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Comparison of Heart Rate Deflection and Ventilatory Threshold During a Field Cross-Country Roller-Skiing Test

Fabre, Nicolas1; Passelergue, Philippe1; Bouvard, Marc2; Perrey, Stéphane3

Journal of Strength and Conditioning Research: November 2008 - Volume 22 - Issue 6 - p 1977-1984
doi: 10.1519/JSC.0b013e3181874ae9
Original Research

Fabre, N, Passelergue, P, Bouvard, M, and Perrey, S. Comparison of heart rate deflection and ventilatory threshold during a field cross-country roller-skiing test. J Strength Cond Res 22(6): 1977-1984, 2008-This study was to assess whether the point of deflection from linearity of heart rate (HRd) could be an accurate predictor of ventilatory threshold (VT2) during a specific cross-country roller-skiing (RS) test. Ten well-trained cross-country skiers performed a maximal and incremental RS test in the field and a standardized maximal and incremental treadmill running (TR) test in the laboratory. Values of oxygen uptake (o2) and heart rate (HR) were continuously recorded during all exercises by a portable breath-by-breath gas exchange measurement system and a wireless Polar monitoring system, respectively. The VT2 and HRd points were individually determined by visual analysis during RS. Maximal o2 (o2max) and HR were higher (p < 0.05) during TR (67.1 ± 7.3 ml·min−1·kg−1 and 196.0 ± 14.1 bpm, respectively) compared with RS (64.2 ± 7.3 ml·min−1·kg−1 and 191.5 ± 13.1 bpm, respectively). However, a high correlation (r = 0.94, p < 0.01) between TR and RS o2 max was observed. Paired t-tests showed no significant differences in HR (183.6 ± 15.1 vs. 185.2 ± 13.9 bpm) and o2 (55.5 ± 7.1 vs. 55.8 ± 6.1 ml·min−1·kg−1) at intensities corresponding to HRd and VT2 during the RS test, respectively; Pearson product-moment correlation coefficients demonstrated significant relationships for HR at the HRd and VT2 points (r = 0.99, p < 0.001) as well as for o2 (r = 0.95, p < 0.001). Our results indicate that the specific incremental RS test is effective in eliciting HRd in the field for all skiers and is an accurate predictor of VT2. These findings give very interesting practical applications to cross-country coaches and skiers to evaluate and control specific aerobic training loads.

1Laboratoire d'Analyse de la Performance Sportive, Département STAPS, Université de Pau et des Pays de l'Adour, Tarbes, France; 2Centre de Biologie et de Médecine du Sport de Pau, Centre Hospitalier de Pau, Pau, France; and 3EA 2991 Efficience et Déficience Motrices, Montpellier, France

Address correspondence to Nicolas Fabre, fabre.nicolas@aliceadsl.fr.

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Introduction

To optimize aerobic training loads in endurance sports, relevant, reliable, and individual markers of intensity and duration are required. Laboratory tests are usually used in this way. However, these costly tests are not always specifically adapted to training constraints; they require individual setups and, thus, a lot of time; and, sometimes, the ergometer used is not always specific to the activity. This last issue is particularly true for cross-country skiing, where the upper-limb propulsion seems essential for skiing performance (3,14,20,28). Consequently, many coaches have been interested in ski-specific laboratory testing, such as upper-body ergometry, to simulate the arm movement of double poling technique (3,20,36). Yet, this experimental situation is rather far from the real conditions encountered during cross-country skiing. In 1996, Rundell (27) was one of the first to evaluate cross-country skiers with roller-skis on a large, motorized treadmill. He showed that peak physiological parameters during a treadmill roller-skiing test were significantly lower than those during a treadmill running test.

