The goal of this study is to evaluate the response of physiological variables to acute normobaric hypoxia compared to normoxia and its influence on the lactate turn point determination according to the three-phase model of energy supply (Phase I: metabolically balanced at muscular level; Phase II: metabolically balanced at systemic level; Phase III: not metabolically balanced) during maximal incremental exercise. Ten physically active (VO₂max 3.9 [0.49] l·min^-1), healthy men (mean age [SD]: 25.3 [4.6] yrs.), participated in the study. All participants performed two maximal cycle ergometric exercise tests under normoxic as well as hypoxic conditions (FiO₂ = 14%). Blood lactate concentration, heart rate, gas exchange data, and power output at maximum and the first and the second lactate turn point (LTP₁, LTP₂), the heart rate turn point (HRTP) and the first and the second ventilatory turn point (V_ETP₁, V_ETP₂) were determined. Since in normobaric hypoxia absolute power output (P) was reduced at all reference points (max: 314 / 274 W; LTP₂: 218 / 184 W; LTP₁: 110 / 96 W), as well as VO_2max (max: 3.90 / 3.23 l·min^-1; LTP₂: 2.90 / 2.43 l·min^-1; LTP₁: 1.66 / 1.52 l·min^-1), percentages of P_max at LTP₁, LTP₂, HRTP and V_ETP₁, V_ETP₂ were almost identical for hypoxic as well as normoxic conditions. Heart rate was significantly reduced at P_max in hypoxia (max: 190 / 185 bpm), but no significant differences were found at submaximal control points. Blood lactate concentration was not different at maximum, and all reference points in both conditions. Respiratory exchange ratio (RER) (max: 1.28 / 1.08; LTP₂: 1.13 / 0.98) and ventilatory equivalents for O₂ (max: 43.4 / 34.0; LTP₂: 32.1 / 25.4) and CO₂ (max: 34.1 / 31.6; LTP₂: 29.1 / 26.1) were significantly higher at some reference points in hypoxia. Significant correlations were found between LTP₁ and V_ETP₁ (r = 0.778; p < 0.01), LTP₂ and HRTP (r = 0.828; p < 0.01) and V_ETP₂ (r = 0.948; p < 0.01) for power output for both conditions. We conclude that the lactate turn point determination according to the three-phase-model of energy supply is valid in normobaric, normoxic as well as hypoxic conditions. The turn points for La, HR, and V_E were reproducible among both conditions, but shifted left to lower workloads. The lactate turn point determination may therefore be used for the prescription of exercise performance in both environments.

• The lactate turn point concept can be used for performance testing in normoxic and hypoxic conditions

• The better the performance of the athletes the higher is the effect of hypoxia

• The HRTP and LTP are strongly correlated that allows a simple performance testing using heart rate measures only.