Altitude and hypoxic training in swimming

Altitude/hypoxic training is nowadays a common practice among swimmers although its benefits are still controversial in scientific literature. Traditional altitude training (“live high-train high”) is still the most frequently used method in swimming, even though from a physiological perspective the “live high-train low” strategy appears to be a more promising alternative. While acute hypoxia deteriorates swimming performance, chronic hypoxia may induce acclimatization effects, mainly through the acceleration of red blood cell production, which could improve aerobic capacity and therewith performance upon return to sea level. Other potential benefits such as improved exercise economy, enhanced muscle buffer capacity and pH regulation, and improved mitochondrial function have also been postulated. In order to get a better picture of the potential usefulness of altitude and hypoxic training in swimming this chapter will (i) briefly review the acute and chronic effects of hypoxia, (ii) describe traditional and current methods of altitude/hypoxic training, (iii) discuss the scientific evidence on the effects of altitude/hypoxic training on sea level swimming performance, and (iv) give some practical guidelines for altitude/hypoxic training. Introduction Checking the records of several swimming organizations and altitude/hypoxic training facilities around the world it becomes clear that altitude/hypoxic training plays an important role in preparing elite and subelite swimmers all over the world [59, 57, CAR Sierra Nevada 2008 (personal communication)]. They devote considerable amounts of time, effort, and material resources to train in real or simulated altitude, with the expectation of improved performance at sea level. Unfortunately, there is a remarkable lack of controlled studies on altitude training in swimming in the scientific literature, and the scientific evidence supporting most approaches to altitude/hypoxic training in general is inconclusive [59, 57]. Moreover, in spite of the important amounts of research carried out over the last decades the physiological mechanisms through which altitude/hypoxic training should be effective Truijens M.J., Rodríguez, F.A. : Altitude and hypoxic training in swimming. Dins: Seifert L. , Chollet D, Mújika I (eds.), World Book of Swimming: From Science to Performance, Chapter 20. Hauppauge, New York: Nova Science Publishers Inc., 2011, pp. 393-408. [ISBN 978-1-61668-202-6]. in enhancing performance are still controversial. Field observations and also research studies show that altitude/hypoxic training may work for some athletes and not for others [8]. It has been estimated that an Olympic swimmer should improve his or her performance by about 1% within the year leading up to the Olympics to stay in contention for a medal [37] interestingly, a recent forthcoming meta-analysis concluded that the expectable performance benefit from altitude/hypoxic training for elite athletes can be as high as 1.6% [6]. Perhaps a worthy strategy if a medal is just some tenths of a second away. The right question today may not be if altitude training works, but how, when, and for whom it works. This chapter aims to provide the reader with an overview of peer-reviewed scientific research on altitude training in swimming for the improvement of performance at sea-level, together with a physiological rationale for altitude training in general. Hypobaric and normobaric hypoxia Altitude is defined as ‘the circumstance of a reduced partial oxygen pressure in ambient air’. This condition can be created by a decrease in barometric pressure, leading to a reduction in the inspired partial pressure of oxygen, known as hypobaric hypoxia [5], or by a decrease in the inspired oxygen fraction without changes in barometric pressure; this is known as normobaric hypoxia [4]. Hypobaric hypoxia can be obtained by 1) an ascent to natural/terrestrial altitude, and 2) at sea level using a hypobaric chamber. Examples of devices that provide normobaric hypoxia are nitrogen houses, hypoxic tents, and special breathing apparatuses. Noteworthy, recently it has been shown that these two types of hypoxia do not evoke identical physiological responses. Hypobaric hypoxia leads to greater hypoxemia (decrease of partial O2 pressure in the blood), hypocapnia (less CO2 partial pressure), blood alkalosis (increase pH), and a lower O2 arterial saturation, compared to normobaric hypoxia. These physiological differences could be the consequence of an increase in pulmonary dead space ventilation, probably related to the barometric pressure reduction [47]. Acute and chronic effects of altitude/hypoxic exposure and training Having the definition of altitude in mind the obvious problem the human body has to overcome when at altitude is the maintenance of an acceptable high scope for aerobic metabolism in the face of reduced oxygen availability in the atmosphere. In general, on acute exposure to hypoxia the human body reacts immediately with an integrated reaction of both the autonomic nervous system and cardiovascular system to overcome the drop in arterial oxygen content. Increased ventilation, sympathetic neural activity, cardiac output, and diuresis, among other mechanisms, constitute the acute response to hypoxia. Maximal values for heart rate and cardiac output are either similar to sea level values or slightly reduced. Maximal anaerobic capacity is reported to be unchanged. If the hypoxic stimulus is maintained (i.e. chronic hypoxia) a complex multisystemic response is developed, leading to complete or partial acclimatization to hypoxia within a few days or weeks (figure 1). The most prominent adaptation that has been observed with continuous altitude exposure that has the clearest link to improved sea-level performance is an increase in red blood cell mass (RCM), which increases the oxygen-carrying capacity of the blood and improves aerobic power [29]. Although some studies in elite athletes have failed to show an increase in RCM with chronic Truijens M.J., Rodríguez, F.A. : Altitude and hypoxic training in swimming. Dins: Seifert L. , Chollet D, Mújika I (eds.), World Book of Swimming: From Science to Performance, Chapter 20. Hauppauge, New York: Nova Science Publishers Inc., 2011, pp. 393-408. [ISBN 978-1-61668-202-6]. altitude exposure [2], the sum of experimental evidence in favor of this response is quite compelling. Several other adaptations to long term hypoxic exposure have been reported in literature. Conflicting evidence exists for changes in anaerobic capacity with altitude acclimatization. Some studies have reported that buffer capacity of skeletal muscle may be increased [31], even with discontinuous altitude exposure [18], which may lead to improvements in anaerobic capacity, whereas other studies reported no change in anaerobic capacity after acclimatization [28]. Furthermore, it is suggested that hypoxic training could induce local adaptations at the molecule (augmented transcription for HIF1-alpha as well as increased mRNA for myoglobin and vascular endothelian growth factor) and muscle level (increased myoglobin and oxidative enzymes) that would be beneficial for performance [51]. Figure 1. Summary of the purported physiological mechanisms involved in the use of hypoxia for performance enhancement. Modified from Rodríguez 2007 [44]. The above mentioned adaptations and the degree in which they take place depend on several factors, some characterizing the “dose” of hypoxia (e.g. degree of hypoxia, duration of the exposure to hypoxia), some related to training (e.g. training goals, training program, normoxic or hypoxic training), and some related to nutrition and clinical status (e.g. iron stores, diet, oxidative stress, immune function). This disparity makes research on this topic particularly complex. Another important limitation is the fact that not all subjects respond the same to a certain combination of factors. In fact, ↑ Chronotropism ↑ Inotropism