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Intermittent hypoxia and training

Intermittent Hypoxia and Training: Methods, Strategies, and Results Ferran A. Rodríguez and Josep Lluís Ventura Institut Nacional d’Educació Física de Catalunya, Av. de l’Estadi, s/n (Anella Olímpica de Montjuïc), 08038 Barcelona (Spain). Correspondence to: F.A. Rodríguez; tel +34 93 4255445; fax +34 93 4263617; email: [email protected]

SUMMARY During recent decades altitude training has been used to improve athletic performance. In practice, the hypoxic stimulus can be attained by several means: 1) environmental or altitude (hypobaric) hypoxia occurs upon ascent to moderate to high altitude (mountains or flights) as barometric pressure decreases with altitude; 2) simulated altitude generally stands for artificial hypobaric hypoxia in low-pressure hypobaric chambers; and 3) normobaric hypoxia can also be attained artificially by modifying the oxygen concentration of inspired air using various technologies. Evidence of physiological adaptations to hypoxia open many possibilities for enhancing athletic performance in competitive sports at sea level, although this issue is highly controversial. This paper reviews a number of methods and strategies currently used to enhance athletic performance by means of intermittent exposure to hypoxia. Results reported in the literature are discussed and compared in an attempt to elucidate the characteristics of effective hypoxic programs.

HYPOXIC METHODS IN SPORT Although the benefits of altitude training are highly controversial in scientific literature, this type of training has become part of the athletic preparation of many elite and subelite athletes all over the world. Altitude training centers have been set up in many countries and bring together an increasing number of athletes from various sports disciplines. However, these centers are only located in mountain areas and technical, logistic, and financial problems arise when athletes, coaches, and physicians have to spend the long sojourns needed if benefits are to be derived from training at altitude. Consequently, researchers have been studying alternative means and strategies of inducing altitudinal effects. Hypoxia can be either hypobaric (low PO2 caused by a naturally or artificially decreased air pressure), or normobaric (low oxygen concentration in inspired air or FIO2 at normal barometric pressure). Environmental or altitude (hypobaric) hypoxia occurs upon ascent to moderate to high altitude (mountains or flights) as barometric pressure decreases -107-

FA Rodríguez, JL Ventura

with altitude. Simulated altitude implies artificial hypobaric hypoxia in low-pressure, hypobaric chambers. Normobaric hypoxia can be attained using various technologies by decreasing the oxygen concentration of inspired air. Table 1 summarizes the hypoxic methods and strategies most commonly used in sports. Table 1. Hypoxic methods commonly used in sports (Rodríguez, 2002). Method

Physical principle

Type of hypoxia

Facilities / Common strategiesa

Moderate to high altitude

Natural reduction of atmospheric and O2 pressure (↓PO2)

Hypobaric, continuous or intermittent

Altitude resorts in mountains

Hypobaric chamber

Artificial reduction of atmospheric and O2 pressure (↓PO2)

Hypobaric, intermittent

Hypobaric chambers (decompression or low pressure chambers)

Hypoxic gas mixtures Artificially decreased O2 concentration in inspired air (↓FIO2)

Continuous or intermittent sojourns at altitude (LH-TH, LH-TL, LL-TH)

Intermittent exposure, passive (LH-TL) or combined with training (LL-TH)

Normobaric, intermittent

Hypoxic gas mixture (cylinders)

Houses, portable chambers or tents with external N2 addition to atmospheric air

Hypoxic houses, portable chambers and tents

Artificial ↓FIO2 by N2 addition to atmospheric air


Respiratory hypoxic devices

Artificial ↓FIO2 by O2filtering membranes in atmospheric air


Intermittent exposure, usually with training (LL-TH)

Intermittent exposure, usually during sleep or resting time (LH-TL) Portable respiratory devices (with breathing masks) producing a hypoxic gas mixture Intermittent exposure, usually combined with training (LL-TH)

a

L= live; T = train; H = high; L = low (e.g. “Live High-Train Low” = LH-TL)

Intermittent hypoxia (IH) can be defined as periodic exposure to hypoxia which lasts from minutes to days and is repeated over several or more days, and is interrupted by return to normoxia or less hypoxic conditions. Thus, the term is used to differentiate intermittent hypoxic stimuli from a single hypoxic stimulus, or continuous chronic hypoxia (CH) (Powell and Garcia, 2000). The practical reasons for IH being an alternative to “conventional” altitude acclimatization or altitude training (CH in the mountains) are: 1) the availability of an artificial environment located at low altitude or sea level, and 2) the more intense but shorter hypoxic stimuli needed, which are considered to be safer, more compatible with normal living conditions, and with a low risk of producing acute mountain sickness (AMS) in unacclimatized subjects. The expression "short-term IH" has been used to denote IH exposure for several hours (from 5 to 13 h following one week of CH) (Richalet et al., 1992). We have used this term to stress short duration IH compatible with normal life and exercise training at sea level which is maintained for a few days or weeks (Rodríguez et al., 1998, 2000). Many of the physiological effects of IH are comparable to those stimulated by chronic hypoxia. For more information readers are referred to a recent review by Powell -108-


