Contribution of psychosocial and mechanical variables to physical performance measures in knee osteoarthritis.

The goal of our research group is to gain a better understanding of the sources of mobility limitations in people with knee osteoarthritis (OA). To address mobility in knee OA, our research approach aims to consider personal, pathophysiological, impairment, and societal factors and how these factors interact through the use of a combination of quantitative and qualitative methods. This particular study investigated the role of psychosocial and mechanical variables related to OA pathophysiology in physical performance measures used to examine people with knee OA. Approximately 33% of independently living Americans aged 63 years and older have radiographically confirmed or symptomatic (or both) OA of the knee. (1) Osteoarthritis of the knee is the single greatest cause of chronic disability among community-dwelling older adults in the United States. (2) Given the high prevalence of knee OA, the expected increase in incidence as the population ages, and the influence of this disease on disability, physical therapists must be prepared to provide effective treatment strategies for people with knee OA.

The functional consequences of knee OA are profound because of its high prevalence and the related lower-extremity mobility limitations. (2) The activities most commonly reported as difficult by people with knee OA include walking, climbing stairs, and transferring. (3) In a sample of 1,769 older adults, those with knee OA (n=318, 18.4%) were twice as likely to report difficulty in walking a mile, climbing stairs, and completing heavy household chores than older adults who are healthy. (2) These mobility tasks, we believe, must be an important component of physical rehabilitation for people with knee OA.

The mobility limitations related to knee OA may result from a combination of psychosocial attributes and pathophysiological factors. The theoretical framework used in this study (Fig. 1) contains psychosocial factors (depression, anxiety, and poor self-efficacy) and mechanical factors (obesity and poor knee strength [force-generating capacity of muscle]) shown to be important in knee OA. Physical therapists are trained to promote improved physical performance in people with knee OA by addressing predominantly mechanical factors related to pathophysiology, such as strength and body weight. However, most clinicians also acknowledge that psychosocial factors have a profound effect on the outcome of treatment. In people with knee OA, the role of these psychosocial factors and the relative importance of psychosocial issues and mechanical pathology in physical performance are unknown.

[FIGURE 1 OMITTED]

Psychosocial variables may influence the physical capacity of people with knee OA. People with knee OA are more likely to report psychosocial problems, such as depression and anxiety. (4) Summers and colleagues (5) were the first to demonstrate that psychological variables influence the perception of functional impairment experienced by people with knee or hip OA. In 65 people with knee or hip OA, depression and coping skill scores were strong predictors of self-reported functional impairments reported on the Sickness Impact Profile. (5) The authors concluded that the influence of depression and anxiety on performance may be mediated by increasing the perception of pain in people with knee OA. (5) No studies directly linking depression and anxiety with physical performance measures in this population were found.

Subsequent studies (3,6,7) demonstrated that poor self-efficacy affects performance. Self-efficacy refers to a person's beliefs in his or her capabilities to organize and execute the actions required to achieve a wide range of goals (8); for example, self-efficacy can be applied to academic, social, and mobility skills. The level of self-efficacy has been shown to be important in the performance of physical tasks in people with knee OA. High self-efficacy significantly decreased the odds (odds ratio=0.79 per 5-point increment on the Western Ontario and McMaster Universities Osteoarthritis Index; 95% confidence interval=0.67-0.93) of having a poor perception of physical functioning and decreased the odds of performing poorly on a sit-to-stand activity after 3 years in 257 people with knee OA. (9) Self-efficacy for stair climbing had a moderate relationship (r=.53) with actual performance of stair climbing in 480 older adults with knee pain, suggesting that measurements of self-efficacy provide some information on performance but do not fully explain performance. (10)

Mechanical factors are important triggers of the biological degradation of articular cartilage and the underlying subchondral bone. (11,12) Mechanical loading can be influenced by muscle strength; improved muscle strength is thought to have a protective effect. Indeed, studies involving muscle strengthening in people with knee OA have shown improvements in the performance of functional tasks, such as walking. (13,14) Obesity is thought to increase joint loading, (15,16) and increased joint loading has been associated with increased varus alignment, (17,18) more severe radiographic grades of OA, (17) greater joint space narrowing, (17) and greater deviation from the ideal mechanical axis of the lower extremity. (18) However, no studies elucidating the relationship between obesity and physical performance measures were found.

