Classical cross-country (XC) skiing involves as many as five different techniques, in addition to those employed in curves and downhill. During a race, the skier adapts these techniques to the topography, snow conditions (e.g., friction) and skiing speed, taking into consideration his/her individual technical competence and physical capacity (Bergh and Forsberg, 1992; Bilodeau et al., 1992; Norman and Komi, 1987; Nilsson et al., 2004; Smith, 1992). Accordingly, the technique employed by different skiers on any given section of a course may differ extensively.
In the case of classical XC skiing, diagonal stride (DIA) and double poling (DP) are the techniques most often utilized on steep uphill and flat terrain, respectively, with kick double-poling (DPK) often being employed on moderate uphill terrain (Gopfert et al., 2013; Pellegrini et al., 2013). DIA is characterized by alternate poling thrusts synchronized with a kick by the opposite leg. DP is performed without kicking, so that all propulsion is generated via the upper body through the poles. DPK involves a DP motion followed by a kick involving one leg, with the kicking leg typically alternating from cycle to cycle.
During the past three decades, DP has become more and more extensively utilized and today its successful usage determines the outcome of classical XC races (Holmberg et al., 2005; Sandbakk and Holmberg, 2014; Stoggl and Holmberg, 2011; Welde et al., 2017). This gradual shift to DP is probably a response to better preparation/grooming of ski courses along with marked improvement in equipment (e.g. grip/strap, basket/tip of the pole, as well as gliding properties of the skis). This, in turn, has allowed skiers to perform more specific upper body training, since DP can now be utilized more extensively. Accordingly, XC skiers have increased their upper-body strength and endurance and thereby substantially improved their technique (Holmberg, 2015; Stoggl and Holmberg, 2011).
The pronounced increase in racing speed during recent decades (Sandbakk and Holmberg, 2014; Stoggl et al., 2008), along with the more extensive use of DP (Hebert-Losier et al., 2017; Stoggl and Holmberg, 2016; Stoggl and Holmberg, 2011), motivate more detailed examination of cycle characteristics and the utilization of the major techniques by elite skiers on different sections of classical XC distance races involving various types of terrain. Pellegrini and co-workers (2013) have demonstrated that when roller skiing on a treadmill, DP is the preferred technique at low inclines (up to 2[degrees]). At moderate inclines (2-3[degrees]), their skiers switched to DPK and all employed DIA on inclines greater than 6[degrees]. In that investigation, no skiers employed DP on an incline steeper than 4[degrees], while at a fixed incline of 2[degrees] DIA could only be used at speeds of 6-14 km/h. These researchers described hypothetical thresholds for transitions between techniques based on pole forces (when 5.5 N/kg was reached, the skiers switched to another technique) and leg thrust time (when this time became less than 0.11 s, this leg action was no longer sustainable).
Furthermore, Stoggl and co-workers (2007) have demonstrated that during a simulated XC sprint race with roller skis on a treadmill, the faster skiers employ longer and fewer total cycles and tend to utilize DP and DPK more extensively. Recently it was demonstrated that elite male skiers use DP and DPK to a greater extent during the first half of a XC skiing distance race, while they switch to DPK during the second half while the slower skiers do the opposite (Welde et al., 2017).
On XC race courses, more than 50% of the total time is spent skiing uphill and uphill performance is thus considered to be the major determinant of success (Bergh and Forsberg, 1992; Bilodeau et al., 1996; Bolger et al., 2015; Mognoni et al., 2001; Norman and Komi, 1987; Sandbakk et al., 2011; 2016). Although a number of biomechanical analyses of XC sprint events have been published recently (e.g. Andersson et al., 2010; Stoggl et al., 2007; Zory et al., 2006), all such analyses performed during actual distance races date back to the 1980-90's (Bilodeau et al., 1996; Norman and Komi, 1987; Norman et al., 1989; Smith et al., 1996). One consistent finding has been that better overall racing performance is associated with greater cycle length and faster cycle velocity on flat (Bilodeau et al., 1996; Norman and Komi, 1987; Smith et al., 1996) and uphill terrain (Bilodeau et al., 1996; Norman and Komi, 1987), where the cycle rate of faster skiers is either lower than (Norman and Komi, 1987) or similar to that of slower skiers (Bilodeau et al., 1996; Smith et al., 1996).
