Cardiorespiratory and Metabolic Responses During Graded Exercise in Normobaric and Hypobaric Hypoxia

Abstract

Background:

The study investigated submaximal exercise responses during an acute exposure to normobaric hypoxia (NH) versus hypobaric hypoxia (HH) focusing on different exercise intensities.

Methods:

Eight recreationally trained male subjects (age 23 ± 3 years) performed submaximal cycling exercise at three different intensity levels (100, 150, and 200 W) in NH (simulated altitude 3150 m) and HH (terrestrial high altitude, 3150 m) in a cross-over study design. Cardiorespiratory parameter, blood lactate concentration, and ratings of perceived exertion were determined at each intensity level.

Results:

Cardiorespiratory parameters, arterial oxygen saturation, and ratings of perceived exertion did not differ between NH and HH except for the higher ventilatory equivalent for oxygen in HH compared to NH (25.9 ± 1.3 vs. 24.6 ± 1.0 at 100 W, 28.0 ± 1.6 vs. 27.1 ± 1.6 at 150 W, 32.1 ± 3.9 vs. 31.3 ± 3.6 at 200 W, p = 0.03). Blood lactate concentration tended to be higher in HH compared to NH (1.8 ± 0.9 mmol/L vs. 1.7 ± 0.8 mmol/L at 100 W, 3.2 ± 1.8 mmol/L vs. 2.8 ± 1.6 mmol/L at 150 W, 6.0 ± 3.1 mmol/L vs. 5.5 ± 3.0 mmol/L at 200 W, p = 0.08) with a significant interaction effect for exercise intensity (p = 0.02).

Conclusions:

Cycling during acute exposure to NH appears to result in equivalent cardiorespiratory responses to HH. The more pronounced lactate accumulation in HH should be a topic of future research.

Introduction

The application of hypoxia using simulated high-altitude conditions (e.g., via normobaric hypoxia (NH) rooms or via facemask) is an established tool to optimize athletes' performance at sea level or to preacclimatize endurance athletes and mountaineers before going to high altitude. For example, sprint training in NH showed promising results to improve high-intensity intermittent running performance in team-sport athletes (Hamlin et al., 2018).

In addition, repeated training sessions in NH have been proposed to optimize endurance performance at high-altitude competitions (Chapman et al., 2013). However, the effectivity of NH exposures to prevent AMS and to optimize high-altitude performance is still matter of debate (Millet et al., 2012; Mounier and Brugniaux, 2012) and it was postulated that hypobaric hypoxia (HH) is much more effective compared to NH (Fulco et al., 2013).

In fact, several studies compared NH and HH conditions focusing, for example, on symptoms of acute mountain sickness (Roach et al., 1996; DiPasquale et al., 2016), sleep quality (Heinzer et al., 2016; Saugy et al., 2016b), or resting physiological parameters (Loeppky et al., 1997; Hemmingsson and Linnarsson, 2009). However, only a few experiments investigated potential differences in exercise performance (Saugy et al., 2016a) or physiological exercise responses (Miyagawa et al., 2011; Faiss et al., 2013).

On the one hand, Saugy et al. (2016a) reported about 7.5% more time to complete a maximal 250 kJ time trial in HH compared to NH. On the other hand, Faiss et al. (2013) found no differences in arterial oxygen saturation and heart rate and only slightly higher tidal volumes and minute ventilations during low-intensity exercise (50% of peak power output [PPO]) in NH compared to HH. In addition, Miyagawa et al. (2011) reported no differences in arterial oxygen saturation and ventilatory parameters during low-intensity exercise (50% of maximal oxygen uptake). Therefore, it could be speculated that differences between NH and HH might at least partly depend on the intensity of physical exercise.

