High rate of muscular oxygen utilization facilitates the development of hypoxemia during exercise at altitude. Because endurance training stimulates oxygen extraction capacity, we investigated whether endurance athletes are at higher risk to developing hypoxemia and thereby acute mountain sickness symptoms during exercise at simulated high altitude. Elite athletes (ATL; n = 8) and fit controls (CON; n = 7) cycled for 20 min at 100 W (EX 100W ), as well as performed an incremental maximal oxygen consumption test (EX MAX ) in normobaric hypoxia (0.107 inspired O 2 fraction) or normoxia (0.209 inspired O 2 fraction). Cardiorespiratory responses, arterial Po 2 (PaO 2 ), and oxygenation status in m. vastus lateralis [tissue oxygenation index (TOI M )] and frontal cortex (TOI C ) by near-infrared spectroscopy, were measured. Muscle O 2 uptake rate was estimated from change in oxyhemoglobin concentration during a 10-min arterial occlusion in m. gastrocnemius . Maximal oxygen consumption in normoxia was 70 ± 2 ml·min −1· kg −1 in ATL vs. 43 ± 2 ml·min −1· kg −1 in CON, and in hypoxia decreased more in ATL (−41%) than in CON (−25%, P < 0.05). Both in normoxia at PaO 2 of ∼95 Torr, and in hypoxia at PaO 2 of ∼35 Torr, muscle O 2 uptake was twofold higher in ATL than in CON (0.12 vs. 0.06 ml·min −1 ·100 g −1 ; P < 0.05). During EX 100W in hypoxia, PaO 2 dropped to lower ( P < 0.05) values in ATL (27.6 ± 0.7 Torr) than in CON (33.5 ± 1.0 Torr). During EX MAX , but not during EX 100W , TOI M was ∼15% lower in ATL than in CON ( P < 0.05). TOI C was similar between the groups at any time. This study shows that maintenance of high muscular oxygen extraction rate at very low circulating PaO 2 stimulates the development of hypoxemia during submaximal exercise in hypoxia in endurance-trained individuals. This effect may predispose to premature development of acute mountain sickness symptoms during exercise at altitude.

whenever unacclimatized persons rapidly ascent to altitude, they are at risk to developing high-altitude illness. At low to moderate altitudes, the illness appears in the form of rather benign symptoms of acute mountain sickness (AMS) such as headache, nausea, and gastrointestinal distress, fatigue, dizziness, and sleeping difficulties. At higher altitudes, high-altitude cerebral and pulmonary edema may develop, causing more severe and potentially lethal physiological and physical dysfunction. High-altitude illness obviously is also implicated in impairment of physical working capacity at altitude (3, 50). But physical activity by itself can also aggravate or even trigger the symptoms, especially when exercising at high altitude (7, 46, 49, 50). The pathophysiology of high-altitude illness is not yet fully understood. However, reduced arterial partial oxygen pressure (PaO 2 ) due to lower inspired Po 2 clearly is playing a pivotal role (4, 41, 46). Accordingly, the degree of hypoxemia caused by acute exposure to hypoxia (HYP) also yields a reasonable prediction of the susceptibility to AMS (3, 4, 30, 32, 65). A PaO 2 decrement may occur due to either the reduction of barometric pressure inherent to altitude (hypobaric HYP), or to the lower inspired O 2 fraction (FiO 2 ) existing in normobaric hypoxic facilities used to simulate altitude at sea level. It is still a matter of debate though whether hypobaric and normobaric HYP really similarly affect high-altitude illness and functional capacity at altitude (14, 20, 34).

