# Simulated Altitude Training and Sport Performance: Protocols and Physiological Effects
## 1 Introduction  
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. Therefore, this study explores the physiological mechanisms and effects of hypoxia environment training and further provides training protocols that match the characteristics of sports to enhance training and athletic performance.  
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].  
Recent studies have shown that a simulated hypoxia environment can help improve the cardiorespiratory endurance of endurance-running athletes. It has been shown that the systematic reduction in arterial oxygen saturation (SpO\({}_{2}\)) during hypoxia training can trigger various biochemical and structural changes in skeletal muscle, which are beneficial to cellular and muscular oxidation processes [13;14]. It has also been found to increase power performance [15; 16; 17; 18; 19; 20; 21], hematological adaptations, anaerobic threshold [22], and muscle conditioning for the transcription of selected genes [14;20]. [PERSON] and [PERSON] [23] point out that when training in a hypoxia environment, one should monitor SpO\({}_{2}\), to evaluate whether it impairs or facilitates adaption to various training methods. Our study explored the mechanisms and effects of various training protocols in simulated hypoxia environments on athletic performance.  
In order to further investigate the training protocols required for athletes with different sport-specific characteristics in different hypoxia environment settings and training protocols, this study reviewed various studies on the hypoxia training protocols of (1) Hypoxia high-intensity interval training, (2) Incremental hypoxia training, (3) Hypoxia submaximal exercise training and combined training, and (4) Exposure to a hypoxia environment in the recovery and sleep states. This study aimed to identify the potential training effects to consider in the development of training protocols within the context of hypoxia environment training to enhance athletic performance. Furthermore, we prepared a table of simulated altitude protocols and physiological effects in the Supplementary Table S1.