Riders come in all shapes and sizes, levels of ability, and physical and mental challenges, and are generally very focused on their sport. Riding, in particular dressage, has particular demands on the body. Many riders have suffered a number of crashing falls, broken bones, haematomas, which each lead to compensating patterns, which in turn easily build into their normal habits.
Many people confuse position with posture. Your posture is an ongoing dynamic process that expresses itself in the way you stand, sit, or lie on the floor. In other words, posture expresses itself in every position of the body.This is why people tend to have tension in the same muscle groups regardless of the position they are in. If you observe your posture on the floor, you will notice basic postural tendencies that are expressed in any position.
Taken from Frank Wildman’s book The Busy Person’s Guide to Easier Movement