Physical therapists may use outcome measures to objectively (that is, without distortion by personal views) examine clients and to document change in a client's functional status over time. (19) Objective measurements that investigate difficulty in the performance of walking, stair climbing, and transferring would best reflect the functional limitations experienced by people with knee OA. (2) However, the relative importance of psychosocial factors and mechanical pathology to objective physical performance measures has not been studied. It is unclear whether psychosocial factors, such as self-efficacy, anxiety, and depression, contribute more or less to physical performance than mechanical factors, such as strength and obesity, in people with knee OA. Therapists sometimes suspect that, because of psychosocial issues, certain people with OA do not improve in physical status despite concerted treatment efforts to improve mechanical variables, such as strength and obesity. Understanding whether psychosocial or mechanical variables have the greatest effect on performance will help to elucidate whether psychosocial issues should be considered during treatment. In addition, understanding which factors have a greater relationship with performance will provide some meaning to physical performance measures in people with knee OA. The purpose of this study was to evaluate the relative contributions of psychosocial and mechanical variables to physical performance measures in people with knee OA.

Method

The data reported here are part of a larger research project in which subjects made 2 visits of approximately 2 hours each, 1 week apart. All measurements reported here were obtained on the second visit. Data from the first visit, a kinematic and kinetic gait analysis, will be reported elsewhere.

Subjects

Fifty-seven subjects participated in this research study. However, upon evaluation of radiographs collected for another aspect of this research project, the data from 3 subjects were excluded because of the presence of predominantly lateral-compartment knee OA. These subjects were excluded because studies of interventions and theories of mechanical pathology have suggested that medial-compartment knee OA may involve a disease process different from that of lateral-compartment knee OA. (20,21)

This study population consisted of a convenience sample of community-dwelling adults who were over the age of 50 years ([bar.X]=68.3, SD=8.7) and who had physician-diagnosed medial-compartment knee OA (n=54). The physicians were family physicians in all except 2 cases, in which an orthopedic surgeon made the diagnosis. Radiographs were taken at the beginning of the study to confirm the presence of OA in the medial compartment. Subjects were recruited by use of a free community newspaper that is circulated to more than 55,000 homes. Recruitment continued for 1 year.

Of the 54 subjects included in the study, 32 were women, and the left limb was studied in 29 cases. In cases of bilateral knee OA (n=26), the more painful knee was tested. As a group, the subjects were highly educated (years of full-time-equivalent formal education: [bar.X]=14.9, SD=4.3).

No subject had undergone corrective surgery or had had a hip or ankle condition in the ipsilateral limb. Before enrollment in the study, all subjects were screened for medical conditions that could be exacerbated by the protocol, such as unstable heart disease. Subjects had an average of 2.5 comorbidities. Comorbidities were defined as conditions that required treatment for more than 3 months by a physician. The most common comorbidities were hand OA, heart disease, low back pain, and hypertension.

All subjects provided written informed consent before enrollment in the study. Table 1 summarizes the descriptive data collected on the 54 subjects.

Physical Performance Measures

Three physical performance measures were completed. The Six-Minute Walk Test (SMW) was used to quantify walking ability. The SMW yields reliable (intraclass correlation coefficient=.96) and valid data (22) and is an inexpensive clinical tool that involves recording the distance that subjects cover while walking indoors at their own pace for 6 minutes. Subjects are free to stop or use a mobility aid to complete the walking task, making this measure clinically useful. The SMW measurement was recorded indoors in a well-lit, 25-m, tiled hallway. The score recorded was the total distance traveled during 6 minutes. Subjects were instructed to "walk as quickly and safely as you can for 6 minutes."

