The result of the study showed a high correlation between tests with a mean difference of 0.03 m/s. To our knowledge, no studies have evaluated test-retest reliability for maximal walking speed in this target group. According to previous studies, ICC values for similar target groups vary between 0.79 and 0.94 for habitual walking speed.28,29 One study has evaluated the reliability of maximal walking speed on a GaitRite showing an ICC of 0.97; the tests were, however, performed during the same day.30 Taken together, the difference in measurement, distance, and speed limits comparability.
ICC is a relative measure of variation within subject in relation to the variation between subjects and takes into account the systematic error and is the recommended choice in these types of analyses.24 Absolute reliability has been suggested to be measured with standard error of measurement31 or Bland and Altman's “95% limits of agreement25,32 because it shows the difference between the 2 measurements. The advantage with Bland and Altman's method is that it also calculates the standard deviation of the difference between measurements. In this study, the Bland and Altman's analyses showed variance of −0.33 to +0.27 m/s. This may seem elevated, but the frail subjects' day-to-day variability must be considered. In addition, the performance of maximal walking speed has been shown to be related to several factors such as age, gender, leg extensor power, standing balance, and physical activity.33
Another explanation of the difference between the 2 measurements could be due to the instructions from the test leader. When the test leader encouraged the subject to walk as fast as possible an initial reduction of walking speed was in some cases observed, but maximum speed was soon reached. This effect should be considered when performing the test. Also, the subjects included in this study were frail with several diagnoses and many of them were dependent on personal activities of daily living. This might influence each subject's daily condition and thus also the differences between the 2 test occasions. Similar findings have been reported in a study evaluating the reliability of Timed Up and Go. The study showed that the slower the subjects performed the test, the more the variance increased.34 Another explanation of the difference between tests 1 and 2 could be the instructor's tone of voice, body language, and feedback.
In health care and rehabilitation settings, knowledge of an older person's physical performance is of utmost importance to deliver appropriate care. The emphasis on individual targeted exercise programs for frail older people19 requires valid and reliable instruments. The variance of −0.33/+0.27 m/s needs therefore to be taken into account if the test is used to evaluate, for example, a physical exercise program. A change within the limits cannot be regarded as a true change. This is of importance both in a clinical setting as well as in randomized controlled trials. Another clinical implication is that walking speed has been suggested to be used as a functional “vital sign” to determine outcomes such as functional capacity, discharge location, and the need for rehabilitation.35
The major limitation of the study is the rather small sample size in relation to the analyses of 95% limits of agreement. According to Altman,36 the sample size should be large enough to allow the limits of agreement to be estimated well; otherwise, there might be a risk of too much variance in ranges. Thus a sample size of at least 50 but preferably larger is desirable.36 This might also explain the results of this study.
Maximum walking speed tests in institution-dwelling people aged 74 years and older, with several different diagnoses, shows high reliability. The method is easy to perform in a clinical setting at a minimal cost. The method can be recommended for use in this group before and after a training period. However, the mean difference −0.03 m/s and Bland and Altman's 95% limits of agreement of −0.33 to +0.27 m/s needs to be taken into account when evaluating the effect of a training period.
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evaluation; physical function; physical performance; test-retest; screening