How do I train a horse to accept unusual rider movements and behaviors during sessions?

Therapeutic riding participants use their horses' backs in ways that differ significantly from conventional riding, and a horse that has only been ridden by able-bodied riders in conventional positions may react to the unusual sensations, movements, and behaviors of therapeutic riding participants before it has been specifically prepared to accept them. Participants may exercise their arms and legs across the horse's back, lie forward or sideways along the horse's neck and back, make unexpected sounds, move suddenly and unpredictably, and interact with the horse in ways that no conventional riding lesson would produce. A systematic approach to unusual position desensitization begins with a knowledgeable, experienced rider deliberately creating the types of unusual movement that therapeutic riding participants produce — lying forward along the neck, extending arms sideways, sitting sideways, leaning back toward the croup — while the horse is at a walk in a controlled environment. The horse's response to each unusual position is observed and the position is maintained or removed based on whether the horse is processing it calmly. Sudden sounds and movements are perhaps the most challenging aspect to prepare for, because they cannot be entirely anticipated or eliminated in session. A horse that has been desensitized to sudden, loud vocalizations, unexpected arm movements, and the weight shifts of a rider losing balance briefly is a horse that will respond to these events with calmness rather than alarm. Building that broad acceptance requires deliberate, varied desensitization work that covers as many of the possible participant behaviors as the trainer can anticipate and simulate.