and Garcia (2000). The adaptations that have been examined can be summarized as follows: 1) altitude acclimatization (ventilation, hypoxic ventilatory response, arterial oxygen saturation; 2) hematological responses and adaptations (EPO, erythropoiesis); and 3) effects on physical performance and aerobic capacity (athletic performance, exercise tolerance, lactate and ventilatory threshold, V& O 2 max ). These adaptations depend on several parameters, some of which characterize the “dose” of IH, while others are related to physical training and clinical status of the subjects. These parameters include the degree of hypoxia (simulated altitude), the duration of acute exposures (single hypoxic stimuli), the duration of the IH program, the type of IH training (passive exposure or hypoxic training), the workload characteristics, the fitness or competitive level of the subjects and their clinical condition, etc. This heterogeneity makes research on this topic particularly complex.

IH AND SEA-LEVEL PERFORMANCE Altitude training has been widely incorporated into the training regimes of elite athletes in recent years. However, the issue of whether this training enhances sea level performance remains controversial (Wolski et al., 1996). The reason for this may be related to the fact that most benefits from adaptive mechanisms at altitude are counterbalanced by a reduction in training workload caused by the same factor that produces the adaptations: the hypoxic environment. When appropriate control groups and adequate experimental design have been used, athletes living and training at moderate altitude do not show an advantage over those with equivalent training at sea level (Adams et al., 1975). More recently, trained and elite runners living at moderate altitude (2,500 m) and training at a lower altitude (1,250 m), known as the “living high-training low” strategy, improved their sea-level running performance because of an increase in red cell mass volume and V& O 2 max after a permanent sojourn of four weeks (Levine & Stray-Gundersen, 1997; Stray-Gundersen et al., 2001). Great variation was observed between athletes, but those who responded best to the living high-training low schedule showed the largest erythropoietic response, the increase in V& O 2 max being proportional to the increase in hematocrit (Chapman et al., 1998). However, whether this kind of approach can be considered IH or CH is open to debate since subjects are permanently exposed to a certain degree of hypoxia and spend more than 20 hours per day in moderate hypobaric hypoxia. Nevertheless, the “living high-training low” approach has led to the use of several new altitude training devices designed to artificially create hypoxic conditions in a laboratory or in domestic environments (Table 1). Here we review the main results of the most common methods currently used in sports, namely normobaric and hypobaric intermittent hypoxia, and hypoxic training.

Normobaric IH The effect of intermittent normobaric IH using hypoxic apartments has been investigated in endurance athletes (runners, cyclists, triathletes, and skiers) in conjunction -109-


with sea-level training. These studies, which follow the “live high-train low” paradigm, typically simulate an altitude environment equivalent to approximately 2,000 to 3,000 m, in which athletes live for 8 to 18 hours a day and complete their training at sea level or at very low altitude (600 m). In a recent review of twelve studies carried out mainly in Finland, Sweden, and Australia, Wilber (2001) concludes that only some showed changes in EPO levels, reticulocyte counts and red blood cell mass, while others failed to demonstrate significant alterations in hematological indices (Table 2). Only one study showed a small but significant increase in V& O 2 max (3%). However, only two found a significant improvement in post-altitude performance in 40 km cycling and in a 400 m time trial, respectively, and only the latter included a sea-level control group. Wilber concluded that these large discrepancies may be caused, in part, by differences in methodology, the hypoxic stimulus that athletes were exposed to and/or the training status of the athletes. Moreover, he highlighted the inconsistency of the results from this particular strategy. Table 2. Synopsis of comparative results after training combined with various intermittent hypoxia (IH) interventions (see text for explanations and references). Sea-Level Training Hypoxic training Normobaric IH Hypobaric IH Hypobaric IH (12-18 h/d, 10-25 d, (1.5-5 h/d, 9-21 d, (1-2 h/d, 9-21 d, 2,000-3,000 m) 4,000-5,500 m) 2,300-5,500 m) EPO release Reticulocytes PCV (Htc) [Hb] Red cell mass VO2max Anaerobic threshold Anaerobic power/capacity Performancea Remarks

↑ ↑↑ ↑= ↑↑= ↑= ↑ = ↑ ? ↑= = ↑= ? ↑ ? ? =↑ ↑= LH-TL model LH-TL model Time/hypoxia/training Time/hypoxia/training level gradients? level gradients?