Knowledge on intensity-dependent differences in exercise responses between NH and HH could explain diverging outcomes of previous research studies (Miyagawa et al., 2011; Faiss et al., 2013; Saugy et al., 2016a). Furthermore, the interpretation of graded exercise testing results could be affected, if differences between NH and HH are related to the intensity level. Although of physiological interest and practical relevance, potential intensity-dependent differences were not topic of previous NH versus HH comparisons. Therefore, the present study aimed to investigate whether there are differences in submaximal exercise responses during an acute exposure to NH versus HH. We hypothesized that the magnitude of these potential differences depends on the intensity of the submaximal exercise.

Materials and Methods

Subjects and baseline testing

Potential study participants were male students of the University Innsbruck (Austria) and were recruited by personal contacts and social media. Volunteers completed a routine health screening using an adapted physical activity readiness questionnaire (PAR-Q). Medical clarification by a physician was undertaken if the PAR-Q has identified specific issues that require further investigation. Exclusion criteria were preexisting acute or chronic diseases, regular smoking of more than five cigarettes per day, habitual residence >1500 m, previous high-altitude exposure >2500 m within the last 2 weeks or sleeping altitude >2500 m within the last 4 weeks.

The first eight persons without exclusion criteria comprised the study population. Participants were informed on the study procedures and the potential risks and gave written informed consent. The study protocol was approved by the Board for Ethical Questions in Science of the University of Innsbruck (certificate of good standing 18/2017). Participants included into the study conducted a maximal cycle ergometry (Cyclus 2; RBM, Germany) in normoxia to determine maximal aerobic power. After a 5-minute warm-up at 80 W, exercise testing started at 100 with 50 W steps every 5 minutes until subjects' subjective exhaustion. PPO was defined as the last completed work rate plus the fraction of time spent in the final uncompleted work rate multiplied by 50 W (Stepto et al., 1999). Cardiorespiratory parameters were measured continuously using an open spirometric system (Oxycon mobile; CareFushion, Germany) that was calibrated according to the manufacturer's guideline before each test. Peak oxygen consumption was defined as the highest 30-second average during the test. About 1 minute after cessation of cycling capillary blood sample was taken from the ear lobe to determine maximal blood lactate concentration (Biosen C-line; EKF, Germany). Participants' demographic and anthropometric characteristics and the results of the exercise test are shown in Table 1. Based on the relative peak oxygen consumption the participants are classified as recreationally trained (performance level 2) according to the classification of De Pauw et al. (2013).

Table 1.

Demographic and Anthropometric Characteristics and Maximal Parameters During Cycle Ergometry of the Study Participants (n = 8)

Age, years	23 ± 3
Height, cm	181 ± 6
Weight, kg	79 ± 9
Peak power output, W	275 ± 48
Relative peak power output, W/kg	3.5 ± 0.6
Peak oxygen consumption, mL/min	3839 ± 594
Relative peak oxygen consumption, mL/min/kg	48.9 ± 7.6
Maximal heart rate, bpm	190 ± 9
Maximal blood lactate concentration, mmol/L	11.3 ± 2.2

Values are means ± SD.

SD, standard deviation.

Study design

Study participants performed three submaximal exercise tests with a standardized procedure described below. The first test was conducted in normoxia and took place at the Department of Sport Science of the University Innsbruck (Austria, 590 m). This test served as familiarization and provided reference values and therefore was not included into the statistical analysis because cardiorespiratory changes from normoxia to hypoxia are well documented and not topic of the present study (Pighin et al., 2014). The second and the third test were performed in NH or HH with four participants starting in NH and four participants starting in HH (cross-over study design). The tests were separated by at least 7 days.

Tests in normoxia took place at the Department of Sport Science of the University Innsbruck (Austria, 590 m). Tests in NH were conducted in a hypoxic chamber (LowOxygen, Germany) in Bad Aibling (Germany, 490 m) adjusted at an inspiratory fraction of oxygen of 15.0% corresponding to an simulated altitude of about 3150 m. The chamber had a dimension of about 5 × 6 m and the hypoxic system provided a high flow to keep the inspiratory fraction of oxygen constant and to avoid an excessive increase of inspiratory fraction of carbon dioxide. HH testing took place at terrestrial high altitude in a quiet room of the restaurant Jochdohle at the Stubaier Glacier (Austria, 3150 m) after passive ascent. Participants ascended by cable car from 1695 to 3170 m within about 15 minutes, which was followed by a few-minute walk down to the restaurant Jochdohle where the tests were performed.

