## 4 Discussion  
This systematic review aimed to compile the present evidence regarding resistance training in hypoxia and update knowledge of the methods used and their effectiveness in developing strength and muscle hypertrophy. The results concord with previous systematic review results [2]. This manuscript confirms that, in most cases, resistance training under hypoxic conditions significantly improves strength and muscle hypertrophy. However, some documents do not report substantial differences compared to training in normoxia. This suggests that a greater quantity and quality of scientific evidence should be reported until convincing results are obtained regarding the effectiveness of this training. It should be noted that some studies with more significant improvements in the percentage of change in the hypoxia group have highlighted several methodological similarities, so recommendations can be made to improve the quality of future studies and thus permit the comparability of results, as well as to encourage carrying out a meta-analysis on this topic when the volume of documents allows strong conclusions.  
It is known that muscular strength training leads to structural or neural adaptations and that these improvements influence muscle growth and the effectiveness of the muscle in generating force [33; 34]. In addition, strength training contributes to sports performance [6; 22; 24; 27; 29; 30; 31] and is also beneficial to health [15; 16; 17; 18; 19; 20; 21; 23; 25; 26; 28; 32]. Resistance training leads to morphological muscle adaptations such as hypertrophy and increased strength due to changes in muscle fibre diameter, protein synthesis of myofibrils and increased anaerobic capacity [7; 33; 34]. In addition, it causes changes in metabolic characteristics such as mitochondrial synthesis, increases in lactate tolerance, and improvements in oxidative function and muscle endurance capacity [7].  
Therefore, strength training induces these changes due to the metabolic stress it generates since the energy pathways used in this type of training generate an anabolic situation that, in turn, causes an increase in anabolic signalling proteins, giving way to the creation of metabolites that promote myofibrillar protein synthesis. Consequently, these muscle proteins balance with satellite cells and bind to the muscle fibre. This balance is only achieved when the protein synthesis rate exceeds its breakdown because of the work carried out by satellite cells in muscle hypertrophy [7; 35].  
Hypoxic training on muscle hypertrophy and strength development is an issue that has become relevant in recent years. Part of the theory is associated with greater metabolic stress than normoxic training since there is more dependence on the anaerobic metabolism, which contributes to muscular adaptations [24; 36]. Training in hypoxia has been postulated as a factor that stimulates capillary growth through increased nitric oxide production and greater vasodilation, as well as increased expression of the vascular endothelial growth factor gene (VEGF). This factor is induced by HIF-1; these signals, due to the activation of the HIF factor, affect the expression of a greater number of genes. Most of these genes have functional relevance in muscle tissue adaptations and are related to erythropoiesis, angiogenesis, pH and glycolysis regulation [7; 16].