Purpose: Recently, a novel model derived from the critical power (CP) concept was developed to determine W` balance (W`BAL) during variable intensity exercise. The purpose of this study was to investigate the effect of hypoxia on the efficacy of the W`BAL model during high-intensity intermittent exercise. Methods: Eleven trained, male cyclists (mean±SD; age 27±6.6 years, VO2peak 4.79±0.56) completed a maximal incremental ramp test and a 3 min "all out" test to determine end power (EP) and work performed above EP (W`EP). On another day an intermittent test to task failure was performed. All procedures were performed in normoxia (NORM) and hypoxia (HYPO; FiO2 ˜ 0.155). The experimental condition was single-blinded, randomized and counter-balanced, and the W`BAL model was used to calculate the minimum W` (W`BALmin) achieved during the intermittent test. W`BALmin in HYPO was also calculated using model parameters derived in NORM (N+H). Results: In HYPO there was a significant decline in VO2peak (4.79±0.56 vs 3.93±0.47 L.min-1; P<0.001) and EP (353±46 vs 319±49W; P<0.001), whereas no change occurred for W`EP (12.6±4.1 vs 13.3±5.3kJ; P=0.34; NORM vs HYPO). The change in VO2peak was significantly correlated with the change in EP (r = 0.72; P<0.05). There was no difference between NORM and HYPO for W`BALmin (1.7±0.9kJ vs 1.3±0.8kJ). The N+H analysis revealed a gross overestimation of W`BALmin (8.3±3.2kJ) and compared with HYPO (P<0.001). Figure 1 shows an example of modeled W`BAL for a typical subject. Conclusion: The W`BAL model behaves similarly in hypoxic conditions equivalent to ˜2450m as previously reported for normoxia, but only when the model parameters (CP and W`) are determined under the same environmental conditions as the performance task is completed. The practical application of the W`BAL model for altitude training and performance monitoring, thus requires CP and W` to be measured at altitude.
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