Laboratory running (4,9,22) or cycling (21) tests were previously used to determine maximal oxygen uptake and training intensities in cross-country skiers. But upper-body has become increasingly important in modern cross-country ski racing, with the coming of the skating technique in the 1980s and the sprint discipline, but also with the increase of the fractional use of double poling (DP) during cross-country classical technique races. So specific cross-country skiing tests using upper-body are now required to precisely evaluate elite skiers. Consequently, many coaches have been interested in ski-specific laboratory testing, using upper-body ergometer to simulate the upper-body movement of DP technique (2,13,16,23,28) or using roller skis on a large treadmill (6,7,11,16,18,19). Rundell (18,19) was one of the first to evaluate cross-country skiers with roller skis on a large motorized treadmill. He observed that peak physiological parameters during treadmill roller skiing test were significantly lower than those during treadmill running test, underlining the need of specific testing protocols. In the same way, Mygind et al. (16) showed, in elite cross-country skiers, that the maximal oxygen uptake measured using a specific DP ergometer was significantly correlated with performance unlike the maximal oxygen uptake measured during running.
Today, there is no doubt about the usefulness of specific testing protocols for cross-country ski activity compared with standard running or cycling tests. To summarize, 2 main kinds of specific protocols are used. First, protocols involving DP motion with specific ergometers (2,13,16,23,28) or on treadmill-but usually used for biomechanical analysis (11,12)-and, second, protocols involving complete technique (i.e., skating or diagonal stride [DS] technique) on treadmill (6,7,11,16). To our knowledge, the comparison among these specific testing protocols in relationship to the level of performance of skiers is poorly documented.
Therefore, the present study aimed to propose a new incremental DP test on treadmill and to verify the capacity of this specific test to predict cross-country ski performance. Data obtained were also compared with results of an incremental and maximal test carried out by the same athletes but with a different protocol involving the DS technique to know which test appears like the best predictor of the racing performance.
Experimental Approach to the Problem
Given the large contribution of the upper body in cross-country skiing, evaluations with ski-specific testing might be more appropriate. This point is largely documented in the literature evaluating the differences between standard running or cycling tests and specific cross-country skiing tests. On the contrary and to our knowledge, no data are available concerning the relationship between different specific cross-country skiing test and the level of performance on snow of the skiers. In this way, physiological results of 2 different tests on treadmill with the DP technique on the one hand and with the DS technique on the other hand were therefore studied by examining Student's t-test, and their relationships with the skiers' level of performance were evaluated using the Pearson correlation.
Ten world-class cross-country female skiers from the national Italian ski team were evaluated. They were asked to refrain from ingesting caffeine or alcohol for at least 12 hours before testing and to eat a light meal 2 hours before testing. The study protocol complied with the Declaration of Helsinki for human experimentation and was approved by the university's ethical committee for human research. Possible risks and benefits were explained, and written informed consent was obtained from each subject before their participation. Mean subject characteristics are presented in Table 1.
The 2 following tests (i.e., DP and DS tests) were carried out in a randomized order and during a short period (i.e., 2 weeks) to minimize the effects of training. This study was carried out during October when skiers had already reached a high volume of training especially in cross-country roller skiing. During the tests, subjects used the same pair of roller skis (Ski Skett Nord CL, Sandrigo, Italy) and their own poles for the classical technique.
At the beginning of each test, the skier was secured by a safety harness, which was connected to an emergency brake suspended from a metal bracket above the treadmill. Each skier was fully familiarized with roller skiing on treadmill.
Double Poling Cross-Country Roller Skiing Test
The DP technique involves simultaneous arm/pole action on both sides synchronized with a trunk flexion and is typically used under fast conditions. Propulsive forces derive strictly from trunk and arm activity.
Double poling test was performed during roller skiing on a large motor-driven treadmill (belt dimensions of 2.5 × 3.5 m; Rodby, Sodertalje, Sweden) with a constant inclination of 4%. The start speed was fixed at 10 km·h−1 and increased by 0.5 km·h−1 every 30-second work period until exhaustion.
Diagonal Stride Cross-Country Roller Skiing Test
The DS technique is a cyclic movement pattern close to the running or walking locomotion, involving a right and a left step combining into a full stride, arms, and legs moving in opposition. Diagonal stride is primarily a technique for climbing.
Diagonal stride test was performed during roller skiing on the treadmill with a constant speed of 9 km·h−1. The start slope was fixed at 4% and increased by 2% every 3-minute work period until exhaustion.
