Exercise intolerance at high altitude (5050 m): Critical power and W′

The relationship between work rate (WR) and its tolerable duration (tLIM) has not been investigated at high altitude (HA). At HA (5050 m) and at sea level (SL), six subjects therefore performed symptom-limited cycle-ergometry: an incremental test (IET) and three constant-WR tests (% of IET WRmax, HA and SL respectively: WR1 70 ± 8%, 74 ± 7%; WR2 86 ± 14%, 88 ± 10%; WR3 105 ± 13%, 104 ± 9%). The power asymptote (CP) and curvature constant (W′) of the hyperbolic WR–tLIM relationship were reduced at HA compared to SL (CP: 81 ± 21 vs. 123 ± 38 W; W′: 7.2 ± 2.9 vs. 13.1 ± 4.3 kJ). HA breathing reserve (estimated maximum voluntary ventilation minus end-exercise ventilation) was also compromised (WR1: 25 ± 25 vs. 50 ± 18 l min−1; WR2: 4 ± 23 vs. 38 ± 23 l min−1; WR3: −3 ± 18 vs. 32 ± 24 l min−1) with near-maximal dyspnea levels (Borg) (WR1: 7.2 ± 1.2 vs. 4.8 ± 1.3; WR2: 8.8 ± 0.8 vs. 5.3 ± 1.2; WR3: 9.3 ± 1.0 vs. 5.3 ± 1.5). The CP reduction is consistent with a reduced O2 availability; that of W′ with reduced muscle–venous O2 storage, exacerbated by ventilatory limitation and dyspnea.