This treadmill roller-skiing test seems most appropriate for cross-country skiing, but this kind of large, motorized treadmill is very scarce around the world and takes time to practice. So, a roller-ski testing session in the field could be the best answer for evaluating cross-country skiers. By comparing a laboratory running test and field roller-skiing test, Vergès et al. (34) highlighted the importance of exercise and protocol specificity for intensity prescription. However, their protocol was too long (four 4-km timed loops) to determine accurate training intensity zones. One of the most popular field tests remains that proposed by Conconi et al. (11). These authors have reported that heart rate (HR) as a function of speed is not linear up to the maximum and that the speed corresponding to the deflection point from linearity of HR (HRd) also corresponds to the speed of the second lactic or ventilatory threshold (VT2). Although contested for physiological and methodological reasons (6,7,15-18,30-32), numerous studies (1,5,8,9,15,21) report that Conconi et al.'s HRd point, when observed, does represent a useful standardized intensity for aerobic testing and training. Conconi et al.'s HRd point already has been observed during incremental tests with cyclists (8), runners, rowers (9,13), swimmers (10), and cross-country skiers (13). In the latter, the HRd point has been compared successfully to the second lactate threshold using a “fixed distance” stage protocol (i.e., speed increased every 140 m). However, this kind of protocol has been targeted as an explanation for the occurrence of an HRd point (16). In fact, with this type of protocol, the time interval of each stage decreases progressively (especially near the end of the test) to the extent that the circulatory system cannot effectively adapt to the increasing workload. This will be manifest as a lagging in HR response and visually observed as an HRd point. This continuous decrease of the stage duration with ineffective cardiovascular adaptation implies that the HRd point may be an artifact of the protocol (see Bodner and Rhodes [4] for a review).

Thus, the present study was designed to assess whether Conconi et al.'s HRd point may be an accurate predictor of ventilatory threshold during a specific cross-country roller-skiing test (RS) with a “fixed time” stage protocol, which is less contested for methodological reasons and easier to carry out than a “fixed distance” stage protocol. To do this, we compared relevant physiological parameters recorded with a portable gas analyzer and HR system at HRd/VT2 during a field incremental RS test. Maximal values during RS were also compared with the maximal values obtained during a traditional treadmill running (TR) laboratory test.

During cross-country ski session training, the skiers are unable to determine their training intensity using speed (as, for example, a runner on a track might) because of the variations of snow conditions and field profiles. They can only use HR or exercise perception to assess exercise intensity. So, the HRd point could be useful for setting training parameters at or near VT2. Moreover, this test would be relatively simple to perform with roller-skis, noninvasive, and inexpensive, and it could be used within the training regimens of the skiers.

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Methods

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. We hypothesized that a field RS test with HR monitoring may be an accurate method for aerobic testing and training intensity determination in comparison with gas exchange measurements. In this way, we proposed using Conconi et al.'s HRd point as a predictor of VT2 during such an RS test. To verify the usefulness of the specific RS test, the HRd point and the VT2, recorded simultaneously during RS in trained cross-country skiers, were compared by examining the Student's t-test, Pearson correlation, and limits of agreement.

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Subjects

Ten well-trained cross-country skiers (10 men, regional to national class) volunteered to participate in this study. Participants were asked to refrain from ingesting caffeine or alcohol for at least 12 hours before testing. They were asked 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 local ethics 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.

Table 1

Table 1

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Experimental Protocol

The two following tests (i.e., field and laboratory tests) were carried out in a random order and during a short period (about 10 days) to minimize the possible effects of training. This study took place during October, when skiers had already reached a high volume of training, especially in cross-country roller-skiing.

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Field Cross-Country Roller-Skiing Test

The study was performed on a 3-km asphalt roadway that was free of sharp turns (start test altitude: 1134 m; end test altitude: between 1208 and 1288 m, according to skiers' performances). The roadway surface was maintained free of rocks and other debris during the testing. The grade of the hill was constant (8.0 ± 1.0%). Wind velocity was measured with an anemometer (V-C5190 Skymaster, Vion Meteo Concept, France) and was always less than approximately 3.4 m·s−1 during all tests. All testing was performed during the month of October at a mean ambient air temperature of 20.5 ± 4.0°C. The subjects were familiar with all roller-skiing techniques and routinely used them during practice while roller-skiing or skiing on snow. The same roller-skis (Swenor Carbonfibre, Norway) were used by all subjects. Cross-country skiers used their own poles and shoes.

