A horse's hamstrings are a group of muscles that extend the horse's hip, let the horse kick and help the horse move to the side. An injury to the hamstrings affects the horse's gait, its ability to move laterally and causes discomfort.
Just as in humans, horses can pull hamstrings if these muscles aren't warmed up using slow exercises before beginning harder or faster work. Injuries also occur from not varying the work the horse is performing. Other causes of hamstring injuries are slipping on a washroom floor or in a trailer, straining up a hill or over a jump or tripping on an uneven surface.
One of the strongest clues that your horse has a hamstring injury is limping. The limping takes the form of a staggered forward walk where the horse tries to put more weight and spend more time on the uninjured legs. Other signs are a decrease in activity, refusal to stand on the injured leg and warmth and swelling of the injured leg. Veterinary technology uses thermographic readings to determine the extent of an injury.
The best care for a horse with an injured hamstring is to let it rest. This doesn't mean complete inactivity, but letting the horse roam the arena or pasture at its own pace without a saddle or rider. Icing the injury for not more than 15 minutes three times a day can ease discomfort and swelling. Massage will stimulate blood flow to the injury and also make the horse feel better, but avoid massage until the injury is no longer tender (about three days).