Answer

By Your Horse

Dressage Rider and Trainer Claire Lilley says:

It sounds as though your horse is very weak through his back. Horses often react to tension in their back muscles by either wanting to wee or poo. They will stand with their hindlegs out behind them asthough they are going to wee, so maybe your horse is doing this to ease his aching back.
If your horse has a history of lumbar problems, spend time lungeing him and working him in-hand to strengthen his back muscles before you start ridden work again. Start by working him in-hand, then you can begin to lunge him in walk and a steady trot. As he gets stronger, try walking him over single poles around the school. Once he can cope with this, trot him over the poles. When he can do this comfortably he should start to stretch his neck forwards and downwards. If he can’t, or has the wrong muscles – thick under the neck, a fat tummy and no muscles on his haunches – then a chambon may help him to get the idea.
It will take about a month to get the first signs that his back muscles are building. You’ll see long band-like muscles twitching along his back under where the saddle will go. You need to be patient and not ride him until he has the right muscles. If you get any adverse reactions it may well be a sign that he’s in pain, so you’ll need to consult your vet.