This one is very closely related to a question that came into my inbox, so if you are reading this you know who you are :-).
Ordinarily most riding horses learn to turn left or right through pressure and release (or negative reinforcement). Early on in breaking in the rider opens the rein in the desired direction, causing pressure to increase on the bridle. When the young horse makes the correct turn the pressure is released and the young horse learns to repeat the behaviour, and form a habit of turning in response to lighter aids that indicate the pressure will increase if they don’t turn. But, if the young horse, or even the older one, is in anyway emotionally aroused, unbalanced (due to their own lack of gymnastic ability, and/or their rider’s poor posture) or anticipating pain (present or past), then the usual order of events may fail to come about.
The key to pressure and release or negative reinforcement is that the behaviour coinciding with the release is that which is reinforced and repeated! In a horse more motivated to regain balance, escape a greater fear, reach a greater reward, or escape from pain than to relieve some bridle pressure, then they may well run through the increased rein contact and get reinforced for a behaviour other than making the turn. That can very quickly become the habit, in response to strong negative emotions triggered by the increased bridle pressure. The horse will also become alert to other warning signals in the environment that predict that strong bridle pressures will occur, and so can become wound up in advance, primed and ready to perform escape and avoidance behaviour.
The keys to rectifying such issues are to eliminate pain through veterinary examination and necessary treatment. It is often advantageous to use physiotherapy (ACPAT) to help further reduce pain, to re-educate balance and movement, and have a positive impact in the musculoskeletal system. Making sure you have a good seat and balance will also help as in our frustration to get a response we can unbalance the horse and help maintain the problem.
It will of course, also help this to change the horse’s behavioural and emotional responses to the bridle and your aids; emotional state and arousal always impacts posture, and not always in desirable, healthy ways. One place to begin is with the horse in a calm state, and to practice using the aids in a way that enables the horse to respond softly and easily until this way of responding is more habitual. The next step might be to add an increase in challenge, something that causes more excitement to the horse such as a faster gait, a change of location, or inclusion on a jump. The idea is to introduce the challenge while the horse is calm and responsive, practise the new way of responding in the presence of the challenge such as one turn over the jump and go back to schooling to maintain or regain calm before jumping again. This way the horse begins to learn that the jump (or other challenge to their emotional state) comes when they are calmer, and that they are returned to this quieter way of going before being challenged again. This helps to prevent the horse from over arousal and going over the threshold that drives them to more extreme escape and avoidance behaviour.
Using this approach helps with emotional management, but still uses pressure and release as the primary motivation for compliance with the rider’s aids. This can mean that there’s not a lot in it for the horse, and can leave them still feeling negative emotions, even if they are calmer ones. The addition of positive reinforcement (a reward such as scratching and/or titbit) can improve their motivation to perform and change their way of going to something that is more optimistic and less defensive. For steering and contact related issues, rewarding the horse when they are relaxed and in the channel of your aids is one approach, target training your horse and having an assistant call them to their target for a reward (even over a jump) can seriously increase their motivation to pay attention and respond to your signals provided they are called to the target as they (attempt) to respond to you. The net result of this is a new and optimistic association with the ridden situation, and experience of positive emotions helping to improve the performance and the horse and rider experience.