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THE CONTROL OF POSTURE INVOLVES TWO INTERRELATED GOALS, equilibrium (balance) and orientation, crucial for most tasks of daily living. Balance control maintains the body in stable equilibrium to avoid falls. Orientation aligns the body segments with respect to each other and to the world, such as maintaining the head vertical. Both balance and orientation use several different types of control: automatic postural responses, anticipatory postural adjustments, postural sway in stance, sensory integration for a body schema, orientation to vertical, and dynamic stability during gait.

To appreciate the complexity of maintaining balance and orientation, imagine that you are waiting tables on a tour boat. You have a tray full of drinks to be delivered to a table on the other side of the rolling deck. Even as your mind is occupied with remembering customer orders, unconscious but complex sensorimotor processes for controlling postural orientation and balance allow you to move about in an efficient and coordinated manner without falling. As you cross the rolling deck, your brain rapidly integrates and interprets sensory information and adjusts motor output to maintain your balance and the upright orientation of your head and trunk, as well as stabilize the arm supporting the tray of full glasses. Sudden unexpected motions of the boat evoke automatic postural responses that prevent falls. Before you reach out to place a glass on the table, your nervous system makes anticipatory postural adjustments to maintain your balance.

Somatosensory, vestibular, and visual information are integrated to provide a coherent sense of the position and velocity of the body in space with respect to the support surface, gravity, and visual environment. Since the surface is unstable and vision is not providing earth-stable information, your dependence on vestibular information is greater than usual. Your head is kept stable while your trunk motions and walking pattern adjust for disequilibrium caused by the moving surface. You notice that both your voluntary tasks and your balance control deteriorate when trying to attend to both goals.

Equilibrium and Orientation Underlie Posture Control

Postural equilibrium refers to the ability to actively stabilize the upper body by resisting external forces acting on the body. Although the dominant external force affecting equilibrium on earth is gravity, other inertial forces and external perturbations must also be resisted. Depending on the particular task or behavior, different sets of muscles are activated in response to or in anticipation of disturbance to equilibrium.

Postural orientation refers to the ability to actively align body segments, such as the trunk and head, with respect to each other and to the environment. Depending on the particular task or behavior, body segments may be aligned with respect to gravitational vertical, visual vertical, or the support surface. For example, when skiing downhill, the head may be oriented to gravitational and inertial vertical, but not to the visual or support surface references that ...

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