IJCS | Volume 31, Nº4, July / August 2018

325 de Souza e Silva et al. Cardiorespiratory optimal point in soccer players Int J Cardiovasc Sci. 2018;31(4)323-332 Original Article (VE) and oxygen and carbon dioxide partial fractions were expressed every 10 seconds, and their mean values for each minute of CPX were then calculated. Determining maximal oxygen consumption and ventilatory threshold The VO 2 max was considered the highest value at a givenminute of CPX. The VTwas visually determined as the point at which an interruption in VE's curve linearity and a sustained increment in VE/VO 2 ratio occurred, being described as the percentage of VO 2 max at that velocity. In addition, the velocity and the VO 2 at which the VT occurred were recorded. Determining the cardiorespiratory optimal point The COP was obtained by identifying the lowest VE/ VO 2 ratio at a given minute of CPX, being, thus, a non- dimensional value. In addition, the VO 2 and the running velocity in the ramp protocol at that point were recorded. Statistical analysis Data distribution was assessed by use of the Shapiro- Wilk normality test. Continuous variables with parametric distribution were expressed as mean ± standard deviation (SD), being compared by use of the unpaired Student t test or ANOVA and post-hoc Bonferroni test, when appropriate. Continuous variables with non-parametric distribution were expressed as median (interquartile range) and compared by use of Mann-Whitney test or Kruskal-Wallis test, when appropriate. Categorical variables were expressed as percentage of the frequency and compared by use of the chi-square test. The coefficients of variation of the variables COP, VT and VO 2 max, obtained by the ratio between standard deviation and mean, were calculated. Pearson correlation was used to test the association between COP and other ventilatory variables. The statistical calculations were performed using the Stata14 ® software, adopting a significance level of 5%. Ethical considerations All soccer players underwent the assessment willingly, having read and signed the specific written informed consent before the CPX, and having authorized the use of their data for scientific research. The retrospective analysis of data was previously approved by the Ethics Committee on Research of the institution. Results Table1describes themajordemographic characteristics, and the resting spirometry and CPX results of the soccer players. Age, weight, height and body mass index (BMI) ranged from 16 to 36 years, from 57.5 to 102.0 kg, from 163.3 to 196.3 cm, and from 19.3 to 29.6 kg.m -2 , respectively. COP, VT and VO 2 max showed a parametric distribution (p > 0.05), with values ranging from 13.1 to 25.3, from 61.8 to 92.7% of VO 2 max, and from 45.0 to 76.2 mL.kg -1 .min -1 , respectively. The coefficients of variation for COP, VT and VO 2 max were 16.1%, 10.7% and 10.0%, respectively. On average, COP, VT and VO 2 max occurred at the velocities of 10.0 ± 1.0, 14.3 ± 1.1, and 18.7 ± 0.9 km.h -1 , respectively (p < 0.01). When stratified by their field positions during the match (Table 1), the only characteristics that differedwere weight and height, with goalkeepers showing the highest values for both variables (p < 0.01). The BMI, however, was similar among the soccer players of different field positions (p = 0.86). Regarding CPX, goalkeepers achieved the lowest VO 2 max values relative to their body weight (mL.kg -1 .min -1 ) (p = 0.01) and reached the COP at a higher HR and percentage of VO 2 max than the players of other field positions (p < 0.01). However, the values of COP (p = 0.41) and VT (% of VO 2 max) (p = 0.42) did not differ according to the soccer players’ field positions. The coefficients of correlation between COP and VO 2 max (mL.kg -1 .min -1 ) and between COP and VT (% of VO 2 max) were 0.032 (p = 0.65) and -0.003 (p = 0.96), respectively, evidencing the low association between those variables. Figure 1 shows those data. Discussion During an exercise training with progressive intensity increase up to the voluntary maximum, the relationship between VE and VO 2 is nonlinear, 20 and the curve that illustrates that relation has a U shape, suggesting higher ventilatory efficiency (lower VE/VO 2 ) at submaximal exercise levels when compared to rest and to the highest exertion intensities. Based on that, COP was described as the lowest VE/VO 2 value at a given minute during an incremental exercise, representing the time point with the lowest amount of ventilation per liter of oxygen to be consumed, which is the best integration of the relationship between circulation and respiration. 15 Recent studies have shown the clinical applicability of the COP for the diagnostic and prognostic assessment