The capacity of an athlete to use an important part of his maximum oxygen uptake (V̇O2max) is a key issue for performance in long duration sports. This capacity, also called endurance, can be estimated during laboratory or field incremental tests, by determining the anaerobic threshold of an athlete. The work intensity corresponding to this threshold appears to be one of the most important parameters to be determined during a testing session. Then, knowing precisely this intensity, in terms of speed (i.e., for runners), power (i.e., for cyclists), and/or heart rate (i.e., especially when speed and power are unusable as during crosscountry ski activity because of snow conditions and field profile changes), athletes can perform some specific training at this intensity to improve their endurance capacity and, therefore, their performances.
During an incremental test to exhaustion, the anaerobic threshold can be determined by examining the changes in blood lactate concentrations and/or in the evolution of the ventilatory parameters. Measuring blood lactate requires invasive procedures, such as capillary, venous, or arterial blood sampling, and the lack of accuracy to detect temporal metabolic changes during graded exercise may be a major limitation (6). The ventilatory method has been proposed to circumvent these problems routinely (23) and is now considered as the standard for clinical evaluation and research settings (3,5,16). The physiological basis of this noninvasive procedure is supported by the observation of an increase of ventilatory drive, consequent to respiratory compensation, for metabolic acidosis (i.e., respiratory compensation point [RCP]).
However, it has been shown that the interaction between limb movements and breathing leads to coordination (also termed “entrainment”), particularly during arm propulsion exercises such as wheelchair propulsion (2), rowing (19), or roller-ski skating (10) and, more precisely, that the locomotor rhythm entrains the breathing one and not the reverse. Moreover, it has been already observed that neuromechanical locomotion-linked respiratory stimuli appear stronger than peripheral chemoreceptors-linked respiratory stimuli during steady-state rowing (11) and cycling (18) exercises. In the light of the foregoing observation, we can easily hypothesize that the strong locomotor-linked neural (increased drive from the phasic impulses from the working limbs and body position) and mechanical (cramped position) respiratory drives, already described during roller-ski skating exercise (10), can sufficiently perturb the ventilatory pattern observed during typical incremental running or cycling test (i.e., increase of ventilatory drive consequent to respiratory compensation for metabolic acidosis) to make it hazardous or impossible to determine the anaerobic threshold through the ventilatory method. Therefore, this observational study aimed to compare the arm propulsion and the breathing rhythms during an incremental roller-ski skating test to exhaustion, on a breath-by-breath and poling-by-poling basis, to question the validity of the RCP determination during exercises mainly involving the upper body in the propulsion. These results could be of great interest for practitioners and/or coaches in that they help adapt the protocol of their testing sessions.
Experimental Approach to the Problem
This study was carried out to question the validity of the ventilatory method to determine the anaerobic threshold (RCP) during an incremental roller-ski skating test to exhaustion because of the strong link already observed between respiration and locomotion during upper-body exercises. To attempt this issue, 3 blinded reviewers were asked for the individual determination of the RCP by visual analysis of the different ventilatory parameters, and the interobserver reliability was examined to verify the validity of the ventilatory method. Then, to test our hypothesis (i.e., disturbance of the ventilatory pattern by mechanical constraints on the respiratory apparatus), the trend of the poling parameters (peak poling force and poling frequency [Pf]) and that of the ventilatory parameters (tidal volume [VT] and breathing frequency [Bf]) at each stage of the incremental test were compared. We hypothesized that the evolution of the Pf and Bf on the one hand and that of the VT and the poling force on the other hand were really superposable during the incremental test, clearly showing the relationship between poling and breathing patterns.