Submaximal exercise testing

Tests were performed on a cycle ergometer (Cyclus 2; RBM) with a commercially available road bike frame. Saddle position was adjusted before the baseline test in normoxia according to the individuals' body height and kept identical for all tests. Participants rested in a seated position for 30 minutes before being transferred to the cycle ergometer. Cardiorespiratory resting parameters were determined during a 30-second period before submaximal exercise testing started with a 5-minute warm-up at 80 W. Subsequently, participants cycled at three subsequent submaximal exercise intensities (100, 150, 200 W), each for 5 minutes. The order of the three intensities was identical for all tests and there was no break between the stages. Heart rate (chest belt; Polar, Finland), arterial oxygen saturation (SpO₂, finger pulse oximetry; Nonin), and ventilatory and gas-exchange parameters were continuously measured during the test (Oxycon mobile; CareFushion). Mean values of a 2-minute steady state phase (minute 2:00 to 4:00 to avoid interference with other measurements) of each stage were calculated and used for analyses. Energy expenditure at rest and for each intensity was calculated using the formula of Brouwer (1957) to estimate gross efficiency (GE) and net efficiency (NE) for each intensity level (Reger et al., 2013). Difficulty breathing and lower limb discomfort according to the Borg scale (Borg, 1982) were recorded and capillary blood samples were taken from the ear lobe to determine blood lactate concentration (Biosen C-line; EKF) during the last minute of each intensity. After each test in NH and HH ambient temperature and humidity, atmospheric pressure and inspiratory fractions of oxygen and carbon dioxide were measured and inspiratory partial pressure of oxygen was calculated (Table 2).

Table 2.

Ambient Conditions During the Test in Normobaric and Hypobaric Hypoxia

	Normobaric hypoxia	Hypobaric hypoxia
Temperature, °C	22.9 ± 1.5	21.8 ± 1.0
Relative humidity, %	33.6 ± 1.1	27.9 ± 0.6
Atmospheric pressure, hPa	962 ± 1	704 ± 1
Inspiratory fraction of oxygen, %	15.1 ± 0.1	20.9 ± 0.0
Inspiratory fraction of carbon dioxide, %	0.09 ± 0.02	0.03 ± 0.00
Inspiratory partial pressure of oxygen, hPa	145 ± 1	147 ± 0

Values are means ± SD.

Statistics

Statistical analyses were performed using SPSS 24.0 (IBM, Austria). Mean values and standard deviations were calculated and data were checked for normal distribution using the Shapiro-Wilk-test. Since data were normal distributed ANOVAs for repeated measures with two within-subject factors (intensity × condition) were conducted to detect general differences between NH and HH and intensity-dependent interactions. Additionally, partial eta square as an effect size was calculated. Cardiorespiratory resting parameters in NH and HH were compared by paired t-tests. The level of significance was set at p < 0.05.

Results

Cardiorespiratory resting parameters are presented in Table 3 with no significant differences between NH and HH. Cardiorespiratory and metabolic parameters, difficulty breathing and lower limb discomfort during submaximal exercise are shown in Table 4. Minute ventilation did not differ between conditions across all intensities, whereas breathing frequency tended to be higher (p = 0.06) and tidal volume tended to be lower (p = 0.09) in HH compared to NH. Additionally, inspiration time was significantly lower, especially at low and moderate intensities (100 and 150 W), and ventilatory equivalent for oxygen was higher in HH compared to NH (p < 0.01 and p = 0.03, respectively). There was no significant intensity × condition interaction for any ventilatory parameters. GE, NE, heart rate, and arterial oxygen saturation revealed no significant main effect for condition and no significant intensity × condition interaction. Blood lactate concentration tended to be higher in HH compared to NH (p = 0.08) with a significant interaction effect for exercise intensity (p = 0.02). Difficulty breathing and lower limb discomfort did not show any differences between NH and HH.