Values of minute ventilation (E), carbon dioxide output (co2), and oxygen uptake (o2) were continuously measured by a portable breath-by-breath gas exchange measurement system (Cosmed K4b2, Rome, Italy). Gas analyzers were calibrated before each test with ambient air (O2: 20.93% and CO2: 0.03%) and a gas mixture of known composition (O2: 16.00% and CO2: 5.00%). An O2 analyzer with a polarographic electrode and a CO2 analyzer with an infrared electrode sampled expired gases at the mouth. The Cosmed K4b2 system is lightweight (800 g) with the main sample unit attached to the chest and the battery pack on the back. The face mask, which had a low dead space (70 ml), was equipped with a low-resistance, bidirectional digital turbine (28-mm diameter). This turbine was calibrated before each test with a 3-L syringe (Hans Rudolph Inc., Dallas, TX, USA). Face masks allowed subjects to simultaneously breathe with mouth and nose for more comfort. Heart rate (HR) was continuously measured via a wireless Polar-monitoring system (Polar Electro Oy, Kempele, Finland) and synchronized with the Cosmed system.
Three experiment reviewers determined individually the second ventilatory threshold by visual analysis of the breakpoints of ventilatory equivalent of carbon dioxide (E/co2), ventilatory equivalent of oxygen (E/o2), and minute ventilation (E) changes over time (24,26).
To verify the capacity of the tests to predict performance, peak oxygen uptake (o2peak), oxygen uptake at anaerobic threshold (o2Th), peak treadmill speed, and treadmill speed at anaerobic threshold (Speak and STh, respectively) were measured during the test and correlated with the level of performance (expressed thanks to the Italian ski federation ranking [FISI points]: better is the level of performance of a skier, lower is his total FISI points).
The ratio (in %) between o2peak during DP and o2peak during DS was also calculated.
Paired t-tests were used to discern any significant differences between both incremental tests. The Pearson product-moment zero-order correlation coefficient demonstrated any significant relationship. Linear regression was calculated to show relationship between level of performance (FISI points) and physiological variables and speed reached during tests. For all statistical comparisons, the level of significance was set at p ≤ 0.05. Values presented are expressed as mean ± SD.
Comparison of Submaximal and Maximal Data During DP and DS Tests
HRpeak, HRTh,Epeak, and o2Th were significantly higher during DS than during DP (p < 0.001 for HRpeak, HRTh,Epeak and p < 0.05 for o2Th), whereas no difference was observed for o2peak (p = 0.1) and ETh (Table 2).
No difference was observed in the percentage of o2peak at anaerobic threshold between DP and DS (o2Th = 86.7 ± 4.4% and o2peak = 88.9 ± 3.6% during DP and DS, respectively).
The value of o2peak DP/o2peak DS ratio reached 94.7 ± 9.1%.
Relationship Between the Level of Performance (FISI Points) and Maximal and Submaximal Physiological Variables and Speed Reached During DP and DS Tests
At maximal intensity, Speak and o2peak only during DP were significantly correlated to FISI points (r = −0.88, p < 0.001 and r = −0.77, p < 0.01, respectively; Figure 1).
Peak values achieved during DS were not correlated with the level of performance.
At intensity corresponding to anaerobic threshold, STh only during DP and o2Th during DP and DS were significantly correlated to FISI points (r = −0.88, p < 0.001; r = −0.63, p < 0.05; and r = −0.65, p < 0.05, respectively; Figure 2).
Finally, o2peak DP/o2peak DS ratio was also significantly correlated with the level of performance (r = −0.77, p < 0.01; Figure 3).
Giving the particularity of the cross-country skiing activity with the use of both upper and lower limbs, a specific physical evaluation is needed in order to be more precise in the determination of optimal training intensities. In fact, it has been reported that, for a valid evaluation of athletes' capacities, discipline-specific tests should be used: considering cross-country skiing, Mygind et al. (16) proposed that such tests should involve the whole muscle mass corresponding to this particular physical activity. The transposition and interest in terms of training prescription and performance prediction of running tests vs. roller skiing tests for nordic skiers are thus raised (18,19,25). Yet trainers and athletes are looking for the best indicators of optimal intensity during prolonged effort. Consequently, numerous specific tests are now available (2,7,11,13,16,18,19,23,28). The present study was mainly designed to assess whether one test is more accurate than another one to predict skiers' performance. Thus, we compared a DP test and a DS test, both performed on a motorized treadmill.
At maximal intensity, we found higher values for HRpeak and Epeak during DS. These results are in agreement with the results obtained by Hoffman et al. (10) during roller skiing with DP and DS at 1.7% grade. This finding could be explained by the greater part of active muscles involved during DS (upper and lower limbs acting together to propel forward the skier), although Holmberg et al. (11) recently showed the important contribution of the legs to DP performance in elite skiers. However, Watts et al. (27) found no difference between DP and DS. Surprisingly and in contrast to the previous study (27), the difference in o2peak between DP and DS did not reach statistical significance (p = 0.1). Two points could explain this result. First, the skiers evaluated in the present study were among the best world skiers and the upper-body training takes an important part of their total training, may be more than at the date of the previous study. Second, we are aware that both protocols were different (DP test was originally though only for biomechanical analysis but we decided to record continuously expired gas). In particular, DP was really shorter than DS (Table 2) and one could say that short duration protocol could lead greater values in maximal oxygen consumption. However, no difference in o2peak has been reported with a 1-minute or a 3-minuite stage duration protocol during cycle ergometry (3,15,17). These authors observed only a lower peak power and HR response. This last point could also explain our difference in HR value between DP and DS.