These earlier studies involved either male or female skiers, with no direct sex comparisons. In this context, Sandbakk and colleagues (2014) found that sex differences were more pronounced while using the DP technique. Moreover, Hegge and co-workers (2016) underlined the observation that larger differences in power output between men and women emerge when a greater contribution from the upper body is required. Consequently, the largest sex differences during a XC competition are expected to be observed in connection with sections of the course where DP is the predominant technique employed.
The current study is part of a larger project in which the effects of fatigue (Welde et al., 2017), track topography and sex differences on kinematic variables during a real XC skiing distance competition are being analyzed in detail. The aim of this current study was to compare the techniques employed on various inclines by world- (faster skiers: FS) and national-class (slower skiers: SS) male and female XC skiers during a distance race to their cycle characteristics and performance. Our hypotheses were that 1) longer and faster cycles, but no difference in cycle rate are associated with better performance; 2) performance uphill is more closely related to race outcome than performance on the flat and intermediate sections; 3) the most pronounced sex differences occur on flat and intermediate terrain, especially while using the DP technique, when the contribution from the upper body increases; and 4) faster skiers utilize the DP technique to a greater extent. Methods
This study was conducted in connection with the 10-km and 15-km classical XC races for women and men, respectively, at the Norwegian National Championships held in Troms0, in 2016. The study was pre-approved by the NSD Data Protection Official for Research in Norway and the subjects fully informed about its nature before providing their consent for us to use their data.
The race course was composed of two 5-km tracks (track A and track B). The men skied track A twice and track B once (A-B-A), for a total racing distance of 15 km, while the women skied track A and B once each (A-B), for a total racing distance of 10 km. The two tracks involved approximately equal distances on uphill, flat and downhill terrain, as well as total climbs of 149 and 185 m, maximal changes in elevation of 72 and 76 m, and maximal climbs of 42 and 38 m, respectively. The competitors were video recorded on four different types of terrain: flat (mean incline -0.3[degrees], range within the site: -0.4 to 1.4[degrees]), intermediate (mean 3.5[degrees], range 2-5[degrees]), uphill (mean 7.1[degrees], range 6.4-7.9[degrees]) and steep uphill (mean 11[degrees], range 9-13[degrees]) at distances of 0.8, 1.2, 2.1 and 7.1 km from the start, respectively.
A grooming machine prepared the course on the evening prior to testing and the weather conditions during the race were stable (no wind, air temperature +1[degrees] C, snow temperature 0[degrees] C, relative humidity 86%), with no problems choosing the optimal wax (base and violet klister for grip, high-fluor paraffin wax combined with fluor powder for glide).
Participants and data analysis
Following the race, 82 of the 202 participants (140 men, 62 women) were classified on the basis of their finishing times as faster skiers (FS: the top-placed 20 women and 20 men) or slower skiers (SS). To obtain relatively homogenous slower groups, the very slowest (i.e., starting with the skier who finished 0.5% (males) or 1.0% (females) slower than the skier who finished immediately before him/her) were excluded to leave 21 men and 21 women as SS. The FS (including four who ranked among the top 10 in the World Cup in 2016 and four medalists at World Championship or Olympic Games) all had finishing times within 8% (men) or 11% (women) of the winner's, whereas the SS skiers were 10-16% (men) and 14-22% (women) slower than the winners.
The cycle characteristics of the skiers on the four different sections were determined with the Kinovea 8.25 software and the total racing time (performance) for each provided by the official timing system (SIWIDATA, Merano, Italy).
Video cameras (Sony HDR-PJ810E, Sony corp., Tokyo, Japan) set at 50 Hz with a shutter speed of 1/500 s recorded the skiers at high resolution (1920 x 1080 progressive scan). Each camcorder was positioned perpendicular to the track 1 m above the ground on top of tripods placed on custom-made wooden platforms, leveled with an electronic inclinometer, and recorded the skiers in a sagittal plane from a distance of 12-25 m. The ski tracks were centered in the field of vision and the focus and zoom set to cover at least three cycles of movement per section filmed. The flat and intermediate sections were both 22 m and the two uphill sections 12 m in length.
For purposes of calibration, four red poles were placed on the video-taped sections of the track prior to the race, two at the beginning and two at the end, creating a regular rectangle enclosing the section on both sides (Fig. 1). A fifth pole was set exactly in the middle of the side of this rectangle closest to the camera. The distances between each pair of poles were determined with a measuring...