Table 3.

Cardiorespiratory Parameters and Blood Lactate Concentration at Rest in Normobaric and Hypobaric Hypoxia

	Normobaric hypoxia	Hypobaric hypoxia	p
Minute ventilation, L/min	11.9 ± 3.1	11.5 ± 1.7	0.71
Breathing frequency, 1/min	13.9 ± 5.6	11.9 ± 4.4	0.19
Tidal volume, L	0.94 ± 0.35	1.02 ± 0.32	0.35
Ventilatory equivalent for oxygen	25.4 ± 3.9	25.4 ± 3.7	0.99
Ventilatory equivalent for carbon dioxide	30.9 ± 3.5	29.7 ± 3.1	0.32
Oxygen uptake, mL/min	424 ± 105	395 ± 73	0.50
Arterial oxygen saturation, %	88.3 ± 4.4	91.8 ± 3.8	0.16
Heart rate, 1/min	77.3 ± 9.9	78.3 ± 10.4	0.81
Blood lactate, mmol/L	0.9 ± 0.2	1.1 ± 0.2	0.05

Values are means ± SD. p-Values refer to paired t-tests between normobaric and hypobaric hypoxia.

Table 4.

Cardiorespiratory and Metabolic Parameters, Difficulty Breathing, and Lower Limb Discomfort During Submaximal Exercise Intensities in Normobaric and Hypobaric Hypoxia

