However, no significant change in pVO2max following training was found. When comparing the baseline values, there were no significant differences and a moderate, non-significant correlation between the observed and predicted VO2max values. However, there were significant differences and a trivial, non-significant correlation between the observed and predicted VO2max values. Y-27632 mw Furthermore, the SEE increased from 8.9% to 10.5% of observed VO2max from baseline to post-training. In addition, Bland-Altman plots revealed wide limits of agreement at pre and post time

points, indicating wide individual error. The significant trends between the difference of the 2 methods (y-axes) and the mean of the 2 methods (x-axes) of the Bland-Altman Plots at both time points suggested a greater overestimation of VO2max within individuals who had observed values lower than the group mean. Therefore, the HRindex was not suitable for tracking changes in VO2max in female soccer players following 8-weeks of endurance training

and resulted in a wide range of individual prediction error at both time points. The HRindex equation was developed by Wicks et al. (2011) as a simple method for predicting oxygen uptake with the ratio of exercise HR to resting HR. The equation was developed from 220 group mean data sets extracted from 60 published exercise studies and apparently explained 99.1% of the variation in oxygen uptake in the study (Wicks et al., 2011). Unfortunately, cross-validation analyses were not performed (Wicks et al., 2011). Future study was warranted to establish prediction errors for individuals and specific groups. Two previous investigations are available that determined the accuracy of the HRindex Method among groups of non-athletic men (Esco et al., 2011; Haller et al., 2013). Esco et al. (2011) showed large limits of agreement when comparing VO2max

determined in the laboratory and predicted via the HRindex equation in a large sample of college-age men. Haller et al. (2013) demonstrated that the HRindex Method significantly underestimated VO2max and also produced large individual prediction errors across various exercise testing protocols in a group of aerobically fit, young men (Haller et al., 2013). The Cilengitide current investigation was the first to establish the accuracy of the HRindex method in female athletes and to determine its suitability for tracking changes in VO2max following training. According to the Fick equation, oxygen consumption is the product of cardiac output (Q) and an arteriovenous oxygen difference (a-vO2diff). An increase in VO2max following training has been shown to be a result of an increase in both of these components (Powers and Howley, 2012). However, the primary contribution of an increase in VO2max between the central (i.e., Q) and peripheral (i.e., avO2diff) components depends on training duration (Ekblom, 1968).