Prevalence of functional trait and distal limb asymmetries and their effects on equine performance

  • Kirsty Leśniak

Student thesis: Doctoral Thesis


The human-horse relationship in modern society is multifaceted and not replicated with any other species; domesticated or otherwise. Those involved in the equine industry have a personal, and social responsibility to be aware of the impact of their actions on the welfare of horses both within their care, and beyond. Furthermore, they need to be accountable for their actions to industry stakeholders and to wider society. Areas in need of greater consideration include performance horse selection, routine prophylactic interventions (e.g., farriery interventions) and the implementation of appropriate breeding strategies, including a greater regard for the breeding-in of incidental characteristics when targeting for performance traits (e.g., poor hoof horn quality in thoroughbreds when breeding for speed).
Within a range of species, including humans, asymmetry has been proposed as an indicator of genetic quality through association with factors including disease resistance, mate choice and performance potential. The aims of this thesis were to: a) establish the existence of, and difference between, the magnitude and direction of distal limb asymmetry within competitive and noncompetitive equine populations; b) verify the role of the equine hoof as a smart structure in compensating for asymmetries of the limb; and; c) determine whether a ‘normal’ level of directional asymmetry exists within the distal limb and hoof of the horse, irrespective of competitive standing. This thesis proposes that previously reported equine bilateral trait asymmetries are not, as had been postulated, associated with either the locomotory directionality of the competitive discipline, or indeed their associated selective breeding strategies. Similar patterns of distal limb asymmetry were confirmed in event horses, to those previously reported in racehorses. Whilst some event horses are indeed derived from racing bloodstock, the demands of the two disciplines are significantly different, thereby dispelling the discipline demand theory of asymmetry development. While a slightly longer outside limb might be of biomechanical advantage to the racehorse, this asymmetry presentation would not pose a biomechanical advantage in other disciplines, such as eventing, where an equal performance on both reins is desired. Furthermore, the identification of the same patterns of asymmetry in a non-competition population implicates the asymmetries as being present at a species level, as opposed to at a breed level. Interestingly, the amalgamated findings of the contributing papers, suggest that whilst bilateral distal limb trait asymmetry is present in the horse at a species level, horses with a proven superior athletic ability present with a weaker presentation of these asymmetries, and a greater affinity for symmetry. The asymmetries investigated are not of a magnitude likely to have a significant negative effect on biomechanics, and are therefore, theorised to reflect internal disturbances; though of what, and at what structural level, is yet to be determined. This thesis has, however, been able to confirm that the pliable equine hoof does not compensate for distal limb asymmetries by counteracting for the shorter limb with a longer hoof. Instead, it is suggested that hoof conformation is impacted by the loading imbalance caused by the asymmetries. To compound this further, the asymmetries of the hoof increase within an increase in the size of the horse, suggesting larger horses, are subject to both greater, and more imbalanced loading forces; an area warranting further investigation in relation to the impact on both performance and welfare.
The field of equine asymmetry research is still in an embryonic state compared to that in other species, or compared to research in other areas of equine science. The findings within this thesis do, however, provide a significant contribution to the area, but in doing so have identified a number of associated research questions; not least the pattern of distal limb and hoof asymmetries in the developing horse and identification of contributing factors, including laterality.
Date of Award2021
Original languageEnglish
SupervisorMatt Parker (Supervisor)

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