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