Swimming has been used for over four decades to help maintain a good musculature in horses with tendon and ligamentinjuries. Despite the clinical advantages of swimming pool training, which reduces stress on the joints, the biomechanics of horse swimming is still not fully understood. In this study, two measurement systems are used to characterize the swimming of 4 horses (to date). The first is a motion capture system consisting of 6 underwater cameras and 28 markers on the horse (7 for each limb), allowing to calculate the joint angles from the shoulder and hip until the 4 fetlocks during the swim. The second is a set of 9 inertial measurement units (IMUs) placed on the head, the sternum, the withers, the 18th thoracic vertebrae, the tuber sacrale and the 4 cannons, which follow the translational movement and angular displacement of these body parts. Four of those IMUs were placed with skin markers on the cannons, providing a basis for comparing the two methods. The 3D data produced by the motion capture system allow to compute the different joint movements on 96 records (to date). Preliminary results (on a single record to date) give the following joint range of motion: shoulder: 100-125°, hip: 60-120°, elbow and stifle: 50-150°, carpus: 50-200°, tarsus: 40-170°, front and hind fetlock:100-180°. In conclusion, this study is one of the first to validate the technological feasibility of 3D motion capture of horse swimming.