? ↑= ↑= ↑= ? = ↑= ↑ ↑= LL-TH model Training volume & intensity reduced

↑: Increase; =: No change; ?: Not reported a Post-altitude performance at sea level

Hypobaric IH The use of hypobaric IH is based on the observation that brief exposures to relatively high levels of hypoxia stimulate the release of EPO (Eckardt et al., 1989). It is assumed that this reaction, when maintained for an appropriate period, stimulates erythopoiesis, and ultimately enhances V& O 2 max and/or endurance performance. In a series of studies in our laboratory, we assayed a short-term intermittent IH model with a higher degree of hypoxia compared with most studies using normobaric IH (i.e. 1,5 to 5 h of simulated altitude equivalent to 4,000 to 5,500 m) and shorter chronic -110-


exposure (2-3 weeks). In a study with 17 trained climbers exposed to IH in a hypobaric chamber over 9 days, 3 to 5 h per day, at a simulated altitude of 4,000 m to 5,500 m (Rodríguez et al., 1999), we observed a significant increase in exercise time (3.9 %), maximal pulmonary ventilation (5.5 %), and lactate threshold, indicating an improvement in aerobic endurance, paralleled by an increase in hematocrit, red blood cell count, reticulocytes, and hemoglobin concentration. Similar results were observed in a longer exposure (17 days), together with a non-significant increase (6.2%; p=0.07) in V& O 2 max (Casas et al., 2000). In a third study using shorter duration of stimuli (90 min, 3 days per week for 3 weeks), we detected similar hematological adaptations as a result of the triggering effect of the hypoxic stimulus on EPO production, but no significant changes in maximal aerobic power or performance were observed (Rodríguez et al., 2000). More recently, in a controlled study (Rodríguez et al., 2002), 16 highly trained swimmers of national level (8 sea-level controls included) combined sea-level training with exposure to IH over 10 days (3 h per day) in a hypobaric chamber (4,000 to 5,500 m). Only the IH group significantly increased V& O 2 max (4 and 7% increase in males and females, respectively), and improved performance, peak V& O 2 and oxygen kinetics in a 200 m timetrial. Again, these changes were also paralleled by increased hematocrit (5%) and hemoglobin concentration. From these results, it can be concluded that an adequate “dose” of hypoxic stimulus to effectively stimulate erythropoiesis and enhance aerobic performance in trained subjects requires relatively high levels of hypoxia (over 5,000 m) of sufficient duration (over 3 h per day during 2-4 weeks) combined with sea-level training.

Intermittent Hypoxic Training The opposite to the “living high-training low” approach is living at sea level and intermittently training under hypoxic conditions. This has been called “living low-training high” strategy and, more recently, intermittent hypoxic training (IHT). Only a few studies have addressed this strategy and they have shown inconsistent results. One compared moderate IHT with training in normoxia (Terrados et al., 1988). A group of competitive road cyclists trained for 3-4 weeks, 4-5 sessions per week, for 105150 min, combining continuous and intermittent cycling at a simulated altitude of 2,300 m. Work capacity at altitude and sea level was increased more by IHT than by training at sea level. This improvement was paralleled by decreased exercise blood lactate concentration, increased capillarization, and decreased glycolytic capacity in leg muscles, but V& O 2 max remained unchanged in both training conditions. In another study with competitive triathletes, neither were effects on maximal aerobic capacity, V& O 2 max or erythropoiesis observed after 3 weeks of usual training schedule combined with 3 sessions per week, each of 1.5 h, in which they trained at a simulated altitude of 4,000 m (Vallier et al., 1996). In a similar study (Meeuwsen et al., 2000), 16 elite triathletes trained at a simulated altitude of 2,500 m for 2 hours at 60-70% of heart rate reserve over 10 days. No significant increments in hemoglobin, hematocrit or V& O 2 max were observed. However, significant improvements in anaerobic power (5%) and anaerobic capacity (40%), measured by a 30-second Wingate -111-


test, were detected in the IHT group in a second test after 9 days. In constrast, in trained climbers exposed to IH in a hypobaric chamber over 9 days (4,000 to 5,500 m) (Rodríguez et al., 1999), no differences were detected between a group that was exposed to a combination of hypoxia during rest and low-intensity exercise on a cycle ergometer and another group exposed only to passive hypoxia (at rest). These results indicate that hypoxia alone was responsible for the changes.

CONCLUSIONS The effects of IH on performance are inconclusive for every method and strategy used, perhaps because of differences in the methods of assessment, “dose” of the hypoxic stimulus, training level, performance status of subjects, and individual response. Further studies that address the underlying physiological adaptations and the optimal doseresponse strategies for performance enhancement in elite athletes are required. Most commercial claims are not supported by research. However, upcoming evidence improves our understanding of the physiological adaptations and effects of IH and opens new possibilities for its use in competitive sports. Whether IH is a valid alternative to conventional altitude training remains an open question.

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