Nine male elite crosscountry skiers from the national Italian ski team, involved during winter in European and/or World Cup crosscountry and/or Winter Olympic ski races, participated in this study. Their mean ± SD of their age was 29.1 ± 3.7 years, height was 1.78 ± 0.04 m, body mass was 71.7 ± 5.6 kg, V̇O2max was 78.2 ± 5.2 ml·min−1·kg−1, and performance level (International Ski Federation [FIS] points from Distance FIS race) was 19.9 ± 14.2 (a skier's rank is relative to a 0-point standard established by the top-ranked skier in the world). They were asked to refrain from ingesting caffeine or alcohol for at least 12 hours before testing, to eat a light meal 3 hours before testing, and to not perform exhaustive training for at least 48 hours before testing. The study protocol complied with the declaration of Helsinki for human experimentation and was approved by our University ethical committee for human research. The possible risks and benefits were explained, and written informed consent was obtained from each skier before participation.
The skiers carried out an incremental crosscountry roller-ski test with the V2 skating technique. This skating technique uses a symmetrical double-pole plant during which weight is transferred to each ski (i.e., 2 pole plants for 1 leg cycle, Figure 1B). This test was performed while roller skiing on a large motor-driven treadmill (belt dimensions of 2.5 m × 3.5 m, Rodby, Sodertalje, Sweden) at a constant slope of 3°. The start speed was fixed at 10 km·h−1 for 3 minutes and then increased by 1 km·h−1 every 1- minute work period until exhaustion. This study took place during October when the skiers had already reached a high volume of training especially in crosscountry roller skiing. During the test, the subjects used the same pair of roller skis (Ski Skett Nord CL, Sandrigo, Italy).
At the beginning of the 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 a treadmill.
The values of minute ventilation ( V̇E), Bf, VT, carbon dioxide output V̇CO2, and oxygen uptake (V̇O2) were continuously measured by means of a portable breath-by-breath gas exchange measurement system (Cosmed K4b2, Rome, Italy, Figure 1A). This system has been shown to be accurate and reliable during field and laboratory tests (8,12). 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: 16.04% 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 light weight (800 g) with the main sample unit attached to the chest and the battery pack placed 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 (Cosmed). Face masks allowed the subjects to simultaneously breathe with their mouth and nose, for more comfort.
The force exerted by the athletes and directed along the poles was continuously measured by a light weight single-axial load cell (Deltatech, Sogliano al Rubicone, Italy) mounted inside standard poles (Diamond Storm 10 Max; OneWay, Helsinki, Finland) under the handgrip (Figure 1A). Instrumented poles were available at multiple lengths of 2.5 cm so that each athlete could use his preferred pole length. Analog signals from the force transducer were sampled at 200 Hz by means of a data acquisition board (NI DAQ-PAD-6016, 16 bit; National Instruments, Austin, TX, USA) (4). Poles instrumented with force transducers were dynamically calibrated before the test of each participant with a load cell used as reference (546QD; DSEurope, Milan, Italy). The calibration was made by pushing the instrumented pole against the reference load cell, simulating 8–12 poling cycles. A linear regression between the values measured on the reference cell and the values obtained for pole force transducer was calculated. When the square coefficient of regression was >0.995 (value arbitrarily chosen), the slope and intercept coefficient were used to convert electrical signal values of the pole load cell into force data. If the square coefficient of regression was <0.995, the calibration procedure was repeated (17).
Respiratory Compensation Point Determination
Three blinded reviewers were asked to individually determine the RCP by visual analysis of the breakpoints of the ventilatory equivalent of carbon dioxide (V̇E/V̇CO2), ventilatory equivalent of oxygen (V̇E/V̇O2), and V̇E changes over time. The second increase in V̇E with a concomitant rapid increase in V̇E/V̇O2 and V̇E/V̇CO2 corresponded to RCP (15,21-23). These experts were chosen because of their significant amount of experience in RCP determination (>300 tests analyzed per year) and because they were absolutely unaware of the objectives of the study.
The poling cycle was defined as the period from the start of the pole ground contact to the start of the subsequent pole ground contact. The pole plant was identified on force data as the first point above a force threshold of 10 N. The pole take-off was identified as the first point below this force threshold. This value was chosen as the threshold because we had verified it to be the minimum value above the level of typical noise recorded during the recovery phase in our data and as also shown by Pellegrini et al. (17). For each cycle, the Pf and the maximum poling force (F peak) were determined.