During exercise the circulating PaO 2 largely reflects the balance between pulmonary oxygen diffusion on the one side, vs. muscular extraction on the other side. In addition, ventilation-perfusion mismatching, right-to-left shunting, as well as possible diffusion limitation might also influence PaO 2 . Hypoxemia is likely to occur whenever the degree of peripheral arterial deoxygenation due to tissue O 2 utilization exceeds the capacity for pulmonary reoxygenation. During exercise, obviously the bulk of peripheral oxygen extraction is accounted for by contracting muscle mass (21, 29). In normoxia (NOR), as a rule, pulmonary oxygen diffusion readily copes with muscular oxygen extraction. This translates into stable PaO 2 values, even during exercise at workloads eliciting maximal rate of whole body oxygen utilization (V̇o 2 max ). In contrast, in HYP, alveolar O 2 diffusion rate is limited by drop of the alveolar-to-arterial Po 2 gradient [Δ(A-a)Po 2 ] due to the fall in ambient Po 2 (10, 60). Against the face of such pulmonary O 2 diffusion limitation, high rates of exercise-induced oxygen utilization are likely to rapidly turn into hypoxemia. Moreover, the theoretical model developed by Piiper and Scheid (38) point outs that alveolar-arterial O 2 diffusion during exercise in HYP might be even more limited than is expected. Combining the pulmonary diffusion constant (D), O 2 solubility of the arterial blood (B), and the pulmonary blood flow rate (Q̇), one obtains the D-to-BQ̇ ratio as classically described by Piiper and Scheid (38). A decreased ratio implies an increased diffusion limitation, as is to be expected during exercise when both B and Q̇ increase, while D might even drop if interstitial pulmonary edema develops (55). Thus individuals with high muscular oxygen extraction capacity conceivably may have lower exercise tolerance at altitude because of premature drop of PaO 2 and possible early triggering of AMS.

It is well established that O 2 extraction capacity is higher in oxidative type I muscle fibers than in the more glycolytic type II fibers (16). In addition, consistent endurance exercise training, apart from producing type II to type I transitions (5, 17, 18), markedly upregulates oxidative energy turnover in either fiber type (13, 17). Thus it has been extensively documented that aerobic training increases mitochondrial volume density and oxidative enzyme activity. Such adaptation in conjunction with elevated capillary density and intramyocellular myoglobin content (6, 11, 13) translates into elevated diffusive capacity for O 2 in muscle cells (28, 61). Hence elite endurance athletes represent a population at the upper limit of muscular O 2 extraction capacity, which, more than less-trained populations, is predisposed to be afflicted by high-altitude illness and dysfunction in HYP. In fact, even at sea level, some endurance-trained athletes involving in maximal exercise experience a reduction in PaO 2 and hence arterial hemoglobin saturation (SaO 2 ), termed “exercise-induced arterial hypoxemia” (12, 39). Consistent with the above, V̇o 2 max drop at altitude is exaggerated in individuals with high V̇o 2 max at sea level, i.e., endurance-trained individuals (35, 36, 62, 66). Furthermore, testimonials by mountaineers indicate that the odds often are on the most explicit endurance-trained crewmembers to developing high-altitude illness and premature dysfunction during extreme altitude expeditions.

Against this background, we hypothesize that endurance-trained individuals in severe HYP, compared with less-trained individuals, can maintain a higher rate of muscular O 2 extraction during exercise-induced arterial hypoxemia. We expected such a mechanism to induce a greater drop of PaO 2 for a given submaximal workload in the former, and thereby increase the incidence of AMS symptoms. Therefore, we compared PaO 2 and muscular and cerebral oxygenation status via near-infrared spectroscopy (NIRS), between young fit individuals and world-class endurance athletes during submaximal and maximal exercise in either NOR and normobaric HYP equivalent to 5,300-m altitude (FiO 2 0.107). In addition, to evaluate muscular O 2 extraction capacity during development of extreme hypoxemia, we also evaluated changes in m. gastrocnemius oxygenation status and O 2 extraction rate during an arterial occlusion in the aforementioned normoxic and hypoxic condition. To the best of our knowledge, this is the first study to evaluate the effect of ambient HYP on the capacity for muscular oxygen extraction in world-class endurance athletes.

