The physiological effects of the hypoxia environment training protocol have not yet been fully elucidated or examined comprehensively from a holistic perspective. To further understand the sports performance benefits of different hypoxia environment training protocols, the height of the simulated environment, hypoxia concentration, training protocol, duration, and exercise ability level are all still under investigation.

The concept of high-altitude or hypoxia training to improve sea level sports performance has existed for more than decade. Researchers and trainers have proposed different training protocols and strategies, such as live high-training high, live high-training low, and another highland training protocol that has received attention in sports science and training protocols because of the continuous development of equipment to simulate hypoxia environments, which is the live low-training high (LL-TH) protocol [1]. LL-TH is a common training protocol used to improve athletes’ performance in sports [2]; in this type of training, athletes or individuals who want to improve their performance in sports, or exercise, choose to live at lower altitudes, while exposing themselves to a lower oxygen environment during training.

Hypoxia is a state in which oxygen intake is reduced, resulting in oxygen levels in the body’s tissues falling below the normal range [3,4]. Hypoxic training, also known as altitude training, is a common way to enhance sports and exercise performance [2,5]. If tissue oxygen demand exceeds supply, a cascade of intracellular events is activated increasing the expression of hypoxia-inducible factors (HIFs). Hypoxia-inducible factors (HIFs) are transcriptional factors and key regulators of the cellular response to hypoxia [6,7], and increase hemodynamic function by oxygen transport and utilization ability [5]. Also, hemoglobin levels have a linear relationship with intermittent hypoxia [8]. An increase in hemoglobin mass leads to easier transport of oxygen [9,10,11], and an adaptive response to high-altitude hypoxia [12].

High intensity interval training is used to increase cardiopulmonary endurance and enhance sports performance, involving short- to moderate-duration exercises (10 s to 5 min) at high intensity, surpassing the anaerobic threshold [24]. Considering that the type of sports training, intensity, and level of exercise of the training target may affect physiological adaptations and hematological changes to improve sports performance, exposure to a hypoxia environment is a more common and widely used training and testing protocol. Intermittent hypoxia interval training (IHIT), or periodic exposure to hypoxia, is also known as LL-TH, and is defined as a protocol in which hypoxia and normoxia alternate during a periodic training session, which may last from a few minutes to several hours of hypoxia exposure, and which is repeated over a period of days to weeks. Hypoxia high-intensity interval training (HHIT) is a combination of an intermittent hypoxia environment and HIIT, and is commonly used in research and interval training. Sprint interval training, a form of HIIT training, involves brief exercise periods lasting approximately 10–15 s at near maximum intensity, followed by recovery intervals of less than 30 s, requiring maximum effort within a short period of time, through the use of repetitive anaerobic testing on a bicycle ergometer. This interval sprint training protocol can improve VO₂max, skeletal muscle oxidative capacity, and endurance performance in two weeks with a total of six “all-out” training sessions [25]. In HIIT, 95% of the lactate threshold load was considered. In this training protocol, athletes perform high-intensity short-duration exercises in a simulated or actual hypoxia environment, alternating between high-intensity (80–90% HR) and low-intensity (60–70% HRR) exercise and recovery exercise [26,27,28]. This hypoxia exposure is an effective protocol to improve skeletal muscle oxygen utilization capacity and exercise performance, characterized by repetitive sprints of high-intensity exercise at maximum workload, with consideration of short recovery times. Several studies related to hypoxia high-intensity interval training (HHIIT) have shown that participants, whether they were trained athletes [1,19,21,29,30,31], high-physical-activity performers [22,32], or untrained individuals [13], were able to increase their maximum oxygen intake after more than two weeks of intermittent hypoxia interval training. The studies with relevant post hoc analyses have shown that HHIIT interventions are more effective than normal oxygen interventions in interval training [7].

In addition to the more common high-intensity hypoxia interval training, another protocol of hypoxia training is to gradually decrease oxygen concentration and increase exercise intensity. In a study related to oxygen concentration tapering training, Czuba et al. [1] investigated the effect of incremental hypoxia interval training on the aerobic capacity and endurance performance of cycling athletes. This training consisted of 3 weeks of intermittent hypoxia training, three times per week. Each session consisting of four sets of all-out 30 s sprint intervals, accompanied by a 90 s low-intensity recovery state, simulating an altitude of 2500–3000 m. Additionally, the FIO₂ was adjusted as the number of training days decreased, from 17% on day 1 to 14% on day 10; the SpO₂ value was controlled at 88% on days 1–2, 84% on days 3–4, and 82% on days 5–10. The VO₂max increased by 3.98% in the incremental hypoxia interval training group, and so did the lactate threshold, power output, and average speed, while no differences in hematological variables were found in the trained cyclists during the 3 weeks of incremental hypoxia training.

