*2.1. VO2max*

VO2max is the maximum oxygen uptake of the human body that defines the maximal amount of energy accessible by aerobic metabolism at peak exercise [13]. It is a standard for quantifying CRF [14] and may reflect the limits of the cardiopulmonary system to maximal exercise. The term VO2max implies an individual's physiological limit that is achieved and sustained for a specified period during maximal effort [15]. Athletes of different sport disciplines present with a wide range of VO2max, so for better inter-individual comparisons, it is better to express it as percent-predicted value or in millilitres of oxygen per kilogram of body weight per minute (mL/kg/min) [15,16]. Since the ideal body weight could be entirely different between disciplines, it makes sense to use fat-free mass instead of body weight for interdisciplinary comparison of athletes.

Expected values differ between male and female athletes at any given age and on different exercise test modalities. Accurate interpretation of VO2max should be made with the knowledge of what is expected for an individual athlete. To facilitate sports counselling, it is critical to have validated reference values in target population [14]. Athletes have higher amounts, usually more than 120% of the predicted VO2max of healthy untrained individuals, therefore, the interpretation of the results have to be done cautiously, as it might mask some latent disorders or potential physiological impairments. The time course of VO2 recovery after exercise is an essential parameter that must be considered in the athletic population [17]. In highly trained athletes, recovery of VO2 is more rapid, and just as depicted in Figure 1, the athlete with a higher exercise economy has a faster VO2 recovery rate than his counterpart with less efficient cardiovascular function.

**Figure 1.** Athletes with different exercise economy as shown by VO2/workload responses to incremental exercise.

There are considerable differences in the VO2max of individuals, and numerous genetic variants have been found to be associated with these variations. Studies have reported that genetic components and inheritance account for 44 to 72% of the baseline VO2max (mL/kg/min) in sedentary subjects [18]. Exercise training improves cardiorespiratory fitness by about 10–25% in previously sedentary individuals [15,19]. This improvement varies greatly between individuals even with a standard exercise training program. Genetic factors determine almost 50% of this VO2 response to training [19–21]. The presented evidence implies the importance of baseline VO2max measurements to find talented young athletes and to consult with athletes about their maximum achievable performance in different sport disciplines.

### *2.2. Ventilatory Threshold (VT)*

During incremental exercise, there is a point at which muscles and blood lactate increase due to the rate of lactate production being higher than disposal [22]. The metabolic rate at which excess carbon dioxide (CO2) develops proportionally to the muscle and blood bicarbonate decreasing rate as a consequence of buffering metabolic acidosis is the ventilatory threshold [22]. This excess CO2 makes VE increase more steeply relative to the increase in VO2 [13,15]. Therefore, VT is a point, at which VCO2/VO2 slope becomes steeper; the ventilatory equivalent for oxygen (VE/VO2) begins to increase while the ventilatory equivalent for carbon dioxide (VE/VCO2) remains stable [23] (Figure 2).

**Figure 2.** The ventilatory equivalents for oxygen (VE/VO2) and carbon dioxide (VE/VCO2) and their association with first and second VT which form four training zones during an incremental CPET. VT: ventilatory threshold, RCP: respiratory compensation point.

VT is expressed as VO2 (mL/kg/min) or percentage of VO2max and compared with VO2max, is better correlated to athletic endurance performance [24,25]. It usually occurs at 45% to 65% of VO2max in healthy untrained subjects [15,23] and at a higher percentage (close to 90% of VO2max) in highly endurance-trained athletes [25]. It has been shown that after training, there is an increase in VO2 at VT by about 10–25% in sedentary individuals [15]. The weighted mean heritability of submaximal stamina and endurance test performance is 49% and 53% respectively [18]. This evidence shows the importance of genetic predisposition for VT and submaximal endurance performance and its potential application in identifying talents in sports.

It has recently been proposed that the lactate threshold (LT) could be used to set the training load in resistance exercises [9]. Resistance training promotes muscle hypertrophy, strength and power, and the intensity of exercise is the most important component in this way. Studies have identified the LT in strength training exercises at intensities ranging from 27% to 36% of a maximum repetition (1RM) [9]. Exercise at this level of intensity could be optimal training in sports requiring strength and power. To verify the association between LT and VT, investigators identified the VT with positive correlation, agreement and at the same intensity of exercise with LT during an incremental resistance exercise test [26,27]. Although there exists a slight difference between VT and arterial blood lactate accumulation with VT occurs earlier in dynamic exercise [22,28], yet VT can measure both the endurance and resistance performance in athletes.

The modality of exercise test (treadmill or cycle) and the population under investigation potentially influence the VT response to exercise [15]. In trained subjects, VT is significantly higher on the treadmill than cycle ergometer but not in untrained individuals [29]. This difference should be taken into consideration, especially when we want to make comparisons among athletes in different sports disciplines. In a study on 29 male competitive triathletes, Hue O et.al. showed that the VT values on treadmill running were lower than the values reported for elite distance runners [30].