To investigate stair climbing, the time required to ascend 5 steps, turn around, and descend 5 steps was used. This stair-climbing task (STR) has been shown to have test-retest reliability of .88 (type of statistic not reported). (23) Both handrails were available, but subjects were asked to use only one handrail during the test. The STR measurement was recorded indoors in a well-lit, low-traffic stairwell, and the total time required to complete the task was used as the score. Subjects were instructed to "climb up and down 5 stairs as quickly and safely as you can."

The Timed "Up & Go" Test (TUG), a modification of the Get Up and Go Test, assesses mobility and balance in older adults. (24) Using a chair with armrests, subjects were asked to stand up from a chair, walk 3 m, turn, walk back, and sit down quickly and safely. The reliability (intraclass correlation coefficient=.99), content validity, and predictive validity of TUG scores have been established. (24) The TUG score was recorded indoors in a well-lit, tiled, low-traffic hallway with the distances clearly marked. The score was the time required to complete the task. Subjects were instructed to "on the word 'go,' stand up, walk 3 m to the marked line, turn, walk back to the chair, and sit as quickly and safely as you can." Each subject practiced this activity once before the average score from 2 trials was recorded.

Independent Variables

Psychosocial variables. Three questionnaires were used to determine levels of self-efficacy, depression, and anxiety. The Arthritis Self-Efficacy Scale was used to determine self-efficacy for managing pain, function, and other health-related variables. (25) The questionnaire uses a visual analog scale in which a higher score indicates greater self-efficacy, a positive result. Three scores result from this questionnaire: The Pain Self-Efficacy subscale (PSE) consists of 5 questions, the Functional Self-Efficacy subscale (FSE) consists of 9 questions, and the Other Self-Efficacy subscale (OSE) consists of 6 questions related to managing fatigue, frustration, and activity. levels. Test-retest reliability coefficients (r) of .85 to .90 have been reported for these subscales. (25) In our research study, the subscales were considered separately.

Depression was assessed with the Center for Epidemiologic Studies--Depression (CES-D) Scale. The CES-D Scale is a 20-item self-report Likert-type scale developed to identify depression in the general population. (26) Unlike the score in the Arthritis Self-Efficacy Scale questionnaire, a higher score in the CES-D Scale questionnaire indicates a greater level of depressive symptoms, a negative result. The scale emphasizes affective components, such as mood, guilt, worthlessness, helplessness, loss of appetite, and sleep disorders. Reliability coefficients of .85 to .90 (type of statistic not reported) have been reported in general and patient populations, (26) and the scale has been shown to yield valid data in people with arthritis. (27) A score of 16 or greater on this scale indicates that the subject likely experienced some depression over the past week. (26) A score of 7 has been reported in the general population. (26)

Finally, the State-Trait Anxiety Inventory (STAI) was used to investigate anxiety with 2 self-administered scales. (28) A higher score on the combined scales indicates a greater level of anxiety symptoms, a negative result. Twenty questions with responses on a 4-point scale address how an individual feels at a given moment. This section reflects state anxiety, the transitory emotional state of an individual characterized by consciously perceived feelings of tension. Trait anxiety is assessed by 20 questions that inquire about how a person generally feels. The validity and test-retest reliability (r=.73-.86) of data for this scale have been established, and normative data have been published. (28,29) On the STAI, older adults with generalized anxiety score approximately 93 points, and older adults who are healthy score approximately 58 points. (29)

Mechanical variables. The body mass index (BMI; kilograms per square meter) was calculated from measured height and weight. The BMI is a standard, widely used measure to indicate levels of obesity, in which "overweight" is classified as a BMI of equal to or greater than 25 kg/[m.sup.2]. (30) Measurements of height and weight were recorded while the subjects were barefoot and wearing shorts and a shirt.

