1. Introduction
Cross-country (XC) skiing is a winter endurance sport with competitions held in hilly terrain with utilization of multiple sub-techniques within the two main techniques, classical and skating [
1,
2]. The racecourses were designed with approximately one third uphill, one third flat, and one third downhill terrain, leading to ~50% of the overall time being spent in the uphill sections, and ~35% and ~15% in the flat and downhill terrain sections, respectively [
1,
3,
4]. Here, previous studies have shown that work intensity is highest in the uphill sections with performance in this terrain being most important to overall performance in classical and skating time-trials both in distance [
3,
4,
5,
6] and sprint competitions [
7,
8]. Moreover, skiers commonly reduce their speeds throughout time-trial competitions (i.e., positive pacing strategy) [
3,
9,
10,
11], although higher ranked skiers are better able to maintain speed compared to lower-ranked skiers [
9].
In XC skiing, each terrain section typically lasts between 10 and 90 s [
1,
10], in which the skiers must effectively select and change between various sub-techniques at different speed and incline combinations [
1]. In recent years, the development of combined global navigation satellite systems (GNSS) and inertial measurement units (IMU) has demonstrated a potential to provide concurrent performance analyses of speed and sub-technique selection during training and competitions [
12,
13,
14]. Although the distribution of sub-techniques has previously been extensively investigated in the classical technique [
12,
14,
15,
16,
17], there is currently limited research into the skating technique [
18]. Moreover, skiing speeds are maintained by optimal adjustments of cycle rate (CR) and cycle length (CL) within each sub-technique [
2], where previous studies have shown CL to be an important determinant of classical time-trial performance [
16]. However, whether the same kinematical patterns and performance determinants apply to skating time-trials requires further elucidation.
XC skiing requires a large aerobic energy contribution, with 70–95% of the total energy expenditure during competitions derived from aerobic energy sources [
2], although the terrain-dependent speed and intensity fluctuations elicit an interaction between aerobic and anaerobic energy systems [
1,
2]. Accordingly, the maximal oxygen uptake (VO
2max), lactate threshold (i.e., fractional utilization of VO
2max), and gross efficiency (GE) are key performance determinants in XC skiing [
2]. Laboratory-based testing of skiers is commonly used to monitor training-induced changes and to predict performance based on test batteries typically including performance indicators such as VO
2max and/or peak oxygen uptakes (VO
2peak) and threshold-derived measures [
11]. In this context, recent studies have demonstrated significant associations between different laboratory tests and performance in a classical time-trial [
8], a sprint skating time-trial [
8], and a roller-ski skating time-trial [
11]. However, laboratory-based performance-determining factors have not yet been associated with an actual on-snow skating time-trial competition. Moreover, inconsistent findings are observed in the above-mentioned studies, likely explained by variations in heterogeneity, statistical power, and performance levels across study samples [
11]. This emphasizes a need for better understanding the role of laboratory-based performance determinants in XC skiing.
In addition to laboratory testing, field-based tests are commonly applied in XC skiing. While such tests are easy to perform and may have high ecological validity, they are associated with higher influence of external factors and lower test-retest reliability [
11]. In a recent study by Talsnes et al. [
11], both an uphill running and roller-ski double-poling time-trial revealed moderate to large correlations with XC skiing performance. However, the significance of on-snow field-based tests to XC skiing performance has not yet been investigated.
Therefore, the purpose of the present study was to examine the contribution of time in different terrain sections and sub-technique distribution to the overall performance in a 10 km skating time-trial competition, as well as the relationships to laboratory and field-based performance determinants in XC skiers.
4. Discussion
The present study examined the contribution of time in different terrain sections and sub-technique distributions to overall performance in a 10 km skating time-trial competition, as well as the relationships to laboratory and field-based performance determinants in XC skiers. The main findings were as follows: (1) time in uphill terrain was the main contributor to overall performance, (2) the skiers adopted a positive pacing strategy with reduced speeds in all terrains throughout the competition, (3) G2 and G3 were the predominant sub-techniques constituting 62% of the overall time, with increased utilization of G2 throughout the competition, (4) higher total and lean body mass were associated with better performances, and (5) out of the performance-determining factors, VO2 and power at 4 mmol·L−1 Bla, VO2peak, and TTE during roller-ski skating in the laboratory, as well as performance in a 3 km on-snow uphill skating field test, had the strongest associations with time-trial performance.
