The Effect of Normobaric Hypoxia in Middle- and/or Long-Distance Runners: Systematic Review

SIMPLE SUMMARY: Exposure to normobaric hypoxia, that is, breathing air with a decreased proportion of oxygen without changing the atmospheric pressure, is a resource used in endurance runners with the aim of increasing their athletic performance. This systematic review describes the effects of different hypoxia programmes, exposure time and duration of the intervention on some haematological and sporting performance parameters. It has been shown that prolonged exposures to hypoxia are necessary to increase haemoglobin values. However, shorter sessions (less than three hours) are effective in increasing time to exhaustion. This review may be useful for planning the training of runners subjected to normobaric hypoxia sessions. ABSTRACT: Background: The use of normobaric hypoxia can bring benefits to sports performance because it improves haematological parameters and/or physical activity tests. Our objective was to conduct a systematic review so as to analyse the methods used in hypoxia and to detect its effects on middle- and/or long-distance runners. Methods: Research was conducted using five electronic databases (PubMed, SportDiscus, Cochrane Library, Scopus and PEDro) until December 2021. The methodological quality of the included studies was assessed using the PEDro scale. Results: Having analysed 158 studies, 12 were chosen for the qualitative and quantitative synthesis. A significant improvement on time until exhaustion was detected, and oxygen saturation decreased after the intervention. There were no significant changes in the 3000-metre time trial or in the haematocrit percentage. The changes in percentage of reticulocytes, heart rate, maximal heart rate, lactate concentration and erythropoietin were heterogeneous between the different research studies. Conclusion: short exposure (less than 3 h to normobaric hypoxia significantly increases the time to exhaustion). However, longer exposure times are necessary to increase haemoglobin. Altitude and exposure time are highly heterogeneous in the included studies.