The principal objective of this study was to test the hypothesis that high intensity hypoxic training improves sea-level performances in well-trained swimmers more than the equivalent training in normoxia. METHODS: Sixteen well-trained collegiate and Masters swimmers (10 women, 6 men) completed a 5 week training program, consisting of three high intensity training sessions in a flume and supplemental low or moderate intensity sessions in a pool each week. Subjects were matched for gender, performance level and training history, and assigned to either hypoxic (HYPO; FIO2=15.3%, simulated altitude of 2500m) or normoxic (NORM; FIO2=20.9%) interval training in a randomized, double-blind, placebo controlled design. All low and moderate intensity training was done in a pool under normoxic conditions. The primary measures of performance were 100m and 400m freestyle time-trials. Laboratory outcomes included maximal O 2uptake (VO 2max), anaerobic capacity (accumulated O 2deficit), swimming economy, and hematological parameters. RESULTS: Significant (p=0.02 for 100m, p<0.001 for 400m) improvements were found in performance on both the 100m (NORM: -0.7 sec, {95% conf limits +0.2 to -1.7 sec}, -1.2%; HYPO: -0.8 sec {-0.1 to -1.5 sec}, -1.1%) and 400m freestyle (NORM: -3.6 sec {-1.8 to -5.5 sec}, -1.2%; HYPO: -5.3 sec {-2.3 to -8.3 sec}, -1.7%). These percent changes were nearly 5 fold greater than the mean of individual percent differences between two 100m time trials (0.26%) in a similar group of well-trained swimmers. There was no significant difference between groups for either distance (ANOVA interaction p=0.91 for 100m and p=0.36 for 400m). VO 2max was improved significantly (NORM: +0.16 l/min (¤0.23), +6.4% (¤8.1); HYPO: +0.11 l/min (¤0.18), +4.2% (¤7.0)). There was no significant difference between groups (P=0.58). CONCLUSION: Five weeks of high intensity training in a flume improves sea-level swimming performances and VO 2max in well-trained swimmers. There was no significant, additive effect of hypoxic training (15.3% oxygen equivalent to a simulated altitude of 2500m) under the conditions of this experiment.
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