## 4 Discussion
Our results showed that for both collegiate towers and collegiate cyclists, the correlation between HR and VO2 during exercise testing for each exercise modality was different and that estimating the VO2 max as the VO2 corresponding to age-predicted maximal HR using the correlation formula was lower in the exercise testing with ACE than in that of CE. These results suggested that exercise capacity assessed by exercise testing with ACE is underestimated, regardless of the training status of the upper limbs. Previous studies reported that the difference in VO2 max obtained between exercise testing with ACE and CE was more modest in individuals with well-trained upper limbs [11; 16]. These studies compared VO2 max at the end of exercise testing for each exercise modality. Therefore, it may be inappropriate to directly compare the results of our study that examined the correlation between HR and VO2 during exercise testing with theirs; however, our study suggests that exercise capacity is underestimated in the case of using results obtained from exercise testing with ACE, even in individuals with trained upper limbs. In future studies, to determine the extent to which VO2 max is underestimated in the exercise testing with ACE in various populations may lead to the development of a prediction equation for VO2 max obtained from exercise testing with CE based on the results from exercise testing with ACE. A few limitations of the present study should be noted. First, it concerns the background of the subjects. The subjects had a short history of competition. Six of the nine rowers (66.7%) had been competing for less than a year. In the cyclists, one subject (12.5%) had been competing for less than one year and five (62.5%) had been competing for one to two years. The subjects recruited in the present study are considered to be in the process of developing exercise training adaptation. Further studies should be conducted with more trained athletes. Additionally, the percentage of body fat in the rowers was significantly lower than that in the cyclists. The difference in body composition could have had a significant impact on the main research findings. The second point concerns how to obtain VO2 max in the present study. In order to achieve our study aim, it was needed to examine whether the correlations between the HR and VO2 formula during exercise testing were equivalent between ACE and CE. Therefore, VO2 max, which we used in the present study, were estimated values obtained from the formula, not actual measurement data. On the other hand, actual measurement values of VO2 max reflect more biological, physiological information than the estimated values. In fact, previous studies have reported that peak VO2 during lower-limb exercise can be predicted with high accuracy using peak VO2 obtained from upper-limb exercise and other parameters. Using actual measured VO2 max during exercise testing with ACE could have been more useful in predicting VO2 max during exercise testing with CE. Third, this study did not evaluate hemodynamic response and sympathetic nerve activity during exercise testing. Future studies are needed to clarify whether these effect the differences in the correlations between HR and VO2 depending on the exercise modalities. Finally, FMD data of the brachial artery measured as an indicator of the training status of upper limb were not significantly different between rowers and cyclists. Exercise training induced arterial remodeling such as decreased wall thickness and dilation of vessel diameter to adapt to increased blood pressure and blood flow during exercise [32; 33]. Some studies suggest that this remodeling may be responsible for the lower FMD response in athletes [34; 35]. In the present study, however, there were no statistically significant differences in wall thickness and baseline diameter of brachial artery between the two groups (\(p\) = 0.754 and 0.151, respectively).