# Simulated Altitude Training and Sport Performance: Protocols and Physiological Effects
## 2 Hypoxia Environment Training Protocols
### Hypoxia Submaximal Exercise Training and Combined Training  
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 [PERSON] 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 [PERSON] 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. [PERSON] 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% VO2 max) 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 VO2 max 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 [PERSON] and [PERSON] [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 VO2 max, 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.