Time-course of muscle fatigue development during intense exercise in hypoxia and normoxia

This study is the first to determine how hypoxia affects human muscle fatigue kinetics and metabolic perturbations during intense dynamic exercise. Using randomized, single-blinded crossover designs, three trials of two-legged knee extensions were performed under hypoxic (HYP, FiO₂ 0.135) and normoxic (NOR) conditions. Trial 1 (n = 8): quadriceps femoris twitch force (F_tw) was measured before, during, and after 4 min intense exercise followed by exhaustive exercise. Maximal voluntary contraction (MVC) was measured pre- and post-exercise. Trial 2 (n = 8): muscle lactate and pH were determined before and after 4 min intense exercise. Trial 3 (n = 6): blood was sampled frequently from the femoral artery and vein during intense exhaustive exercise. Dynamic F_tw decreased more (P < 0.05) in HYP from 60s of exercise and onwards. After 4 min, isometric F_tw decreased more (P < 0.05) in HYP, whereas MVC was similar between conditions. At exhaustion, isometric F_tw and MVC were similar between conditions despite HYP exercise time being 55 ± 17% of NOR (P < 0.01). Muscle lactate and pH in- and decreased more (P < 0.001), respectively, after 4 min in HYP. Exercise-induced blood metabolites disturbances were largely unaffected by hypoxia. Conclusively, moderate hypoxia accelerated muscular fatigue from 60s and onwards. Hypoxia caused higher muscle but not blood lactate and H⁺ accumulation rates.