Discipline-associated locomotor coordination and asymmetry patterns in trained Quarter Horses: a cross-sectional study

Locomotor asymmetry in horses is traditionally interpreted as a potential indicator of pain-related lameness. However, objective gait analysis has shown that measurable asymmetries may also occur in horses without overt clinical lameness. Whether trained horses from different athletic disciplines show discipline-associated asymmetry patterns under standardized straight-line conditions remains unclear. This study examined locomotor asymmetry and vertical displacement asymmetry in Quarter Horses competing in three Western disciplines: barrel racing, reining, and ranch sorting. Ninety adult Quarter Horses were initially recruited, and 57 horses were retained after signal quality control and stride validation, with 19 horses per discipline. Horses were evaluated in-hand at walk and trot on a straight 100 m sandy track. Locomotor data were recorded using ProMove-mini inertial measurement units from the Equi-Pro system at 200 Hz, positioned on the head, withers, sacrum, and limbs. Vertical displacement asymmetry was assessed using Mindiff and Maxdiff of the head, withers, and sacrum. Interlimb asymmetry was evaluated using the Symmetry Index, Asymmetry Ratio, and absolute side-to-side difference. Discipline effects were tested using horse-level averaged values, with discipline as the main fixed factor and gait velocity included as a covariate when appropriate. Static morphometric traits were analyzed using principal component analysis, and exploratory morpho-functional associations were assessed using cross-validated Elastic Net models. Discipline-associated differences were observed in selected variables of vertical displacement asymmetry, spatiotemporal, angular, and vertical acceleration. Reining horses showed greater head and withers vertical displacement asymmetry than barrel racing horses, whereas ranch sorting horses showed greater hindlimb absolute side-to-side differences in acceleration and angular range-of-motion variables. Several effects were gait- and segment-dependent, indicating that asymmetry patterns were not uniform across locomotor domains. Morphometric principal components did not clearly separate horses by discipline, and morpho-functional models exhibited limited global explanatory power. Elastic Net models identified moderate associations only for selected outcomes, with cross-validated R² values ranging from 0.35 to 0.42. Trained Quarter Horses from different Western disciplines exhibited discipline-associated patterns of locomotor and vertical displacement asymmetry when evaluated under standardized in-hand conditions. Static morphometric variation showed limited and outcome-specific associations with locomotor variables, suggesting that conformation alone did not explain the observed locomotor differences. These findings support the interpretation of locomotor asymmetry as a multidimensional biomechanical phenomenon that should be considered in relation to discipline context, clinical assessment, and individual locomotor history.