Similar locomotor patterns across Quarter Horse disciplines inform lameness assessment

BACKGROUND: Lineage-based selection is central to the Quarter Horse industry and is often justified by the assumption that morphology determines locomotor function. However, objective evidence linking static conformation to discipline-specific locomotor behaviour under standardised conditions remains limited. OBJECTIVES: To investigate whether morphological differences among Quarter Horse disciplines are associated with distinct locomotor patterns or whether locomotor function converges across disciplines despite morphological variation. STUDY DESIGN: Cross-sectional. METHODS: Barrel racing, reining, and ranch horses underwent detailed morphometric assessment and inertial measurement unit (IMU)-based gait analysis at the walk and trot under standardised straight-line conditions. Multivariate approaches (principal component analysis, UMAP, k-means, and Gaussian mixture models) were used to evaluate relationships between morphology and biomechanical outputs, including stride timing, limb excursion, and axial kinematics. RESULTS: Morphometric differences varied across disciplines, and PCA revealed three components explaining 90.6% of the variance. Biomechanical variables overlapped across groups, and no model consistently identified discipline-specific locomotor patterns. Greater within-discipline variance than between-discipline centroid distances in biomechanical PCA space supported functional convergence (Ranch Sorting: 12.48 vs. 5.64; Barrel Racing: 10.41 vs. 4.77; Reining: 7.71 vs. 2.28). Axial range of motion and vertical acceleration variables were similar across groups. CONCLUSIONS: Under standardised walk and trot conditions, Quarter Horses across disciplines exhibit broadly similar locomotor patterns despite measurable morphological differences. Static morphology alone does not uniquely predict locomotor function, and neuromuscular control and training exposure likely play a central role in shaping movement patterns. Objective biomechanical assessment provides a complementary framework for evaluating equine athletic function alongside traditional lineage-based selection. Discipline-specific biomechanical expectations should not replace normal movement patterns in clinical gait assessment, and individual baseline evaluation remains essential for interpreting objective locomotion data in Quarter Horses, particularly to distinguish lameness from physiological sources of asymmetry.