	Intensity, W	Normobaric hypoxia	Hypobaric hypoxia	Difference (%)	p (partial eta square)
	Intensity, W	Normobaric hypoxia	Hypobaric hypoxia	Difference (%)	Condition	Intensity	Interaction
Minute ventilation, L/min	100	46.6 ± 4.0	47.3 ± 2.2	+1.8 ± 5.8	0.77 (0.01)	<0.01 (0.96)	0.98 (<0.01)
	150	65.7 ± 5.9	66.0 ± 4.3	+0.9 ± 7.0
	200	93.6 ± 11.4	94.2 ± 12.6	+0.7 ± 8.2
Breathing frequency, 1/min	100	23.9 ± 3.5	25.8 ± 3.4	+8.1 ± 10.5	0.06 (0.41)	<0.01 (0.82)	0.82 (0.01)
	150	28.9 ± 5.3	30.3 ± 5.1	+5.1 ± 5.9
	200	35.6 ± 6.1	36.7 ± 8.2	+3.1 ± 15.2
Tidal volume, L	100	1.98 ± 0.34	1.87 ± 0.32	−5.0 ± 7.0	0.09 (0.36)	<0.01 (0.93)	0.52 (0.07)
	150	2.33 ± 0.37	2.23 ± 0.37	−4.2 ± 5.0
	200	2.65 ± 0.25	2.64 ± 0.44	−1.0 ± 10.4
Inspiration time, seconds	100	1.13 ± 0.20	1.06 ± 0.20	−6.1 ± 8.0	<0.01 (0.70)	<0.01 (0.79)	0.41 (0.11)
	150	1.00 ± 0.25	0.93 ± 0.20	−6.3 ± 4.5
	200	0.81 ± 0.16	0.81 ± 0.24	−1.1 ± 15.9
Mean inspiratory flow, L/s	100	1.76 ± 0.16	1.78 ± 0.14	+1.4 ± 6.7	0.45 (0.08)	<0.01 (0.93)	0.91 (<0.01)
	150	2.38 ± 0.26	2.42 ± 0.22	+2.4 ± 6.8
	200	3.35 ± 0.50	3.39 ± 0.59	+1.0 ± 7.4
Inspiration to total breath time ratio, %	100	44 ± 3	45 ± 4	+0.8 ± 4.7	0.63 (0.03)	<0.01 (0.64)	0.28 (0.17)
	150	46 ± 3	45 ± 3	−0.8 ± 2.5
	200	46 ± 2	47 ± 4	+1.3 ± 3.2
Ventilatory equivalent for oxygen	100	24.6 ± 1.0	25.9 ± 1.3	+5.4 ± 4.2	0.03 (0.53)	<0.01 (0.84)	0.47 (0.09)
	150	27.1 ± 1.6	28.0 ± 1.6	+3.1 ± 2.4
	200	31.3 ± 3.6	32.1 ± 3.9	+2.7 ± 6.2
Ventilatory equivalent for carbon dioxide	100	27.9 ± 1.5	28.0 ± 1.4	+0.6 ± 4.9	0.57 (0.05)	<0.01 (0.55)	0.73 (0.02)
	150	27.9 ± 0.8	28.1 ± 1.4	+0.9 ± 4.1
	200	30.0 ± 1.7	30.4 ± 2.6	+1.3 ± 6.3
Oxygen uptake, mL/min	100	1807 ± 137	1735 ± 84	−3.7 ± 5.4	0.18 (0.25)	<0.01 (0.99)	0.89 (0.02)
	150	2323 ± 135	2265 ± 100	−2.3 ± 5.7
	200	2886 ± 75	2831 ± 145	−1.9 ± 5.2
Gross efficiency, %	100	16.2 ± 1.1	16.6 ± 0.6	+3.3 ± 5.1	0.23 (0.20)	<0.01 (0.96)	0.88 (0.01)
	150	18.4 ± 1.1	18.8 ± 0.6	+2.1 ± 5.8
	200	19.4 ± 0.6	19.8 ± 0.9	+1.8 ± 5.0
Net efficiency, %	100	21.1 ± 1.9	21.4 ± 1.2	+2.2 ± 9.9	0.69 (0.02)	<0.01 (0.63)	0.92 (<0.01)
	150	22.4 ± 1.4	22.6 ± 1.1	+1.5 ± 9.1
	200	22.6 ± 1.0	22.8 ± 1.3	+1.2 ± 7.7
Heart rate, 1/min	100	120.3 ± 10.2	120.0 ± 9.5	±0.0 ± 6.9	0.72 (0.02)	<0.01 (0.96)	0.78 (0.02)
	150	140.8 ± 14.0	139.5 ± 13.5	−0.7 ± 5.5
	200	160.3 ± 17.2	159.2 ± 15.5	−0.5 ± 2.9
Blood lactate, mmol/L	100	1.7 ± 0.8	1.8 ± 0.9	+9.6 ± 28.5	0.08 (0.38)	<0.01 (0.78)	0.02 (0.51)
	150	2.8 ± 1.6	3.2 ± 1.8	+14.7 ± 33.7
	200	5.5 ± 3.0	6.0 ± 3.1	+11.5 ± 16.9
Difficulty breathing	100	9.4 ± 1.4	9.4 ± 1.3	+1.3 ± 16.1	0.94 (<0.01)	<0.01 (0.91)	0.76 (0.03)
	150	11.8 ± 2.1	11.6 ± 1.5	+0.8 ± 16.2
	200	14.3 ± 1.8	14.5 ± 0.9	+2.8 ± 11.1
Lower limb discomfort	100	9.4 ± 1.8	9.4 ± 1.4	+1.2 ± 13.3	0.54 (0.06)	<0.01 (0.94)	0.61 (0.05)
	150	12.4 ± 2.7	12.3 ± 2.0	+1.1 ± 15.1
	200	16.1 ± 2.1	15.6 ± 2.0	−2.9 ± 7.4

Values are means ± SD. Differences are calculated as relative changes from normobaric to hypobaric hypoxia. p-Values (partial eta square) refer to ANOVA main effects for condition (normobaric vs. hypobaric hypoxia), intensity (100, 150, 200 W), and interaction (intensity × condition).

