Physical Function is defined as one's ability to carry out various activities, ranging from self-care (activities of daily living) to more challenging and vigorous activities that require increasing degrees of mobility, strength or endurance. Measuring physical function is important as physical function often serves as an indicator of current health status and may predict future health and other outcomes. Currently there is no singular gold standard for the assessment of physical function.
Measurement of physical function is complex as it contains multi-dimensional constructs. A range of both self-report and performance-based measures have been used to assess physical function. Performance-based measures of physical function are defined as assessor-observed measures of a subject's performance of specific physical tasks, typically measured in controlled environments. These measures typically involve the completion or timing of basic strength, balance, or mobility tasks. Performance-based measures, as a full battery of tests, or gait speed alone predict subsequent disability.
Selecting a Performance-Based Measure:
Performance-based measures of physical function should be chosen based on whether they were designed and have been used with people similar to the people to be measured. In other words, the measure that you chose should be appropriate for your target population. In addition, one must be cognizant of limitations in measurements, such as ceiling or floor effects. These issues can usually be avoided by selecting measures that have been demonstrated to provide meaningful information about people who are similar to those being measured. Ceiling or floor effects, meaning a large number of individuals receive the maximum or minimum score limit the ability of a test user to show change. Measures for which a sizable proportion of those measured perform at the ceiling or floor level typically fail to provide meaningful information. Other issues to consider are practicality of the instrument, methods of scoring, and format of the results of reported score.
The population of interest is extremely important when deciding which performance-based measure to use. It is important that measurement tool be chosen based on whether it was designed and has been used with people similar to the people to be measured.
Face validity indicates whether a measure appears to have been designed to measure what it is supposed to measure. With regard, to physical function, face validity, while contributing to the validity of the data obtained with a measure, is not represented by the outcome of a statistical test but the judgment of the tester that the measure has been used under similar conditions of measurement.
Practicality should be considered when choosing a test. Factors to consider are: (1) the time needed to administer the test, (2) the experience needed by the person administering the test (eg, professional or technical), (3) whether administering the test requires prior experience or formal training, (4) the equipment needed, (5) the format of the test (self-report or performance based), (6) the method of scoring (eg, manually or computer-assisted), and (7) the format of the resulting measurement.
For more information on selecting a proper physical function assessment tool, please see:
Van Swearingen JM, Brach JS. Making geriatric assessment work: selecting useful measures. Physical Therapy. 2001;81: 1233-1252.