4.1. Analyses of the Skating Time-Trial Competition
The nearly perfect correlation found between time in uphill terrain and overall time-trial performance expands upon previous research demonstrating similar associations both in distance- [
3,
4,
5,
6] and sprint time-trials in XC skiing [
7,
8]. Nearly perfect and very large correlations between time in flat and downhill terrains and the overall performance were also found, but the stepwise multiple regression analyses showed that most of the variance was shared with the contribution from uphill performance. In fact, only 2% of the remaining variance in the stepwise regression was explained by time in flat and downhill terrains, demonstrating high multi-collinearity between the independent variables. Altogether, these findings emphasize a significant contribution of all terrain types with the best performing skiers being generally faster in all terrains. However, uphill-specific performance was clearly the main determinant of 10 km skating time-trial performance in a group of national-level male skiers.
The skiers in the present study adopted a positive pacing strategy with a ~9% speed reduction from lap one to lap three. These findings are consistent with previous research indicating the use of a positive pacing strategy during time-trial competitions in XC skiing [
3,
9,
10,
11], although it has been demonstrated that higher-ranked skiers demonstrate a more even pacing compared to lower-ranked skiers [
9]. The observed speed reductions were distributed across all terrain types but most of the reductions in speed from lap one to lap three occurred in the uphill terrain sections, in line with previous findings during a classical time-trial competition among female skiers [
4]. However, the speed reductions found in the present study were somewhat higher than the typically 2–4% decreases in speed seen over the second half of time-trials among skiers [
9]. This may in part be explained by differences in racecourses and/or the competition altitudes (e.g., ~1650 m.a.s.l in the present study) which may have increased the demands for a more even pacing strategy [
30]. Alternatively, this may be related to differences in pacing expertise between study samples. Interestingly, a recent intervention by Losnegard et al. [
31] demonstrated that skiers with a fast-start pattern improved skating time-trial performance by adopting a slower start and thus more even pacing. Therefore, considering the relatively large speed reductions found in the present study, it is likely that these skiers would have benefited from adopting a more even pacing strategy.
4.2. Analyses of Sub-Technique Selection and Kinematical Patterns
The G2 and G3 were the most predominant sub-techniques during the skating time-trial competition, constituting 20% and 40% of the overall time, respectively. Moreover, only 7% was G4, whereas 10% and 16% of the overall time were spent in the G5 (skating without poles) and G7 (tuck position) sub-techniques, respectively. These findings are comparable with the sub-technique distribution found during a sprint skating time-trial [
7], although the minor (~1–2%) distribution of G6 (turning technique) was not included in the present study due to classification errors. Interestingly, the 16% time spent in the G7 (tuck position) in the present study was higher than previously found during a sprint skating time-trial [
7] and in classical time-trial competitions [
15,
16]. These differences are likely explained by variations in the degree of ascent and descent between different racecourses. However, considering that ~61% of the overall time was spent in uphill terrain where the largest performance differences occurred, developing the G2 and G3 sub-techniques (62% of the overall time) should be prioritized in the training process of skiers to improve skating time-trial performance.
Coinciding with the reduced speeds throughout the time-trial competition, the skiers reduced their utilization of G3 and increased the utilization of the G2 sub-technique from lap two to lap three, and particularly from lap one to lap three. The speed reductions and corresponding changes in sub-technique selection further coincided with reductions in CL within G2 and G3 from lap one to lap three. These findings agree with those found in the double-poling technique during a 10 km classical time-trial competition, demonstrating reduced CL throughout the competition [
16], as well as more laboratory-based findings, emphasizing CL as an important performance determinant in the skating technique [
1,
2]. Overall, speed reductions seem to be related to the use of “lower gears” and reduced ability to generate propulsion and CL, indicating that the use of “higher gears” and long CL are important determinants of skating time-trial performance and are also associated with a more even pacing strategy.
