Effect of Water Depth on Limb and Back Kinematics in Horses Walking on a Water Treadmill

Water treadmill (WT) exercise is frequently used for training/rehabilitation of horses. There is limited study into the effect of water depth on limb/back kinematics warranting investigation. The objective was to determine the effect of walking in different water depths, at the same speed, on limb/back kinematics measured simultaneously in a group of horses. Six horses (age:15 ± 6.5 years) completed a standardized WT exercise session (19 minutes duration; speed:1.6 m/s; water depths: 0.0/7.5/21.0/32.0/47.0 cm). Ten waterproof light-emitting-diode tea-light-markers and reflective-spheres were affixed to the skin at predetermined locations; inertial-measurement-units were fixed to the poll/withers/left and right tubera coxae (TC)/sacrum to determine range-of-motion (ROM) changes of these locations. Univariable-mixed-effects-linear-regression-analyses were carried out, with a significance value of P ≤ .05. At maximum carpal/tarsal flexion during swing, regression analyses showed a clear and consistent nonlinear increase in carpal and tarsal flexion at increasing water depths (P < 0.0001 for both variables). As water depth increased there was a significant increase in thoracic spine flexion-extension ROM (P < 0.0001 at all thoracic sites) and increased dorsoventral and mediolateral ROM of the sacrum/left and right TC (P < 0.001 for all variables) as water depth increased. Results suggest that horses responded to an increase in water depth until a threshold depth was reached when the biomechanical response levelled off, and there was increased pelvic roll. In conclusion, changes in limb kinematics brought about by relatively modest increases in water depth at walking speed of 1.6 m/s are sufficient to induce significant changes in back/pelvic movement highlighting key issues with relevance for WT program design.