It takes enormous lumbar strength for a horse to carry itself in an uphill frame with a rider on its back for any length of time. Asking for too much too soon can cause tension, resistance, and resentment. Allow frequent stretching breaks.
In a good, productive stretch - active energy travels over the horse's topline and into the rider's hands. If you don't feel at least a little power coming from behind up into your connection, your stretch is not correct!
“You need to produce a walk. So work at the walk as well as the trot and canter. Get a good feeling of the body working in the walk. The walk is a mirror of the training of the horse.” ~ Christoph Hess
In the sport of Eventing, ensuring that our horses are truly fit for our level of competition is one of the best ways to prevent unnecessary injuries.
When riding Dressage, we want our horses to reach and push forward through the neck into the rein connection. This happens as a result of the horse's back engaging and lifting... similar to a nice round bascule over a jump.