Hemoglobin, maximal heart rate and work capacity, as well as time to exhaustion are significantly improved with combined hypoxia and aerobic exercise training in healthy adults: a meta-analysis

Abstract

Abstract Funding Acknowledgements Type of funding sources: None. Background/Introduction Hypoxic conditions have been used for a variety of reasons and have been implemented in multiple ways in an attempt to improve physiologic performance. Hypoxic conditions are administered via real or simulated altitude or by breathing lower levels of oxygen via facemask to improve different physiologic processes. Maximal work capacity (MWC) and time to exhaustion (TTE) are important factors related to sports performance. It is unclear how aerobic exercise (AE) with hypoxia effects MWC, TTE, and maximal heart rate (MHR) as well as hemoglobin (Hgb) and hematocrit (Hct) concentrations. Purpose Examine the effects of chronic AE with and without hypoxia on MWC, TTE, MHR, Hgb, and Hct. Methods A comprehensive search using PubMed, Cochrane, and PEDro was performed using a variety of keyword permutations on "hypoxia" and "aerobic exercise" in healthy adults. Inclusion criteria were studies with both a control and experimental group in which chronic AE was performed with and without hypoxia and MWC and TTE were provided in Watts and seconds, respectively. Study quality was assessed using the PEDro scale. Statistical analyses were performed using R with sub-analyses examining changes in Hgb, Hct, and MHR. Results A total of 16 studies met the inclusion criteria yielding a total sample size of 389 healthy adults. Study quality ranged from 4 to 6 out of 10 (mode=5). Significant tests for the overall effect of AE with hypoxia compared to AE without hypoxia on MWC and TTE were found under random-effects models (Z=2.83, p<.001, 95% C.I. = 0.29-1.60 and Z=2.94, p<.001, 95% C.I. = 0.80-4.00, respectively). A significant test for the overall effect of AE with hypoxia compared to AE without hypoxia on Hgb was also found (Z=2.06, p=.03, 95% C.I. = 0.01-0.60; fixed-effects model), but a non-significant overall effect on Hct was observed (Z=1.24, p=.21, 95% C.I. = -0.19-0.85; random-effects model). A significant test for the overall effect of AE with hypoxia compared to AE without hypoxia on MHR was also found (Z=2.84, p<.001, 95% C.I. = 0.19-1.04). Substantial heterogeneity was observed in all analyses (I2 range of 64% - 89%). Conclusions Chronic AE with hypoxia elicits a significantly greater increase in MWC and TTE compared to AE without hypoxia in healthy adults. The significant improvement in Hgb and increased MHR provides a rationale for the improvement in MWC and TTE associated with chronic AE combined with hypoxia. Improvements in MWC and TTE have the capacity to improve sport performance. Further investigation of AE with and without hypoxia on specific sports and on other physiologic measures is warranted.