crosscountry skiing was used in several studies of diagonal stride (Komi 1985; Komi & Norman 1987). The papers combined electromyographic measurement with high speed filming and force measurement. The force-time curves included in the papers were resolved into components allowing comparison of normal and propulsive forces through the stride. Several conditions were tested, but not illustrated systematically: uphill slopes of 2.5 and 11 degrees and 3 velocities (2.7, 3.1 and 4.8 m/sec). Komi and Norman (1987) sought to determine whether an eccentric, preloading period preceded thrust phase in the diagonal stride. Based on hip, knee, ankle and elbow angular velocity curves and observed muscular activity patterns, they concluded that preloading was occurring with one of two elite skiers analysed. The end of preloading was found to coincide with the peak vertical and propulsive force. In addition, Komi's (1987) review included several graphic illustrations of other force plate experiments. These included a vector analysis of the direction of kick force under 3 waxing conditions and a comparison of poling forces at 3 slopes (2.5,5.5 and 11 degrees) and 3 waxes. Unfortunately, most of this graphic information was not elaborated upon in text. Briefly summarising this research, methods have been developed for the measurement of the forces involved in the classic techniques, and several papers have indicated the force measurements and conditions that might be studied. Currently, however, no published research of skiing forces has systematically varied conditions of velocity, slope, wax or skill level. The relationships involved have yet to be clarified.

4. Skating Technique Kinematics The revolution of technique in crosscountry skiing became apparent when Bill Koch won the World Cup Series in 1982 in large part because of his adaptation of and strength in the marathon

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skate. Within 3 years, skating had completely changed the nature of crosscountry ski racing. By the 1985 World Championships, the marathon skate, along with other new skating techniques, dominated racing. The biomechanical studies of crosscountry skiing have consistently trailed behind the leading edge of technique development through this period. A kinematic analysis of the marathon skate (Smith 1985) was completed as skiers supplanted that technique with the VI skate as the primary mode. Most subsequent studies have concentrated on the VI skate, but skiers have continued to innovate. For example, on flat terrain the VI timing is often adjusted to what has been called the 'open field' or the 'Gunde' skate (after Gunde Svan, a World Cup winner). Smith (1985) systematically compared the marathon skate technique with the double pole with stride technique as a means of estimating the relative effectiveness and the method of velocity control in each case. In either case, skiers were found to increase their horizontal velocities primarily by stride rate increases but little adjustment of stride length. The proportions of the stride phases were found to remain relatively stable across a range of velocities (5.05 to 6.06 m/sec) for the marathon skate. For a given intensity, the marathon skate was found to be significantly faster than the double pole with stride. Gervais and Wronko (1988) compared the marathon skate kinematics on snow skis with those on training devices (roller skis and 'Rollerblade' skates). Results of the study were presented only in terms of significant differences between devices without summary tables of kinematic characteristics. Initial kinematic analysis of the VI skating technique began with the 30km World Cup race at Biwabik, Minnesota, in December 1985. Two sites were filmed and analysed. McNitt-Gray et al. (1986) completed a temporal analysis of the VI skate on relatively flat terrain while Dillman and Schierman (1986) examined a steep uphill section (approximately 30% grade). In the latter study, 10 elite male racers were characterised in terms of cycle velocity, length and rate. Similar to classic technique analyses, the racers exhibited quite similar tempo, but

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Strong side poling 49.4%1

Weak side poling 48.0%

a

100 Percentage of full cycle

Fig. 4. The VI skating cycle involves strong and weak side skating and poling phases (with permission from Smith et al.

