Basic terminology

Balance, a skill required to maintain upright posture, has both orientation and stability components.

Figure 8.1. Illustration of the CoM and BoS in normal standing (a) and when using an assistive device (b).

Figure 8.1. Illustration of the CoM and BoS in normal standing (a) and when using an assistive device (b).

Orientation involves interpreting sensory information and using it to maintain the appropriate alignment of the body with respect to the environment and the task. The body may be aligned using several references, including gravity, the support surface, the visual environment, and internal representations. Postural stability, or equilibrium, involves controlling the center of body mass (CoM) relative to the base of support (BoS) to resist perturbations or allow functional movement. CoM refers to the point at which the entire mass of the body may be considered to be concentrated (Winter, 1990). BoS is the region bounded by the points of contact between body segments and the support surface. For example, during unsupported stance, the BoS consists of an area under the base of foot support and when the subject is using an assistive device, the BoS includes the combined area under both the assistive device and the feet (Fig. 8.1). For static equilibrium, the

CoM must be kept over the BoS. The boundaries of the region in which the CoM can be moved safely without changing the BoS are referred to as the limits of stability. For dynamic equilibrium, such as during walking, the CoM must be controlled both within and outside a changing BoS (Winter, 1990). During dynamic tasks such as locomotion or arising from a chair, the limits of stability depend on the momentum, as well as the position, of the body CoM (Pai et al., 1994).

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