*3.4. Respiratory Compensation Point (RCP)*

With increasing exercise intensity above the VT, the lactate production rate gets higher, and a point is reached when bicarbonate is no longer able to counteract exerciseinduced metabolic acidosis. In the isocapnic buffering region, bicarbonate is decreasing with no evident hyperventilation. Then, there is an exponential increase in blood lactate concentration and an excess CO2, whereas the increase in VO2 remains linear. The second breakpoint in the ventilation response to exercise is where the peripheral chemoreceptors invoke hyperventilation, which is identified as the second VT or RCP [10,13,43,44].

According to the physiological changes at RCP, there is an inflection of VE versus VCO2 and also VE/VCO2 versus workload, so as depicted in Figure 2, the second VT is identifiable by the nadir of the VE/VCO2 to workload curve [43,44]. Both VE/VO2 and end-tidal O2 pressure (PETO2) increase while there is a deflection point on the PETCO2 trajectory [45]. It is usually achieved at around 70–80% of VO2max or 80–90% of peak HR during incremental exercise [13,43]. In a recent systematic review and meta-analysis, it has been demonstrated that there is a highly significant correlation between RCP and critical power, with the power output at CP being 6% lower than RCP [10,43,45].

The second VT or RCP can be expressed as VO2 (mL/kg/min) or percentage of VO2max. Workload consistent with RCP (Watts) might also provide a scale to compare the ability of different athletes to comply with higher intensities of exercise. Anaerobic capacity is an essential parameter in the performance of athletes, especially those who participate in sports with sudden bursts of high-intensity activity [4]. Therefore, a longer duration of exercise at a constant workload within the CP or at the RCP level (VO2 slow component) could be an advantage for such sportspersons.

Table 2 outlines the effects of training on each CPET variable in well-trained athletes compared to their ordinary counterparts.


**Table 2.** Key CPET parameters in elite athletes.

RCP: respiratory compensation point, VT: ventilatory threshold, OUES: oxygen uptake efficiency slope, ↑↑: quite more, ↑: more, ↔: no difference, ↔↓: equals or less, ↓: less.

#### **4. Role of CPET in Diagnostic Workup**

Many athletes experience some occasional vague symptoms such as exertional dyspnea, chest discomfort, and fatigue during their sports career in which the etiology could be cardiovascular, pulmonary, or muscular. Evaluating the physiological response of body organ systems to exercise provides valuable information on potential underlying ailments. In clinical practice, CPET is used to detect latent diseases and can help to differentiate cardiac and pulmonary problems. In elite endurance athletes, expiratory flow limitation (EFL) is assumed to be very frequent, with a prevalence of up to 40% in males and 90% in females [46]. The assessment of the flow-volume loop during CPET would reveal the presence and magnitude of EFL, further clarify a pulmonary mechanism for the symptoms and provide resolution of disease severity [16].

Assessment of ventilatory reserve and efficiency in addition to the standard haemodynamic and ECG monitoring provide insight into probable physiological abnormalities. The ventilatory reserve is the ratio between peak VE on a CPET and the maximum amount of air that can be breathed within one minute by a voluntary effort at rest termed the maximal voluntary ventilation (MVV), which is often measured in 15 s and multiplied by 4 [11,47]. Abnormal ventilatory reserve, which is VE/MVV ≥ 0.8 along with abnormalities in FEV1 and peak expiratory flow (PEF) are indicative of pulmonary limitations [11], but athletes with superior cardiovascular function can demonstrate some degrees of EFL and low ventilatory reserve with normal lung function tests [47]. Since EFL can be a cause of hypoxemia on exertion, pulse oximetry (SPO2) should also be measured throughout the CPET process. Ventilatory efficiency parameters [VE/VCO2 slope and PETCO2], reveal cardiopulmonary coupling and function, and when abnormal, may indicate subclinical ventilation-perfusion abnormalities as a possible mechanism for exertional symptoms [11,16]. Electrocardiographic and/or hemodynamic abnormalities like a hypertensive response to exercise or a slow recovery period might reveal a cardiovascular source for the symptoms.

