Background: Water treadmill (WT) exercise is a popular modality for the training and rehabilitation of horses. However, evidence-based literature regarding the use of WT exercise, particularly using inclines, is lacking.Objectives: The aim of this study was to assess the effect of recurring inclined WT sessions on equine epaxial muscles development.Methods: Six horses completed 24 sessions of 15 minutes of WT activity over four weeks. Horses walked with water at mid-cannon level at a treadmill incline of 4%. Back traces were measured at three and seven centimetres ventral to the dorsal midline at T5, T9, T14 and T18, prior to the first session (W0) and weekly for 4 weeks (W1-4). Results: Overall the back traces demonstrated progressive increases in muscle development (p<0.05), starting at W2 up to W4. At three centimetres ventral to the dorsal midline the most to least significant increases in gross muscle development were at T18, T5, T9 and T14, respectively and when measured at seven centimetres ventrally, the most to least significant increases were demonstrated at T5, T18 and T14 . It was noted that increases in thoracic back profile musculature were mainly observed within two to four weeks of the WT intervention.Conclusions: It has been concluded that repeated WT exercise on an incline setting has a significant effect on the rate and size of growth of equine thoracic back profile musculature. Muscle hypertrophy due to resistance training in the WT starts at 2 weeks within the programme, and it progresses as exercise continues to be performed.