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Riding a horse involves coordination, balance, strength and patience. As you progress, you work towards higher goals and raise your expectations of you and your horse. suddenly, you can feel your horse making decisions, changing balance, speed and focus, and you never noticed these things before. One of the biggest transitional changes if the use of the outside rein.
In beginning riding lessons, we learn to look left and open the left rein to turn left. Simple, right? As you progress, you work towards riding the horse underneath you instead of just pointing the horse's nose where they need to go.
It's about the whole horse
One you have mastered cantering and some small jumps, its time to re-evaluate how the horse moves. In order to carry a rider, a horse needs to have balance. We can assist their balance by driving them into a turn rather than pulling them into a turn. We begin to learn to use the outside aids. First you apply an outside leg pressure, to encourage and push them, drive them, where you want to go. For some horses that is all that is needed. Most lesson horses, however, have learned to "tune out" some commands, in case it was a mistake on the novice rider's part. For horses that have "tuned out" the outside leg, they need an additional aid. We see these extra-wide turns and call this "bulging out"
A horse that bulges isn't naughty, simply unbalanced and not focused and directed
To prevent bulging out, use an outside, bearing rein. Often in a bulging horse, we see the rider's outside hand shift forward, similar to a small turn made while riding a bicycle. This really encourages bulging... when you release a rein aid, the energy and forward motion can only escape in that direction. We don't want the horse's energy to escape out towards their outside front leg. This would mean an imbalance. Instead we need to capture that energy. Sit up, drive the horse forward and collect the energy with an outside bearing rein, which is pressure straight back towards your outside hip. This creates a wall of pressure on the outside of their body, connecting the driving outside leg with a collecting outside hand, so that the horse engages that entire side of their body.