Vertical movement asymmetries in 3-year-old SWB horses and their associations with scores in Young Horse Tests

Abstract: To improve equine performance and welfare, many attempts have been made to investigate associations between performance, longevity, orthopaedic health, conformation and talent. Causal relationships between these qualities supported by strong scientific evidence are lacking. This is partly due to absence of objective, accurate and repeatable metrics. The Swedish Warmblood is a sport horse bred for performance and to improve performance so called “young horse tests” (YHT) are arranged each year. They provide insight to the young horse population’s conformation and talent. Linear profiling is a recently introduced method thought to increase objectivity and measurability for visual assessment of conformation, gait, jumping and behavioural qualities in the young horse tests. Increased objectivity is also achieved in gait assessment research through kinematic tools. Such tools, offering objective measurements of vertical movement asymmetry of the head and pelvis, have proven useful in detection of subtle lameness in the horse. Recent studies using these tools have shown a high prevalence of vertical movement asymmetries in horses considered sound, including foals. The aim of this study was to investigate possible associations between scores in the YHT and the degree of vertical movement asymmetry in 3-year-old Swedish Warmblood horses using an objective gait analysis tool. Additionally, possible differences in vertical movement asymmetry between horses bred for different equestrian disciplines were investigated. A total of 116 three-year-old Swedish Warmblood horses were recruited to the study at five YHT events. The horses were recorded at the trot on a flat surface using a smart phone gait analysis application. Each horse was assigned to either of the categories jumping, dressage, all-round or thoroughbred, based on their pedigree. Scores from the YHT were retrieved from the Swedish Warmblood Association (SWB) and transformed into 18 factors. From the four asymmetry values retrieved from the motion measurements, three mean asymmetry values per trial were calculated for each individual: forelimb asymmetry, hindlimb asymmetry and total asymmetry. The total asymmetry, i.e. sum of all asymmetry values, was significantly (p <0.01) lower in horses bred for dressage than in horses bred for show jumping. Correlation coefficients between asymmetry values and YHT score factors were calculated using Pearson’s method. Seventeen significant (r  ≥0.20) correlations were found and subsequently used to construct linear models. In these models, fourteen YHT score factors significantly (p ≤0.05) explained their respective asymmetry value. These factors consisted of one jumping factor, three conformational factors and ten gait related factors. The coefficient of determination was generally low (adjusted r2 <0.10). Overall, what is considered to be good gaits in walk and trot seems to be associated with a lower degree of vertical movement asymmetry. There are however some limitations to the study. The study population is small (n = 116), and potentially skewed in the sense that it likely includes the best developed 3-year-old SWB horses. Also because of novel data handling, the practical implications can’t be fully determined. The results do all the same raise interesting questions about human conception of subtle gait asymmetries and the importance of vertical movement symmetry to performance in the Swedish Warmblood and should be further investigated.