The duration of the breakover — a crucial phase in the gallop of a horse — appears largely unaffected by the type of shoe worn, researchers have found. However, they did find some interesting dynamics at play in the barefoot state.
In the stride cycle of a horse, there is a period when the hoof pushes off from the ground surface and rotates through an angle of about 90 degrees before it is lifted off. This time period is known as the hoof breakover.
The study, reported in the journal Animals, enrolled 13 retired Thoroughbred racehorses at the British Racing School in Newmarket, England. The horses, aged 6 to 20, were in regular work, including gallop training, and were normally used for jockey education.
Using slow-motion video footage, Kate Horan and her fellow researchers measured the breakover duration of the galloping horses at a range of speeds on two surfaces — artificial and turf — in four shoeing conditions: aluminium, barefoot, GluShu and steel.
Hooves from different limbs were assessed separately, at galloping speeds ranging from 24 to 56kmh.
Increasing speed was linked with decreasing breakover duration, and this trend was more enhanced in the hindlimbs than in the forelimbs at high galloping speeds.
Breakover duration was found to be significantly longer on the turf track for both leading and non-leading forelimbs and hindlimbs.
In contrast, shoeing condition was found to affect breakover duration only in the non-leading hindlimb, where it also showed a significant interaction with speed.
Evaluation of the shoe–speed interaction across four speed categories for the non-leading hindlimb revealed that shoeing condition had only a limited influence on hoof breakover durations over the central speed range, with pairwise comparisons revealing few significant differences.
However, in the barefoot state, the breakover time was often significantly faster at high gallop speeds and significantly longer at low gallop speeds, when compared to the other three shoeing conditions.
“It was hypothesised that greater storage and release of energy from a more expanded barefoot hoof may accelerate breakover at higher gallop speeds, and a less predictable landing orientation for a lighter distal limb may drive more variable and longer breakover times at lower speeds,” they said.
Breakover duration tended to shorten with increasing speed, although hindlimb hooves were found to have a greater relative decrease in breakover duration compared to the forelimb hooves at gallop speeds beyond about 45kmh.
“We proposed that this may be linked to a greater propulsive effort required from the hindlimbs,” they said.
The results pointed to the transfer of more bodyweight to the hind end at higher galloping speeds, consistent with the biomechanical strategies previously reported in the galloping cheetah and greyhound.
Looking at the role of speed, the study team said the reduction in breakover duration observed with increasing gallop speed was consistent with observations in the walk and trot, which highlight breakover duration to be a primarily speed-dependent variable.
Discussing the role of surface, the authors noted that ground conditions are commonly the dominant influence on hoof biomechanics, with longer breakover durations recorded on the turf track in the study compared to the artificial surface for all hooves.
“It is possible that the turf versus artificial surface differences masked more subtle effects of shoeing on breakover duration, particularly in the forelimbs and the leading hindlimb,” they said.
“There are several possible explanations for the surface differences. First, the artificial surface may have offered more grip, as was perceived by the jockeys, and thus allowed the horses to push-off more effectively and quickly.
“Second, artificial surfaces have a high level of elastic deformation and it is likely that this surface returned more energy to the hoof than the turf through elastic rebound, thereby potentially accelerating breakover.
“Third, it is possible that the softer nature of the artificial track meant the toe rotated into the surface more easily.”
They said preliminary hoof accelerometry data from this group of horses indicate that peak accelerations are higher on the turf compared to the artificial track at impact, which is suggestive of a generally softer artificial surface.
The researchers noted that the turf surface appeared to be more irregular, compared to the frequently harrowed artificial track, and it is plausible that a more uneven surface would result in non-simultaneous impact of the medial and lateral heels.
Discussing the shoeing state of the horses, the authors offered their views on their finding that the greatest number of significant differences occurred between the barefoot hooves and the other three shoeing conditions. As noted earlier, the barefoot condition was associated with longer breakover durations at slow speeds and more rapid breakover at higher speeds, relative to the shoed horses.
The researchers noted that the British Racing School horses used in the study were commonly left barefoot behind and only shod in front. “Therefore unfamiliarity to the barefoot condition, as might be expected in a racing Thoroughbred, would be unlikely to explain the results here.
“At increasing speeds, heel expansion is expected to increase, likely reflecting the increase in vertical ground reaction force at mid-stance.
“In the barefoot condition, it is plausible that a greater percentage of potential energy would be stored, as the heels expand under load to a greater extent, relative to the shod conditions. The subsequent release of this energy may accelerate breakover as gallop speeds increase.
“In contrast, shoeing can reduce medio-lateral heel movement, and therefore is likely to lessen this effect for the three shod conditions.”
At lower gallop speeds, when less potential energy is stored in the hoof, perhaps hoof conformation and shoe shape become more important, they suggested.
“The more bevelled edge of the shoes, compared to the hoof edge, could act to increase the rate at which the hoof rolls over.”
Alternatively, the mass of the lower limb may be relevant. A reduced lower limb mass in the barefoot and aluminium-shod horses would lower the energetic cost of locomotion, and may permit increased movement variability.
The researchers said their findings may have implications for minimising the risk of injury and improving performance in galloping Thoroughbreds.
“This work demonstrates that speed, ground surface and shoeing condition are important factors influencing the galloping gait of the Thoroughbred racehorse.”
The study team comprised Horan, Peter Day, Sean Millard and Thilo Pfau, all with the Royal Veterinary College in England; James Coburn, Dan Harborne, Liam Brinkley and Henry Carnall, with James Coburn AWCF Ltd in Bury St Edmunds, England; Lucy Hammond and Kieran Kourdache, with the British Racing School; and Mick Peterson, with the University of Kentucky.
Horan, K.; Coburn, J.; Kourdache, K.; Day, P.; Harborne, D.; Brinkley, L.; Carnall, H.; Hammond, L.; Peterson, M.; Millard, S.; Pfau, T. Influence of Speed, Ground Surface and Shoeing Condition on Hoof Breakover Duration in Galloping Thoroughbred Racehorses. Animals 2021, 11, 2588. https://doi.org/10.3390/ani11092588