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RESEARCH ARTICLE |
a Epidemiology, Demography, and Biometry Program, National Institute on Aging, Bethesda, Maryland
b The Johns Hopkins University Medical Institutions, Baltimore, Maryland
c Geriatric Department "I Fraticini, " National Research Institute (INRCA), Florence, Italy
d Centers for Disease Control, National Center for Health Statistics, Hyattsville, Maryland
Eleanor M. Simonsick, Epidemiology, Demography, and Biometry Program, National Institute on Aging, 7201 Wisconsin Avenue, Suite 3C-309, Bethesda, MD 20892 E-mail: simonsie{at}gw.nia.nih.gov.
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Abstract |
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To better understand disablement and transitions from impairment to disability, discrete valid measures of functional limitation are needed. This study reports the development and criterion-related validity of scales that quantify severity of upper and lower extremity functional limitation.
Methods.
Data are from 3,635 cognitively intact community-dwelling women aged 65 years and older and 1,002 moderately to severely disabled participants in the Women's Health and Aging Study. Scales assessing severity of upper and lower extremity functional limitation were constructed from commonly available questions on functional difficulty. Criterion-related validity was evaluated with self-report and performance-based measures.
Results
The upper and lower extremity scales range from 0 to 6 and 0 to 9, respectively. Scale scores were well distributed in the disabled group and discriminated limitations in the broader community. For both scales, rates of difficulty for all ADL and IADL increased (p < .001) with increasing severity score, and percent able and mean performance on respective upper and lower extremity tasks decreased (p < .01).
Discussion.
These scales, constructed from commonly used self-report measures of function, provide discrete measures of upper and lower functional limitation. Because these scales are distinct from measures of disability and impairment, their use should facilitate increased understanding of the disablement process.
INVESTIGATING the disabling process has become a major focus of gerontological research; a conceptual framework has been developed and several complementary models of the pathway leading from pathophysiology to disability have been proposed (Fried, Herdman, Kuhn, Rubin, and Turano 1991
; Lawrence and Jette 1996; Nagi 1965, Nagi 1976, Nagi 1991; Verbrugge and Jette 1994). A common feature and key component of these models is functional limitation, the state between physical impairment and disability. Functional limitation is considered a state of high risk for disability (Fried et al. 1991; Guralnik, Ferrucci, Simonsick, Salive, and Wallace 1995; Harris, Kovar, Suzman, Kleinman, and Feldman 1989), but also one in which the greatest benefit from intervention may occur (Pope and Tarlov 1991). To more fully understand the disabling process and transitions to and from functional limitation, researchers need to (a) adopt a uniform definition of functional limitation, (b) consistently apply this definition in selecting and operationalizing measures, and (c) develop reliable and valid scales that distinguish gradations of severity of functional limitation (Clark, Stump, and Wolinsky 1997; Johnson and Wolinsky 1993; Kelly-Hayes, Jette, Wolf, D'Agostino, and Odell 1992; Pope and Tarlov 1991).
Nagi 1965, Nagi 1976 developed a model of disablement similar to the World Health Organization model (World Health Organization 1980) but that explicitly distinguishes an interim state between impairment and disability, labeled functional limitation (Nagi 1991). This conceptualization of functional limitation is accepted widely (Fried et al. 1991; Lawrence and Jette 1996; Schroll 1994; Verbrugge and Jette 1994) and has been adopted by the Institute of Medicine (Pope and Tarlov 1991). Functional limitation occurs at the level of the organism and constitutes "restriction or lack of ability to perform an action or activity in the manner or within the range considered normal that results from impairment" (Pope and Tarlov 1991, p. 79), such as lifting and carrying 25 lb; whereas disability, "inability or limitation in performing socially defined activities and roles" (Pope and Tarlov 1991, p. 79), such as shopping for groceries, occurs at the societal level. Rarely have these definitions been applied strictly in analyzing the state between physical impairment and disability. Although the term functional limitation is used frequently, the measures to which it is applied often include indicators of disability in the operational definition (e.g., Boult, Kane, Louis, Boult, and McCaffrey 1994; Ensrud et al. 1994), which prohibits examination of links between functional limitation and disability (Stuck et al. 1999). Further, substantial variation in the measurement of functional limitation restricts cross-study comparisons (Boult et al. 1994) and can lead to inconsistent results and thereby impede progress in understanding the disabling process (Johnson and Wolinsky 1993; Landerman and Fillenbaum 1997).