For each stage of the incremental test, the ventilatory determinants (i.e., Bf and VT) and the poling signals (i.e., Pf and F peak) were averaged and synchronized.
Values presented are expressed as mean ± SD. When at least 2 reviewers identified a point that could correspond to RCP, interobserver reliability was examined using a 2-way mixed model for absolute agreement of intraclass correlation coefficient (ICC). The values of peak poling force and VT were normalized (z-score) to be compared together. The assumptions of normality and sphericity were verified. As data distribution fails the normality test, 2-way (stages × frequencies and stages × F peak/VT) analyses of variance (ANOVAs) for repeated measurements on ranks were chosen to test for differences between poling and breathing patterns at each stage. When the overall difference was found, the Tukey post hoc test was used for specific comparisons. Statistical significance was accepted at p ≤ 0.05. Data were analyzed with the Statistical Package for the Social Sciences (SPSS v15.0, SPSS Inc., Chicago, IL, USA).
At least 1 reviewer was unable to identify a point, which could correspond to the RCP in 6 of the 9 skiers, and at least 2 reviewers agreed with the impossibility of determining it in 4 of the 9 skiers. When at least 2 reviewers identified a point, which could correspond to the RCP (i.e., in 5 of the 9 skiers), the interobserver reliability for the time at RCP, as assessed by the ICC, revealed poor reliability coefficients (ICC [95% confidence interval] = 0.46 [−0.40 to 0.86], p = 0.36) (Figure 2).
The 2-way ANOVAs for repeated measurements on ranks showed no significant difference between Bfs and Pfs during the last 9 stages (p values ranging from 0.06 to 0.97, Figure 3). As shown in Figure 4, the skiers used either a strictly 1:1 or a 1:2 Bf to Pf ratio when the intensity of the exercise was moderate.
The evolution of VT seemed to follow exactly the same trend as F peak did. As observed in Figure 5, when normalizing (i.e., z-score) VT and F peak values, a significant difference was observed between these parameters only during the first stage (p = 0.04). Thereafter, these 2 curves ran almost parallel with no significant difference (p values ranging from 0.18 to 0.99).
This study aimed at questioning the validity of the ventilatory method to determine the RCP during an incremental roller-ski skating test to exhaustion. The results of this study clearly showed the difficulties for the precise determination of RCP with no evident breakpoints in the parameters traditionally studied (i.e., V̇E/V̇CO2, V̇E/V̇O2, and V̇E changes over time) and with a poor interobserver reliability. These findings make the use of the ventilatory method risky for RCP assessment during an incremental roller-ski skating test to exhaustion.
This phenomenon can be explained by the tight coordination between breathing and locomotor rhythms already reported for activities such as hand-rim wheelchair or rowing, with an increase in respiratory frequency in concert with an increase in arm movement frequency (2,19,20). More specifically, during constant-load roller-ski skating exercise, it has been shown that arm poling drove ventilation, suggesting that the mechanical influences (i.e., rhythmic trunk tilting with a strong tightening of the abdominal muscles and neural impulses from the moving limbs) are predominant compared with the metabolic ones (10). These authors reported that the V2 skating technique (i.e., 2 double-pole plants for 1 leg cycle) led to a relative hyperventilation compared with the V2A skating technique (i.e., 1 double-pole plant for 1 leg cycle) for the same amount of V̇O2. They explained this observation as resulting from a significantly higher Pf using the V2 skating technique than when using the V2A technique. In the same way, Holmberg et al. (13,14) also observed a high correlation between Pfs and Bfs during double poling exercises. This strong relationship has been explained by biomechanical factors such as the rhythmic trunk tilting and a strong tightening of the abdominal muscles during the poling motion (10,13,14). Additionally, the high activation of some of the accessory respiratory muscles during each poling cycle has also been proposed as an explanation (14).
