Exercise performance is highly related to various factors that can be altered through altitude/hypoxic training, including erythropo iesis, ex ercise econo my ( d efined as the amount of energy spent per unit of distance), metabolic response, aerobic exercise capacity, and capillary density (Levine and Stray-Gunderson, 1997; Park et al., 2017; Saunders et al., 2004). Therefore, many sports scientists have been studying various altitude/hypoxic training programs aiming to enhance the exercise performance of athletes; currently, altitude/hypoxic training is recognized as a useful training method for athletes to improve their performance potentially in subsequent competitions at sea level (Brocherie et al., 2015; Millet et al., 2013; Sinex and Chapman, 2015).
Altitude/hypoxic training aiming to improve athletic performance can be classified into four types: living high-training high (LHTH); living low-training high (LLTH), such as intermittent hypoxic exposure, continuous hypoxic training, interval hypoxic training (IHT), and repeated sprint training in hypoxia (RSH); living high-training low (LHTL); and living high-training low and high, such as LHTL + IHT or RSH (Brocherie et al., 2015; Millet et al., 2013; Wilber, 2007). To date, LHTL is widely recognized as a relatively effective altitude/hypoxic training method for improving athletic performance at sea level, despite ongoing debate (Bonetti and Hopkins, 2009; Brugniaux et al., 2006; Garvican et al., 2011).
The potential underlying mechanisms for improved exercise performance following LHTL include increase in erythropoiesis and blood oxygen transporting capacity (Hauser et al., 2017; Levine and Stray-Gunderson, 1997; Wehrlin et al., 2006). This translates into increased maximal oxygen consumption (V[O.sub.2]max) while residing in an altitude/hypoxic environment, and there is a possibility that the increased exercise intensity in competitions and enhanced V[O.sub.2]max will be maintained at sea level (Levine, 2002; Schmitt et al., 2006; Wilber, 2007). LHTL improves the sea-level running performance of trained elite runners because of adaptation to hypoxia (increase in erythropoiesis and V[O.sub.2]max) and maintenance of training velocity at sea level, most likely accounting for the increase in running velocity at V[O.sub.2]max (Brugniaux et al., 2006; Garvican et al., 2011). These benefits are primarily attributed to the increase in erythropoiesis that occurs while residing in an altitude/hypoxic environment, provided that the "dose" of hypoxia is adequate (Dehnert et al., 2002; Levine and Stray-Gundersen, 2006; Wehrlin and Marti, 2006; Wehrlin et al., 2006). However, several studies have indicated that LHTL does not increase erythropoiesis or exercise performance (Ashenden et al., 1999a; Clark et al., 2004; Hahn et al., 2001). Ashenden et al. (1999a) demonstrated that 12 nights of exposure to moderate normobaric hypoxia (e.g., at an altitude of 2,650 m) is not sufficient to stimulate erythropoiesis. Hahn et al. (2001) reported that failure of moderate hypobaric hypoxia (e.g., at an altitude of 2,500 m) for 23 days to stimulate erythropoiesis and V[O.sub.2]max improvement is probably attributable to an insufficient duration of exposure (8-11 hours daily). In addition, Clark et al. (2004) reported that exercise economy, muscle markers of lactate metabolism, and pH regulation were unchanged after 20 consecutive nights of normobaric hypoxic exposure (fraction of inspired oxygen [[F.sub.I][O.sub.2]], 16.27%). These previous studies have indicated that LHTL is affected not only by enhanced erythropoiesis but also by improved exercise economy, increased storage of glycogen, utilization of fatty acids, muscle buffer capacity, skeletal muscle oxygenation, and cardiovascular function (Garvican et al., 2011; Gore et al., 2001; Park and Nam, 2017; Saunders et al., 2004). Therefore, to assess the effectiveness of LHTL in improving athletic performance, various physiological mechanisms, such as exercise economy, muscle buffer capacity, skeletal muscle oxygenation, cardiovascular function, and erythropoiesis, must be considered.
Another issue that has been raised regarding the effectiveness of LHTL in improving exercise performance is the optimal dose of hypoxia. Levine and Stray-Gundersen (2006) recommended using hypoxic environments at altitudes ranging from 2,000 to 3,000 m to stimulate erythropoiesis and to avoid any detrimental effects, such as acute mountain sickness, leukocyte count alteration, ventilator response impairment, and desaturation. Brugniaux et al. (2006) sought to determine the influence of factors, such as range and duration of residence in/exposure to the altitude/hypoxic environment, on the balance between the beneficial effects and potentially detrimental effects of LHTL. They suggested that during LHTL, the altitude should not exceed 3,000 m; residence should be continued for at least 18 days; and exposure should be at a minimum of 12 hours per day.