4.3. Laboratory- and Field-Based Performance Determinants
The moderate to large inverse relationship between total mass, lean mass, and time-trial performance indicated that the heaviest skiers with most lean mass were the fastest skiers in both overall time and time in different terrains. These findings are somewhat in conflict with previous studies demonstrating no correlations between body mass and body composition measures and XC skiing performance [
7,
11]. However, comparable associations have been found between lean mass and performance among both female [
32] and male skiers [
33], and particularly in sprint XC skiing [
34,
35]. The reason for the present findings may be related to the advantages of having better upper- and lower-body strength for propulsion among the skiers with most lean mass, which is supported by the same associations found between lean mass in the arms and performance in flat and downhill terrain. This is further supported by Larsson and Henriksson–Larsèn suggesting the importance of lean body mass in the arms to XC skiing performance in junior male skiers [
33]. Another explanation may be the fact that some of the lower performing skiers in the present study had a background in running, and thus a lower body and lean mass before transferring to XC skiing in a talent transfer initiative as previously described by Talsnes et al. [
25].
The laboratory-derived performance indicator (TTE), as well as both absolute and body-mass normalized VO
2peak while roller-ski skating, demonstrated moderate to very large correlations with overall time and time in all terrains of the time-trial competition. These findings agree with previous studies, emphasizing the importance of a high aerobic energy turnover in XC skiing [
2,
4,
11]. Moreover, both VO
2peak and TTE displayed a larger correlation with time on lap three compared to lap one, in which the latter is in line with a previous study investigating 10 km biathlon sprint competition [
36]. This indicates that better performing skiers use different pacing strategies than their lower performing counterparts, and that incremental testing while roller-ski skating in the laboratory is relevant for determining performance at the end of an on-snow skating time-trial competition. The large to very large correlations found between VO
2 at 4 mmol·L
−1 Bla and time-trial performance probably reflect the higher VO
2peak values found among the better performing skiers, as no significant correlations were found between fractional utilization (%VO
2peak) and the time-trial performance. The higher VO
2 at 4 mmol·L
−1 Bla probably also explains the moderate to large correlations found between power output at 4 mmol·L
−1 Bla and time-trial performance. Overall, the present findings emphasize the validity of laboratory-derived performance and physiological measures in XC skiing, which can be used to determine performance and monitor training progress. However, the test-retest reliability and actual ability to detect training-induced changes across time using such tests should be examined further.
The 3 km on-snow uphill skating test demonstrated large to very large correlations with overall time and time in all terrains of the time-trial competition. These findings are consistent with a recent study demonstrating significant correlations between uphill field tests in both running and double poling with XC skiing performance in a group of national-level male skiers [
11]. The large correlations found between the field test and the time-trial performance are likely explained by the fact that both tests were performed on snow in the same area, and under the same external conditions. This was further strengthened by the largest correlations found between the uphill skating test and performance in uphill terrain and lap three of the time-trial competition. Although associated with higher influence of external factors and lower test-retest reliability (e.g., wind, temperature, and snow friction) than standardized laboratory-tests, the present study demonstrates that performance in an uphill skating field test showed very large associations with performance and provides an easy-to-use practically relevant field test.
4.4. Methodological Considerations
Whilst care was taken to ensure scientific quality, the present study included some limitations. First, there was a 3 week gap between the competition and the other tests. However, the participants were highly trained elite athletes, had a stable performance level during this period and similar training regimes and preparation before all tests. Second, the participants’ motivation to perform such extensive testing and differences in skis and equipment may have influenced the results, although the latter was minimized by using the same ski wax and professional waxer for all participants. Third, correlation analyses only provide information about the validity of different laboratory- and field-based performance tests. Hence, the test-retest reliability and actual ability to detect training-induced changes across training cycles using such tests should be examined further.