Figure 1.

Panel A: Diagrammatic representation of oxygen uptake (VO₂) in red and carbon dioxide output (VCO₂) in blue as a function of work. Panel B: Diagrammatic representation of minute ventilation (V_E) as a function of VCO₂. VCO₂ was measured at the inflection point in V_E. Panel C: Diagrammatic representation of VCO₂ as a function of VO₂. Metabolic threshold (VO₂θ) was determined by measuring the VO₂ at which the first inflection point in VCO₂ occurred. Ventilatory threshold (V_Eθ) was determined by measuring the VO₂ at the corresponding value for VCO₂ derived from Panel B. Panel D: Diagrammatic representation of heart rate (f_C) as a function of VO₂. f_C at VO₂θ, which served as the boundary between Zone 1 and Zone 2, was determined by measuring the f_C at the corresponding value for VO₂ derived from Panel C. The identical process was employed to obtain the f_C at V_Eθ, the boundary between Zone 2 and Zone 3.