METHODS Subjects. Elite endurance-trained athletes (ATL, n = 8) and fit healthy subjects who were not involved in consistent long-term endurance training (CON, n = 7) volunteered to participate in the study after having been informed in detail of the experimental procedures. Some general characteristics of the subjects are given in Table 1. All subjects were nonsmokers and were diagnosed to be healthy via a medical screening, including a medical questionnaire, a resting electrocardiogram, and a standard clinical examination. They were instructed not to change their dietary and training habits throughout the study period. None had been exposed to altitudes higher than 1,500 m during the 6 mo preceding the study. One week before the start of the study, the subjects participated in an incremental (60 W + 30 W per 3 min) exercise test on a cycling ergometer (Avantronic Cyclus II, Leipzig, Germany) to determine their V̇o 2 max (Table 1). Furthermore, subjects were asked not to participate in any strenuous exercise from 2 days before the experimental sessions. The study protocol was approved by the local Ethics Committee (K.U. Leuven) and was in accordance with The Declaration of Helsinki. All subjects signed an informed consent. Table 1. General characteristics and leg skinfolds of the subjects CON ATL Age, yr 24.0 ± 1.9 25.4 ± 1.8 Height, cm 181.9 ± 2.7 180.0 ± 2.6 Body weight, kg 67.0 ± 1.9 68.7 ± 2.0 Body mass index, kg/m2 20.2 ± 0.2 21.2 ± 0.2 V̇ o 2max , ml·kg−1·min−1 43.4 ± 2.2 70.0 ± 1.7* Quadriceps skinfold, mm 6.4 ± 0.5 5.4 ± 0.3 Gastrocnemius skinfold, mm 6.6 ± 0.9 4.2 ± 0.3* Experimental protocol. The subjects reported to the laboratory on two occasions to participate in an experimental session, according to a randomized crossover study design. Procedures for both sessions were identical. However, one session was done in NOR (FiO 2 0.209 O 2 ) and one session was done in normobaric HYP (FiO 2 0.107O 2 , ∼5,300-m altitude). All experiments were performed in a normobaric hypoxic facility (Sporting Edge, Leicestershire, UK) with fully automated control of FiO 2 , ambient temperature (18.5°C), and relative humidity (50%). The normoxic and hypoxic sessions were administered in random order and were interspersed by a 2-wk interval. Each experimental session started with an arterial occlusion experiment with the subjects lying on a bed. An orthopedic cuff was wrapped around the left leg just above the knee, and NIRS optodes were fixed on m. gastrocnemius (see below) during an isometric extension. After 5 min of rest, the orthopedic cuff was inflated to 350 Torr within 2-3 s by means of a compressor to occlude the circulation to the lower leg for 10 min. After the occlusion, the subjects rested for another 5 min on the bed. The NIRS signal was registered continuously throughout the experiment. Following the occlusion experiments, the subjects rested for 10 min in a comfortable chair. Thereafter an exercise protocol on a cycling ergometer (Avantronic Cyclus II, Leipzig, Germany) was started. The subjects consecutively performed a 5-min warming-up at a load of 50 W, a 20-min constant-load exercise at 100 W (EX 100W ), and a maximal incremental test (EX MAX ; 100 W + 30 W/min). Following the incremental test, the subjects recovered for 5 min at 50 W, where after they returned to a comfortable chair for a final 15-min rest period. At the end of each experimental session, the presence and severity of symptoms suggestive for the presence of AMS were assessed using the Lake Louise Questionnaire. The symptoms “headache,” “gastrointestinal distress,” “fatigue,” and “dizziness” were scored on a 4-point scale (0–3). Measurements. Heart rate (HR) (RS600, Polar, Kempele, Finland), SaO 2 , and cerebral as well as muscular oxygenation status of the right m. vastus lateralis were continuously measured throughout the experiment. Hemoglobin saturation was measured by pulse oximetry (N600X, Covidien, Mansfield, MA) using a sensor placed 2 cm above the left eyebrow. Cerebral and muscular tissue oxygenation status were monitored by NIRS (see below). Furthermore, capillary blood samples (5 μl) for blood lactate determination (Lactate Pro, Arkray, Kyoto, Japan) were collected from an earlobe at regular intervals before, during, and after exercise. Arterial blood samples were collected from the radial artery just before and at the end of the 20-min constant-load exercise bout using a self-filling blood-gas syringe (Radiometer Pico 70, Copenhagen, Denmark). Arterial blood samples were immediately analyzed for pH, PaO 2 , arterial Pco 2 (Pa CO 2 ), and bicarbonate concentration using an automated acid-base laboratory (Radiometer ABL 510, Copenhagen, Denmark). From the start to the end of the constant-load cycling bout, as well as during the maximal incremental exercise test, O 2 uptake (V̇o 2 ) and CO 2 output (V̇co 2 ) were measured using an open-circuit ergo-spirometry system (Cortex Metalyzer II, Leipzig, Germany). The alveolar partial pressure of oxygen (PaO 2 ) was calculated using simultaneously measured PaO 2 , body temperature, and the respiratory exchange ratio (RER), as previously described (63): P A O 2 = F 1 O 2 × ( P ATM − P H 2 O ) − P A CO 2 × [ F 1 O 2 + ( 1 − F 1 O 2 ) / RER ) ] where P ATM is atmospheric pressure, and P H 2 O is water pressure. The P a CO 2 so calculated was used to calculate alveolar-arterial oxygen pressure difference during both rest and exercise. where Pis atmospheric pressure, andis water pressure. The Pso calculated was used to calculate alveolar-arterial oxygen pressure difference during both rest and exercise. NIRS measurements and analysis. The principles, limitations, and reliability of NIRS measurements have been extensively discussed elsewhere (9, 15, 23). We used the Niro-200 NIRS instrument (Hamamatsu, Japan) for the purpose of this study. NIRO-200 applies three different wavelengths of near-infrared light and provides real-time information on the dynamic balance between O 2 supply and O 2 consumption. The so-called “tissue oxygenation index” (TOI), as determined by spatially resolved spectroscopy, is a valid parameter (9, 40) to assess the fraction of O 2 -saturated tissue hemoglobin and myoglobin content, reflecting tissue oxygenation status. Furthermore, oxyhemoglobin (Δ[HbO 2 ]), deoxyhemoglobin (Δ[HHb]), and total hemoglobin concentration changes (Δ[tHb], where Δ[tHb] = Δ[HbO 2 ] + Δ[HHb]) are estimated using the Beer-Lambert law. Considering the large variability of muscle fiber composition and that uniform differential path length factor values are not available for peripheral skeletal muscles, no differential path length factor was used. Therefore, we report raw NIRS data expressed in millimoles per centimeter to be able to readily compare with data from other literature reports. Two pairs of NIRS probes, each consisting of one light emitter and one light detector, were used. For the cerebral measurements, a probe was fixed 2 cm above the right eyebrow to face the frontal cortex. For the measurements in muscle tissue, during the occlusion experiment a probe was placed centrally on the muscle belly of the left m. gastrocnemius lateralis during a submaximal plantar flexion of the ankle. For the exercise protocol, the probe was transferred to the right m. vastus lateralis at one-third of the imaginary line connecting the upper lateral border of the patella with the spina iliaca anterior superior. The probes were fitted in a dark-colored plastic spacer with fixed interoptode distance of 4 cm and attached to the skin using double-sided adhesive tape. Elastic noncompressive bandages were used to keep the probes in place during the experiment. Before positioning of the probes, the skin was shaved to exclude interaction of hair as a chromophore. The contour lines of the muscle probe were drawn on the skin with a permanent marker to allow identical repositioning of the probe during the second session. Importantly, skinfolds were <10 mm in all subjects on both m. gastrocnemius and m. vastus lateralis, which guaranteed NIRS signals sampled to largely represented absorption of near-infrared light in muscle tissue and not in subcutaneous fat. In addition, skinfolds were similar between CON and ATL (Table 1). A successful arterial occlusion fully blocks vascular flow with no blood influx nor efflux at the tissue distal to the occlusion site. Thus total blood and hemoglobin content in the occluded area (Δ[tHb], where Δ[tHb] = Δ[HbO 2 ] + Δ[HHb]; see below) are supposed to be constant. In such setting, the rate of decrease of [HbO 2 ] vs. increase of [HHb] are perfectly matched. Therefore the rate of muscular oxygen extraction is reflected by the rate of transformation of [HbO 2 ] to [HHb]. Using a hemoglobin-to-oxygen molar ratio of 4 and a skeletal muscle density of 1,04 g/ml, muscle tissue oxygen extraction can be expressed as milliliters per minute per 100 g (56, 57). Transformation of [HbO 2 ] to [HHb] during the initial 30 s after inflation of the cuff was taken into account to determine muscle oxygen consumption. Statistical analysis. Before statistical analysis, NIRS data were preprocessed with a Butterworth filter in customer level made mathematical software (Matlab, The Mathworks). Furthermore, mean values for HR, SaO 2 and gas exchange, and NIRS parameters were calculated for 1) the last 5 min of the rest period; 2) the last 5 min of the constant-load cycling bout (EX 100W ); and 3) the final minute of the maximal incremental exercise test (EX MAX ). All data were assessed for normality of data distribution using a Kolmogorov-Smirnov test. The data so obtained were analyzed using a two-way repeated-measures analysis of variance (ANOVA; Statistica 9.0, Statsoft, Tulsa, OK). A planned contrast analysis was used for post hoc comparisons (Bonferroni), when appropriate. Contrast analysis was also used to perform specific preplanned comparisons. A probability level (P) ≤ 0.05 was considered statistically significant. All data are expressed as means ± SE.