Researchers have used both self-report and performance-based approaches to estimate prevalence and severity of functional limitation (Guralnik, Ferrucci, et al. 1995; Harris et al. 1989; Kelly-Hayes et al. 1992; Lawrence and Jette 1996). Although performance-based measures have several advantages (Guralnik, Branch, Cummings, and Curb 1989), they can be impractical in some settings because they are time consuming, require face-to-face contact, and are seldom used exclusively. Thus, for developing summary indices of functional limitation, we have focused on self-report measures, which are more widely available in large-scale, longitudinal studies of older adults.
Self-report methods typically ascertain inability to perform, need for assistance, and/or the presence and sometimes amount of difficulty or tiredness associated with a variety of functions. These functions are distinct from basic and instrumental activities of daily living (ADL and IADL) and are typically represented by physical tasks such as climbing stairs, walking, grasping and handling, and lifting and carrying (Nagi 1976; Rosow and Breslau 1966). Even though capacity to perform these functions has been ascertained in several large-scale epidemiologic studies and national surveys (e.g., Established Populations for Epidemiologic Studies of the Elderly, U.S. Bureau of the Census Survey of Income and Program Participation, Longitudinal Study on Aging), few researchers have published attempts to develop and validate summary scales of severity of functional limitation using these or similar data (Avlund, Davidsen, and Schultz-Larsen 1995; Clark et al. 1997). Although several composite indices of physical function exist (Avlund et al. 1995; Feinstein, Josephy, and Wells 1986; Langlois et al. 1996; Laukkanen, Heikkinen, Schroll, and Kauppinen 1997), most include ADL and IADL as well as functional tasks involving both the upper and lower extremities and thus are not distinct measures of functional limitation (Johnson and Wolinsky 1993). In addition, many scales simply sum the number of tasks that present difficulty (e.g., Clark, Stump, and Wolinsky 1998; Lawrence and Jette 1996; Lichtenstein, Dhanda, Cornell, Escalante, and Hazuda 1998). Rarely has degree of difficulty with individual functions been utilized in scale construction, despite evidence that amount of difficulty is associated with increasing age and disability in ADL (Foley, Berkman, Branch, Farmer, and Wallace 1986; Guralnik, Fried, Simonsick, Kasper, and Lafferty 1995) and decreased physical activity (Simonsick, Guralnik, and Fried 1999). Simple counts can pose limitations for tracking decline over time, because persons with even minor difficulties may lie at the most severe end of the scale (Feinstein et al. 1986). Moreover, because a lot of difficulty in one area may have similar if not greater implications for function than minor difficulties in two areas, simple counts may misclassify severity of limitation.
Our primary objective in this article is to report the development of separate scales that quantify severity of functional limitation in the upper and lower extremities using commonly available self-report measures of functional difficulty. These scales are intended to have a broad range in order to detect small but potentially meaningful amounts of change (Feinstein et al. 1986). Our second objective is to establish the criterion-related validity of these scales using self-report measures of disability and performance-based measures of physical impairment, functional limitation, and disability. Because the scales are intended to capture functional limitation, we expect the scales to show a stronger association with other measures of functional limitation than with measures of disability and impairment.
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) were eligible to participate. Table 1 lists the functional domains and tasks representing each domain. More complete details of the study objectives, design, and screening approach have been published elsewhere (Guralnik, Fried, Simonsick, Kasper, and Lafferty 1995; Kasper, Shapiro, Guralnik, Bandeen-Roche, and Fried 1999). In brief, of 5,316 women selected, 4,137 (78%) were contacted and screened for eligibility. Of the screened population, 3,635 (88%) were cognitively intact and were administered the physical functioning questions. Of the total population screened, 1,409 (34%) were study eligible and 1,002 (71% of those eligible) participated by completing an interviewer-administered questionnaire and physical assessment and nurse-administered physical examination in the home.
