Posture is the position of a person's body while standing or sitting. Posture is determined by the person's ability to maintain balance against the force of gravity, which constantly pulls the body downward. The action of certain muscles keeps the body upright. These muscles include those that keep the back, hips, and knees from bending. Because of the constant give and take of the pull of gravity and of muscle action, posture is a dynamic, ever-changing state.