Artificially induced altitudes aerobic and anaerobic performance differences on double-poling ergometer in elite young cross-country skiers

Background

Currently, the methodology for preparing the best cross-country skiers focuses on the importance of strength training, not just for sprints (Hébert-Losier et al., 2017) but also for long-distance tracks (Sandbakk & Holmberg, 2014) and on the utilization of higher altitudes between 1 800 and 2 200 m above sea level ASL (Suchý, 2012). For testing (Fukuda et al., 2014; Hegge et al., 2015) and training of specific power endurance abilities in cross-country skiing, the SkiErg^® Concept 2 device (Morrisville, VT, USA) was utilized. Since 2011, this simulator has been the standard for testing racers enrolled in the Youth Sports Centers of the Czech Ski Association. The rise of the strength-oriented concept of cross-country skiing is determined primarily by the sliding ability of skis, races taking place on artificial snow, and track design. It is not just World Cup (WC) races that are currently ceasing to be raced on natural snow, but many other races also take place on “firn” (almost ice) snow. Tracks are increasingly demanding to make races more attractive. Therefore, skiers with advanced strength have been more successful in recent years.

Double-poling cross-country skiing (hereinafter double-poling) has recently been discussed not only for long-distance races but also for easier (i.e., generally older) WC tracks. Under suitable conditions, some skiers could complete classic-technique long-distance WC races using double-poling (e.g., Dario Cologna and Petter Northug Jr. at Davos WC in the 2015/2016 season). The advantage of much more strength-intensive double-poling compared to classic cross-country skiing is the ability to complete the entire cross-country race without kick wax. Such skis are faster on downhill and flat sections, and even slightly inclined. In the 2016/2017 season, the Fédération Internationale de Ski (FIS) reacted to this rise in double-poling by shortening the pole length for classic cross-countries, with the rules now defining a length (max. 83% of the racers’ height in ski boots) is less favorable for double-poling. Furthermore, juries of certain races can now define what are called “classic zones” or “no double pole technique zones,” where double-poling is prohibited (www.fis-ski.com). Nevertheless, double-poling has become a vital part of the preparation, with treadmill and double-poling ergometer being the most suitable training tools to measure this ability.

Several studies have been published on utilizing higher altitudes (1 800 2 400 m ASL) to increase physical endurance, along with competition results at low altitudes (Millet & Schmitt, 2011; Suchý & Opočenský, 2015; Wilber, 2004). A meta-analysis by Bonetti and Hopkins (2009) confirmed that after a three-week sojourn at natural altitude, top athletes, on average, increased their performance upon returning to low altitudes by 5.2%. Higher altitudes place greater demands on training management than on training at sea level, especially for adult elite athletes (Burtscher et al., 2018). Therefore, redefining the exercise intensities obtained at low altitudes before the onset of acclimatization processes for the first days of stay and altitude training is essential. Heart rate (HR) and lactate concentration (LC) in capillary blood may be significantly higher at the same exercise intensity as at sea level, which is generally called the “lactate paradox” (Hochachka et al., 2002; Lundby et al., 2000). Long training camps at altitude, essentially for the whole preparatory period and even part of the competition period, are necessary for top skiers (Chrástková & Suchý, 2011). Subsequently, due to the frequent absence of snow at low altitudes (Dong & Menzel, 2020), cross-country skier training at the start of the season is increasingly shifting to higher altitudes of 1 800 to 2 200 m ASL. This has negative socio-economic consequences. Thus, to a limited extent, alternatives have been used at artificially induced altitudes. Several studies have described its positive effects (Treff et al., 2022; Pupiš et al., 2014; Wilber, 2004). Artificial higher altitudes used for improving physiological parameters can be induced by masks, oxygen tents, or “Alpine houses.”

For sports training, a simplified distinction of three primary exercise intensities is used: aerobic, anaerobic, and critical (Bunc, 1990; Keul et al., 1978). With regard to the importance of indicating changes in acute performance at various altitudes and exercise intensities before the onset of acclimatization processes, we focused on this issue in our study. For verification, we utilized the SkiErg machine because only a few studies have been conducted, despite its wide application in the field and beyond. We identified only a single study (Zinner et al., 2015) that dealt with performance changes in artificially induced altitude using double-poling ergometer.

This study aimed to determine the impact of artificially induced altitude (2,000 m ASL) on acute performance changes in aerobic and anaerobic threshold levels. For testing, imitation of double-poling was used on elite cross-country skiers via a sport-specific test on the ski double-poling ergometer. We focused on the lactate concentration (LC) and heart rate (HR) values for aerobic and anaerobic intensities. We hypothesized that the differences in these parameters at both altitudes would be statistically significant and have a medium effect size.

Methodology

Intervention

A timeline of the experiment is shown in Figure 1. In total, three imitated double-poling tests were performed using the double polling ergometer machine and a hypoxic generator over three consecutive days:

• 1. The graded stress test was modified according to Buskirk et al. (1967) and Faulkner et al. (1968) to determine individual AE and ANE values.

• 2. Non-standardized aerobic and anaerobic intensity test at low altitude (500 m ASL).