In Bonetti et al. [16]’s study of 10 sub-elite male short-distance sprint light-boat athletes who underwent 3 weeks of incremental hypoxia treatment, while being connected to the system via a face mask, the incremental hypoxia treatment used Wood et al. [33]’s protocol. The incremental hypoxia concentration experiment followed a specific design, starting at 12% in the first week, reducing to 11% in the second week for 6–10 days, and reaching 10.9% in the third week. Pre- and post-tests were conducted, including an incremental step test to assess peak power, VO₂max, exercise economy, lactate threshold, average burst force over 500 m, and short-distance sprints of 5 × 100 m. Throughout the experiment, intravenous blood samples were collected for analysis of hemoglobin concentration, hematocrit, ferritin, erythrocyte sedimentation rate, and white cell count. The findings demonstrated a 6.8% improvement in peak power, an 8.3% enhancement in mean repeat sprint power, and a 3.6% rise in hemoglobin concentration. However, no notable benefits were observed in lactate threshold, mean 500 m power, VO₂max, or exercise economy.

In non-athletes, incremental hypoxia training was performed in the non-athletic progressive intensity study by Nishiwaki et al. [34]. The first phase consisted of 30 min low-intensity continuous exercise (e.g., walking) at 60–70% of maximum HR; the second phase consisted of eight sets of 2 min high-intensity exercise with 1 min rest between sets at 85–95% of maximum HR. It was observed that in the hypoxia group of women with menopause, the endothelium-dependent vasodilation (FMD) increased from 4.4 ± 1.8% to 6.4 ± 2.0% after 10 days of hypoxia training, and the vasodilatory reactivity improved. The physiological benefits of hypobaric hypoxia training in postmenopausal women mainly involve improving the physiological parameters of FMD.

Compared to high-intensity intermittent exercise in a hypoxia environment, progressive hypoxia exposure training can bring about changes and improvements in exercise performance, cardiorespiratory fitness, and anaerobic fitness; however, there are only few studies related to high-intensity intermittent and sub-intensity training. Therefore, if we consider the physiological adaptations associated with exercise, progressive intensity and hypoxia training protocols may provide the time needed to adapt to training, allowing the individual to adapt to physiological changes and bring about better athletic performance; however, more research is needed to further investigate the relevant training protocols.

As mentioned previously, HIIT and progressive exercise intensity/incremental hypoxia training can provide physiological and performance benefits. Submaximal exercise is a state after completing aerobic exercise training. The blood lactate concentration is lower when exercising at the same absolute intensity [35]. This type of training is another form of hypoxia training. The study conducted by Park et al. [36] investigated the effects of simulated 3000 m hypoxia combined with exercise at 70% of maximal heart rate (HRmax). This condition led to increased anaerobic metabolism and oxygen supply to muscles due to limited oxygen delivery capacity. Moreover, the study found evidence of greater metabolic stress in healthy men, characterized by decreased SpO₂, VO₂, and oxygen pulse, as well as an increased respiratory exchange ratio. Additionally, notable changes were observed in skeletal muscle oxygenation responses.

In another study examining submaximal exercise training in hypoxia, conducted by Jung et al. [37], it was found that endurance submaximal ergometer exercise load at 80%, performed under hypoxia (simulated 3000 m), results in increased metabolic and cardiac responses compared to exercise under normoxia. The hemodynamic function, including parameters such as oxygen uptake, oxygen pulse, and cardiac output, increases. Hemodynamic and autonomic nervous system (ANS) functions also improve, and these improvements do not have adverse effects on immune function.

Overall, submaximal exercise training in hypoxia has shown several effects. It has led to increased anaerobic metabolism in skeletal muscle tissue due to limited oxygen delivery capacity, enhanced oxygen supply to the muscles, and greater metabolic stress. The latter is characterized by decreased SpO₂, VO₂, and oxygen pulse, along with an increased respiratory exchange ratio [15]. Additionally, observable changes have been noted in skeletal muscle oxygenation responses [37]. However, it appears that submaximal exercise training in hypoxia does not have a significant impact on hemorheological properties.

