## 1 Introduction  
Water treadmill (WT) exercise is becoming increasingly popular in the conditioning [1, 2, 3, 4] and rehabilitation [5, 6, 7, 8] of equine athletes. The decision to incorporate WT exercise into a conditioning or rehabilitation program is often made to leverage the benefits of water properties [9]. When a horse is immersed in water, buoyancy acts against the horse's body weight [10] and reduces vertical acceleration and vertical ground reaction forces [11]. Additionally, the hydrostatic pressure can enhance the horse's ability to maintain postural control [5]. Finally, the drag force opposing the horse's forward movement modifies the kinematics of a horse's limbs [4; 12; 13; 14; 15; 16] and back [4; 16; 17; 18; 19] and increases energy expenditure during exercise [2; 20; 21; 22; 23]. WT exercise protocols vary based on the combination of water depth, belt speed (and consequently the horse's gait), exercise duration, and frequency of application, producing different effects on the horse working in water. Therefore, like any other aspect of a horse's exercise regimen, WT exercise protocols should be tailored to address specific individual exercise objectives [4]. If the goal of a WT exercise is to encourage the horse to take fewer, longer strides with increased swing duration and enhanced lumbar flexion, walking in fetlock-depth water is recommended [18; 24]. To reduce limb protraction, walking in carpal-depth water is advised [4; 16]. To increase limb flexion and back vertical displacement, walking in stifle-depth water is preferable [4; 15; 17]. It can be observed that water depth is the primary factor influencing WT exercise outcomes. However, increased belt speed promotes greater craniocaudal movement of the head and neck, potentially negating improvements in limb and back kinematics, particularly at higher water depths [4]. Excessive craniocaudal movements of the head and neck indicate that belt speed is too high, at any water depth [25]. Therefore, from a kinematic perspective, high belt speed exercises in water should be avoided [4]. On the other hand, to progressively develop a horse's cardiovascular and muscular fitness, it is recommended to gradually increase energy expenditure [26]. Energy expenditure during the work on the WT was previously investigated using indicators of physiological responses, such as heart rate (HR) [2; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29] as well as hemoglobin concentration (HGB) [20], lactate concentrations (LAC) [20; 21; 22; 23; 26; 30], creatine phosphokinase (CK) activity [28; 30], aspartate aminotransferase (AST) [30], and lactate dehydrogenase (LDH) [30] in blood. However, it is important to highlight that the primary purpose of these hematological and biochemical indicators is to differentiate pathological alterations from physiological ones. While their variations within normal reference ranges may aid in assessing physiological responses to effort on the WT [2; 20; 21; 22; 23; 26; 27; 28; 29; 30], they do not necessarily indicate an improvement in the horse's health or wellbeing. These studies on energy expenditure showed that the WT exercise represents a low- to moderate-intensity aerobic activity [2; 20; 21; 22; 23; 26; 27; 28], and the greatest workload can be achieved by increasing the resistance posed by the water. To date, this increased resistance has been achieved by increasing water depth [2; 4; 23; 26] or by increasing the treadmill belt speed [22; 23; 26], not by introducing the water counterflow of an artificial river (AR). One may observe that even a single WT exercise session increases muscle activity, indicating greater muscular workload against the water's resistance [31]; however, exercise intensity changes with the combination of WT settings. Walking in fetlock-depth water is initially associated with the horse adapting to this form of locomotion by attempting to avoid contact with the water, effectively'stepping over the water surface' [13; 15; 18].