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; Wolinsky and Johnson 1991), others have considered it, as we did here, to represent upper extremity limitation (Bild et al. 1993; Lichtenstein et al. 1998; Satariano and Ragland 1996). The lower extremity functional limitation scale uses two items from the mobility/exercise tolerance domain—walking for 1/4 mile and walking up 10 steps without resting—and two items from the baseline interview, which also represent this domain—walking across a small room and stooping, crouching, and kneeling. Note that we excluded two items in this domain from the lower extremity functional limitation scale—getting in and out of bed or chairs and doing heavy housework—because they tap dimensions of disability.
Self-report measures of disability.
As described previously, self-reported disability was ascertained in both the screening and baseline interviews. Even though most activities involve both upper and lower extremity function, for economy in analysis and presentation we categorized activities by their primary component, such that the upper extremity disability items include dressing, bathing, preparing meals, turning a key in a lock, and eating; items representing lower extremity disability include doing heavy housework, getting in and out of bed or chairs, and using the toilet; and two items represent both upper and lower extremity disability—doing light housework and shopping. To examine the criterion-related validity of the functional limitation scales, we used the disability items individually and dichotomized them as any versus no difficulty. Even though using the telephone was a screening item, we did not include it in these analyses, because it is primarily an indicator of cognitive function (Wolinsky and Johnson 1991). Because only 10 of the 1,002 WHAS participants would not have qualified for the study if using the telephone were not a screening item and 8 of these women had vision problems, we saw no need to exclude them from the analyses.
Performance-based measures of impairment, functional limitation, and disability
The interviewer-administered physical assessment and nurse-administered clinical examination included several performance-based measures of strength-related impairment, functional limitation, and disability. Detailed measurement procedures can be found in Simonsick and colleagues 1997. The upper extremity impairment measures included grip (Jamar adjustable grip hand dynamometer, model BK-7498, Fred Sammons, Inc., Burr Ridge, IL) and pinch strength (0–60-kg pinch gauge, model 81441, Adaptability, Colchester, CT). Three tests assessed participants' upper extremity functional limitation: (a) ability to lift a 10-lb water jug using both arms from the lap to eye level to up over the head, (b) capacity to fully internally and externally rotate the left and right shoulders, and (c) time needed to pick up and place 10 pegs in a pegboard with the dominant hand. Two tests captured upper extremity disability: ability and time to (a) put on and button a blouse and (b) pick up a key and open a lock. Lower extremity impairment measures included knee extensor and hip flexor strength determined from hand-held dynamometry (Nicholas Manual Muscle Tester, model BK-7454, Fred Sammons, Inc., Burr Ridge, IL). A standardized battery, consisting of measured walks, a standing balance test, and five repeated chair stands (Guralnik et al. 1994) tapped lower extremity functional limitation.
For all performance measures, participants too weak or functionally limited to perform the test safely were excluded from testing. For example, for both knee extensor and hip flexor strength measurement, participants who had undergone knee or hip surgery within the preceding 3 months and those who did not have sufficient range of motion to do the strength measures were not tested. Thus, for the strength and some of the performance measures, we report both the percentage of participants able to do the component and the mean performance of those tested. The values for grip, pinch, knee extensor, and hip flexor strength represent the average of the best trials for the right and left sides.
