## 4 Discussion
However, the previous study was conducted on Germans, and it is difficult to directly compare them with our studies with a different VO2 max [36]. Meanwhile, another previous study of 30 Asians showed a correlation of 0.80 as a result of comparing with the actual measured VO2 max using the previously developed VO2 max prediction equation [37]. In this previous study, Bland-Altman analysis was performed, and the average difference between the measured VO2 max value and the predicted VO2 max value was 2.26 mL/kg/min, and the upper and lower lines were 7.73 mL/kg/min and \(-\)3.21 mL/kg/min. In our study, as a result of Bland-Altman analysis, the average difference between the measured VO2 max value and the predicted VO2 max value was 0.020 mL/kg/min, indicating a lower average. However, the upper and lower lines were 8.756 mL/kg/min and \(-\)8.716 mL/kg/min. This seems to be because more subjects were used than in the previous study. Moreover, the average age of the subjects in the previous studies was 25.7 years (\(\pm\)1.9), lower than that in our study (41.69 \(\pm\) 12.66). In our study, the predictors in the PACER estimation equation were age, gender, height, weight, and the number of PACER counts (r = 0.847, SEE = 3.732 mL/kg/min). Although the predictors were different from those in our study, a previous study conducted a validity study in Asian adults [38]. Previous validation studies on Asians were conducted with gender, age, BMI, and PACER counts as predictors (r = 0.88, SEE = 3.0 mL/kg/min). They showed a similar level of effectiveness as that in our study, but the range of age was limited to between 18 and 23 years. Our study was conducted on a wider range of ages. Moreover, in our study, the ICC between the measured and predicted VO2 max in the PACER test was 0.910, the highest among the three submaximal exercise tests. Among the three submaximal exercise tests, the treadmill test showed the lowest correlation in the multiple regression model (r = 0.762). It also showed the lowest correlation between the measured VO2 max and the predicted VO2 max by the estimation formula (ICC = 0.847; r = 0.762). In the Bland-Altman analysis results, the difference between the measured VO2 max value and the predicted VO2 max value on the treadmill test was \(-\)0.108 mL/kg/min. The upper and lower lines were 8.975 mL/kg/min and \(-\)8.795 mL/kg/min. In contrast, the PACER test showed the highest multiple correlation values (r = 0.847). As a result of Bland-Altman analysis, the PACER test showed the lowest difference between the measured VO2 max and the predicted VO2 max values (\(-\)0.010 mL/kg/min). The upper and lower lines of the PACER test were 7.272 mL/kg/min and \(-\)7.290 mL/kg/min. The YMCA step test values were 8.756 mL/kg/min and \(-\)8.716 mL/kg/min. In our previous study results, the multiple correlation coefficient was 0.784 when measured on the treadmill test, and the multiple correlation coefficient of the YMCA step test was 0.75 to 0.78, which was similar to that in our current study. These results were similar to the results in the current study, which analyzed multiple correlations between the three tests using the same variables and subjects. The main result of this study was that a valid VO2 max predictive model was developed and compared by applying the treadmill test, YMCA step test, and PACER test to the same subjects. The three exercise tests had significant validity as in previous studies. In particular, the PACER test showed the best validity among the three submaximal exercise tests. However, there were some limitations to this study. First, although the results of our study indicated that the equations were suitable for predicting cardiovascular endurance among healthy Korean adults aged 19-64 years, it remains doubtful whether the measured three submaximal exercise tests are useful to subjects of health condition, ages, races, and other risk factors. Three tests may not be appropriable for use in all populations.