The strength of the quadriceps femoris (QUAD) and hamstring (HAM) muscles was measured by use of a Biodex System 3 isokinetic dynamometer. * Each subject completed a set of 5 submaximal practice trials of knee flexion and extension before measurements were obtained. Then, 5 maximum-effort trials of concentric knee flexion and extension at 60[degrees]/s were performed. Verbal encouragement with the words "kick" and "pull" were given. This speed of concentric contraction was used because the majority of studies evaluating strength in people with knee OA have used 60[degrees]/s and 120[degrees]/s. (13,31,32) Some subjects in our study were unable to achieve a true isokinetic phase at 120[degrees]/s; therefore, we did not use the data from the trials conducted at 120[degrees]/s. To maximize the reliability and validity of the strength assessments, the data were "windowed" to remove the acceleration and deceleration phases of movement, thereby removing impact artifacts. (33,34) The reliability (r) of windowed data for concentric knee flexion and extension has been reported to be .90 to .96 with Biodex systems. (35) The 5 peak windowed values for flexion and extension were averaged for each subject.

Protocol

The following sequence of measurements was used with every subject to allow a 5- to 10-minute rest period between physical activities, during which a questionnaire was completed: SMW, PSE-FSE-OSE, TUG, CES-D Scale, STR, STAI, BMI, QUAD/HAM. The same tester (MRM), a registered physical therapist with 6 years of experience solely in adult and geriatric orthopedic practice, carried out all performance measures and collected and scored all questionnaires.

Data Analysis

First, Pearson correlation coefficients were calculated for the physical performance measures (SMW, TUG, and STR) and all of the psychosocial and mechanical variables. Because 55 coefficients were calculated, a Bonferroni correction set the significance level for these correlations at a P value of <.001. Second, a repeated-measures analysis of variance was carried out to investigate the effect of fatigue over the 5 maximun-effort strength trials. Third, a stepwise linear regression was performed with the SMW as the dependent measure. The 3 mechanical variables (BMI, QUAD, HAM) and the 5 psychosocial variables (PSE, FSE, OSE, CES-D, and STAI) were independent variables. The subscales of the Arthritis Self-Efficacy Scale were considered separately. Next, the same statistical technique was applied with the TUG and the STR as the dependent measures. In all stepwise regressions, the stepping-method criteria required an F value of 0.05 or greater for inclusion in the model and an F value of 0.10 or less for removal from the model. To ensure that the regression results were not affected by multicollinearity of variables, an analysis of multicollinearity was performed. (36) In addition, the centered leverage values were analyzed to determine whether implausible outlier values were included in the data set for the SMW, TUG, and STR regressions. The SPSS version 11 statistical package ([dagger]) was used to complete these analyses.

Results

Table 1 provides the means, standard deviations, and ranges of values obtained for the physical performance measures and the psychosocial and mechanical variables. One subject did not complete the strength testing because of a medical event related to asthma. A repeated-measures analysis of variance revealed that fatigue was not a factor over the 5 strength trials of knee extension and flexion (P=.61 and P=.76, respectively).

The subjects had a mean BMI of 28.6 kg/[m.sup.2] (SD=5.1), indicating that, as a group, the subjects were overweight. The mean ratio of QUAD to HAM muscle strength was 0.52 (SD=0.19). The responses on the CES-D Scale and STAI measures indicated that the levels of depression and anxiety found in the general population were lower than those found in people with psychiatric disorders.

Correlation coefficients (r) for the FSE and all physical performance measures ranged between absolute values of .68 and .72 (P<.001) (Tab. 2). The STAI data were not significantly related to data obtained for the performance tasks, and the correlations (r) between the CES-D Scale data and data obtained for the performance tasks ranged between absolute values of .33 and .37. Correlation coefficients (r) for the mechanical variables (BMI, QUAD, HAM) and the physical performance measures ranged between absolute values of .35 and .52.