DISCUSSION The present experiments for the first time clearly demonstrate that elevated muscular O 2 extraction capacity, against the face of impaired pulmonary gas exchange, puts highly endurance-trained individuals at risk to develop excess hypoxemia during submaximal exercise at high altitude. We show that muscular O 2 extraction rate in NOR is approximately twofold higher in highly trained endurance athletes than in less-trained controls. This higher O 2 extraction rate is maintained at the very low PaO 2 values (28–34 Torr) typically produced by exposure to extreme altitude (22). In addition, a given absolute workload in HYP, which corresponds to an even lower percentage of V̇o 2 max in athletes than in controls, produces a greater drop of PaO 2 in the former compared with the latter, yet this is not associated with more explicit symptoms of AMS. The extent of PaO 2 drop resulting from an acute exposure to HYP in general is well correlated with the susceptibility to AMS: the greater the Po 2 drop, the higher the incidence of AMS symptoms (3, 4, 32, 41, 46). Nonetheless, some authors did not find a relationship between the degree of hypoxemia and the occurrence of AMS (33, 37, 45). At rest in healthy individuals, the magnitude of arterial O 2 desaturation is largely regulated by the inspired Po 2 (19, 47). However, the degree of hypoxemia resulting from a given reduction of inspired Po 2 is exaggerated by exercise, because the contraction-induced rate of muscular O 2 extraction often exceeds the capacity for pulmonary O 2 diffusion (12, 19, 25, 39, 46). Other mechanisms impairing the efficiency of pulmonary gas exchange, such as a patent foramen ovale or intrapulmonary arteriovenous anastomoses (IPAVA), may also be involved. It is also well documented that the effect of endurance training to enhance muscular O 2 extraction is many-fold greater than the concomitant small increment in pulmonary oxygen diffusion capacity (8, 60). Against this background, we postulated that elite endurance athletes, compared with less-fit individuals, can maintain higher rate of muscular O 2 extraction, even at the very low circulating PaO 2 levels inherent to exercise at high altitude. Hence athletes conceivably are more exercise intolerant due to premature development of both exercise-induced hypoxemia and symptoms of AMS. To test this hypothesis, we compared high-level endurance athletes with age-matched fit, healthy subjects during submaximal and maximal exercise in either NOR, or in normobaric HYP equivalent to ∼5,300-m altitude. In the same conditions, we also assessed O 2 extraction rate in m. gastrocnemius during an acute hypoxic challenge induced by arterial occlusion. In line with our hypothesis, we clearly demonstrate that endurance athletes, compared with less-trained individuals, are able to maintain higher rate of muscle O 2 extraction during hypoxemia. As a result, they exhibit a greater drop of PaO 2 for a given submaximal exercise rate. Still, within the conditions of the present study, we could not document higher incidence of AMS symptoms during hypoxic exercise in the endurance athletes. Muscle O 2 consumption in HYP. Exercise at extreme altitude can decrease extracellular Po 2 to values as low as ∼20 Torr, which largely eliminates the gradient for muscle O 2 diffusion (10, 22, 25). However, higher capillary density, together with elevated oxidative capacity due to both higher fraction of type I fibers and endurance training adaptation (6, 11, 13, 17), conceivably may facilitate O 2 extraction in such condition. To test this assumption, we used NIRS to compare hemoglobin deoxygenation rate in m. gastrocnemius between elite endurance athletes and just-fit control subjects during an arterial occlusion of the lower leg. A cuff above the right knee was inflated to 350 mmHg within seconds. Such occlusion should cut all afferent and efferent blood flow to and from the tissue distal to the cuff (15, 56, 57). The constant values measured for m. gastrocnemius tHb content (see Table 6) prove that the occlusions were successful in any experimental condition. As mitochondrial oxidative metabolism in the occluded segment persists, HbO 2 is continuously transformed into HHb, and the initial rate of transformation is directly proportional to the rate of muscular O 2 utilization. As shown in Fig. 3, starting from an PaO 2 of ∼95 Torr in NOR (see Table 6), [HbO 2 ] in m. gastrocnemius rapidly decreased upon occlusion. Oxygen extraction rate in fit controls (∼0.06 ml·min−1·100 