Scale Construction
Our objective was to create scales with good face validity as indicators of severity and a scoring range of at least 6 up to 10 points (Krosnick and Fabrigar 1997), with the scores well distributed in a disabled population. For upper extremity, we gave equal weight to the three representative functions—using fingers to grasp and handle, raising arms up over the head, and lifting and carrying 10 lb—because they bear similar importance to independent functioning and have little overlap in terms of underlying impairments. For the first step of scale construction, we used the full range of difficulty scoring (0–4) for each item and created a simple sum of the amount of difficulty reported for each task, resulting in a scale ranging from 0 to 12. Because less than 20% of the disabled women had scores above 4, in the second step we collapsed categories to yield a 7-point scale ranging from 0 to 6: (a) scores from 0 to 5 were retained, (b) scores of 6 and 7 were recoded to 5, and (c) scores from 8 to 12 were recoded to 6. In the third step, we reviewed the patterns of difficulty across the items at each severity level, then adjusted scale scoring to improve face validity. Specifically, we recoded (a) scores from 5 to 4, when difficulty was no greater than "some" on any item; (b) scores from 5 to 6, when "unable" was reported for one item and "a lot of difficulty" was reported for another; and (c) scores from 6 to 5, when "unable" was reported for one item and "some difficulty" was reported for the others. In this way, a slightly higher weight was given to a lot of difficulty or inability to do a task than to a little or some difficulty. Table 3 describes the categories in detail; the scoring algorithm is available on request.
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) even among well-functioning elderly persons (Berkman et al. 1993) and is not critical to daily functioning, we counted only inability as indicative of limitation. Table 6 describes the categories in detail; the scoring algorithm is available on request.
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) because it takes into account inability to do the test. We present the mean scores on the individual components, each ranging from 0 to 4, and the summary score, which ranges from 0 to 12. For the statistical analysis, we used the chi-square test for trend for dichotomous variables, for example, ADL difficulty or not and able to do test or not. Pearson correlation coefficients were calculated for all continuous measures, such as strength and timed performance, and Spearman correlation coefficients were calculated for ordinal variables. All analyses were performed using SASpc version 6.12 (SAS Institute, 1988). |
Results |
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). These associations are also consonant with findings from an empirical analysis of the disablement pathway (Lawrence and Jette 1996). The relatively weak relationship between scale scores and strength is supported by reports that the association between strength and functional limitation exists only below a specific threshold of weakness (Buchner, Larson, Wagner, Koepsell, and De Lateur 1996; Ferrucci et al. 1997; Rantanen et al. 1998). Severity of limitation scale scores are most strongly associated with the performance-based measures of functional limitation, as expected. For the upper extremity, the correlations are weaker (-.26 to -.34), but the actual tasks, lifting 10 lb while seated to eye level and then up over the head, full internal and external rotation of both shoulders, and placing 10 pins in a pegboard in less than 60 s, are more difficult to perform than the functions to which the self-report items refer (i.e., lifting and carrying 10 lb, raising arms up over the head, and using fingers to grasp and handle). Also, because the upper extremity scale assesses global upper extremity limitation, measures that assess hand function (e.g., grip strength and pegboard) show a less consistent decrease in performance with increasing scale score, particularly at the lower range of the scale, where severe difficulty using the fingers is rare. The magnitude of association is greatest between the lower extremity functional limitation scale and the lower extremity performance battery, with the self-report scale explaining more than 42% of the variance in the performance-based measure. Thus, it appears that self-report of functioning that uses several levels of difficulty with basic functions may be a viable substitute measure of lower extremity functional limitation when performance-based testing is not possible.
As another check on scale validity, we examined associations between the functional limitation scales and the opposing performance measures of functional limitation. Despite the fact that 70% of the disabled cohort had both an upper and a lower extremity limitation, these analyses provide moderate support for the specificity of the scales, in that (a) correlations between the upper extremity scale and lower extremity performance tests are substantially weaker than those between the lower extremity scale and lower extremity performance: -.17 versus -.49, -.18 versus -.54, and -.27 versus -.60 for standing balance, repeat chair stands, and walking speed, respectively; (b) correlations between the lower extremity scale and upper extremity performance items are weaker than those between the lower extremity scale and lower extremity performance; and (c) correlations between the upper extremity scale and upper extremity performance are generally stronger than the correlations between the upper extremity scale and lower extremity performance. However, correlations between the lower extremity scale and upper extremity performance tests, although weak, are similar to those between the upper extremity scale and upper extremity tests: .33 versus .20, -.25 versus -.26, and -.34 versus -.34 for pegboard, shoulder rotation, and 10-lb lift, respectively.
