**3. Further Key CPET Parameters**

#### *3.1. Oxygen Pulse*

The oxygen pulse (O2 pulse) is the ratio of VO2 in mL/min and HR in beats/min, expressed as mL/beat [23]. According to the Fick equation, VO2 = (HR × SV) × C(a-v)O2, where SV is stroke volume and C(a-v)O2 is the arterio-venous oxygen difference. Thus, the O2 pulse provides an estimate of stroke volume and peripheral vascular perfusion/extraction response to exercise [15]. Normal values at rest range from 4–6 mL/beat and increase up to 10–20 mL/beat at maximal exercise [23]. Athletes demonstrate a 10%–15% increase in ventricular cavity size and enhanced cardiac filling in diastole that augment their stroke volume (SV) compared with individuals of similar age and size [12,31]. They reveal increased mitochondrial oxidative capacity and capillarity within the skeletal muscle, which results in higher C(a-v)O2 during exercise [32]. As a result, O2 pulse is higher in trained athletes, but the reference values in this population are yet to be determined.

Central [SV] and peripheral [C(a-v)O2] adaptations to exercise result in higher O2 pulse in trained subjects. As mentioned earlier, in calculating the O2 pulse, VO2 should be in mL/min. So, for making reliable comparisons between athletes, the weight and height of the athletes have to be taken into account. Therefore, the authors of the present paper recommend calculating the O2 pulse in relation to body surface area (BSA). Since the relative contribution of SV to cardiac output is paramount during the early and intermediate phases of exercise [4,15], the amount of O2 pulse/BSA at submaximal levels (e.g., 50% or 75% of the VO2max) demonstrate more central (cardiac, i.e., SV) adaptations and the maximal value shows both central and peripheral (cardiovascular and muscular perfusion/extraction) adaptations to exercise.

Different patterns of adaptations according to O2 pulse response to incremental exercise are displayed in Table 1. This approach could be a precise manner to compare athletes with different conditioning levels. It guides sports physicians and athletic trainers to distinguish elite athletes and to prescribe the appropriate training program to focus on central (cardio-pulmonary) or peripheral (skeletal muscles) structures based on the standard requirements for any sports discipline.


**Table 1.** O2 pulse response of four athletes with different adaptation patterns to incremental exercise in comparison with a reference athlete

Athlete A has a better central and peripheral adaptation to exercise than the reference athlete. Athlete B has better central but lower peripheral adaptation and Athlete C might has better peripheral adaptation than central. Athlete D is worst in both central and peripheral adaptations. Reference athlete: RA, ↑: more than RA, ↑↔: more or equals to RA, ↓↔: less or equals to RA, ↓: less than RA.