Discussion

This study provided no evidence for intensity-dependent differences in submaximal exercise responses at different intensity levels between NH and HH—except for blood lactate concentrations. Therefore, the proposed hypothesis could be accepted for blood lactate concentrations but had to be rejected for all other parameters. With respect to general condition effects, we found a slightly higher ventilatory equivalent for oxygen and a shorter inspiration time in HH compared to NH. The observation, blood lactate accumulation over the different intensities was more pronounced in HH compared to NH, could affect the comparability of graded exercise tests with lactate diagnostic and the derived exercise prescriptions in NH and HH.

Several studies compared cardiorespiratory exercise responses in HH and NH in different settings. For example, Netzer et al. (2017) compared submaximal exercise responses during a simulated treadmill hike with a real ascent to Mauna Kea and found about 20 bpm higher heart rates and about 7% lower arterial oxygen saturation values in HH at a comparable altitude (3100–3200 m). While these results are in contrast to our findings, the authors stated that the observed differences are at least partly caused by the specific conditions during the real mountain hike, that is, impaired walking economy due to uneven surface, group dynamic aspects, and increased solar radiation (Netzer et al., 2017). In our study, exercise was performed on a cycle ergometer under well-standardized conditions in a closed room and without presence of other persons except the scientific staff. Therefore, differences in power output or in exercise efficiency and group dynamic effects can be excluded. Also, Faiss et al. (2013) reported no differences in SpO₂ during cycling exercise at about 50% of PPO in HH compared to NH at about 3000 m altitude. Interestingly, they found slight differences in ventilatory parameters, that is, a slightly higher ventilation, accompanied by a lower end-tidal partial pressure of carbon dioxide, after 8 hours and more of exposure but not acutely after 1 hour. In addition, Miyagawa et al. (2011) determined ventilatory parameters and SpO₂ during exercise at about 50% peak oxygen uptake in normoxia, HH, and NH (altitude 3200 m) in a laboratory study setting. Exercise testing was performed acutely after 60–90 minutes exposure time and no differences were detected between HH and NH (Miyagawa et al., 2011). The reported findings support the hypothesis that no differences in ventilation and SpO₂, at least for the range of exercise intensities tested in this study, exist between HH and NH if exposure time is less than 1 hour (Debevec and Millet, 2014).

Despite no significant condition effect in minute ventilation, a higher breathing frequency—mainly driven by a shorter inspiration time—and a lower tidal volume were detected in HH compared to NH potentially indicating slightly different ventilatory patterns. Although the differences missed statistical significance (p = 0.06 and p = 0.09, respectively) and must therefore be interpreted with caution, partial eta square values (0.41 and 0.36, respectively) were in a similar range to other significant findings in this study. Therefore, a larger sample size might have resulted in significant condition effects for these two parameters. However, this assumption remains speculative based on the present data and should be topic of future research.

Mean inspiratory flow and inspiration to total breath time ratio showed no significant condition effect (NH vs. HH). This observation indicates that neural ventilatory drive and the timing of the breathing pattern are the same during an acute hypoxic exposure independent of normobaric or hypobaric conditions. Referring to studies of Mekjavic et al. (1987) and Mekjavic et al. (1991), Burtscher (2014) suggested that HH could have an effect on the timing component of breathing, which was not observed in NH. However, this hypothesis is not supported by the results of this study.

The observation that blood lactate concentration tended to be higher in HH compared to NH was not reported in previous studies. Saugy et al. (2016a) determined blood lactate concentration at the end of maximal time trials in NH and HH with no differences between conditions. However, these results are not comparable to ours since participants had—in contrast to our setting—a lower mean workload in HH compared to NH derived from the impaired time trial performance in HH.