The continuous incremental field test involved roller-skiing with diagonal stride technique (classical) at a constant grade (i.e., 8%) beginning at 5 km·h−1 and increasing 1 km·h−1 every 1 minute until exhaustion. The speed was fixed by an experimenter set just before the skier on a bicycle with a calibrated speedometer (Sigma Sport BC 800, Germany). The test was finished when subject was unable to maintain the imposed speed.

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Laboratory Treadmill Running Test

On another day, the same subjects performed an incremental running treadmill test (Training Treadmill S2500, HEF Techmachine, Andrézieux-Bouthéon, France) (altitude: 210 m). The test began at a walking pace (5 km·h−1), followed by four increments of running at zero grade at 1-minute intervals (7, 9, 10.5, and 12 km·h−1); thereafter, speed remained at 12 km·h−1, but slope increased by 2% at 1-minute intervals until volitional fatigue.

To be certain that subjects reached their maximal oxygen uptake (o2max), the following criteria were used: a respiratory exchange ratio above 1.1, a measured maximal HR higher than 95% of theoretical maximal HR (220 − age), and an inability to maintain treadmill pace. All criteria were achieved during each test except the maximal HR during RS, which was lower than 95% of theoretical maximal HR in two subjects (subjects 9 and 10).

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Physiological Measurements

Values of minute ventilation (e), carbon dioxide output (co2), and oxygen uptake (o2) were continuously determined during all exercises 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%; CO2: 0.03%) and a gas mixture of known composition (O2: 16.00%; 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, Tex). Face masks allowed subjects to simultaneously breathe with the mouth and nose, for more comfort. Each subject's HR was continuously measured via a wireless Polar monitoring system (Polar Electro Oy, Kempele, Finland) and was synchronized with the Cosmed system.

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Data Analysis

The HRd point on the HR/skiing speed curve corresponds to the point where the curve loses its linearity and starts to plateau (11,12). From the visual analysis of the curves, two blinded investigators individually determined the HRd point (Figure 1). If a difference was observed between the results of the two investigators, a third one was asked to do the analysis (this situation happened one time).

Figure 1

Figure 1

Three other reviewers individually determined VT2 by visual analysis of the breakpoints of the ventilatory equivalent of carbon dioxide (e/co2), ventilatory equivalent of oxygen (e/o2), and e changes over time (33,35) (Figure 1).

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Statistical Analyses

Paired t-tests were used to discern any significant differences between the means for the physiological variables at the HRd point and at VT2 during the RS test, on the one hand, and between the maximal physiological variables obtained during the field and the laboratory tests, on the other hand. The Pearson product-moment zero-order correlation coefficient demonstrated any significant relationship. Linear regression was calculated to show the relationship between the TR and RS tests for dependent variables. For all statistical comparisons, the level of significance was set at p 0.05. The values presented are expressed as mean ± standard deviation (SD).

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Results

Comparison Between Maximal Data During Treadmill Running and Roller-Skiing Tests

Table 2 summarizes peak physiological variables during the TR and RS tests.

Table 2

Table 2

Subjects' o2max and HRmax values were higher (p < 0.05) during TR (67.1 ± 7.3 ml·min−1·kg−1 and 196.0 ± 14.1 bpm, respectively) compared with RS (64.2 ± 7.3 ml·min−1·kg−1 and 191.5 ± 13.1 bpm, respectively). There was a high correlation (r = 0.94, p < 0.01) between TR and RS o2max (L·min−1) (Figure 2). As shown in Figure 2, RS o2max was consistently and appreciably lower than TR o2max (intercept of the regression equation of 1.70; SE = 0.33; p = 0.002). Also, the correlation showed that 88% of the variability of RS o2max could be explained by the TR o2max (r 2 = 0.88; SEE = 0.10). No significant difference was observed for emax between the two tests.

