“Negotiating a circle involves a change of direction, and a change in direction causes inward acceleration toward the center, which is called centripetal acceleration,” said Sarah Jane Hobbs, PhD, of the University of Central Lancashire Centre for Applied Sport and Exercise Sciences, in the U.K.
Hobbs gave a one-hour presentation on locomotion in circles during the Centaur Biomechanics Virtual Equine Sports Science Summit on Oct. 3.
The mechanical equation for centripetal acceleration takes both radius and velocity into consideration, but by different amounts, Hobbs explained. “So the speed of the turn will be more influential than how sharp the turn is,” she said.
Applied to the horse, this means the faster he goes around a curve, the greater centripetal force he must create to make the turn. He does this through “pushing outward” with his body, she said. “The outward push can be achieved by changing limb position, body position, or muscular effort, and the effect of these changes can have an influence on the horse’s balance,” said Hobbs.
Horses also tend to lean inward—some more than others, depending on their muscular development and discipline, with dressage horses leaning less than racehorses, for example. The lean makes the horse “shorter,” in that his vertical height from ground to withers is reduced, which is important for clinicians to keep in mind as a possible compounding effect on lameness exams.
How Circling Affects Lameness and Lameness Exams
How a horse copes with a turn can affect his gait, making him appear more or less sound than he would be on a straight line. “Circles can exacerbate lameness on the outside forelimb, probably due to increased vertical ground reaction force (the way the ground pushes back against the horse’s feet), or on the inside hind limb, possibly due to altered pelvic and limb posture,” said Hobbs.
Meanwhile, subtle outside hind-limb lameness might appear improved on a curve. “It could actually be masked on a circle due to the hind-limb asymmetry that you would find normally during turning,” she explained.
Horses might also look lame on turns even if they’re not, she added. “Circles can give the impression of inside forelimb or inside hind-limb lameness, with a head nod on the outside forelimb or hip hike on inside hind limb,” said Hobbs.
High-Speed Turns: Should Tracks Be ‘Banked’?
At high speeds, the centripetal force needed to make a tighter turn can be so great that the horse slows down—an important consideration for racetrack, training track, and cross-country course designs, said Hobbs. When the ground is soft, horses can dig their hooves in better for these curves—but with the countereffect of having a longer stance time (when the foot is on the ground). When the ground is hard, horses must compensate for the centripetal force in other ways to keep from falling, either by slowing down or changing their positions in ways that could, in certain circumstances, lead to musculoskeletal injury or excess wear and tear.
“Banking” tracks might be a solution to help horses negotiate high-speed turns, said Hobbs. Banked tracks are common in sports such as car and motorcycle racing, with raised surfaces at curves to keep pilots from skidding off the track.
“The mechanics of turning are less demanding when negotiating banked turns, which is a consideration for food for thought for track designers,” she said.
A likely biomechanical consequence of banked tracks, however, would be increased speed. In high-speed turns with no banking, “horses have to slow down because they are limited by the amount of force they can produce to make the turn,” said Hobbs. “If you did start to bank those tracks, you’d probably see differences in speed.”
The logistics of maintaining a banked track, however, could make it very difficult to actually have horses racing on them, she added.