Upper-body asymmetry of 19 elite dressage horses: trot in hand on the straight versus collected- and extended trot during a standardized dressage test
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Asymmetry in motion of the dressage horse is frequently debated among clinicians, riders, trainers and judges. The aim of the current study was to objectively describe the vertical upper-body movement asymmetry of owner-sound elite dressage horses in trot in hand on the straight and collected- and extended trot during a standardized dressage test. Differences in protraction and retraction of the fore- and hindlimb pairs were also analysed. Linear mixed models were performed for all variables with horse ID as random effect and trot condition as fixed effects. There was a significant increase in ROM of the head (p<0.01), withers (p<0.01) and pelvis (p<0.01) for both extended trot and collected trot compared with trot in hand. For collected trot vs. trot in hand, an increase was seen in HDmin (p<0.01), HDmax (p<0.01), WDmax (p<0.01) and PDmin (p=0.04). For extended trot vs. trot in hand, there was an increase in HDmin (p<0.01), HDmax (p<0.01) and WDmax (p=0.04). No significant differences were found for any upper-body parameter in collected trot vs. extended trot. DiffProtraction of the front limbs increased in both extended trot vs. trot in hand (p<0.01) and collected trot vs. trot in hand (p<0.01). An increase in DiffRetraction of the front limbs was seen for extended trot vs. trot in hand (p=0.02) and extended trot vs. collected trot (p=0.02). Factors that could have affected movement asymmetry under saddle include the increased ROM, the rider’s aids, the rider’s weight, the rider’s static or dynamic postural asymmetry, the horse’s head-neck position and the horse’s laterality pattern. Regarding trot in hand measurements, horses showed asymmetry values greater than the currently recommended symmetry thresholds. This raises the highly important question: should all of our dressage horses, based on their degree of upper-body asymmetry, be classified as lame? Or should the current symmetry thresholds be re-evaluated, at least for highly trained sports horses? These asymmetries namely might also be the result of laterality, conformation, shoeing or training. The main conclusion of this study was that no elite dressage horse will trot perfectly symmetrical, neither in a natural moving trot in hand on the straight nor in collected- and extended trot under saddle. It should be investigated to what degree detectable movement asymmetries in dressage horses can be directly attributed to orthopaedic pain or natural asymmetry in the horse’s locomotor pattern. Furthermore, studies investigating the long-term effect of asymmetrical motion on the development of musculoskeletal injuries would be appropriate as part of prevention and early recognition of orthopaedic disease in the dressage horse.