According to the World Health Organization (WHO) 1980 definition, disability consists in any restriction or lack of ability (resulting from impairment) to perform an activity in the manner or within the range considered normal for a human being . The new WHO model (2001) gives to the term disability a broader, more general connotation, but nonetheless confirms the importance of activities as the constitutive elements of a person's functioning . Disability and activity are viewed in relation to the person as a whole. For example, a heart condition (according to the 1980 WHO model) might be defined as a lessening (or malfunctioning) of one body part. Difficulty going up stairs likewise represents a deficit with respect to an activity (since a whole person, only, can climb stairs). A deficit in one or more activities defines a malfunctioning of the whole person and is therefore called a disability. Usually, the definition of function is misunderstood. In the context of rehabilitative medicine, a recent definition ,  - namely, energy or information exchange - is useful. For although this definition is general, it makes an important distinction between physiologic functions (breathing, nerve conduction, etc.), and functions that take place between the person and environment - in other words, activities.
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