Figure 2

Figure 2

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Comparison Between Point of Deflection From Linearity of Heart Rate and Ventilatory Threshold Points During Roller-Skiing Test

Individual data for relevant physiological variables at the HRd and VT2 points during the RS test are represented in Table 3.

Table 3

Table 3

At the HRd point, HR and o2 represented 95.8 ± 2.2 and 86.6 ± 6.3% of the maximum values, respectively. At VT2, HR and o2 represented 96.7 ± 1.3 and 87.1 ± 5.4% of the maximum values, respectively.

Paired t-tests showed nonsignificant t-values for HR corresponding to the HRd point (183.6 ± 15.1 bpm) and HR corresponding to VT2 (185.2 ± 13.9 bpm, p > 0.05) and for o2 corresponding to the HRd point (55.5 ± 7.1 ml·min−1·kg−1) and o2 corresponding to VT2 (55.8 ± 6.1 ml·min−1·kg−1, p > 0.05). Pearson product-moment correlation coefficient demonstrated significant relationships for HR at the HRd point and HR at VT2 (r = 0.99, p < 0.001, Figure 3A), and for o2 at the HRd point and o2 at VT2 (r = 0.95, p < 0.001, Figure 3B).

Figure 3

Figure 3

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Discussion

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. (20) have proposed that such tests should involve the whole muscle mass corresponding to that 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 (26,27,34). Yet, trainers and athletes are looking for the best indicators of optimal intensity during prolonged effort. Although controversial, Conconi et al.'s method is often used to fulfil that purpose.

Thus, the present study was mainly designed to assess whether Conconi et al.'s HRd point could be an accurate predictor of ventilatory threshold during a specific cross-country roller-skiing test. At first, we needed to propose on the field a maximal, continuous, and incremental RS test with comparison with a maximal TR test. During RS, we found significant lower peak physiological variables (i.e., HR and o2) compared with those measured during TR. This is consistent with the results observed by Rundell (27) with trained cross-country skiers. However, in another study, Rundell (26) has identified similar peak o2 values in elite women biathletes during treadmill roller-ski skating and treadmill running. In our study, the relatively low peak o2 values are probably a consequence of a lower ski-specific training of our subjects (regional to national class) compared with elite skiers (26). Moreover, the associated lower HRmax and o2max values in RS compared with TR suggest that the skiers may have ended the test before achieving true o2max in RS, perhaps because of early muscle fatigue. Yet, achievement of a o2 plateau before volitional termination of the RS and TR tests in all skiers indicates that the skiers did really achieve o2max during each protocol. However, the lower HRmax during the RS test is evidence of a local, ski-specific muscle fatigue involved in the RS test termination (26). Another plausible explanation of these lower peak values could be an effect of the exposure to moderate hypoxia during the RS test. In fact, RS was carried out at an altitude ranging between 1134 and 1288 m at the highest. Squires and Buskirk (29) have concluded that o2max in physically well-conditioned persons is reduced by 4.8% during acute exposure to 1219 m and above. However, a high correlation of o2max (r 2 = 0.88) between the standardized running test and the RS test could permit us to consider RS as a valid field test compared with the standardized laboratory test.