In fact, intensity dependent increase in blood lactate concentration was significantly more pronounced in HH in our study indicating altered lactate kinetics in HH compared to NH. The specific design with the progressive increase in workload over three intensities without breaks between stages may have contributed to our results indicating a stronger lactate accumulation in HH compared to NH. However, it has to remain speculative if a higher lactate production or/and a lower lactate removal have caused the steeper increase in blood lactate concentration in HH. The higher blood lactate concentrations in HH could be a consequence of an increased beta-adrenergic stimulation of glycolysis in HH compared to NH (Mazzeo et al., 1991; Brooks et al., 1992; Reeves et al., 1992). The assumption of a higher relative carbohydrate oxygenation is supported by the slightly, but not significantly, lower oxygen uptake and, thereby, could explain the higher ventilatory equivalent for oxygen we detected in HH compared to NH.

Of course, several other mechanisms, including an impaired efficiency or a reduced plasma volume, can cause higher blood lactate concentrations. However, an impaired cycling efficiency is very unlikely based on our results in GE and NE and a reduced plasma volume should have translated in an increased heart rate for the same intensity level and oxygen uptake, which was not detected in our study. From a practical point of view, the observed differences in blood lactate concentrations should be taken into account when applying lactate threshold concepts, especially fixed blood lactate thresholds, during exercise testing in NH and HH.

This study had several limitations. (1) We selected fixed absolute exercise intensities not considering differences in body weight and exercise performance. Thereby, inter-individual differences in the hypoxia-related decrease in exercise performance were not taken into account. Both issues could have resulted in different relative exercise intensities, that is, percentages of individual maximal oxygen uptake, between the subjects. However, the impact of these two aspects seem to be of limited relevance since we selected a homogenous group of regularly physical active individuals and the three intensities covered a broad range of relative intensities from 37% ± 7% of PPO at 100 W to 75% ± 13% of PPO at 200 W. In addition, subjective perceived exertions at the three exercise levels varied from 9 (very light) to 12 (somewhat hard) and 16 (hard) at the subjective scale of Borg (1982). Nevertheless, relative exercise intensities (e.g., based on percentage of peak oxygen consumption), including the individual hypoxia-related performance reduction, would have reduced the inter-individual variability in exercise responses.

(2) HH testing was performed at terrestrial high-altitude after passive ascent by cable car. Although differences in ambient conditions were small (Table 2), we cannot completely exclude that the slightly lower temperature and humidity in HH have affected physiological exercise responses.

(3) Conclusions with respect to potential differences in maximal aerobic power or endurance performance between NH and HH cannot be drawn based on the reported submaximal exercise responses.

(4) The sample size of this study was relatively small and no a priori power analysis was conducted. Although some significant effects were found, the study might be inadequately powered to detect more subtle, still practically relevant effects. Therefore, further study is required to confirm the results in a larger sample size.

Conclusions

The results of this study suggest small differences in the ventilatory pattern between NH and HH during submaximal exercise within the first hour of exposure. These differences do not affect minute ventilation and arterial oxygen saturation and are not accompanied by condition-specific subjective ratings of difficulty breathing and lower limb discomfort. The level of exercise intensity seems not to influence potential differences in submaximal exercise responses between NH and HH except of blood lactate concentrations. The intensity-dependent differences in blood lactate concentrations indicate altered lactate kinetics in HH compared to NH that may be caused by a stronger beta-adrenergic tone in HH compared to NH. Endurance performance testing with lactate diagnostic might produce diverging results in NH and HH and derived thresholds may not be interchangeable. Future studies should focus on time-dependency of potential ventilatory and metabolic differences and their impact on acclimatization effects in NH and HH.

Footnotes

Authors' Contributions

All authors have substantially contributed to the project. M.F. designed the study, collected and analyzed the data, and prepared the article. S.P. and L.R. collected data and reviewed the article. V.M. contributed to data analyses and to the article preparation. All authors have reviewed and approved the article before submission.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

The project was supported by Wintersport Tirol AG & CO Stubaier Bergbahnen KG (Neustift, Austria).

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