Based on previous findings, we established the following hypothesis: 21 days of LHTL involving more than 12 hours per day of residence at 3,000 m ([F.sub.I][O.sub.2] = 14.5%) in a normobaric hypoxic environment and more than 4 hours per day of training at sea level will improve exercise performance by enhancing erythropoiesis, exercise economy and metabolism, and hemodynamic function during sub-maximal exercise compared with LLTL in moderately trained competitive runners.
The participants were male, competitive, moderately trained, middle- and long-distance runners (n = 24) registered with the Korea Association of Athletics Federations. They were randomly assigned to perform either LLTL (n = 12) or LHTL (n = 12) according to their body composition, V[O.sub.2]max, and 3,000 m time trial performance (Table 1). All 12 participants in the LHTL group resided together in environmental chambers comprising a 14.5% oxygen normobaric hypoxic environment. The 12 participants in the LLTL group resided together in a room at sea level. Other than oxygen concentration, the remaining environmental conditions during residence were the same (temperature = 24 [+ or -] 2[degrees]C; humidity = 60 [+ or -] 2%) for the LHTL and LLTL groups. During the 21-day intervention period, the weekly routine for all athletes included 6 days of training and 1 day of rest. The participants received information on the study purpose and methods. They provided written consent for participation after receiving sufficient explanations regarding the experiment and understanding the possible adverse effects prior to the start of the study. This study was approved by our Institutional Review Board (7001355-201510-HR-090) and was conducted in accordance with the provisions of the Declaration of Helsinki.
The experimental design is shown in Figure 1 and consisted of the following: 7-day preintervention period (i.e., 3 testing days and 2 rest days between testing days) at sea level, during which baseline testing was performed; 21-day living and training session in the assigned environmental condition; and 7-day postintervention period at sea level. During the 3-day testing sessions, hematological parameters, exercise economy and metabolism, hemodynamic function, and exercise performance were evaluated.
On the first testing day, hematological parameters, including erythrocyte count, hemoglobin (Hb) concentration, hematocrit (Hct), reticulocyte count, erythropoietin (EPO) concentration, mean corpuscular volume (MCV), mean corpuscular Hb (MCH), and MCH concentration (MCHC), were measured between 7:00 and 8:00 AM in the rested state. Thereafter, the body composition was measured. Subsequently, the V[O.sub.2]max was measured to evaluate exercise performance in the afternoon. On the second testing day, the following parameters were measured during a 30-minute bout of submaximal cycle ergometer exercise: exercise economy; metabolism indicators, including oxygen consumption (V[O.sub.2]) and blood lactate concentration; skeletal muscle oxygenation, including oxygenated Hb and myoglobin ([O.sub.2]Hb), deoxygenated Hb and myoglobin (HHb), and tissue oxygenation index (TOI); and hemodynamic function, including heart rate (HR), stroke volume (SV), end-diastolic volume (EDV), end-systolic volume (ESV), and cardiac output (CO). Exercise intensity was set at individual cycle ergometer exercise load values corresponding to 80% maximal HR (HRmax) obtained during the preintervention period. On the third testing day, performance in a 3,000 m time trial was measured.
After the preintervention period, the participants were randomly assigned to one of two groups according to their initial body composition, V[O.sub.2]max, and 3,000 m time trial performance (Table 1): LHTL group (21-day LHTL, which included >12 hours per day of living in a normobaric hypoxic environment at an altitude of 3,000 m and training at sea level [n=12]) and LLTL group (21-day LLTL, which included living and training at sea level [n=12]). The athletes in the LHTL group stayed in a 10x14x3 [m.sup.3] environmental chamber (Submersible Systems, Huntington Beach, CA, USA). The 3,000 m altitude ([F.sub.I][O.sub.2]=14.5%) normobaric hypoxic environment was simulated by introducing nitrogen into the environmental chamber using a nitrogen generator (Separation & Filter Energy Technology Cooperation, Siheung, Korea). The athletes in the LLTL group resided at sea level under comfortable conditions otherwise similar to those for the LHTL group. The daily schedule of the 21-day program is shown in Table 2. The training program was created by coaches based on their experience and does not conform to regular practice. The daily training programs consisted of >4 hours of exercises, including interval rests performed at various intensities, which were expressed as percentages of [HR.sub.max]. As shown in Table 3, the daybreak exercise consisted of a...