Driving rein hand position, or if you are riding in a double, hold your reins Fillis style. If you can’t manage driving rein position, sometimes even changing to holding the reins outside your pinky instead of between pinky and ring finger can be enough of a different sensation to get you to concentrate on listening and communicating better with your horse and with your own body.
I would also say to use the giving the rein exercises from the tests, but also, alternate giving of the reins. Do it everywhere, and several repetitions in a row for at least 2 footfalls per give. Left, right, left, right.
Warm up your canter in jumping seat - sometimes our arm tension comes from a horse’s back tension and if we allow them a little more forward and bouncing while giving their back “room” to come up under the saddle in the warm up it can help.
Make plenty of counter-flexions in warm up and whenever you feel your tightness.
Finally, but sure that your tightness in your arms isn’t coming from a horse who is not listening to lateral leg aids. That is a HUGE one. Horse shuts off to proper bending/sideways aids and we get tight because, omg, weight in hands, hands not allowed to cross withers, aaaaaaaaaccccckkkkk! <- that’s all usually subconscious, but generally how it goes after you analyze it. If this is the case, after your 10min loose rein walk, and before you start your trot warm up, spend a chunk of time on lateral work at the walk. It will help, I promise.
Remember that your tightness and your horse are linked no matter who started it, and worse, it’s your responsibility to fix it, because horses can become accustomed to just about anything they are allowed to become accustomed to … which means you might be in for a challenge if the horse has been allowed by previous owner to remain “locked in” on autopilot, not sufficiently use its abs, become dead to the lateral legs aids, etc.