g−1) matched normal reference values (1, 56, 57). However, in the athletes, O 2 extraction occurred at approximately twofold, a faster rate than in the controls, confirming the markedly higher O 2 extraction capacity existing in the former. Interestingly, when the occlusion was started at a circulating PaO 2 of only 35 Torr in HYP, the athletes maintained a twofold higher O 2 extraction rate compared with the controls. Inspection of the individual values (Fig. 4) shows that six out of the eight athletes exhibited O 2 extraction rates that were higher than the peak value measured in controls. These observations clearly demonstrate that highly endurance-trained individuals are capable of maintaining substantially higher rates of muscular O 2 utilization than less-trained individuals, even during exposure to very low circulating arterial oxygen tensions inherent to exercise at high altitude. It could be argued that the higher O 2 extraction rates measured in the athletes compared with the controls are at least partly due to higher muscle mass in the former. However, body weight, body mass index, as well as leg skinfolds were similar between the groups. Arterial blood-gas content and Δ(A-a)P o 2 . Whenever reduced capacity for pulmonary O 2 diffusion coincides with high rates of peripheral O 2 utilization, maintenance of high arterial O 2 tension becomes critical. We calculated the Δ(A-a)Po 2 to evaluate possible O 2 diffusion limitation. At rest in NOR, Δ(A-a)Po 2 was normal (∼5 Torr) and expectedly (24, 26) decreased to ∼0 Torr at an ambient Po 2 of 81.3 Torr, equivalent to 5,300-m simulated altitude (see Table 2). Stimulation of peripheral O 2 utilization by low-intensity exercise (EX 100W ) increased Δ(A-a)Po 2 to >10 Torr in either group, which suggests impairment of alveolar O 2 diffusion (24, 26) or other conditions, such as right-to-left shunts or ventilation-perfusion mismatching due to a patent foramen ovale or IPAVA. Indeed, Stickland and coworkers (52) have demonstrated that IPAVAs can contribute to elevated Δ(A-a)Po 2 during submaximal exercise in healthy male subjects. Interestingly, Δ(A-a)Po 2 increased more in the athletes than in the controls. This observation contradicts earlier reports stating that pulmonary O 2 diffusion during light to moderate exercise in HYP is independent of training status (2, 44, 59, 64). In the earlier work by Stickland et al. (52), IPAVAs occurred whenever Δ(A-a)Po 2 exceeded 12 Torr. In our present study, Δ(A-a)Po 2 during EX 100W in HYP was ∼16 Torr in the athletes vs. ∼11 Torr in the controls. IPAVAs thus may occur more frequently in highly endurance-trained individuals than in the less trained. Such effect is also compatible with a higher degree of arterial desaturation occurring during submaximal exercise in the former compared with the latter (see Fig. 1). However, the potential importance of such mechanism needs to be addressed in further studies. Despite the fact that the athletes exercised at a markedly lower relative exercise intensity (62.3% V̇o 2 max ) than the controls (77.6% V̇o 2 max ; see Table 3), the former exhibited greater impairment of pulmonary O 2 diffusion, as proven by elevated Δ(A-a)Po 2 . However, we chose to enhance peripheral O 2 consumption by submitting the subjects to a given absolute (cycling at 100 W, EX 100W ) exercise intensity with the express purpose to elicit similar rates of peripheral O 2 utilization in all subjects (see Table 3). As expected, in NOR EX 100W , neither group altered PaO 2 , Pco 2 , acid-base balance, or muscle TOI, yet elicited moderate increments of steady-state HR and pulmonary ventilation (see Tables 2 and 3). Conversely, identical exercise in HYP caused explicit hyperventilation and tachycardia in both groups, substantially lowered PaO 2 and Pa CO 2 , as well as decreased muscle HbO 2 content and TOI. In addition, blood lactate level in fit controls on average increased to as high as ∼8 mmol/l, while the athletes maintained steady-state lactate concentrations at ∼2–3 mmol/l. Interestingly, compared with NOR, during EX 100W in HYP, the athletes increased their V̇o 2 by ∼7% (not significant), while V̇o 2 slightly decreased in the controls. This increase was not due to higher pulmonary ventilation or HR increase, and thus was conceivably due to greater muscular oxygen extraction. Accordingly, the exercise-induced drop in PaO 2 was greater in the athletes than in the controls, yielding a mean exercise Po 2 of ∼28 Torr in the former vs. ∼34 Torr in the latter. As shown in Fig. 