Shoulder-in is a basic work exercise done on two tracks. It can be done at three different paces. It's an excellent exercise for helping the horse become more supple by engaging the hind legs in a relaxed way. Shoulder-in is the happy medium between circle work and straight line work. It helps to solve a lot of problems found in horse riding.
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At the beginning, the simplest is to start off with shoulder-fore. Stand in the track; your horse should have the tip of its nose pointed towards the outside and its hips on the inside. When first starting, it can be helpful to use a wall or barrier in order to use the legs and hands less and thereby help the horse learn the exercise as calmly as possible. The outer hand should slightly spread apart while the inner leg is against the strap and pushing slightly. Your horse should not accelerate or rush. The best is to start at a walk, then to repeat the exercise at a trot once the horse has learned it. This exercise is performed with both hands.
To take the exercise further, after finishing your straight line with shoulder-fore, continue in a large circle, in the same fashion, fold on the outside, hips on the inside. When you finish your circle, move away from the track and try to do a shoulder-in on the inner track and see how that goes without the help of the barrier. Do this work using both hands. Once the horse has learned this, it is ready for shoulder-in work!
Start by doing one turn, with horse bent in a curve. Once your horse is in the right stance, move out of the volte while keeping the same stance as in the circle and make your horse "skid" a bit. Your inside leg against the strap controls the hips; do a slight leading rein, and control the horse with the outer rein. Be careful not to put too much of an angle or fold. The outer front leg and the inner hind leg should be aligned. Start out with a few paces, and then progressively increase the distance over which you do shoulder-in.