Comments on Scale Construction
The scales described in this report represent a compromise between simplicity and comprehensiveness. Although aspects of scale construction were complex, more simplistic approaches did not produce satisfactory results. For example, for the upper extremity scale, we first grouped the response options of "a little and some" (1) and "a lot and unable" (2) to produce a scale ranging from 0 to 6, the same range of the more complex scale presented in this article. The distribution of the simple scale, however, was skewed, because more than 75% of the disabled women had scores between 0 and 2 and only 5% had scores of 5 or 6. Furthermore, we found that the prevalence of difficulty in most of the ADL and IADL items more than doubled between scores of 1 and 2, using the more simplified approach to scale construction (data available on request).
As noted in the Methods section, some studies consider the lifting and carrying item as a measure of lower extremity function (Lawrence and Jette 1996; Wolinsky and Johnson 1991) and others use it to indicate upper extremity limitation (Bild et al. 1993; Lichtenstein et al. 1998; Satariano and Ragland 1996). This item is problematic because it combines activities that involve both the upper and lower extremities and it is unknown whether difficulty lifting and carrying implies difficulty with the lifting and/or holding aspect of carrying or with the walking component of carrying or both. Because factor analyses have yielded inconsistent findings (e.g., Fried, Ettinger, Lind, Newman, and Gardin 1994; Lichtenstein et al. 1998; Wolinsky and Johnson 1991), we conducted our own analysis in the disabled sample. Using principal component analysis with a varimax rotation, we identified two factors. On the first, walking 1/4 mile plus across a room; walking up 10 steps; and stooping, crouching, and kneeling loaded between .66 and .82, and lifting and carrying loaded .63. On the second factor, using fingers to grasp and handle loaded .82, raising arms up over the head loaded .75, and lifting and carrying loaded .22. However, given that factor analysis tends to group items according to their frequency distributions and not necessarily by their underlying constructs, it is unclear whether Factor 1 represents lower extremity function and Factor 2 upper extremity function or whether they represent more frequent and less frequent functional limitations, respectively. The distribution of difficulty on the upper extremity items supports the latter possibility, because 65% report difficulty lifting or carrying, in contrast to 35% for using fingers to grasp or handle and 28% for raising arms up over the head. On the lower extremity tasks, 77% report difficulty walking 1/4 mile; 52% have difficulty walking up 10 steps; and 88% have trouble stooping, crouching, and kneeling.
The WHAS has used the lifting and carrying item to represent upper extremity function in both the screening process and analyses of data for the main study. Because lower extremity function, including walking, is adequately assessed using multiple items, we also chose to use lifting and carrying as a measure of upper extremity function to reduce the likelihood of overlooking limitations in this dimension. In support of this decision, among women aged 70–79 with no difficulty walking for 1/4 mile or up 10 steps without resting, 10% report difficulty lifting or carrying 10 lb and 36% report difficulty with 20 lb.
There are three important points to make regarding the lower extremity functional limitation scale. First, the alternate scale constructed for the screened population that included only two items—walking 1/4 mile and up 10 steps—performed well in the disabled women. From 11 to 18% fell into each category from 0 to 6, with more than 17% having a score of 6. There was a steady increase in the prevalence of difficulty for each of the ADL and IADL items between 0 and 5, with a large jump in prevalence for some activities, including transferring, shopping, doing light housework, and using the toilet, between scores of 5 and 6. In terms of the performance battery, mean scores for each individual test as well as the summary scale declined steadily, as the severity of lower extremity functional limitation increased from 0 to 4, then showed a dramatic decrease between 5 and 6 (e.g., from 5.0 to 2.7 for the summary performance scale score; data available on request).
