Some lameness in horses is not due to pain as some are related to mechanical problems and others are related to nervous system disorders, such as nerve root pain affecting the use of a limb.  Dr. Richard Mansmann indicates in Sporthorsemedicine.com that there is also a condition called rein lameness that is a gait irregularity in which the cause is usually from resistance, not pain.  This condition is best seen and usually only seen when ridden.  The lameness appears as a straight-forward front leg lameness or possibly a mixed front and rear leg lameness; sometime it is called a cat trot.  The gait irregularity is as if the horse is trying to break into a canter from a trot but falls back into a trot while it resists going forward.  When the rider continues, the horse will raise its head as it raises the lead front leg up, making it appear lame.  The horse will then come back to a trot before actually breaking into a canter. 

Early in the syndrome, other people may ride the horse and the horse will be normal and may be sound on the lone line with no rein pressure.  However, Dr. Mansmann says that as the rider continues to ride the horse, the gait becomes imprinted in the horse’s mind and can be difficult to retrain, especially with that specific rider.  Sometimes there may be a history of pain in the past and now the pain is gone yet the irregular gait continues.  Rein lameness is a difficult diagnosis because the veterinarian has to determine if the horse is in physical pain, if it is a mental issue for the horse, or if is it due to the rider.  A regular lameness exam with nerve blocks, a trial of a pain reliever like bute, and trying other riders are all options to diagnose the condition.  If your horse has an abnormal gait that does not respond to rest, call your veterinarian.