Saddle Research Trust
Keynote Presentation Structural and Functional Asymmetry of the Equine Athlete Prof Hilary Clayton
11.40-12.40
Structural and functional asymmetries are a common finding in people and animals. In horses, some of the body organs, including the heart, caecum and colon, are asymmetrically arranged within the body. In the left and right limbs, the long bones may be unequal in length and the fore hooves are often asymmetrical in shape. During locomotion, limb movements and coordination patterns may show subtle differences between the left and right sides even in sound horses. This talk will describe the sources of asymmetries and will explore current knowledge of sidedness patterns in horses. The rhythmic movements of the limbs that characterize and distinguish the different gaits are generated by clusters of nerve cells called central pattern generators in the spinal cord. They drive the rhythmical patterns of flexion and extension of the joints and may give rise to slight asymmetries between movements of the left and right limbs. These are somewhat random and do not have a have a consistent pattern or side preference. The resulting functional asymmetries are regarded as a normal feature of locomotion in able-bodied people and sound animals. They are distinct from the effects of sidedness (handedness or leggedness) which is associated with a specific pattern of preferential use of the left and right limbs. Sidedness is present in a wide variety of species of animals, birds, lizards, fish, and even insects. About 80% of the human population are right leg dominant which means the right leg is more dexterous and is preferred for tasks such as kicking a ball or picking something up with the toes, while the left leg is preferred for support. Horses also have a preferred leg that provides support while stepping into or out of a halt with the other leg. Sidedness in horses is well-recognized by trainers; horses have a preferred canter lead, they turn more easily in one direction, they take more contact on one rein, and so on. Asymmetries in the horse’s movement due to sidedness must be distinguished from those arising from the trainer’s side preference or as a consequence of mild lameness. Research is beginning to unravel equine sidedness patterns and to reveal which asymmetries are present consistently in horses that exhibit a preference for the left or right side [1]. In a group of seven highly-trained dressage horses that met the criteria for soundness at trot, only one moved symmetrically at walk. Five of the seven horses showed asymmetry in the same direction and one horse was asymmetrical in the opposite direction. The five horses with the same asymmetry pattern consistently took a longer step from right fore to left fore, the withers sunk lower during the weight shift from right to left fore and the haunches tracked to the right of the shoulders putting the left hind closer to the midline of the body. Interestingly, almost all the asymmetries were exaggerated by the weight of the rider rather than being improved in response to the rider’s aids. Reference [1] Byström, A., Egenvall, A., Roepstorff, L., Rhodin, M., Bragançança, F.S., Hernlund, E., van Weeren, R., Weishaupt, M.A., and Clayton, H.M. 2018. Biomechanical findings in horses showing asymmetrical vertical excursions of the withers at walk. Plos One (accepted)
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