Effects of training in normoxia and normobaric hypoxia on time to exhaustion at the maximum rate of oxygen uptake

The effects of endurance training in normoxia or in hypoxia on time to exhaustion (T _lim) at the work rate corresponding to peak oxygen uptake (V̇O_2peak) were examined at sea level in 13 healthy subjects. Before and after training the subjects performed the following: (1) incremental exercises up to exhaustion to determine peak oxygen uptake in normoxia (V̇O_2peakN), the percentage of this value at the 4 mmol l⁻¹ blood lactate concentration (V̇O₂4%N) and the work rate corresponding to V̇O_2peakN (Pa_peakN), (2) a 5-min 90% Pa_peakN exercise followed by a 10-min passive recovery to determine the maximal blood lactate concentration (La_max) measured during the recovery, and (3) a T _lim at Pa_peakN. Training consisted of pedalling 2 h a day, 6 days a week, for 4 weeks. Five subjects trained in normobaric hypoxia [HT; partial pressure of inhaled oxygen (P _IO₂) 89 mmHg] and eight subjects trained at the same relative work rates in normoxia (NT; P _IO₂ 141 mmHg). The training-induced improvement of all the measured parameters were closely matched between the HT and the NT (P>0.05). Training increased T _lim by 59.7% [164(40) s]. The value of T _lim was related to V̇O₂4%N and to La_max before and after training. Also, the training-induced improvement of T _lim was related to the concomitant decrease in La_max. It is concluded that: (1) endurance training including continuous high-intensity exercises improves T _lim for exercises performed at the same relative (higher absolute) work rate after training, (2) intermittent hypoxic training has no potentiating effect on T _lim as compared with training in normoxia, and (3) the intra-individual training-induced improvement of T _lim was associated with metabolic alteration in relation to lactate accumulation.