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Loss of balance and falling are problems that affect individuals with a wide range of diagnoses. Physical therapists commonly evaluate balance and use balance training/exercises as either primary or secondary interventions for patients undergoing many types of rehabilitation programs. The purpose of this chapter is to present an overview of key background terms and concepts related to balance, how balance control is normally achieved in humans for a variety of conditions, possible causes of balance impairments, and evidence-based assessments and interventions for enhancing all aspects of an individual's balance control.

Background and Concepts

Balance: Key Terms and Definitions

Balance, or postural stability, is a generic term used to describe the dynamic process by which the body's position is maintained in equilibrium. Equilibrium means that the body is either at rest (static equilibrium) or in steady-state motion (dynamic equilibrium). Balance is greatest when the body's center of mass (COM) or center of gravity (COG) is maintained over its base of support (BOS).

Center of mass. The COM is a point that corresponds to the center of the total body mass and is the point at which the body is in perfect equilibrium. It is determined by finding the weighted average of the COM of each body segment.15

Center of gravity. The COG refers to the vertical projection of the COM to the ground. In the anatomical position, the COG of most adult humans is located slightly anterior to the second sacral vertebra,15 or approximately 55% of a person's height.63

Momentum. Momentum is the product of mass times velocity. Linear momentum relates to the velocity of the body along a straight path, for example, in the sagittal or transverse planes. Angular momentum relates to the rotational velocity of the body.

Base of support. The BOS is defined as the perimeter of the contact area between the body and its support surface; foot placement alters the BOS and changes a person's postural stability.118 A wide stance, such as is seen with many elderly individuals, increases stability, whereas a narrow BOS, such as tandem stance or walking, reduces it. As long as a person maintains the COG within the limits of the BOS, referred to as the limits of stability, he or she does not fall.

Limits of stability. "Limits of stability" refers to the sway boundaries in which an individual can maintain equilibrium without changing his or her BOS (Fig. 8.1).118 These boundaries are constantly changing depending on the task, the individual's biomechanics, and aspects of the environment.159 For example, the limits of stability for a person during quiet stance is the area encompassed by the outer edges of the feet in contact with the ground. Any deviations in the body's COM position relative to this boundary are corrected ...

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