Measurement of muscle performance is a key element of the examination of patients undergoing rehabilitation.1 Although several options are available for measuring muscle performance, the portability and objectivity of handheld dynamometry (HHD) render it a compelling option. The availability of norms2 contributes further to the usefulness of HHD. There is considerable research on the reliability and validity of HHD.3–5 Only a few papers, however, have addressed the responsiveness of measurements of muscle strength obtained using HHD. One of the papers, a systematic review, identified 13 articles that included the data necessary to calculate effect sizes.6 The 79 effect sizes calculated from the data ranged from 0 to 1.36. Nineteen percent of the effect sizes qualified as medium and 11.4% qualified as large. Another systematic review involved calculating minimal detectable changes (MDCs) from data available in 5 studies, 4 of which involved patients.7 The MDCs from the 4 studies involving patients varied considerably (range, 20.5–92.1 N). In a third article, Bohannon8 determined a minimal clinically important difference (MCID) for measures of knee extension force obtained from home care patients. Based on a transition from dependence to independence in the sit-to-stand maneuver, an MCID of 83 N was determined. More and better information than the aforementioned is required if clinicians are to rightly determine whether differences in measurements over time represent “real” changes, as opposed to normal variability.9 Particularly needed is information on MDCs obtained from larger samples of older adult patients. The purpose of this study, therefore, was to describe the responsiveness of HHD by determining MDCs for measurements of knee extension force obtained by HHD from older adult patients in 2 different settings.
This study had 2 components. The first component was based on the assumption that data from multiple studies provide a better estimate than data from a single study. It involved the consolidation of intraclass correlation coefficients (ICCs) from 3 studies describing the test-retest reliability of measurements of knee extension force obtained by the author using HHD.6–8 A weighted mean ICC was determined from 6 ICCs reported in the studies. This involved multiplying the ICCs by the sample sizes in each study, adding these products together, and dividing by the total sample size from all of the studies.10 The second component involved the retrospective retrieval of data from 2 sources. The first source was a database used to examine the relationship between knee extension strength and sit-to-stand performances among acute rehabilitation inpatients.11 That study was approved by the institutional review committee of Hartford Hospital. The second source was a database generated from the initial therapy notes of patients seen by the author in a home care setting. Development and use of the database were approved by the institutional review board of the University of Connecticut. Information retrieved from the databases included basic descriptors of the patients (ie, primary diagnosis, age, gender, height, weight) and measurements of left and right knee extension force obtained by the author with an Ametek (acute rehabilitation) or MicroFET (home care) handheld dynamometer. All force measurements were acquired using isometric make tests, while patients were seated with their legs vertical and the dynamometer was applied perpendicular to the leg just proximal to the malleoli. The mean and standard deviation (SD) of the knee extension force measurements procured at each of the 2 settings were then calculated and used to determine the MDC for each lower limb (MDC95% = [SD × √1 − ICC] × [1.96 × √2]).
Table 1 provides a summary of the patients tested in acute rehabilitation (n = 53) and home care (n = 46) settings. All were older than 50 years. There were more women than men. They presented with a variety of diagnoses, but stroke, cardiovascular, and cancer were the most common. Table 2 summarizes the studies from which ICCs were obtained. The ICCs ranged from 0.84 to 0.95; the weighted mean ICCs calculated from the studies was 0.90. Table 3 reports the means and SDs calculated for knee extension forces from the 2 databases. Table 2 also reports the MDC calculated for each knee. The MDCs ranged from 46.0 to 79.0 N.
This study reports MDC values for knee extension force measured by HHD from 2 heterogeneous samples of patients examined in 2 distinct settings. The MDC values of this study are in the range of those derived from 4 studies of patients (ie, 20.5–92.1 N).8 Given the contributors to the equation for MDC, differences in MDCs should be expected when they are determined for different subject pools. The MDC is influenced by the SD of contributing measurements, which in turn is affected by both the variability of the measurements and sample size. In the present study employing heterogeneous samples, the standard deviations were large and greater for the patients measured in an acute rehabilitation setting than for patients measured in a home care setting. This might be expected as patients able to manage at home are probably more alike functionally than patients in acute rehabilitation whose conditions preclude immediate, or for some even postrehabilitation, discharge home. The MDC is also influenced by the ICC assumed for repeated measures. As the same ICC (0.90) was used to calculate the MDC of measurements in both settings, it follows that the MDC would be higher in the acute rehabilitation setting. Regardless of setting, assuming an ICC of 0.90, a measurement of knee extension force obtained at a later time would have to be at least 46.0 N (10.3 lb) greater than a measure obtained earlier to confidently conclude that a real improvement had occurred. If the lowest ICC (ie, 0.84) used in calculating the weighted mean ICC in this study had been used, the MDC would have ranged from 51.0 to 99.8 N. If the highest ICC (ie, 0.95) used in calculating the weighted mean ICC in this study had been used, the MDC would have ranged from 32.5 to 55.9 N. Whether calculated using the weighted mean ICC or another ICC, the MDCs might be viewed as substantial. They are, nevertheless, much smaller than the change in force accompanying transitions between higher manual muscle test grades (ie, 4, 4+, 5) of knee extension force.12
In addition to the issue of heterogeneity, previously addressed, 3 limitations should be considered in interpreting the results of this study. The first is the populations studied. The populations used in this study involved older patients. Healthy older adults may have MDCs that are quite different. The second limitation is this study's focus on a single muscle group, that is, the knee extensors. The MDCs calculated in this study for the knee extensors may not generalize to other muscle groups. The third limitation of this study is its focus on one measure of responsiveness. Although the MDC does assist the clinician in ascertaining whether an encountered change is real, it does not indicate whether the change is clinically meaningful. Knowing the MCID of a measure may be more important than having a sense of its MDC. Determination of an MCID was not possible in this study, given the information available in the databases used.
The MDCs for knee extension force measurements obtained by HHD in 2 settings ranged from 46.0 to 79.0 N. These values would need to be surpassed if a clinician is to confidently conclude that a real change in knee extension force has occurred between test sessions.
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