Improved running economy and increased hemoglobin mass in elite runners after extended moderate altitude exposure

There is conflicting evidence whether hypoxia improves running economy (RE), maximal O₂ uptake (over(V, ̇)_{O2 max}), haemoglobin mass (Hb_mass) and performance, and what total accumulated dose is necessary for effective adaptation. The aim of this study was to determine the effect of an extended hypoxic exposure on these physiological and performance measures. Nine elite middle distance runners were randomly assigned to a live high-train low simulated altitude group (ALT) and spent 46 ± 8 nights (mean ± S.D.) at 2860 ± 41 m. A matched control group (CON, n = 9) lived and trained near sea level (∼600 m). ALT decreased submaximal over(V, ̇)_O2 (L min^-1) (-3.2%, 90% confidence intervals, -1.0% to -5.2%, p = 0.02), increased Hb_mass (4.9%, 2.3-7.6%, p = 0.01), decreased submaximal heart rate (-3.1%, -1.8% to -4.4%, p = 0.00) and had a trivial increase in over(V, ̇)_{O2 max} (1.5%, -1.6 to 4.8; p = 0.41) compared with CON. There was a trivial correlation between change in Hb_mass and change in over(V, ̇)_{O2 max} (r = 0.04, p = 0.93). Hypoxic exposure of ∼400 h was sufficient to improve Hb_mass, a response not observed with shorter exposures. Although total O₂ carrying capacity was improved, the mechanism(s) to explain the lack of proportionate increase in over(V, ̇)_{O2 max} were not identified.