Individual variation in the reduction of heart rate and performance at lactate thresholds in acute normobaric hypoxia.

Heart rate monitoring and lactate measurements are used to control exercise intensity during training at moderate altitude although there is some uncertainty about hypoxia-induced changes in these parameters at equivalent submaximal exercise intensities compared to normoxia. To study the influence of acute normobaric hypoxia (FiO2 0.15) on heart rate and performance at the individual anaerobic lactate threshold (IAT), at the 4 mmol x l(-1) threshold (AT) and at an intensity requiring 80 % of VO2max measured in the respective environment, 20 endurance-trained male athletes performed an incremental treadmill test in normoxia and normobaric hypoxia. During exercise in normobaric hypoxia, heart rate and velocity were significantly (p < 0.001) reduced with a wide individual variation at the IAT (range: - 1 to - 17 min(-1), - 0.3 to - 3.5 km x h(-1)), at the AT (- 2 to - 13 min(-1), - 0.2 to - 3.3 km x h(-1)) as well as at an intensity requiring 80 % of VO2max (0 to - 18 min(-1), - 1.1 to - 3.7 km x h(-1)). Relative VO2 at the lactate thresholds expressed as a percentage of VO2max was not significantly different compared to normoxia (86 +/- 6 % vs. 84 +/- 5 %, IAT; 90 +/- 5 % vs. 88 +/- 6 %, AT), but also showed a considerable individual variation. In conclusion, heart rate and performance have to be reduced individually to a varying extent during exercise in a hypoxic environment in order to achieve an equivalent intensity compared to exercise in normoxia.