The stepwise linear regression models for the SMW, TUG, and STR are shown in Table 3. The FSE explained 45% or more of the variance in the SMW, TUG, and STR scores, suggesting that the FSE is the major determinant of these performance scores. Mechanical variables, such as the BMI and strength combined, contributed less than 15% to the models. Specifically, a model of the FSE, QUAD, BMI, and PSE explained 62.0% of the subjects' SMW scores. The last 3 variables added a total of 11.4% to this model. The significant factors explaining the variance in the TUG scores included the FSE, which contributed 51.7% of variance to the TUG scores, and the QUAD and BMI measures, which each added another 6% to the TUG score variance. Finally, a model of the FSE and HAM explained 52.7% of the STR scores, with the FSE contributing 45.7%.

With the 2 strength variables (QUAD and HAM muscle strength) excluded, an analysis of the multicollinearity of these regression models showed little correlation between the independent variables, with the smallest tolerance value at .75 and all others above .83, indicating that the regression models were not affected by interrelationships between explanatory variables. (36) To investigate whether QUAD and HAM could be used interchangeably because of the high correlation coefficient (r=.794), we ran each regression for the physical performance measures without one strength variable. In all cases, the subsequent regression model substituted the QUAD with the HAM or vice versa. Each resultant model explained the same amount of variance (within 5%) of the dependent variable as the original regression model presented here.

Analysis of potential outliers with the centered leverage values determined from residual statistics identified no extreme cases (values below 0.21) for the SMW and TUG regressions. For the STR regression, one extreme case was identified: a centered leverage value of 0.52, or 3.2 standard deviations from the mean, for a subject requiring 28 seconds to complete the test. Because this STR score is clinically possible, (37) we retained the data from this subject. Furthermore, analysis of the regression excluding this outlier did not affect the variables selected or the order of variables in the STR regression.

Finally, to confirm that mechanical variables did not share some variance with the psychosocial questionnaires in our regression models, we performed a stepwise linear regression for the SMW, TUG, and STR with only mechanical variables (BMI and strength) as independent variables. A model of HAM and BMI measures explained 30.2% of the SMW scores. Similarly, a model of HAM and BMI measures explained 36.7% of the TUG scores and 32.1% of the STR scores. In all cases, the BMI contributed less than 10%.

Discussion

The purpose of this study was to evaluate the relative contributions of psychosocial and mechanical factors to physical performance measures in people with knee OA. Self-efficacy explained much of the variance in performance in people with knee OA, with contributions from knee strength and body weight as well. The implications for physical therapist practice are that interventions in this population should aim to improve an individual's confidence in performing physical tasks in addition to mechanical strategies, such as weight loss and strengthening. These findings provide some evidence to support the clinical experience that psychosocial issues have a role in determining performance. Figure 2 depicts a proposed theoretical framework based on the results, showing that both mechanical variables and self-efficacy are related to physical performance in people with knee OA. Readers should be cautioned that the results of this cross-sectional study provide only an overview of the variables that influence performance. Longitudinal research is needed to determine whether variables such as self-efficacy can be manipulated to improve performance in people with knee OA.

[FIGURE 2 OMITTED]

Self-efficacy is the confidence that people have in their abilities to perform a specific task. (25) Bandura (8) contended that how people behave is better predicted by their beliefs about their capabilities than by what they are actually capable of accomplishing. In addition to past successful performance, sources of self-efficacy include persuasion and observation of others. Persuasion refers to the act of verbally encouraging people that they possess the capabilities to master a given task. (8) Observing another individual succeed raises the observer's beliefs that they, too, possess the capabilities to master comparable activities. (8) Therefore, it is possible to determine self-efficacy for a task without considering past performance. In some cases, such as first-time child birth, past performance may not be available. From such a perspective, an individual's level of self-efficacy could be completely independent of his or her capabilities for performance.

