## 4 Discussion  
This study was performed to investigate the effect of limb-specific resistance training on both central and peripheral artery stiffness in young adults. It is, to our knowledge, the first to assess the effect of limb-specific long-term resistance training on both central and peripheral artery stiffness in young adults without any clinical disease. Resistance exercise is one type of exercise recommended for improving muscular strength and cardiovascular function and for preventing musculoskeletal diseases, such as osteopenia and osteoporosis [31]. Although resistance exercise is recommended to enhance cardiovascular health, the effect of resistance exercise on arterial stiffness is still controversial. In this study, 8 weeks of URT reduced AIx, but not AorPWV, in young adults. For the same duration, LRT did not change the central artery stiffness measures. It has been documented that change in the resting heart rate after exercise training influences AIx, which may result from the adaptation of the autonomic nervous system to exercise training [32; 33]. In this study, the decrease in the resting heart rate and the systemic norepinephrine concentration (Table 1) following URT may in part explain the reduction in AIx. Furthermore, previous studies imply that an increase in muscular strength after exercise training is associated with a decrease in central artery stiffness. [PERSON] et al. reported that adults who have higher muscular strength present lower central artery stiffness; particularly, those who have higher upper-limb muscular strength tend to have lower central artery stiffness [34]. In response to 8 weeks of URT, 1 RM of upper-limb resistance exercises was improved with a simultaneous increase in arm LBM in this study. The increase in muscular strength accompanying muscle hypertrophy may in part account for AIx reduction after URT. Change in AIx after the resistance exercise was associated with change in serum triglyceride levels. Blood triglyceride concentration measured after overnight fasting is used as a traditional marker of CVD risk. Considering both that endothelial dysfunction is a strong early marker to predict future CVD risk and that vascular endothelial dysfunction is closely related to serum triglyceride concentration, a decrease in serum triglyceride concentration after resistance training may contribute to a decrease in central artery stiffness via enhanced vascular endothelial function [35]. ArmPWV, a measure of peripheral artery stiffness in this study, showed a tendency to decrease in response to URT, although this tendency was not statistically significant. This result is different from the results of previous studies. [PERSON] et al. reported that 8 weeks of single arm-curl resistance exercise increased baPWV, which is another measure of peripheral artery stiffness reflecting mainly lower-limb artery stiffness, in young women [36]. Similarly, 10 weeks of URT increased baPWV in young adults [37]. However, LRT did not influence baPWV in young adults and did not show any negative effects on AorPWV, the gold standard measure of central artery stiffness, in older adults [37; 38]. The acute effects of one bout of resistance exercise on arterial stiffness can be different based on the exercising limbs. [PERSON] et al. reported that one bout of upper-limb resistance exercise increased baPWV, but lower-limb resistance exercise did not increase central artery stiffness in young men [39]. Similarly, in young adults, one bout of lower-limb resistance exercise did not show any negative effect on central artery stiffness [40]. In previous findings, the effects of resistance exercise on arterial stiffness were different according to the participant's age, gender, exercising limbs, training duration, and vascular beds measured for arterial stiffness assessment.