Secondly, and mainly, we aimed to compare the HRd point and the ventilatory threshold during the RS test in the determination of the anaerobic threshold to calibrate the intensity of training. In this way, we compared, during the same incremental RS test, the HR corresponding to VT2 and the HR corresponding to the HRd point, both visually determined by different, independent, and experimented reviewers. Visual analysis is one conventional method for HRd point assessment: Ballarin et al. (2) have reported significant correlations (r > 0.94) between a computer-determined method and visual inspection by experienced observers, and they recommend that visual HRd point analysis could be executed by experienced observers. The high correlation observed between HR at the HRd point and HR at VT2 (r = 0.99, Figure 3A), on the one hand, and between o2 at the HRd point and o2 at VT2 (r = 0.95, Figure 3B), on the other hand, confirms the previous results of Droghetti et al. (13) in various physical activities. Interestingly, these authors report during roller-skiing the validity of the determination of the lactate threshold with the HRd point method for cross-country skiers. The two main differences between the latter and the present study lie in the comparison with the lactate threshold-not with the ventilatory threshold-and with a different incremental protocol. Our “fixed time” protocol seems easier to carry out and is less critical than the “fixed distance” protocol used by Droghetti et al. (see Introduction).

In other respects, in most of the protocols that were carried out regarding the determination of the HRd point and its utility for training, intensity was often set according to the speed corresponding to the HRd point (and not according to the HR at the HRd point). Such an indicator (i.e., speed at HRd) is unusable by cross-country skiers because of the constant variations of snow conditions and field profiles. Passelergue et al. (21) has shown that training at a constant HR corresponding to the HRd point and the regular use of a HR monitor were more appropriate than training at the speed corresponding to the HRd point. This statement gives additional interest to the determination of the HRd point because HR seems like the unique parameter that can be used by cross-country skiers to monitor their training intensity.

Many hypotheses have been made to explain the physiological basis for the HRd point, but this is still not fully understood: a greater release of oxygen from hemoglobin with the augmentation of metabolic acidosis could lead to an augmented o2 by the tissues and, so, to an increase in exercise intensity without the same linear increase in HR (12). Other studies have shown that the parasympathetic drive (24), the intrinsic myocardial function (23), or a greater cardiac wall dimension (19) could contribute to the HRd point at high levels of exercise. Despite this lack of a strong explanation, our result is supported by other studies that have reported that the HRd and VT2 points were significantly related (5,8,25). The high correlation between o2 at the HRd point and o2 at VT2 (r = 0.95, p < 0.001), and the nonsignificant differences between o2 at the HRd point and at VT2, are in line with the results of Bodner et al. (5) and Bunc et al. (8), who also observed high relationships between o2 at the HRd point and o2 at VT2 in cycling (r = 0.87, p < 0.001 and r = 0.72, p < 0.001, respectively, for the two studies) and no differences in o2 and HR at the HRd point and at VT2.

In summary, this study has shown a strong relationship between the VT2 and HRd points during a maximal roller-skiing test on the field. But, further research with steady-state exercise at an intensity corresponding to the HRd point is required to fully validate our findings.

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Practical Applications

From a practical point of view, the results of this study give cross-country skiers and coaches two important key points for training programs. First, skiers can easily use an incremental field roller-skiing test to determine or evaluate their maximal aerobic capacity throughout the year. This test is not expensive, it is easy to carry out (several skiers can perform the test at the same time), it does not require individual setups, and it is specific to the real field athlete's activity. Second, using the HRd point method, coaches (once they have experimented) can precisely and individually determine the intensity corresponding to the VT2 (intensity close to competitive intensity) without very expensive materials. Determination of this intensity is useful for skiers and/or coaches because it is a key factor of performance in cross-country ski races. Indeed, this intensity represents the capacity of the athletes to utilize an important part of their maximum oxygen consumption (i.e., endurance). To improve their endurance capacity and, therefore, their race performances, skiers have to perform some specific training sessions (usually interval training sessions) precisely at the intensity corresponding to VT2 or HRd. Lastly, skiers can regularly reevaluate this specific intensity more often (this is usually done once a year with laboratory tests).

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Acknowledgments

The authors gratefully acknowledge the assistance of all those who took part in this study, particularly all the subjects from the Ski Club Azun and their coach.

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Keywords:

heart rate; oxygen uptake; training intensity; specific test

© 2008 National Strength and Conditioning Association