1989).

the faster skiers had longer cycle lengths. The temporal analysis of McNitt-Gray et a1. (1986) also included data from a later World Cup race on moderate uphill terrain in Oslo, Norway, in March 1986. The analysis, which was preliminary in nature, described the individual skier cycle phases but was without any comprehensive statistical analysis of the V I skate timing. A follow-up study using the films from the Oslo World Cup 4 X IOkm relay race (March 1986) was completed by Smith et al. (1988). The VI technique of 10 elite male skiers was analysed in kinematic terms. The primary characteristics included the following mean values: cycle velocity of 3.23 m/sec, cycle length of 3.84m, cycle rate of 0.84Hz, strong and weak ski angles of 25.3 and 22.7 degrees, and mean CM velocity vector angle of 8.5 degrees. Of these, cycle length, weak ski angle and CM velocity vector angle were significantly related to velocity. The fastest skiers tended to skate with longer cycle lengths, smaller weak side ski angles and smaller CM deviation from the forward direction. In addition, there was a tendency for the cycle thrust phases to be shorter for faster skiers. A more recent race analysis involved the free technique (skating) races of the Calgary Winter Olympics 1988 (Smith et al. 1989). High speed films of the men's 50km and the women's 20km races were obtained at 2 sites involving a steep uphill

(approximately II degrees) and a moderate uphill (about 7 degrees). Kinematic analysis of the steeper uphill site was completed for 20 male and 22 female Olympic skiers. The typical V I skating cycle (fig. 4) involved skating phases of about 57 to 60% of full cycle and poling phases of 48 to 50% of full cycle. The asymmetry of the V I skate is illustrated in figure 5 from the steep uphill site of the men's 50km race at Calgary. The strong and weak side arm and pole positions are typically somewhat different throughout the skating cycle. The strong side pole is usually placed in a more vertical position and with a more forward orientation than is the weak side pole. The strong side poling pattern is associated with arm positions with greater elbow flexion (smaller elbow angles) than typical of the weak side. Knee angles (fig. 5c) did not exhibit such characteristic differences from side to side. As one might expect, the cycle velocities and lengths observed on the steep uphill site of the Calgary races were smaller than for the more moderate terrain of most previous studies. Both ski angles and CM lateral motion tended to be somewhat greater due to the slope. On the steep uphill, neither cycle length nor cycle rate were significantly correlated with cycle velocity which was contrary to other findings relating cycle velocity and length on moderate uphills, but agreeing with the Norman et al. (1989) finding with the diagonal stride on the same slope. No consistent pattern was demonstrated by the skiers; both cycle length and rate contributed to determining faster from slower skiers. Cycle length and rate were significantly negatively correlated. Cycle length and CM lateral motion were positively related while cycle rate was negatively related to CM lateral motion. Ski edging angles were estimated through the skating phases of the VI skate (fig. 5d). The sample of skiers demonstrated no clear pattern of edging between strong and weak sides, however most skiers exhibited a relatively flat gliding ski on one side while the other side was more sharply edged. A temporal comparison of the skating phases observed at the 2 sites found several differences. On the lower angle hill, the poling phases tended to be shorter while the skating phases were of longer duration than on the

281

Biomechanics of Crosscountry Skiing

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Fig. 5. (a) The pole orientations are typically somewhat different from side to side in the VI skate. The strong side pole (.) is usually placed in close alignment with the forward direction while the weak side pole (0) is placed at an angle to the forward direction. (b) Arm positions in the VI skate are typically asymmetrical. The strong side elbow (.) is usually in a more flexed position than the weak side elbow (0). (c) Leg positions in the VI skate are quite similar from side to side. Strong (.) and weak (0) side knee angles closely match each other but are approximatelr half a cycle out of phase. (d) Ski edging is an important factor in the skating motion. Zero degrees corresponds to a flat ski. The strong side skate (.) [occurring during the first half of the cycle] involved relatively flat ski placements while the weak side ski (0) was edged sharply through most of the skating phase (last half of the cycle). Skiers typically edged one side or the other. No clear preference of strong versus weak side edging was observed (used with permission from Smith et al. 1989).