Excessive training load without adequate recovery period exposes elite athletes to an inability to adjust optimally to the overall load. This process can results in overreaching, or in more severe cases overtraining syndrome (OTS), with different indeterminate signs and symptoms accompanying performance decrements and the development of acute illness [48]. Studies have shown that up to 64% of elite athletes experienced OTS at least once [49]. Parasympathetic alterations with bradycardia in endurance sports, and sympathetic alterations with tachycardia and hypertension in explosive and high intensity sports, have been suggested as various cardiovascular responses [50]. It is been advocated that heart rate and blood lactate concentration variations are the two most discriminating factors between overreached and normal athletes [51]. In an experimental study, Le Meur Y et.al. showed decreased cardiac output at submaximal and maximal exercise intensities with lower VO2max and reduced HR and SV values in triathletes after an overload training period [52]. It, therefore, seems that the CPET of elite athletes should be interpreted more carefully.

#### **5. Exercise Prescription**

CPET provides a context for determining a highly individualized training intensity zones for prescribing a structured exercise program. The physiological response to exercise characterizes the first and second VTs and VO2max, which allow for the identification of four intensity zones as it is illustrated in Figure 2 [43]. Heart rate and workload corresponding to each appropriate zone should be used for exercise prescription. Zone 1 consists of all workloads below the first VT, which represents light to moderate-intensity exercise. Zone 2 comprises those workloads between the first and second VT (RCP) equals to moderate to high-intensity exercise. The workloads above the CP that result in VO2max at exhaustion are in Zone 3 constitute high to sever intensity exercise domain. Sprints and all-out efforts above the workloads that allow for the attainment of VO2max are in Zone 4.

Constant workload exercise in Zone 1 brings about a steady-state VO2 that is sustainable for a long duration (>30 min) with only a modest sense of fatigue. It is suitable for the recovery phase of HIIT in athletes. Training in Zone 2 results in VO2 and lactate steady-state conditions and is important in inducing significant improvements in these parameters [24]. The highest workload with steady-state lactate is called critical power (CP), a marker of the upper-limit of sustainable prolonged aerobic exercise [43]. Endurance athletes like marathoners and triathletes benefit from improvements in these parameters.

Exercise training in Zones 3 and 4 might cause VO2 to reach maximum value without steady-state achievement and is better defined by CP concept (Time limit at VO2max). The duration of exercise is variable in these domains and based on the conditioning level of athletes, it would be in the range of 3 to 20 min in Zone 3 and less than that in Zone 4. Given a short exercise duration, these domains can only be used for HIIT programs [53]. There are various HIIT training protocols in the literature [53,54] that precisely characterize the physiological response of the exercise program.

#### **6. Summary and Practical Implications**

CPET has extensive practical applications in athletes, and comprehensive knowledge of exercise physiology in connection with various sports disciplines are essential for the interpretation of the results. Measurement of baseline fitness and the assessment of cardiopulmonary function in athletes suspected of having the cardiovascular or pulmonary disease are common indications for CPET. Revealing more talented athletes along with quantifying the physiologic parameters determinant of sports performance could be a reliable guide for team doctors and coaches.

VO2max and VT are the well-known fitness parameters that depend on several factors, including age, sex, genetic predisposition, and exercise training. Athletes need high values for best performance but with varying importance in different sports.


CPET is a well-accepted method to evaluate the function of body organs integrated into exercise. It could be a standard procedure to measure athletic performance in different sport disciplines but is disregarded in this field. Reference values have to be determined in athletes, so the interpretation of the results and performance differences would be accurately quantified. Hereby we call for further research on a large number of athletes in different disciplines to have comprehensive data for each CPET parameter in athletes.

**Author Contributions:** Conceptualization, all authors; methodology, R.M., C.S. and D.N.; validation, R.M. and M.G.; investigation, R.M., C.S. and D.N.; resources, R.M. and C.S.; data curation, R.M.; writing—original draft preparation, R.M. and D.N.; writing—review and editing, C.S. and M.G.; visualization, all authors; supervision, M.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All the data are available in the main text.

**Conflicts of Interest:** The authors declare no conflict of interest.