Combined training in a hypoxia environment refers to the integration of any two various protocols to optimize performance in sports and exercise. Roels et al. [17] implemented a hypoxia training protocol that involved three weeks of interval training twice a week, along with three sessions of sub-intensity (60% VO₂max) aerobic endurance training; the interval training consisted of three sets of two intervals at a peak power-output (PPO) intensity of 100% for 2 min, with a 2 min rest state between intervals and a 6 min rest state between sets. The results showed that the VO₂max did not improve significantly in either the normoxia training group or the hypoxia training group under normoxia conditions. However, in terms of PPO performance, both the normoxia training group and the hypoxia training group showed a significant improvement of 6.6% and 7.2% under sea level conditions, while only the hypoxia training group showed a significant improvement in power performance under hypoxia conditions (11.3%; p < 0.05). In a study by Park and Lim [2], 20 swimmers were exposed to a hypoxia environment for 6 consecutive weeks for 120 min per day, 3 days per week, with a combination of 30 min of sub-intensity running aerobic training (80% HR max), 10 sets of high-intensity stationary bicycle training for 2 min each (90% HR max), and 80–90% maximal resistance training for the remainder of the time. The results showed significant improvements in VO₂max, anaerobic power output, and 50 m and 400 m freestyle performance, as well as significant increases in growth hormone, insulin growth factor (IGF-1), vascular endothelial growth factor (VEGF), and muscle strength and endurance performance.

Based on the aforementioned research, sub-intensity hypoxia exposure and combined hypoxia exposure training can stimulate changes in skeletal muscle oxygenation responses during submaximal exercise training and enhance power performance in combined training. Unlike most hypoxia training in cardiorespiratory demanding sports, such as marathons, triathlons, and endurance sports, sub-intensity hypoxia exposure and combined hypoxia exposure training are also utilized in sports that require cardiorespiratory fitness performance on the field, such as boxing, taekwondo, and badminton, which are open-skill sports that also demand endurance performance. While high-intensity intermittent hypoxia interval training necessitates physical fitness, sub-intensity and integrated hypoxia exposure training protocols can offer sufficient training duration and physical capacity for specialized technical training in the target sports groups.

The recovery hypoxia training protocol considers exposure to a hypoxia environment during the recovery state or outside the training time, and further observes physiological and sports performance changes. Exposure to a hypoxia environment during the recovery state was also considered during the training period. In a study conducted by Katayama, Sato, Matsuo, Ishida, Iwasaki, and Miyamura [38], 15 runners in middle- and long-distance training, both in the normoxia group and the hypoxia group, were exposed to submaximal exercise in a normoxia environment with a VO₂max of 75%, while the hypoxia group was exposed to a hypoxia environment only during the recovery state in a tent with a hypoxia environment of 12.3% oxygen concentration. The results of the study showed that the recovery hypoxia exposure group showed a 7.0% improvement in maximum oxygen uptake during the recovery state, with a significant correlation at 3000 m. However, no significant changes were observed in hematological parameters.

Similarly, studies related to hypoxia exposure were conducted outside the training time, e.g., Gore et al. [39] and Saunders et al. [40], where the participants were specially trained long-distance athletes, cyclists, triathletes, and long-distance skiers exposed to 14.3% oxygen in a hypoxia environment for 20–23 days of sleep. Saunders et al. [40] showed that training time with progressive 20-day sleep exposure to a hypoxia environment resulted in a 12.4% improvement in oxygen uptake and improved running benefits in long-distance athletes. In the other study, 23 days of sleep exposure to 14.3% oxygen concentration in a hypoxia environment resulted in a significant increase in peak oxygen uptake (VO₂ peak) and a significant increase in oxygen uptake benefit at submaximal training intensities in trained athletes. In contrast to the recovery state and sleep exposure to hypoxia environments, the oxygen benefits of submaximal training intensity were significantly increased [39]. Truijens et al. [41] set the time of daytime rest and relaxation for trained swimmers and long-distance runners, who were exposed to hypoxia environment for 3 h per day, 5 days per week, for 4 weeks, showed a 0.02% improvement in VO₂max. Hypoxia exposure during the recovery and sleep states reduces the interference of training intensity, which means that the training intensity and technical training can be maintained during the training period.