It’s all about movement
I believe there is a real danger that riders perceive the meaning of the word outline to be something that is static. In my opinion this needs this to be revised because movement is by definition not static — in order for your horse to go in any profile you need mobility and flexibility in his muscles and joints.
Some other commonly used terms are similarly confusing, such as ‘ride with a consistent rein contact’. This gives the impression that the weight of the rein should never vary, whereas of course it should absolutely vary. Your horse has to be able to move his tongue and neck vertebral joints and other parts of his body in order to find relief from the rein or leg pressure. We are talking about influencing living tissue, and if your rein or leg contact is static you will be inhibiting him both physical and mentally.
Brain function
Let’s go one step deeper into how to achieve this desired micro mobility or flexibility. It is actually only your horse that can create this. You as his rider can only induce him to function in that way. It is part of his voluntary movement system, which is a result of cognitive function.
For example, it’s not about us as riders physically forcing his neck to flex left or right, it’s about inducing your horse to adopt that action on cue. In order to induce him to do that willingly and repeatedly it will be important that he discovers that when he adopts that specific movement, he is rewarded by a certain amount of pressure reduction. That will be his incentive to respond positively in the future, and is the basis of how ridden horses learn.
Understanding how your horse learns and the biomechanics of voluntary movement will help your training methods become more effective on both a mental and a physical level.
Creating flexibility
I use the term flexible rather than supple because that describes what you want your horse’s joints to do — they flex and extend. I think the word supple is more
related to muscle tone, but this is also important as without it then joints can’t function optimally. This highlights why you must start thinking of all the systems involved in movement.
Walk is a useful gait in which to work your horse to initiate various degrees of the different flexions or stretches. Not only is there a wide range of natural movement in the walk, but as it’s slower it is easier for your horse to appreciate and find a release response, which is critical for his reward. Consistent repetitions of each flexion increase the probability that your horse will embed this release in his memory as a form of reward.
The canter also offers a nice specific lumbar-sacral (behind the saddle) action, boosting his all-round flexibility.