So, the important locomotor-respiratory coupling during activities involving mainly the upper body in the propulsion is well established. However, to our knowledge, this is the first time that this phenomenon has been studied in relation to the determination of the anaerobic threshold thanks to the ventilatory method during an incremental test to exhaustion. We put forward evidence that the determinants of the ventilation (i.e., Bf and VT) are strictly dependent on the poling pattern during an incremental test to exhaustion when the upper body is mainly involved in the propulsion. In fact, except during the first stage, the normalized VT and F peak curves were highly superposable (Figure 5). To sustain the speed increment, the skiers must increase the Pf and/or the forces applied to the poles. Obviously, to obtain this last result, the strength of tightening of the upper-body muscles (mostly), including abdominal and accessory respiratory muscles, increases. Consequently, more important the poling forces are, the more important is the contraction of the respiratory muscles, influencing the ventilatory pump in a more important way (1). In the same way, even if there were significant differences between Bfs and Pfs during the first 4 stages, the Bf was strictly subordinate to the Pf during the entire test. These differences observed during the first stages arose from a more important degree of freedom of the Bf when the exercise intensity was still moderate with the use of either a strictly 1:1 or a 1:2 Bf to Pf ratio (Figure 4).
Thus, this study strongly questioned the validity of the usual recommendation for determining the RCP (15,21,23) and allowed us to give recommendations for the assessment of the anaerobic threshold with ventilatory methods during specific roller-ski laboratory and/or field incremental tests and, more generally, during all exercise modalities involving the upper body in the propulsion. In fact, the physiological basis (see the Introduction) of the ventilatory method appears weak for these particular forms of locomotion. The mechanical constrains imposed on the respiratory apparatus overwhelmed the usually observed metabolic effects (i.e., increase in CO2 concentration, decrease in pH, etc.) on the ventilation. To overcome this issue, a solution could be to reduce the mechanical constrains by fixing the Pf, for example, with an audio signal. This would nevertheless be too restricting for the skier, and the appropriate rhythm would be difficult to choose.
From a practical point of view, the present findings allow us to establish that, when a really tight coordination exists between breathing and locomotor rhythms (this is the case of crosscountry ski skating, but the following idea can also be extended to activities such as rowing or wheelchair propulsion), the determination of the anaerobic threshold, thanks to the ventilatory method, must be used cautiously because it mainly depends on mechanical factors (i.e., compared with the biochemical ones). However, we are aware that we cannot completely affirm that this method is invalid for this activity because we did not directly test this hypothesis (further study is needed to clearly confirm this point). But, considering the crosscountry ski activity, these results taken as a whole and the difficulty met by the reviewers, we propose to single out specific protocols preferably using the diagonal stride technique rather than using the skating or double poling techniques for the determination of the anaerobic threshold via the ventilatory method. With this technique, the arms move alternatively with a lighter trunk tilting than when during double poling or skating, limiting the mechanical constrains imposed on the respiratory apparatus. But, when the skating technique is imposed for testing purpose (i.e., for Nordic-combined skiers and biathletes, who exclusively used this technique during races), the unique method to “properly” determine the anaerobic threshold is the lactate method with sampling at each stage of the incremental test with a minimum stage duration of 3 minutes. Alternative methods, such as heart rate deflection (9) or heart rate variability (7), could also be proposed, particularly during field roller-ski tests, but the use of such methods needs to be more profoundly evaluated.
The authors gratefully acknowledge the assistance of all those who took part in this study, particularly all the subjects from the national Italian ski team and their coaches. The authors wish to declare that they did not receive any funding for this work from any of the following organizations: NIH, Wellcome Trust, HHMI, or other(s).
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Keywords:Copyright © 2012 by the National Strength & Conditioning Association.
respiratory compensation point; breathing; incremental test; locomotor-respiratory coupling; upper body; crosscountry ski