Effects of Fatigue on Stride Parameters in Thoroughbred Racehorses During Races.

Exercise intensity during races is considerably high. To understand how Thoroughbreds adapt to fatigue conditions, stride parameters for the first and second lap of the race (2400-m, turf) were compared. A high-speed video system was set in a right lateral position about 20 m before the finishing post, with a field view width of about 16 m. The stride frequency, the length between each limb (hind step, diagonal step, fore step, and airborne step), and stride length were measured and analyzed using a generalized linear mixed model. Compared with the first lap, the mean ± standard deviation values in the second lap for running speed (17.3 ± 1.3 to 16.0 ± 0.9 m/s, P < .01), stride frequency (2.34 ± 0.08 to 2.21 ± 0.09 strides/s, P < .01) and stride length (7.42 ± 0.52 to 7.25 ± 0.38 m, P = .04) significantly decreased. Furthermore, significant changes (P < .01) were observed in the diagonal step length (2.32 ± 0.34 to 1.88 ± 0.23 m), hind step (1.19 ± 0.09 to 1.26 ± 0.10 m) and airborne step length (2.43 ± 0.25 to 2.61 ± 0.18 m). When controlled for speed, stride frequency (P = .02) and diagonal step length (P < .01) decreased, while the length of the hind step (P < .01), fore step (P < .01), airborne step (P < .01), and stride (P = .02) increased with fatigue in the second lap. These results suggest that horses could not extend their body when fatigued.