Self-efficacy for physical tasks, measured by the FSE subscale of the Arthritis Self-Efficacy Scale, had the strongest relationship with all physical performance measures. The FSE also explained at least 45% of the variance in the SMW, TUG, and STR scores, suggesting that the more certain people were that they could complete physical tasks, the better they performed in walking, transferring, and stair climbing. Other studies also have highlighted the role of self-efficacy in people with knee OA. Gaines and colleagues (7) found a significant relationship (r=-.559) between the FSE and self-reported performance in 29 women with knee OA. No significant relationship was found in the 14 men who participated, perhaps because of the small sample size. In the study by Gaines and colleagues, (7) performance was self-reported as opposed to observed. Harrison (38) found that balance and self-efficacy explained 42% of the variance in physical performance, whereas self-efficacy and pain explained 74% of the variance in self-reported function. Sharma and colleagues (9) showed that high levels of self-efficacy resulted in decreased odds of poor observed performance of a sit-to-stand task over the span of 3 years in 257 people with knee OA. It is important to note that the self-efficacy scores in these other studies (7,9,38) are much lower (indicating poorer levels of self-efficacy) than those obtained in our sample. Although these studies demonstrated that self-efficacy was important in performance, we specifically compared the relative contributions of psychosocial and mechanical variables to performance.

Although depression and anxiety are strong predictors of pain and function among people with knee OA, (5,39) the subjects in this study did not demonstrate anxiety or depression scores appreciably different from those in the general population. Our sample had a mean score of 63 (SD=16) on the STAI. In comparison, older adults with generalized anxiety score approximately 93 on the STAI, whereas older adults who are healthy score approximately 58.29 Similarly, our sample had a mean score of 10 (SD=9) on the CES-D Scale; studies of subjects who were younger and healthy demonstrated scores of approximately 7, (26) and people with depressive symptoms over the preceding week had scores of over 16. (26) As a result, the subjects involved in this research study appear to be unique compared with those in other studies. We speculate that the high level of education in our sample may have contributed to healthier levels of depression and anxiety. (40) Although our correlations between the STAI or the CES-D Scale and the performance measures were weak (r=.12-.37), several studies (23,39,41) have highlighted a role for depression and anxiety in the perceived functional status of people with knee OA. It is possible that depression and anxiety mediate the relationship between perceived and performed physical abilities.

Our study sample included subjects who were, as a group, overweight. Previous studies of knee OA (30,42) included subjects with BMIs higher than those of our sample; our findings are limited to subjects with knee OA and relatively lower levels of obesity. In this study, the correlations between BMI and the SM-W or the TUG were moderate, suggesting that the less obese a subject, the greater the distance that he or she can walk and the faster that he or she can transfer. In addition, BMI contributed 3.5% to the SMW score, suggesting that high body weight has some relationship to the performance of physical tasks, although perhaps not as much as anticipated. One other study showed similar results: Sharma and colleagues (9) found that BMI did not relate to the performance of a sit-to-stand transfer task in 257 people with knee OA and an average BMI of 30.5.

The absolute values of strength and the ratio of QUAD to HAM muscle strength (0.52 [+ or -] 0.19) matched the findings of other researchers. (13,31,32,43) Unfortunately, we found a paucity of established normative strength data, particularly for isokinetic knee flexion and extension in older men and women (44,45); thus, a comparison with normative data was not possible. Our results demonstrated that knee muscle strength had a good relationship with performance (r =.47-.52) in people with knee OA. In addition, QUAD or HAM strength contributed to the variance of how well our subjects performed physical tasks, suggesting that the stronger the subjects, the better their performance. Because of the strong relationship between QUAD strength and HAM strength, these variables could be used interchangeably in the regression models. Supporting our findings, a longitudinal study (9) also demonstrated that knee strength contributed to the performance of physical tasks, but to a lesser degree than psychosocial factors, such as self-efficacy. Strength may be a better determinant of performance if the absolute values of strength are lower, perhaps at a level below a critical threshold for transferring, walking, and stair climbing. Further investigation would be necessary to elucidate whether strength is a more important determinant when absolute values of strength are low. In addition, most studies of strengthening for knee OA have included functionally relevant exercises, such as stair climbing and walking, (14,46-48) making it impossible to discern the contribution of strength versus that of self-efficacy to performance. That is, practicing walking may strengthen knee muscles and may provide a source of experience that can promote improved self-efficacy. These study designs make it impossible to elucidate whether improvements in performance are the result of improved strength or improved self-efficacy, or both. Nevertheless, knee strengthening needs to be an important component of treatment for people with knee OA and our results suggest that the ideal treatment for knee OA also should consider self-efficacy.