1, PaO 2 in seven out of the eight athletes, yet in none of the controls, dropped below 30 Torr during the exercise, which represented a lower percentage of the altitude V̇o 2 max in the athletes (62.3%) than in the controls (77.6%). Taken together, our findings clearly demonstrate that higher muscular O 2 extraction capacity against the face of impaired pulmonary O 2 diffusion capacity facilitates the development of exercise-induced arterial hypoxemia in highly endurance-trained individuals at altitude. Another element that is likely to contribute to the different degree of arterial hypoxemia between the two experimental groups is a difference in their ventilatory hypoxic drive. Indeed, it is well documented that endurance-trained subjects experience a decreased ventilatory responsiveness to HYP and hypercapnia (31, 51). The relative hypoventilation of the athletes compared with the control group during EX 100W in HYP in the present experiments is, therefore, also confirmed by higher Pa CO 2 values (P < 0.05) and lower V̇e rates (P < 0.10) in the former (see Tables 2 and 3). Muscle and brain oxygenation during maximal exercise. Immediately following the submaximal exercise bout, the subjects performed an incremental exercise test to exhaustion (EX MAX ). In fact, high-intensity maximal exercise is atypical for high altitude. Nonetheless, from a physiological perspective, it is interesting to compare maximal exercise responses in severe HYP between endurance athletes and less-fit individuals. Consistent with literature data (35, 36, 62, 66), HYP in the conditions of the present study inhibited V̇o 2 max more in the athletes (minus ∼41%) than in the controls (minus ∼25%). Still, the former maintained 25% higher peak V̇o 2 . The greater HYP-induced impairment of V̇o 2 during maximal exercise in the athletes probably also largely explains the more explicit drop in maximal HR compared with controls (35, 43). Furthermore, EX MAX also caused substantially greater muscular deoxygenation in the athletes than in the controls, both in NOR and in HYP (see Table 3 and Fig. 2). Quadriceps muscle TOI in controls at EX MAX in HYP was similar to that in the athletes in NOR, and TOI in controls during EX MAX in HYP was not even lower than in NOR. However, in HYP, the athletes, but not the controls, were able to decrease muscular deoxygenation to values as low as during a 10-min arterial occlusion (<20% TOI; see Table 2). Accordingly, the rise in HHb vs. drop in HbO 2 fraction at EX MAX was exaggerated in the athletes. We did not measure Po 2 by arterial blood-gas analysis during maximal exercise. However, the drop in Sp O 2 was only slightly higher in the athletes than in the controls (see Table 3). Thus the more negative ΔHbO 2 occurring in the trained muscles at EX MAX conceivably is due to higher rate of muscular O 2 extraction by virtue of higher capillary-to-fiber ratio, elevated capillary blood flow, and stimulation of mitochondrial O 2 utilization producing more gradient for O 2 diffusion (6, 13). Thus athletes not only in NOR, but also in severe HYP, can produce substantially higher (∼35%) workloads than less-fit controls (see Table 3), yet at increased risk to develop more hypoxemia. Indeed, it is the prevailing opinion that hypoxemia, resulting in cerebral O 2 deficit and subsequent arterial vasodilatation, plays a pivotal role in the development of AMS (3, 4, 65, 67). Hence, compared with controls, we expected higher incidence of AMS symptoms in the athletes. In this regard, we also investigated whether differential changes in PaO 2 between athletes and controls would alter cerebral oxygenation status. Throughout the experimental protocol, cerebral TOI changes were less explicit than muscular TOI changes, which is easily explained by the fact that acute HYP disrupts vascular autoregulatory mechanisms to a lesser extent in cerebral compared with skeletal muscle tissue (42, 54). In NOR, cerebral TOI both at rest and during EX 100W were constant and similar between ATL and CON. However, at EX MAX , TOI dropped in ATL but not in CON (see Table 5). By analogy, elite Kenyan long-distance runners at sea level were able to maintain stable cerebral oxygenation during a self-paced 5-km time trial, but not during the final stage of an incremental exercise test (45). In HYP, frontal cortex TOIs consistently were ∼25% lower than in NOR in both of the groups. TOI slightly dropped from rest to EX 100W and further during EX MAX , but with no differences between the groups, which corroborates