Alvarez-Herms, Jesus, Sonia Julia-Sanchez, Hannes Gatterer, Francisco Corbi, Gines Viscor, and Martin Burtscher. Effects of a single power strength training session on heart rate variability when performed at different simulated altitudes. High Alt Med Biol. 21:292-296, 2020. Background: This study assessed heart rate variability (HRV) after a single power strength training session performed at different hypoxic levels. Materials and Methods: Eight physically active subjects (31.1 +/- 4.3 years; 177.6 +/- 3.0 cm; 70.1 +/- 5.2 kg) performed 6 bouts of 15-second continuous maximal jump exercises interspersed by 3 minutes of rest at different altitude levels (total volume of each session: 20 minutes). The normoxic hypoxia levels were FiO(2) low altitude: 20.9%; moderate altitude: 16.5%; and high altitude: 13.5%. Results: Average power output during the jumps was similar for all conditions ( congruent with3150 W). Twenty-four hours before (PRE) and 24 hours after (POST) each training session, HRV parameters (R-R, square root of the mean of the sum of differences between intervals [RMSSD], pNN50, and very low frequency, low frequency, and high frequency) were determined without resulting in significant statistical differences, neither from PRE to POST nor between conditions (p > 0.05). Conclusions: This study showed a negligible perturbation of HRV parameters 24 hours after a single power strength session up to a hypoxic level equivalent to 4000 m. Further studies are needed to determine the hypoxia-dependent threshold and intensities of training loads affecting HRV.