How To Train A Horse to Stop Cross Cantering On The Lunge Line

When I lunge my horse at the canter, he continually cross canters off and on throughout the entire session. Could this be a soundness issue? I have owned him for just over a year and he has not been lame.

Well, this certainly could be a soundness issue and I believe the best way to answer that would be to have a vet take a look at the problem and evaluate your soundness question.

On the other hand it may be just a habitual issue. I have also had a few quite sound horses that have just developed this habit on the lunge line in small circles. Increase the size of the circle as much as practically possible and that should be some help.

You might try this as well: Slowly educate your horse to accept an additional lunge line attached to the outside of the bit, through the ring of the surcingle, and behind his hindquarters to your hand. For example, going to the left, the line would be around the right side, behind to your right hand while you hold the other lunge line in your left hand. Take this very slowly until your horse gets used and comfortable with the line being behind his hindquarters. Once he is comfortable walking and trotting on the circle, begin the canter work. This will give you some added control of his hind end and to some degree replace what your outside leg, displaced slightly back behind the girth, would be doing. Controlling the haunch. Horses sometimes get this habit from swinging out their hind end and this may be a help in preventing this. 
Hope this helps.