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.

Background: The autonomic nervous system, which is composed of the sympathetic and parasympathetic nervous system, is closely related to the cardiovascular system. The temporal variation between each of the intervals between the consecutive “R” waves of an electrocardiogram is known as heart rate variability. Depending on the type of activity, both systems can be activated, and also influence the interval between “R” waves. Currently, with advancements in technology and electronic devices, photoplethysmography is used. Photoplethysmography detects changes in the intensity of reflected light that allow differentiation between systole and diastole and, therefore, determines the heart rate, its frequency and its variations. In this way, changes in the autonomic nervous system can be detected by devices such as the Max Pulse^®. Objective: To determine whether the information provided by Max Pulse^® on autonomic balance and stress is modified after intense physical exercise, thereby determining whether there is a relationship with body composition, and also whether there are differences with respect to gender. Materials and Methods: Fifty-three runners (38.9% female) with a mean age of 31.3 ± 8.1 years participated in the study. Two measurements (before and after intense physical effort) were performed with the Max Pulse^® device. The flotoplethysmography measurement lasted 3 min, and was performed in the supine position. The exercise test was performed on a treadmill. It was initiated at a speed of 6 and 7 km/h for women and men, respectively. Subjects indicated the end of the test by making a hand gesture when unable to continue the test. Results: Autonomic nervous system activity and mental stress values decreased significantly (p < 0.05) in men and women, while autonomic nervous system balance decreased only in women. Physical stress increased (p < 0.05) in both sexes. Conclusions: Intense exercise causes changes in variables that assess autonomic nervous system balance and stress, as measured by a device based on photoplethysmography. The changes are evident in both sexes, and are not related to body composition.

Introduction: Tolerance to breathing in conditions with a decreased oxygen ratio is subject-specific. A normobaric hypoxia tolerance test (NHTT) is performed to assess the ability of each individual, as this may be influenced by genetic or personal factors such as age or gender. The aim of this study is to test the influence of deep breathing on hypoxia tolerance time. Material and methods: A total of 45 subjects (21 parachutists and 24 students) performed two NHTTs at 5050 m altitude (iAltitude). Arterial (SatO₂) and muscle (SmO₂) oxygen saturation were monitored with the Humon Hex^® device. The first NHTT was performed with free breathing, without any instructions; and the second NHTT was performed with wide, slow, diaphragmatic breathing. The NHTT was terminated at the end of 10 min or when a value of less than 83% was obtained. Results: The first NHTT was completed by 38.1% of parachutist and 33.3% of students while the second NHTT was completed by 85.7% and 75%, respectively. In the second NHTT, both parachutists and students had a significantly (p = 0.001) longer duration compared to the first NHTT. SmO₂ and SatO₂ values also increased significantly (p < 0.001) in both groups (p < 0.05). Conclusion: Performing controlled diaphragmatic breathing is successful in increasing hypoxia tolerance time and/or SatO₂ values.

The aim of this work was to analyze the influence of acute normobaric hypoxia on quadricep oxygenation. Muscle oxygen saturation (SmO₂) was measured using near-infrared spectrometry (NIRS) technology during a normobaric hypoxia tolerance test (NHTT). SmO₂ was measured with a Humon Hex^® device. In total, 54 healthy subjects participated, 68.5 of which were males and 31.5% of which were females. They performed an NHTT with the IAltitude^® simulator, breathing air with an FiO₂ level of 11% (equivalent to 5050 m). The maximum duration of the NHTT was set at 10 min, stopping if it reached 83% SpO₂. The initial values (PRE) were compared with those obtained at the end of the test (POST) and after 10 min of recovery. The participants were divided into two groups based on whether (G1) they completed the ten minutes or not (G2). In total, 35.1% of men and 41.2% of women completed the 10 min. In both groups, significant differences were observed in the decrease in SmO₂ values (p < 0.0001) (G1: PRE = 59.5 ± 12.48%; POST = 55.95 ± 14.30%; G2: PRE = 60.06 ± 13.46%; POST = 57.2 ± 12.3%). There were no differences between groups in any of the three periods. Exposure to normobaric hypoxia produces a decrease in quadricep levels of SmO₂ in both sexes, regardless of whether the test is completed. Two patterns appeared: A.—less time and more hypoxia; B. a longer duration and less hypoxia.

Heart rate variability (HRV), the variation of time intervals between heartbeats, reflects the result of the interaction between the autonomic nervous system and the cardiovascular system. Hypoxia is a stressor that causes changes in HRV and a decrease in arterial oxygen saturation (SaO2). The aim of our study is to analyse if the time and frequency domains of HRV are affected after a hypoxia tolerance test in healthy participants as a function of gender. Material and methods: 23 healthy volunteers (11 women and 12 men) with a mean age of 23.08±2.99 performed a hypoxia tolerance test (11%, 5050 m) with the iAltitude simulator. Heart rate and SaO2 were monitored during the test and HRV frequency domain (FD) and time domain (TD) data were obtained. We analysed the five minutes before and after each session. Outcomes: Simulated altitude-induced hypoxia can cause changes in HRV in a group of healthy individuals. Statistically significant differences were also found between before and after the normobaric hypoxia test in the time-domain variables RRm, SDNN, HRm, HR STD, pNN50. In addition, in women statistically significant differences were found in RMSSD, and in the frequency-domain variables HF and LF/HF. While in males, statistically significant differences were found in SDNN. In both men and women, significant differences were observed in RRm, HRm and pNN50. Conclusion: The differences found in HRV after the hypoxia tolerance test indicate that females show a greater activation of the parasympathetic nervous system compared to males. While males showed greater sympathetic activation. Keywords: Heart rate variability, normobaric hypoxia, stress, autonomic nervous system.