On the basis of our findings, therefore, physical therapists need to consider that treatment for knee OA would be improved by including strategies that increase self-efficacy in addition to strengthening knee muscles and producing weight loss. Strategies to improve self-efficacy for physical tasks in people with knee OA exist; for example, the Arthritis Self-Management Program involves 6 weekly, 2-hour sessions taught by a trained layperson and covering pathophysiology, exercise, relaxation, appropriate use of joints, medications, patient-physician communication, and problem solving. (49) People who had knee OA and participated in this program experienced a 15% to 20% decline in pain, a reduced number of physician visits, and improved self-efficacy, even over a 4-year follow-up period. (50,51) However, no study evaluated performance after this program, a direction for future study. It is theorized that education and practice mediate behavior changes in people. (23) These findings reinforce the pivotal role of physical therapists in providing practice and comprehensive education to promote self-management for people with knee OA. The role of self-efficacy in performance likely is substantial.

Some limitations of this study must be considered. In terms of generalizability, most characteristics of our study sample were typical of those of other study samples of subjects with knee OA. The gender distribution (59% female) and strength profiles were typical of people with knee OA. However, the mean score of 7 seconds (SD=3 seconds) on the TUG was consistent with literature indicating that community-dwelling women perform the test in between 6.0 and 11.2 seconds. (52) It is possible that mechanical factors, such as strength and BMI, did not relate more strongly to performance on the TUG, SMW, and STR because our subjects performed well, that is, similar to community-dwelling subjects. Unlike the subjects in other studies, our subjects did not appear to have scores on the CES-D Scale or STAI that were appreciably higher than those in the general population. The subjects were overweight but to a lesser degree than in other studies. The Western Ontario and McMaster Universities Osteoarthritis Index scores (Tab. 1) indicated that our subjects had relatively lower levels of pain and impairment. The self-efficacy scores were considerably higher than those obtained in other studies of people with knee OA. (9,38) Thus, our sample likely was composed of subjects with mild to moderate knee OA. Different results may be found in samples with more severe knee OA.

No information about the duration of illness was recorded; thus, the subjects may have had acute or chronic disease. However, the recruitment process of obtaining a diagnosis was lengthy (mean of 3 months between initial contact and participation), suggesting that the subjects had chronic disease. The same tester recorded scores for all measurements. This tester was aware of the dependent and independent variables because all of the independent variables were considered important at the outset of the study. The order of measurements was not randomized, a factor that may have introduced bias into this study. However, the order of measurements was necessary to ensure adequate rest periods between physical tasks and to ensure that strength measures did not create fatigue before other performance measures. The reliability of data obtained with the measures and questionnaires was not established in this study because the reliability and validity of data obtained with the measures and questionnaires in studies of people with arthritis have been reported elsewhere. Finally, investigators are encouraged to include at least 5 subjects, ideally 10 to 15 subjects, for every independent variable included in regression analyses. (53) In this study, 8 independent variables were used (5 psychosocial and 3 mechanical), suggesting that at least 40 subjects were required, but 80 to 120 subjects would have been ideal. Our study included 54 subjects, a factor that may have resulted in an enhancement of type I errors.

