The effect of an acute angulation of the hind hooves on diagonal synchrony of trotting horses.

High speed cinematography was used to measure the timing and distance characteristics of the trot in five sound horses. Two farriery treatments were compared: (1) normal, in which the dorsal hoof wall was aligned with the pastern axis in all four feet and (2) acute hind hoof angulation, in which the hind toes were allowed to grow relatively longer than the heels, so that the angulation was approximately 10 degrees lower than the normal hind hoof angle for the individual animal. Comparison between the two treatments showed that the acute hind hoof angulation was associated with significant increases in hind limb breakover time (P = 0.001), overreach distance (P = 0.001) and overreach duration (P = 0.017), whereas stride length, forelimb breakover and diagonal distance did not change (P > 0.05). In the majority of horses, limb movements at the start and end of the diagonal stance phase were slightly asynchronous, so that when the hooves were trimmed normally the hind limb more often preceded the diagonal forelimb at impact and lift off. The prolongation of hind limb breakover due to the acute hind hoof angulation was associated with a tendency for the hind limb to lift off later than the forelimb. The difference in lift off sequence between the two farriery treatments was highly significant (P = 0.001). During the swing phase the normal coordination was restored and there was no significant difference in the impact sequence with the normal and acute hind hoof angulations (P > 0.05).