IJCS | Volume 31, Nº4, July / August 2018

329 de Souza e Silva et al. Cardiorespiratory optimal point in soccer players Int J Cardiovasc Sci. 2018;31(4)323-332 Original Article suggests the existence of a plateau in the VO 2 curve, which is not always possible, and it can vary according to the CPX protocol used and the gas sampling or collection interval. 10,11 In addition, VO 2 max depends on performing a truly maximal exercise test, whose determination criteria vary in the literature, being subjective to a certain extent. On the other hand, although the VT does not require a maximal test, it requires a more intense exercise than COP does, and VT measurement is hindered by the existence of several distinct criteria for its identification and/or characterization, which, in a significant percentage of cases, cannot be obtained, limiting its use in clinical practice and sports. 13 In addition, although both VO 2 max and VT can be detected automatically with commercial software, the methods available for that have been developed from varied definitions and algorithms, implying the need for its review by at least one experienced observer, making those measures subjective and widening the potential of high inter- and intraobserver variability. 23,24 In contrast, COP is easily determined from the identification of the lowest value of the VE/VO 2 ratio for eachminute of CPX, not depending, thus, on the interpretation and experience of the observer, and relying on a relatively small effort, because it occurs at relatively low exercise intensities, before the VT. Regarding the COP of the soccer players assessed, some findings are worth noting: 1- as expected, COP was obtained at lower percentage of VO 2 max and velocity than those at the VT; 2- similarly to VT, but opposite to VO 2 max, COP did not differ according to the different field positions of the soccer players; 3- no significant associationwas observed between COP and the variables VO 2 max and VT; and 4- the coefficient of variation of oxygen consumption at the time of the COP was greater than that observed at the VT and VO 2 max. It is interesting to point out that, on average, the COP values found for the soccer players were below the 50 th percentile of the values found for healthy male non-athletes of the same age group in a previous study, 15 and that only eight (4%) soccer players had COP over 22, considered the cutoff point for optimal clinical prognosis, 17 suggesting that those soccer players have a privileged circulation- respiration interaction, probably more economic at the submaximal exercise. However, it is worth noting that the COP values described for non-athletes were obtained from a CPX performed on a lower limb cycle ergometer, with an individualized ramp protocol. Thus, the description of COP in different exercise modalities and protocols should be approached in future studies, because there is evidence that the behavior of some variables obtained in CPX differ depending on the ergometer and protocol used. The running velocity on the treadmill and the exercise intensity represented by the percentage of VO 2 max at which the soccer players assessed in this study reached the COP (10.0 ± 1.0 km.h -1 and 51.3 ± 8.7%, respectively) were lower than the values obtained at the VT by soccer players assessed in other studies, even when compared to those of players of lower athletic performance, who are expected to reach an earlier VT. For example, according to Ziogas et al., 25 soccer players of the first, second and third Greek division submitted to a CPX in the pre- season period reached the VT at a mean velocity of 13.2, 12.6 and 12.3 km.h -1 , respectively. Boone et al., 26 however, assessing 289 soccer players of the first Belgian division, have reported mean running velocities on the treadmill at the VT ranging from 12.7 ± 1.4 in goalkeepers to 14.4 ± 0.7 km.h -1 in center-defenders. Regarding the exercise intensity, Impellizzeri et al. 27 and Helgerud et al. 28 have reported that junior soccer players reached the VT at a mean percentage of VO 2 max greater than 80%. Considering that the running velocity and the exercise intensity at which the VT is reached reflect the training status of the soccer players, future studies should assess whether COP is also useful to differentiate the physical performance of athletes. When comparing the soccer players according to their field positions, goalkeepers, midfielders, left/ right-backs, center-defenders and forwarders did not differ regarding the COP. Manari et al., 29 comparing the VT and the VO 2 max of 450 European elite soccer players of different field positions, have found no differences regarding the VT, similarly to our study’s findings regarding VT and COP. However, similarly to our study’s findings, VO 2 max was lower in goalkeepers. Tonessem et al., 30 assessing 1,545 male soccer athletes, have found small to moderate differences in VO 2 max according to the athlete’s field position, with greater values in the midfielders, followed, in decreasing order, by the defense athletes, forwarders and goalkeepers. Similarly, Balikian et al., 31 assessing 25 professional soccer players, have found lower mean VO 2 max values of goalkeepers (52.68 mL.kg -1 .min -1 ) as compared to the mean values of soccer players of other field positions. However, in contrast to our study’s findings, the mean velocity at which the players reached the VT differed according to their field position, being lower for goalkeepers (12.66 km.h -1 ) and higher for left/right-