This usually happens between the withers and the loin, or in the region that involves the 10th-18th vertebrae. In advanced cases, sometimes parts of the spine even grow together.

"It is thought that the process is only painful in (this) acute phase," she said.

Probably because of uneven weight distribution, horses with longer backs are more likely to suffer from back problems associated with riding. It is possible that horses bred to bear heavy loads, such as Icelandic horses, may be less vulnerable, but de Cocq said she has not studied these horses yet.

Back problems are not the only risk to horses that carry human riders. A recent Journal of the American Veterinary Medical Association study found that some racehorses experience hemorrhaging from the pulmonary artery into the bronchial tubes and windpipe during intense exercise.

The disorder was most commonly found in horses that lost races and trailed the winner by an average of 14 feet.

Half of all thoroughbreds experience this problem, called exercise-induced pulmonary hemorrhage, according to lead author Kenneth Hinchcliff, who is a professor of veterinary clinical sciences at Ohio State University.

He said a number of treatments, including drugs and herbal products, are often administered before racing, but it is not clear if they always work.

For horse riders, de Cocq offered this advice: "Riders should pay attention to the signals a horse can give. For example, 'cold back' (when a horse stiffens its back or negatively reacts after bearing weight), problems with saddling, girthing and mounting the horse may be an indication that there are problems with the saddle or the riding technique."

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