• 3. Non-standardized aerobic and anaerobic-intensity test at artificially induced high altitude (2 000 m ASL).

Figure 1. Timeline of testing on double-poling ergometer.

The individual resistance during the first test was defined based on a graded stress test. During all tests, the subjects upheld the defined intensity of exercise using an electronic display with information on power in watts and heart rate (HR).

All subjects underwent all three tests at the preset intensity. The temperature of the room during testing was 22 – 24 °C. During all the tests, the subjects breathed exclusively through a mask with a supply of air from a hypoxic generator. Before testing, all participants were informed in detail about the technique of using the ergometer, the necessity of constantly checking that the mask was tight, and the necessity of completing the tests responsibly and with the required effort. The subjects regularly used double-poling ergometer during their training. Following recommendations (Pacholek 2021, 2023; Pacholek & Zemková, 2022), the research team members actively motivated all subjects verbally to the same level throughout the tests.

In accordance with the recommendations of SkiErg’s manufacturer, the technique for imitating double poling during cross-country skiing is as follows:

• 1. Standing with hands and feet shoulder-width apart and hands slightly above the head, arms bent at the elbows.

• 2. The subjects drove the handles downward by engaging their core abdominal muscles and bending their knees; they maintained the bend in their arms to keep the handles fairly close to their face.

• 3. They finished the drive with knees moderately bent and arms extended alongside their thighs.

• 4. They extended their arms upward and straightened their bodies to return them to the starting position.

• 5. If technical violations were observed, they were immediately corrected verbally by invigilators.

The machine’s resistance is directly proportional to the force exerted by the athlete; the greater the force expended, the greater the resistance created by the ergometer (measured in watts). This double-poling ergometer (www.concept2.com) utilizes the resistance of air set by a valve, but the amount of air flowing into the gyroscope does not impact the resistance, only the cadence of the pulls. The valve was set at level 7 for the entire period for all subjects. Performance was continually recorded in the ergometer’s memory and portable USB drive, making it possible to analyze the exercise.

The hypoxic generator HYP-100™ (Hypoxico Inc., USA) was used to induce artificial hypoxia. Before testing, the hypoxic generator was tested using a Servomex 1440 device (Servomex Group Ltd., Crowborough, UK) at the Biomedical Lab (Faculty of Physical Education and Sports, Charles University), and the data were read according to the manufacturer’s instructions. An exercise bike, Hause Fit (Ontario, CA, USA), was used for cooling and regeneration. The Polar S810 device (Polar Electro Oy, Kempele, Finland) continuously monitored the HR. LC was analyzed from capillary blood samples obtained from the finger using a SensoStar GL device (DiaSys, Holzheim, Germany). According to the manufacturer, it was measured with a variation coefficient of <2.5% for 24 samples (each with a volume of 90 mg.dl⁻¹). Vital lung capacity was assessed using a MIR Spirobank II analyzer (MIR Inc. Roma, Italy). The manufacturer claimed an accuracy of ±3%, or 50 ml. Anthropometry was performed by an experienced person using the skinfold measurement method. Relative body fat percentage can be estimated by measuring skinfold thickness at five sites: biceps, triceps, subscapular, suprailiac, and medial calf. The measurements are summed, multiplied by two, and the result is compared to age- and sex-specific reference tables to determine body fat percentage. This method is practical, non-invasive, and reliable when performed using standardized techniques (Parizkova 1961).

Results

Individual LC curves, including AE and ANE intensity, were established based on the initial graded stress test on double-poling ergometer. The determined intensities were then verified expertly by the racers’ trainers, who adjusted the intensity for two participants by 5% and 7%. These individuals underwent exercise at defined aerobic and anaerobic intensities at low (500 m ASL) and artificially induced high altitudes of 2 050 m ASL afterward.

Table 2 reports the mean lactate values during aerobic and anaerobic intensity exercise. Table 3 reports the mean HR values in AE, and Table 4 reports the mean heart rate values in ANE.

The differences in the average LC during and following the tests at aerobic and anaerobic intensities ( Table 2) at both altitudes were neither statistically significant (p > 0.05) nor substantively significant. We did not evaluate the LC level after warming up, as it is not sufficiently reliable, although we attempted to manage the course of warming up as a standard.

The average HR values of all tests at high altitudes were consistently statistically significant compared to those at low altitudes (p < 0.05). The substantive significance of the differences was at the boundary of the medium-to-large effect.

The calming kinetics in the first and second minutes after AE and ANE were not statistically significant (p < 0.05) or substantively significant.

Discussion

Our study showed that the high altitude HR values were statistically (p < 0.05) and substantively higher in lowland areas at both the AE and ANE levels. For the average lactate values, no statistically significant or substantively significant differences were reported. The data partially correspond to our prior studies (Suchý, 2012), with the one difference being that the acclimatization of the athletes preceded the tests conducted at high altitudes. The “lactate paradox” (Hochachka et al., 2002; Lundby et al., 2000) was not discovered in our study. This could be because our participants did not undergo maximum-intensity exercise.