Although the alternate scale showed promise, it did not adequately discriminate severity in the most functionally limited women. A major difference between the two scales was the inclusion of "walking across a small room" in the 10-point scale. Although inability to walk across a room is typically used to indicate disability (Branch, Katz, Kniepmann, and Papsidero 1984; Jette, Branch, and Berlin 1990), Nagi 1991 considered even severe mobility restrictions to be a functional limitation. Moreover, we found this item critical for discriminating severity level among those with more severe limitations in lower extremity function. Many women with difficulty walking 1/4 mile also reported difficulty walking across a room. For example, of the 187 women with a lot of difficulty walking 1/4 mile, 52 (28%) also reported some level of difficulty walking across a room; of the 278 unable to walk for 1/4 mile, 62% had difficulty walking across a room. Clearly there exists a range of walking ability among those reporting the same level of difficulty in walking 1/4 mile. Even though the likelihood of difficulty walking across a room increased with increasing difficulty walking 1/4 mile, the correspondence was not perfect; therefore, including walking across a room is necessary to improve discrimination of functional limitation at the upper, most severe end of the scale. Lastly, stooping, crouching, and kneeling, although used only as a dichotomous measure—"able with or without difficulty" (0) and "unable" (1)—serves to disaggregate women at the least severe end of the scale. Without this item, 17.5% of the disabled women scored zero versus 14.7%. This difference is small but nonetheless provides further assurance that persons who score zero on the scale have no major functional limitations.
The questions we used to derive the severity of functional limitation scales include "unable" as a response option. Some have used inability to walk a distance or climb stairs to indicate mobility-related disability (Guralnik, Ferrucci et al. 1995). Even though this is a useful construct for understanding pathways to disablement, under a strict interpretation of Nagi's conceptual framework (1991) inability to walk is more appropriately considered a measure of functioning. Classification of those unable to perform a function can vary with analytic objectives, however. Persons unable to walk can be contrasted with those who are able when the focus is on the threshold between the two, and interpretation of these analyses may be framed in terms of disabled and not disabled. For evaluating heterogeneity of function with a single scale that captures the full continuum of functional limitation, those who are unable, represent the lowest level of functioning, and should be assigned the poorest score, as was done here. This is analogous to performance-based measures of functional limitation, in which persons unable to do a test are assigned the lowest performance score (Guralnik et al. 1994; Seeman et al. 1994).
In the introduction we argued that to more fully understand disablement, researchers must define and measure functional limitation more consistently. This is a challenging objective because model specification from diverse perspectives—medical, rehabilitative, and social science—results in different, although often complementary, interpretations of Nagi's theoretical perspective. For example, Fried and colleagues 1991, Fried and colleagues 1996 have used the term preclinical disability to describe a state of diminished or altered function that falls between impairment and disability, similar to functional limitation. Preclinical disability, however, is operationalized as no report of difficulty with task performance, but self-perceived reduction in frequency of and/or alteration in approach to performing ADL, IADL, mobility, and upper extremity tasks. Similarly, Schultz-Larsen, Avlund, and Kreiner 1992 have examined ability to do activities with or without reduced speed and/or tiredness as a precursor to disability. We focused here on reported level of difficulty in functioning, because questions about difficulty are more widely available in studies of older adults.
Future Directions
We developed the scales described in this article to investigate the causes and course of disability in a cohort of older, moderately to severely disabled women. Although the scales are well distributed and have good criterion-related validity, the findings are limited to, primarily, disabled older women. It is unknown how the scales would (a) distribute in men, (b) relate to self-reported difficulty in ADL and IADL in men, and (c) relate to strength and performance-based measures in both men and women in the broader community. Even though the individual items have good test-retest reliability (Rathouz et al. 1998; Tager, Swanson, and Satariano 1998), reproducibility of the complete scales should also be considered to determine how much change in score constitutes true change.
Despite these caveats, we believe these scales can be useful research tools for understanding (a) how impairments, such as pain and weakness, lead to functional limitation; (b) the conditions (e.g., social, psychological, cognitive, and environmental) under which functional limitations promote disability; and (c) the unique contribution of upper and lower extremity functional limitations to difficulty in activities and tasks that involve both the upper and lower extremities. Because the items we used to construct these scales were included in many large studies and databases, comparable investigations of varied populations addressing several of the issues outlined previously could be undertaken with current data. Lastly, in terms of public health and primary prevention of disability, the availability of self-report-based assessments of severity of functional limitation permits cost-effective screening of persons at risk of disability.
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Acknowledgments |
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Received for publication February 16, 2000. Accepted for publication August 4, 2000.
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