steeper terrain. These combined with longer recovery phases to produce lower cycle rates on the moderate hill than on the steep hill. One aspect of characterising the kinematics of a locomotion pattern involves the mechanisms used in the control of velocity. In crosscountry skiing, several studies of classic techniques have found that cycle velocity is typically controlled through adjustment of cycle (or stride) rates (Gagnon 1980; Smith 1985). The marathon skate (Smith 1985) also displayed a similar relationship: skiers tended to skate with relatively constant cycle lengths across a range of speeds. Increases of speed were produced by higher tempo skating. In an analysis of V I skating, Smith and Nelson (1988) found that for a given intensity level, faster skiers were consistently skating with longer cycle lengths than

slower skiers but with quite similar cycle rates. However, across a range of intensities, both slower and faster skiers were found to increase their velocities by increasing the frequency of skating rather than cycle length. The implications were that cycle length is very important for performance (the faster skiers had the longest cycle lengths) but is not generally the mechanism used to go faster. At any given moment, in a race for example, the skating tempo governs velocity and should be the skier's focus. It is only in long term training that emphasis should be given to cycle length. There the racer should strive to increase the natural skating cycle length.

5. Skating Technique Kinetics Little biomechanical analysis of the skating techniques has included measurement of the forces involved. Both Komi (1987) and Pierce et al. (1987)

282

included a brief kinetic description of skating. Komi (1987) included one set of EMG and force component graphs from what was labeled 'skating (double pole)'. It is not clear which skating technique was analysed. Likewise, Pierce et al. (1987) included a table of peak forces for 3 skiers from a set of 'skate strides'. In both of these papers, the emphasis was on the instrumentation methodology rather than a thorough kinetic analysis of skating. Analysis of various training devices relating to cross country skiing has typically taken the form of kinematic comparison of skiing on a device (roller ski or 'Rollerblade' skate) with skiing on snow (i.e. Baumann 1985; Gervais & Wronko 1988). However, Street (1988) instrumented roller skis and poles in an analysis of VI skating forces. While roller skiing has been shown to differ in some respects from snow skiing, Street's analysis provided at least an initial estimation of what might be found on snow. Four collegiate level skiers were involved in the study which systematically varied skating velocity while observing the applied forces. Three-dimensional methods were used in conjunction with the force measurement to determine directions of the applied forces. The terrain involved was a moderate uphill (7 degrees) similar to previous VI kinematic analyses. The cycle velocities of 2.50, 3.25 and 4.00 m/sec were similar to those observed under race conditions (Smith et al. 1988). Cycle length and cycle rate were both found to increase with the speed of skating (a somewhat different response from that observed on snow by Smith & Nelson 1988). Street (1988) summarised the major kinetic change occurring with increased velocity to be arm production of a larger proportion of total force. Poling forces (40 to 50% of bodyweight) were considerably larger than previously reported for diagonal stride. The skating forces exhibited peak values of about 2 times bodyweight but average values through the skating phase of about 1.2 times bodyweight. Averaged over a complete cycle, the force from each side was approximately 0.8 times bodyweight. Resolution of the skating and poling forces into components showed the relative contributions of each to be quite different. The roller ski forces provided most of the

