## 4 Discussion  
The purpose of this study was to evaluate the validity of VO2 max estimation equations developed using the treadmill test, YMCA step test, and PACER test in the same subjects to more accurately evaluate cardiovascular endurance in healthy Korean adults aged 19-64. The models were significant predictors of VO2 max and had acceptable validity in a large sample of Korean adults. Especially, among the predictive models, PACER had the highest acceptable validity. In a previous study on estimation equations using the heart rate response during the treadmill test [9], a VO2 max estimation equation was developed using heart rates during treadmill exercises in 400 adult men and women aged from 18 to 40 years (r = 0.91). [PERSON] et al., (2007) developed a formula for estimating VO2 max during treadmill exercises for 100 subjects aged 18 to 65 years [30]. In a previous study, the multiple correlation coefficient was 0.94, showing highly significant results, but a maximal exercise test was conducted, and the predictors included speed (mph) and the grade (%) of the treadmill test. In our previous study, as a result of developing VO2 max estimation equations as variables for gender, age, height, body fat, and heart rate (stage 2) during treadmill exercise, the multiple correlation coefficient was 0.784, and the explanatory power of the model was 61.4% [33]. Previous studies showed a higher correlation than that in our study (r = 0.762). However, the subjects in the previous study were limited to relatively young adults [9], and it is difficult to use in the field by conducting the maximal exercise test during the treadmill test [30]. According to [PERSON] et al. (2009), the exercise intensity showing the highest explanatory power in the VO2 max estimation equation was 64-75% of the maximal heart rate [34]. In our study, in stage 3 treadmill exercises, the exercise intensity was 90% (163.16 ppm) of the maximal heart rate (178.87 ppm), which was higher than in previous studies. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Tests**} & \multirow{2}{*}{**B**} & \multirow{2}{*}{**SE**} & \multirow{2}{*}{**\(\beta\)**} & \multicolumn{3}{c}{**95\% CI**} & \multirow{2}{*}{**R**} & \multirow{2}{*}{**R2**} & \multirow{2}{*}{**SEE**} \\ \cline{3-3} \cline{5-8}  & & & & **Lower** & & **Upper** & & \\ \hline \multicolumn{1}{c}{\begin{tabular}{c} Treadmill test \\ \end{tabular} } \\ \begin{tabular}{c} Intercept \\ Slope \\ \end{tabular} & 0.129 & 1.438 & \(-\)2.696 & 2.954 & 0.762 & 0.580 & 4.528 \\ \begin{tabular}{c} YMCA Step test \\ \end{tabular} \\ \begin{tabular}{c} Intercept \\ Slope \\ \end{tabular} & 0.017 & 1.406 & \(-\)2.746 & 2.779 & 0.770 & 0.592 & 4.461 \\ \begin{tabular}{c} PACER test \\ \end{tabular} \\ \
\begin{tabular}{c} Intercept \\ Slope \\ \end{tabular} & 0.047 & 1.067 & \(-\)2.052 & 2.139 & & & \\
\begin{tabular}{c} Slope \\ \end{tabular} & 0.999 & 0.027 & 0.847 & 0.946 & 1.052 & 0.847 & 0.717 & 3.718 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Results of simple regression analysis of the treadmill test, YMCA step test, and PACER test results. Figure 2: [PERSON] plots between the predicted and measured VO2 max values in the three submaximal exercise tests. (**A**): Treadmill test; (**B**): YMCA step test; (**C**): PACER: test. The mean line represents the mean difference (bias) between the measured and predicted VO2 max values. The upper and lower lines are limits of agreement between the measured and predicted VO2 max values. A previous study conducted a YMCA step test on 97 Germans aged 22 to 77 and reported a high correlation (r = 0.828). Between the measured VO2 max value and the predicted VO2 max value estimated using age, gender, and recovery heart rate [35]. In our study, there was a correlation of 0.770 between the measured VO2 max value and the predicted VO2 max value, indicating a lower correlation than in the previous study.