*3.2. Ventilatory Equivalents (VE/VO2 and VE/VCO2)*

Athletic performance requires proper integration of cardiovascular, pulmonary, and skeletal muscle physiology. Physiological deficiency of any of these systems diminishes VO2 and increases ventilatory equivalents [15]. Enhanced mitochondrial function is a result of chronic exercise training [24], and this improvement is valuable for the effective production of adenosine triphosphate (ATP) through oxidative phosphorylation. Inadequate adaptation to exercise reduces the oxidative capacity of skeletal muscles and makes them rely on anaerobic glycolysis for ATP production, which leads to lactic acid accumulation early in exercise. Very deconditioned individuals and patients with mitochondrial myopathy might show the same manifestations but exaggeratedly [33].

Early lactic acidosis is reflected in CPET as a low VT, which is demonstrated by a rapid increase in VE/VO2 and respiratory exchange ratio (RER). Arterio-venous oxygen difference might also be lower in athletes with unfavourable mitochondrial adaptations resulting in a reduced peak O2 pulse. Some studies support the improvement of these parameters in fit individuals by demonstrating a decreased submaximal VE/VO2 and increased peak O2 pulse after exercise training [34,35]. Figure 3 depicts the ventilatory equivalents of two athletes with different mitochondrial adaptations.

**Figure 3.** The ventilatory equivalent for O2 in a well-trained athlete compared with a poorly adapted athlete. As it's evident, the ventilatory threshold is at higher workload with lower VE/VO2 values in well-trained athlete than their less fit peer.

The slope of the VE/VCO2 relationship and PETCO2 during an incremental exercise test represent the matching of ventilation and perfusion within the pulmonary system, and they are determinants of ventilatory efficiency in subjects. Studies have revealed a lack of relationship between ventilatory efficiency evaluated by VE/VCO2 slope and sports performance in athletes [36,37].

The relationship between VO2 and the log scale of VE represents the oxygen uptake efficiency slope (OUES) and expresses the ventilatory requirement for O2 [16,23]. Many investigators found it useful in the evaluation of fitness level, and reference values have been proposed [38], but a broadly accepted threshold to define normal response has not been clearly established. Training induced changes in OUES are variable and not sensitive enough to show the improvement of fitness after training [39].