The results demonstrate that an acute artificially induced altitude of 2000 m ASL among the monitored cross-country skiers led to an increased internal load intensity indicated by HR. The general findings of this test correspond to those of previously published studies. However, these methods do not utilize artificially induced altitudes or a double-poling ergometer. It has been confirmed that adapted individuals have a 3–5% higher HR at a higher altitude during the same load as at sea level (Fuchs & Reiss, 1990; Powel & Garcia, 2000; Pupiš et al., 2014; Truijens et al., 2008; Wilber, 2004). An interesting indicator at altitude without prior adaptation is the average HR value between minutes 6 and 7 for ANP, where the value at the artificially induced altitude of 2,000 m ASL was six beats higher than at sea level. This value confirms the necessity of realizing adaptation processes before commencing training exercises of the same intensity at low altitudes (Millet & Schmitt, 2011; Wilber, 2004). Furthermore, the results confirm that HR is more suitable than LC for monitoring the immediate reaction of a body that is not adapted to higher altitudes in capillary blood. This fact is consistent with publications reporting an increase in HR during maximum load by 10% to 30% in individuals not adapted to altitude (Bonetti & Hopkins, 2009; Millet & Schmitt, 2011; Suchý, 2012; Wilber, 2004).

These results correspond to those reported by Zinner et al. (2015). The study reported changes in performance during artificially induced high altitudes and lowlands using double-poling ergometer. The test design lasted for 3 × 3 min. of maximum intensity exercise, with the subjects also having hypoxic conditions during the break, in contrast to our design. Sandbakk et al. (2015) performed all-out tests on a double-poling ergometer machine in highly trained athletes who were sitting. This testing method differs fundamentally from ours; thus, no comparison is possible. Testing subjects in standing positions is more appropriate for a better simulation of cross-country skiing. Fukuda et al. (2014) used double-poling ergometer to confirm a multicriteria accord of results in 3 × 3 min all-out tests with a maximum load of 300 m, 650 m, and 1 000 m. Therefore, we considered the design of our study appropriate.

The 7-minute AE and ANE performance test segments corresponded to the subjects’ current fitness level. The altitude of 2000 m ASL was selected for its contribution to practical application, and the possibility that results in the tests could have improved with optimization of the technique of pulling, thus degrading the results, can be ruled out because all participants regularly used the ergometer for training and tests. The double-poling ergometer tests are also part of the test battery of the Czech Ski Association.

We did not analyze the intersexual differences in the results because the study was conducted on a small set and the goal was to compare the differences in exercise at low and high altitudes. However, no significant differences in the performance of men and women during the all-out tests on double-poling ergometer were identified by Hegge et al. (2015).

In the structured post-test interviews, the participants did not report any problems with the masks. They also reported no complications related to inhaling relatively dry air from the hypoxic generator. The reason is their experience with this type of mask and the dry air from the functional examinations they undergo several times per annual training cycle.

This study has some limitations. It was impossible to test the impact of acute changes in altitude on the critical intensity. Unfortunately, this intensity could not be measured because of the limited capacity of the hypoxic generator. For this assessment, it is necessary to use (at least) two hypoxic generators and acquire larger sacs that are not supplied by the manufacturer. Information on critical intensity exercise at altitude could have interesting benefits for application, for example, as an input for training management during unplanned shifts to altitude or arriving at races at altitude without prior adaptation. We consider the use of face masks as a limiting factor in the study of equipment. Although we used a very high-quality rubber band, we were forced to attach it to a second rubber band so that it fit snugly. Despite all efforts to affix the mask to the face perfectly, there is a certain risk that for female subjects, the test could have been slightly influenced by the mask not being tight, thus inhaling partially normoxic ambient air. We minimized this risk by performing regular checks. The experts set the initial load values for the graded stress tests. The initial value was set improperly for two subjects, so it was necessary to repeat this test. No statistically or substantively significant change was found immediately before the second test, but in light of the fact that the value being investigated just before the test, and the subjects had been 5 min without the mask providing air from the hypoxic generator, we do not consider this value essential. The AE and ANE were defined based on a sea-level graded stress test. If we were to choose a precise term for the exercise at an artificially induced altitude, then the exercise the athletes completed here should not be called such. For a better understanding, we can use oxygen saturation measurements. Another limitation is the methodology determination of AE and ANE by Buskirk et al. (1967) and Faulkner et al. (1968), which has been questioned in the current literature, for example, by Poole et al. (2021). The “classical” methodology that we applied is frequently used as a part of the training process management in cross country skiing.

Based on our investigation and past studies, we are considering a similar study design at natural altitudes, that is, without the use of hypoxic generators. In this way, the study could be expanded to include the speed of acclimatization processes in confrontation with the drop in strength endurance and purely endurance performance at higher altitudes compared to testing at sea level.

Conclusion

Not-adapted elite young cross-country skiers have higher HR at artificially induced high altitudes than for the same load in lowlands. The absence of alterations in the average LC confirms that it is more appropriate to monitor the HR for altitude acute effect assessment and employ LC only for verification.