Sports Medicine 9 (5) 1990

observed lateral force involved in the side to side skating action. Likewise, the roller ski forces dominated vertical components measured; the skis were the primary means of supporting the body against gravity. On the other hand, the propulsive forces were rather evenly distributed between skis and poles. The strong side pole was the largest contributor to propulsion followed in order of magnitude by weak side pole, strong side ski and weak side ski. Velocity increases were found to derive from increased poling forces more than from increased skating forces. Street (1988) emphasised the following differences between VI skating forces (on roller skis) with published kinetic data from the diagonal stride: the poling forces of the V 1 skate are 2 to 4 times larger than those found in diagonal stride and the duration of the skating thrust is about 70% longer than diagonal. These factors combined with the non-requirement of the ski to stop during a skating thrust result in increased skiing velocities compared with those obtainable using classic techniques. The relationship of skating forces to velocity and uphill grade were explored in a recent study of VI skating (Smith 1989). A lightweight force plate was developed for placement between the ski and binding which measured normal forces to the ski surface. A small (1.3kg) computer-based recording system was carried by the skier and was used to sample and store the analog force signals. In conjunction with 3-dimensional video analysis of skating patterns, the force components of both ski and pole reaction forces were determined throughout skating cycles on uphill grades of 9 and 14% at four velocities ranging from moderate to sprinting pace. Typical reaction forces (resultant) are illustrated in figure 6a. Peak skating forces ranged from about 1.2 to 1.6 times bodyweight. Poling forces were considerably greater than observed in diagonal stride: peak values were typically about 0.5 to 0.6 times bodyweight. The force components from skis and poles were summed to determine the totallateral, propulsive and normal force components (X, Y and Z, respectively) for each cycle (fig. 6b). Both impulse and average force values were determined for each component for each cycle and condition.

Biomechanics of Crosscountry Skiing

Average propulsive force was found to be significantly greater on the steeper slope and tended to increase as skating velocity increased. Poling forces contributed the largest proportion of the propulsive force (about 66%) while contributing less than 20% of the lateral and normal average forces.The function of the legs in V I skating appears to be primarily as support for the body and to induce lateral motion while the function of the arms is primarily propulsive.

283

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(1 250

a

6. Conclusions The development of the skating techniques during the 1980s has revolutionised crosscountry skiing. The 'classic' and 'free technique' designations have changed ski racing in both obvious and subtle ways. While the classic events are raced using the traditional diagonal stride and double poling techniques, the associated equipment has changed due to the influence of skating. For example, poles used in skating are typically at least 10% longer than traditional length poles for diagonal stride, but many skiers have changed to longer striding poles as well. The influences of such changes on technique and on performance have not been thoroughly studied. Biomechanical research of the classic techniques has emphasised diagonal stride on uphill terrain. The effects of varying terrain, velocity, snow surface and/or wax have not been thoroughly studied and are not well understood currently. The forces involved in diagonal striding have been measured by several groups, but little systematic analysis has been published. While the classic techniques are very well established, a great deal ofbiomechanical research is yet to be completed before the kinematic and kinetic relationships are well enough understood to help optimise skier performances. Study of the skating techniques has been concentrated on the V I skating pattern on uphill terrain. While the basic relationship of forces and kinematics has been measured for the VI, numerous relationships are yet to be clarified. Flat terrain is often skied with a modification to the normal V I

o b

20

40

60

80

100

Time (percentage of full cycle)

Fig. 6. (a) The reaction forces of the VI skating cycle derive from poling and skating forces. The strong side skate during the first 60% of the cycle involved poling and skating. while the weak side skate involved only the force from the weak = strong pole; - - - = weak pole; - ' - = side ski. strong ski; .. = weak ski. (b) The total component forces through a V I skating cycle were summed from component forces of each of the skis and poles. The propulsive force (Y component) was typically largest during the strong side skate (first half of the cycle) but included a small non-zero magnitude through the remainder of the cycle ..... = X component; - - - - = Y component; = Z component.

skating pattern ('open field skate'). The effectiveness of such modifications is not known nor is their influence on skating kinematics and kinetics understood. The biomechanist faces a difficult problem in analysing 3-dimensional motion in skiing: movement cycles are often of such length that typical cinematography methods are not useable because of small image sizes. Future studies may need to employ more elaborate schemes involving multiple, consecutive, calibrated fields or perhaps some 3D methodology using images from a panning camera. Alternatively, the development of ski

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treadmill surfaces may provide better experimental control than is possible in field studies. Such methods used in conjunction with force measurement hold considerable promise for future skiing research. However, some significant advances in our understanding of the kinematic and kinetic characteristics of skiing will be necessary before clear application to optimise individual technique is possible.

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