Some horses appear to be sensitive over the hip area. Multiple causes are suspected, including arthritis in the hip area and pain in the lower legs that cause abnormal movement of the back and hips. Riders of these horses with pain on pressure over the hips also reported performance problems, resistance to the rider, and a stiff or irregular gait. Several equine veterinarians at North Carolina State felt it was possible that the length of the horse's toe on the hind feet could be another cause of pain over the hip. Horses with long toes also usually have low heels that are crushed or collapsed. This causes the load bearing area to be further forward of the center of the rear legs. Because of this, horses stand with their rear feet further forward than normal.
Another way of looking at this is the horse stands with the back feet standing up under themselves instead of straight and this also gives the horse a sickle hock appearance. Because of this stance, the muscles of the entire limb are pulled abnormally and can affect the horse's posture and movement. To prove this theory, the vets at North Carolina State performed a study that checked for pain over the horse's hip area and compared that to toe length of the rear feet. It was found that horses with hip pain had longer toes in the rear feet than horses without hip pain, and that pain could be decreased fairly quickly by trimming the toes shorter. X-rays are required to determine the distance the toes need to be trimmed, but in general most light breed adult horses should not have toes longer than 20mm past the apex of the frog. So in most cases if you look at the soles of your horse's rear feet, the end of the toe should not be over 20mm from the apex of the frog.