Evaluation of the movement during trot in circles is commonly used in clinical equine lameness examinations to exacerbate mild or bilateral lameness. Sound horses trotting in a straight line have a high degree of movement symmetry, which decreases with lameness. The aim of this study was to investigate the effect of lungeing and circle size on movement symmetry in sound horses.
Twenty five riding horses categorized sound by inspection on straight line and lungeing were trotted in a straight line and in circles of 8, 10, 12, 14 and 16 metres diameter on both reins. Overall symmetry (S), symmetry of diagonal limb pair loading (A), and symmetry of diagonal limb pair timing (W) were calculated from trunk accelerations measured over the 13th thoracic vertebrae (T13) with a 3-axis accelerometer. Data were analysed via ANOVA with horse as random effect.
S, A, and W showed high degree of symmetry of straight line trot with mean ± SE = −6.0 ± 0.1, 0.01 ± 0.02 and −0.001 ± 0.003, respectively, that decreased with circular trot (P<0.02 in pairwise tests). S and A showed decreasing symmetry with decreasing circle size. For 16 and 8 metres, respectively; S = −5.2 ± 0.1 and −4.7 ± 0.1; A = 0.10 ± 0.01 and 0.16 ± 0.01; W = −0.009 ± 0.002 and −0.012 ± 0.002.
In sound horses, movement symmetry measured over T13 decreased with circular trot and decreasing circle size as shown in earlier studies of movement of the poll and sacrum. The asymmetry mimicked inside diagonal lameness. Effect of circular movement and circle size should be considered in interpretation of lameness signs and asymmetry measured during circular trot.