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How to keep a horse calm while riding 
Take several deep breaths before you get into the saddle. 
Approaching the horse calmly, as a rider, can also help to keep the horse calm. Before you mount the horse, take several deep, long breaths.

Available options: +This will slow down your heart rate, which will also help your horse's heart rate to remain steady and calm. Talk to the horse and stroke him gently.; +Then try to : Put the horse's right heel onto the front knee. Gently align your right foot with the horse's left heel.; +Open your stirrups to the sides to get into the saddle. Change your right leg over the left if necessary to get in the saddle safely.; +When you seat the horse in the saddle, keep your seat stable. Hang your handbag or sling on the side of the seat and allow the horse to negotiate.;
This will slow down your heart rate, which will also help your horse's heart rate to remain steady and calm. Talk to the horse and stroke him gently.