At anaerobic threshold intensity, HRTh and o2Th were also higher during DS. It is remarkable to note that these values were different according to the technique used during skiing. This result gives important key issues for training programs to cross-country skiers and coaches. Generally, during cross-country ski sessions training, skiers are unable to determinate their training intensity using speed like, for example, a runner on a track because of the variations of snow conditions and field profiles. They can only use HR or exercise perception to assess exercise intensity. So, the HR value at anaerobic threshold point could be useful to set training parameters close to the competitive intensity. To improve their endurance capacity (i.e., capacity of the athletes to use an important part of their maximum oxygen consumption) and therefore their race performances, skiers have to perform some specific training sessions (usually interval training sessions), precisely at this intensity. With the increasing part of the upper-body contribution in the performance in modern cross-country ski races (see above), it could be interesting to include some specific training session during DP to increase the upper-body endurance capacity (not only training sessions based on upper-body force or power development, as actually and usually performed). In this way, the specific determination of HRTh during DP is required, given the high difference between DP and DS test (i.e., difference of about 8 b·min−1 at anaerobic threshold).
The last goal of this study was to verify the relationships between the results of both tests and the skiers' racing performance. The main finding of this study was that the DP test appeared like a better predictor of the skier's racing performance than the DS test. Indeed, Speak, o2peak, STh, and o2Th during DP were highly correlated with the performance, whereas, during DS only o2Th was correlated. But, as previously discussed, this disparity could result from the difference in protocol duration. Anyway, to our knowledge, this is the first time that the results of 2 specific tests were compared according to their relationships with the skiers' performance on snow. Usually, capacity to predict performance on snow was compare between a specific cross-country ski test and a standard running or cycling test. In 1991, Mygind et al. (16) already identified a better correlation between performance and maximal oxygen consumption during an incremental DP test on a ski ergometer than during a running test in elite skiers. Rundell and Bacharach (20) found high correlations between performance and o2peak and peak power during DP on arm ergometer in top women biathletes but not in top male biathletes.
Moreover, the major difference between the present study and the previous is that we were also interested in submaximal results (values at the intensity corresponding to the anaerobic threshold) and we observed the same relationships with the performance. Significant relationships between anaerobic threshold and race performance have been documented for several activities (1,8,14). The relationships between performance and DP test observed in the present study are particularly notable that the group evaluated was very homogeneous according to their level of performance.
Finally and interestingly, the o2peak DP/o2peak DS ratio appeared also like an accurate predictor of the racing performance. This ratio emphasizes the capability of the skier to consume oxygen when the major part of the work is produced only by upper-body muscles (during DP) against by the whole body (during DS). Mygind et al. (16) showed that the ratio between o2peak during DP and o2peak during running changed from 87.7% in September to 95.7% in December and falling to 91% post-season in April in 4 elite but not in world-class cross-country skiers. Even if the correlation between this ratio and the performance was not realized in this last study, it seems clear, looking at the changes over 1 year, that this ratio was also related to the skiers' shape level. These values correspond to our finding (i.e., 94.7%), although in the present study, we calculated the upper-body/whole-body ratio and in the study of Mygind et al. they calculated the upper-body/leg ratio. On the contrary, our result was higher than the percentage (i.e., 81%) obtained by Watts et al. (27) comparing o2peak during DP and DS. This discrepancy could be explained by the lower level of the skiers analyzed by Watts et al. (27).
From a practical point of view, the results of this study give important key issues for evaluations and for training programs to cross-country skiers and coaches. Double poling incremental test on treadmill appears like a more accurate test to predict performance than DS test, certainly because this test emphasizes the importance of the upper-body contribution in the cross-country skiing performance. However, given the high differences observed in heart rate responses between both tests, it seems necessary to perform both tests to precisely determine specific techniques training intensities in high-level skiers. Moreover, the combination of both tests permits to obtain an additional good performance indicator.
Last, given that motorized treadmill is very scarce around the world and that nonprofessional skiers have not always the possibility to be tested on laboratory, we suggest that a field test on the model of the cross-country roller skiing field test recently proposed by Fabre et al. (5), but involving DP, could be easily carried out and could give interesting data to predict future performance on the snow.
The authors gratefully acknowledge the assistance of all those who participated in this study, particularly all the subjects from the national Italian ski team and their coaches.
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