It is clear that both horizontal and vertical loadings must be taken into account when trying to fully understand the interaction of the hoof and the ground on racetracks, according to researchers who delved into fatal limb fractures in North American Thoroughbreds.
Researchers, using data from the Equine Injury Database (EID) and Equibase, have taken an in-depth look at the effects of racing surfaces and turn radius on fatal breakdowns in racehorses.
Their findings, reported in the journal Sustainability, provide insights into how key track-related factors relate to patterns of catastrophic leg injuries.
North American Thoroughbred racing is conducted on three types of surfaces — dirt, turf, and synthetic. The tracks are oval, and races are run counterclockwise.
The perimeter of oval tracks range from 1200m to 2000m.
There is considerable variability across the country with regard to racetrack design and geometry, the nature of the track surface, the length of races, and the proportion of race time spent traveling straight versus turning.
Turns are typically banked by 2 to 6%, and the straights have a 1 to 2% crossfall (gradient or slope across the track toward the inside rail) to assist drainage.
The gallop is an inherently asymmetrical gait implying unequal forces and kinematics in the trailing and leading limbs.
While the vertical loading is similar in fore and hind limbs, the hind limbs provide the primary tractive effort.
Turning imposes additional asymmetrical forces on the limbs on the inside and outside of the turn, the researchers noted. Racehorses must lean into the turn so their center of mass is inward of the position of their hooves on the ground.
Studies related to the dynamics of turning at a gallop have shown that horses slow for a turn due to either the inability to produce sufficient force or having insufficient frictional resistance at the hoof–shoe–surface interface.
For their study, Michael Peterson and his fellow researchers looked at the data to seek associations between fatal limb fractures and factors related to the horse, the race and the track for the years 2009 through 2014.
Fatalities are defined under the Equine Injuries Database as events occurring on the racetrack which result in death or euthanasia within 72 hours of the race. The study team focused on fractures involving one or more limbs.
For the period in question, the injury database included 2,356,427 race starts, 75.4% of which were run on dirt, 13.2% on turf and 11.4% on synthetic surfaces.
There were 3954 race-related fatalities associated with fractures of one or more limbs.
They found that injury rates were lower on turf and synthetic surfaces and the pattern of limb injuries in left versus right and fore versus hind limbs were different.
“Regardless of surface, forelimbs were more likely to fracture,” they reported.
Dirt surfaces showed higher rates of forelimb injuries compared to other surfaces, while hind limbs were more likely to experience a fatal fracture on turf than on dirt.
The left fore and right hind limbs were more likely to experience a fatal fracture but only on dirt surfaces.
Males had a higher injury rate than mares, and stallions were higher than geldings.
Age was also a significant factor, with a lower injury rate for horses younger than 3 years old compared to 4 and 5-year-old horses. Carrying a higher weight also reduced the fracture risk.
Races run over 1408m (seven furlongs) or more had a lower fracture risk than those run over shorter distances.
Races in which the purse was $US8000 to $30,000 had a higher risk of fracture compared to those with lower and higher purses.
Sex restricted races had a lower risk than those without sex restriction.
There was a significantly higher risk ratio for fatal limb fracture if the last timed workout was performed on dirt or if the stretch distance in the race was longer than 477m.
The effects of turn radius were assessed on dirt tracks only, with higher risks linked to tighter turns, with the right forelimb at higher risk of fracture on tighter turns, whereas the right hind limb appeared more susceptible to larger turns.
Banking in the turns may also be a factor as well as the speed of the horse in the turns. These should be considered in future studies, they said.
The study team said differences in injury rates by limb on different surfaces are potentially important.
“Fatal fractures in the hind limbs, while overall much less common than forelimb fractures, may be more common on surfaces with better traction.
“Understanding the risk of fracture in the right and left limbs on higher traction surfaces like turf and synthetic is a subject worthy of further investigation.
“With better understanding of limb loading on the different types of surfaces it may be possible to optimize the surfaces for balanced loading across all limbs.
“Based on the findings presented here, it is clear loading in the horizontal plane must be considered in addition to vertical loading in order to fully understand the interaction of the hoof and ground.”
The study team comprised Peterson, Wayne Sanderson and Nurlan Kussainov, all with the University of Kentucky; Sarah Jane Hobbs with the University of Central Lancashire in England; Patti Miles, with the University of Maine; Mary Scollay, with the Racing Medication and Testing Consortium in Lexington, Kentucky, and Hilary Clayton, with Sport Horse Science in Mason, Missouri.
Peterson, M.; Sanderson, W.; Kussainov, N.; Hobbs, S.J.; Miles, P.; Scollay, M.C.; Clayton, H.M. Effects of Racing Surface and Turn Radius on Fatal Limb Fractures in Thoroughbred Racehorses. Sustainability 2021, 13, 539.