earlier reports from other groups (27, 48, 53). Still, compared with CON, the tHb increment produced by EX 100W and EX MAX was blunted in ATL. Interestingly, in ATL, the small increment in tHb during exercise was due to an increase in HHb but not HbO 2 , while in CON the substantial increase in tHb resulted from an increased HbO 2 level. These observations are compatible with earlier findings showing frontal cortex blood flow in athletes to decline toward baseline levels during maximal exercise in HYP (58). Occurrence of AMS symptoms. Along this clinical perspective, a secondary aim of the present study was to explore whether endurance-trained individuals are more susceptible to develop AMS symptoms than less-trained individuals, indeed. However, the present altitude protocol, involving no more than 2 h of acute exposure to normobaric HYP, elicited moderate AMS symptoms only, which may have been an inadequate context to differentiate between the groups. Longer exposure (>12 h) to severe HYP is normally needed to produce more explicit AMS symptoms, and, in addition, hypobaric HYP is a more stressful environment for the development of AMS than normobaric HYP (3, 4, 33). Therefore, reported AMS scores in the present experiments should not be considered as a measure of the severity and establishment of the clinical entity itself, but rather as an index of the rate of appearance and severity of initial AMS symptoms. Whether highly endurance-trained individuals are more at risk for the development of AMS than the less trained thus remains to be established. Implications and future perspectives. The interindividual variability in the susceptibility to AMS is presently an important topic of interest in altitude research. The results from the present study contribute to a better insight into the physiological mechanisms underlying the development of exercise-induced hypoxemia at altitude and susceptibility to AMS. We clearly demonstrate that elevated muscular O 2 extraction rate, together with impaired pulmonary gas exchange, puts highly endurance-trained individuals at risk to developing excess hypoxemia during submaximal exercise in HYP. However, whether this effect is specifically caused by training, or rather reflects a genetic predisposition remains to be established. Limitations and technical considerations. Unfortunately, we could perform neither cardiac nor pulmonary catheterizations during the present experiments. Hence we are unable to exactly quantify the role of mixed-venous Po 2 changes in PaO 2 regulation separately from the other factors involved in pulmonary gas exchange. Furthermore, the NIRS equipment used in the present study protocol uses continuous-wave technology and the Beer-Lambert law to measure Δ[HbO 2 ], Δ[HHb], and Δ[tHb]. Data are thus expressed relative to the “baseline” at the start of the measurement with initial set to “zero” by default. This implies that the measured data only reflect changes in heme oxidation status due to interventions in the experimental protocol, regardless of the initial heme oxidation status. Therefore, it is not possible to make any statement about the total amount of [HbO 2 ], [HHb], or [tHb] before, during, or after the experimental protocol. Conclusions. In conclusion, our present data clearly demonstrate that highly endurance-trained individuals maintain markedly elevated capacity for muscular oxygen extraction, even at the very low arterial oxygen tensions occurring during exercise at high altitude. During submaximal exercise, and against the face of simultaneous limitation of pulmonary oxygen diffusion, this high peripheral oxygen extraction capacity facilitates the development of severe hypoxemia. Based on the present observations, it is tempting to speculate that individuals with a high fraction of oxidative type I fibers, and with a consistent history of endurance training, are at high risk to develop hypoxemia and AMS during exercise at high altitude.

GRANTS This study was supported by Grant OT/09/033 from the Katholieke Universiteit Leuven .

DISCLOSURES No conflicts of interest, financial or otherwise, are declared by the author(s).

AUTHOR CONTRIBUTIONS Author contributions: R.V.T. and P.H. conception and design of research; R.V.T. performed experiments; R.V.T. and P.H. analyzed data; R.V.T. and P.H. interpreted results of experiments; R.V.T. and P.H. prepared figures; R.V.T. and P.H. drafted manuscript; R.V.T. and P.H. edited and revised manuscript; R.V.T. and P.H. approved final version of manuscript.