Kinematics of the equine axial skeleton during aqua-treadmill exercise

Equine aqua-treadmills are increasingly applied within the industry for rehabilitation from injury and training. Research into aqua-treadmill exercise has been increasing yet there are still opportunities to further quantify the effect of water depth on locomotion in order to optimise aqua-treadmill protocols to see improved rehabilitation from injury or to exercise horses more effectively for their chosen discipline. Much of the current aqua-treadmill literature focusses on the effects of water on locomotory parameters in walk, thus providing an opportunity for investigation into the impact of water at trot. Trot is the favoured gait for effective quantification of lameness and symmetry studies so it was anticipated that there may be opportunities to make comparisons to previously published overground data. Effective schooling of horses overground includes training aids, such as side reins, to constructively develop a horse’s way of going and often to assist the horse in maintaining concentration. This provided a further opportunity for investigation into the use of side reins during aqua-treadmill exercise. This project therefore, aimed to quantify the effect of increasing water depths on pelvic and withers movement of horses trotting on an aquatreadmill and to analyse the impact the use of side reins has on these movements. Seventeen sound horses were habituated to aqua-treadmill exercise and subjected to one of two exercise protocols where data were collected either by optical motion capture (Qualisys©) or an inertial sensor system (Xsens©). The exercise protocol involved trotting on the aqua-treadmill at four increasing water depths, that of the third phalanx (P3), mid fetlock, mid third metacarpal (MC3), and mid carpus. Markers for optical motion capture were located on the poll, withers (T4/T5), mid thorax (T13), tuber sacrale, left and right tuber coxae, left and right tuber ischia. Inertial sensors were located on the poll, withers (T4/T5), mid thorax (T13), lumbar vertebrae (L4), tuber sacrale, left and right tuber coxae, and top of the tail (1st coccygeal vertebrae). Data were cut into strides with accelerations double integrated to generate displacement amplitudes, both vertical and mediolateral, for statistical testing. Pitch and roll data from the inertial sensors was also extracted and processed for analysis. Data were processed using custom written scripts (Matlab®) and repeated measures ANOVAs were performed throughout to test for significance with post hoc analysis where appropriate. Water depth was found to have a significant effect on vertical displacement amplitudes of the pelvis and withers with vertical displacements increasing with increasing water depth, and a greater displacement in the pelvis than the withers. Minimum and maximum positions of the pelvis and withers were found to decrease and increase accordingly with increasing water depth, with minimum values decreasing significantly indicating an increase in limb compression during stance. Maximum vertical positions also increased significantly indicating greater maximum lift out of the water as a result of the increased compression. Water depth had no effect on symmetry of horses trotting on an aquatreadmill and no effect on pitch amplitudes. Vertical displacements, pitch and symmetry were not altered with the addition of side reins, suggesting that the adoption of a different head and neck position whilst reaching comparable displacement amplitudes encourages further engagement of back muscles possibly providing stimulus for building greater strength through muscular development. Water depth was found to have no effect on mediolateral displacements of the pelvis or withers but with the withers exhibiting larger mediolateral displacements than the pelvis at lower water depths but reducing to an amount comparable to the pelvis at deeper depths suggesting that deeper water provides a stabilising effect on the front end of the horse. Side reins had no effect on mediolateral displacement amplitudes or on roll amplitudes. Mediolateral flexions of the spine were not affected by water depth or side reins, suggesting that the horse can be worked harder at greater water depths without over stressing the mediolateral capabilities of the spine. Vertical displacements of the pelvis were significantly increased when trotting on the aquatreadmill in a very low depth of water compared to measurements overground but this effect was not seen in the withers suggesting the front end of the horse can efficiently compensate for water depth by flexing at the carpus, although larger pitch amplitudes were reported at the withers suggesting a change in head and neck position to create a ‘jump up’ over the water. Side reins were found to decrease vertical displacement amplitudes in the withers overground but trotting on the aqua-treadmill in a small amount of water counteracted this effect suggesting that the addition of water may counteract a ‘downhill’ effect seen in horses wearing side reins overground. This project suggests that the aqua-treadmill is beneficial at increasing the workload for the horse that may possibly have a corresponding effect of increasing muscle mass, strength and condition, but without detrimental effects to cranial-caudal or mediolateral symmetry patterns and that side reins have a potential benefit in supporting these locomotory patterns. Knowledge of this primary scientific data will better assist professionals working with aqua-treadmills to more effectively benefit the horses with which they work. There is, however, an opportunity for further longitudinal research to further support the effective application of the aqua-treadmill as a tool for rehabilitation and training.