## 4 Discussion
This study highlights the potential for increased resistance created by water to influence horses' physiological responses during water walking, encouraging further comprehensive research on the application of AR in WT exercise. The findings of this study demonstrate the differential effects of the AR mode on WT exercise at two water depths in a group of healthy school horses. Feltock-depth and carpal-depth were investigated based on previous studies indicating that water depths below the carpus are more commonly utilized than those above the carpus [1]. At fetlock-depth water, the only notable difference associated with AR mode activation was lower LDH activity. This may be related to the horses' movement strategy on the WT at this depth. While walking in fetlock-depth water, horses tend to'step over' the water surface to minimize contact with the water [13; 15; 18]. With AR mode enabled, this avoidance strategy may become even more pronounced, mitigating the assumed workload effect associated with increased water resistance. It can be speculated that avoiding water contact, regardless of flow rate, may remain effective at a constant belt speed. However, further kinematic studies [4; 12; 13; 14; 15; 16; 17; 25] are necessary to confirm this hypothesis. At carpal-depth water, more pronounced AR-related differences in physiological responses were detected. During the measurement of the highest workload, HR was higher during the carpal-depth WT+AR session compared to the DT and both fetlock-depth sessions. For the carpal-depth WT session without AR, no differences in HR were observed compared to the fetlock-depth sessions. LAC was elevated during both carpal-depth WT exercises compared to other treadmill exercises. Notably, in the carpal-depth WT+AR session, this increased LAC persisted for 30 min post-exercise, whereas it returned to resting levels at the same time point in the carpal-depth WT session and all other treadmill exercises. LDH activity was also higher immediately post-exercise in the carpal-depth WT+AR session compared to the session without AR. However, the amount of LAC generated in any of the horses after WT exercises in this study does not meet the criteria for lactic acidosis. This suggests that horses were able to compensate for the increased energy expenditure and likely higher oxygen demand [2; 23]. Previous research indicates that increased respiratory rate and enhanced oxygen transport capacity through the mobilization of the splenic RBC reservoir [20] act as compensatory mechanisms against increased blood LAC concentration in horses. However, in this study neither respiratory rate nor splenic contraction were assessed after effort to support this speculation. Although oxygen demand was not specifically measured in this preliminary study, oxygen consumption (VO\({}_{2}\)) has been investigated in recent WT studies [2; 23]. These studies demonstrated that walking at stfile-depth water increases HR and VO\({}_{2}\) compared to DT exercise and fetlock-depth WT exercise [23], suggesting potential conditioning benefits for Thoroughbreds [2]. Given the relatively low-intensity nature of WT exercises [2; 20; 21; 22; 23; 26; 27; 28], such potential conditioning benefits requires walking [2; 23] or even trotting [22; 26] at higher water depths. However, higher depths may pose kinematic challenges and thus be contraindicated [4; 25]. We speculate that horses undergoing carpal-depth WT exercise with AR could achieve similar submaximal physiological responses and cardiorespiratory fitness as those engaged in stfile-depth WT exercise without AR [2; 23]. Therefore, further research utilizing ergospirometry systems [2; 23; 37] or impulse oscillation systems [37; 38] is recommended to verify this hypothesis. The speculation of splenic RBC reservoir mobilization [20] may be supported by the increased RBC levels observed immediately post-exercise in horses walking in the carpal-depth WT +AR session.