Conclusion

Physical therapists, we believe, should use outcome measures to document a change in status in the mobility of people with knee OA. However, understanding what the scores mean also can guide clinical treatment. Selfefficacy, or a person's confidence in his or her ability to complete a physical task, explained much of the variance in data obtained with the 3 physical performance measures in our group of subjects with mild to moderate knee OA. Body weight and strength also explained some of this variance. Therefore, strategies to improve physical performance in people with knee OA should include not only a combination of mechanical treatments for weight loss and strengthening but also psychosocial interventions aimed at improving self-efficacy. In addition, physical therapists evaluating the significance of the SMW, TUG, and STR scores in subjects with knee OA should note that a large part of each score reflects an individual's self-efficacy for physical tasks. Further research is necessary to develop concrete strategies for physical therapists to use in aiming to increase the level of confidence of their subjects with knee OA.

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* Biodex Medical Systems, 20 Ramsay Rd, Shirley, NY 11967-4704.

([dagger]) SPSS Inc, 233 S Wacker Dr, Chicago, IL 60606.

MR Maly, BScPT, PhD, is Post-Doctoral Fellow, Aging & Health Research Unit, University of Toronto, Toronto, Ontario, Canada, and Assistant Professor, Elborn College, School of Physical Therapy, The University of Western Ontario, London, Ontario, Canada. Address all correspondence to Dr Maly at Elborn College, School of Physical Therapy, The University of Western Ontario, London, Ontario, Canada N6G 1H1 ([e-mail]).

PA Costigan, BPHE, PhD, is Associate Professor, Physical Education Center, School of Physical Health and Education, Queen's University at Kingston, Ontario, Canada.

SJ Olney, BScPT, BScOT, PhD, is Full Professor and Director, School of Rehabilitation Therapy, Queen's University at Kingston.

Dr Maley provided concept/idea/research design, data collection, and clerical support. All authors provided writing. Dr Maley and Dr Costigan provided data analysis and project management. Dr Maley and Dr Olney provided fund procurement. Dr Costigan provided facilities. Dr Costigan and Dr Olney provided consultation (included review of manuscript before submission).

The Queen's University Health Sciences Research Ethics Board approved this study.

This study was supported by the Canadian Institutes for Health Research (grant #99034), the Toronto Rehabilitation Institute, and the Natural Sciences and Engineering Research Council.

An abstract of this research was presented at the 2005 Annual Conference and Exposition of the American Physical Therapy Association; June 8-11, 2005; Boston, Mass.

This article was received October 7, 2004, and was accepted May 16, 2005.

Table 1.

Descriptive Subject Characteristics, Physical Performance Measures, and
Psychosocial and Mechanical Characteristics (n=54)

Variable                         [bar.X]   SD      Median      Range

Age (y)                           68.3       8.7    69.0       50-87
Education (y)                     14.9       4.3    14.5        8-30
Weight (kg)                       82.2      15.0    81.4       52-127
Height (cm)                      169.7      10.0   168.0    149.2-190.5
Pain (a) (/100)                   30.3      18.6    26.4      3.2-89.2
Stiffness (a) (/100)              43.3      25.0    39.5      6.5-98.0
Physical function (a) (/100)      34.4      19.6    31.1      3.6-79.9
Gait speed (m/s) (b)               1.2       0.4     1.3      0.4-1.8
Chronic morbidities (n) (c)        2.5       1.3     3          0-6
Six-Minute Walk Test (m)         440       123     451        146-642
Timed "Up & Go" Test (s)           7         3       7          3-15
Stair-climbing task (s)            9         6       7          3-28
Pain Self-Efficacy subscale       63.3      19.9    66.0     11.4-97.4
  (%) (d)
Functional Self-Efficacy          80.7      13.4    82.8     39.7-98.3
  subscale (%) (d)
Other Self-Efficacy subscale      73.3      17.5    77.0     20.8-95.3
  (%) (d)
Center for Epidemiologic          10         9       7          0-38
  Studies--Depression Scale
  (e)
State-Trait Anxiety Inventory     63        16      57         43-123
  (f)
Body mass index (kg/m2)           28.6       5.1    27.9     19.9-43.7
Quadriceps femoris muscle         63.8      29.0    61.0     14.4-134.4
  strength (N-m) (g)
Hamstring muscle strength         34.0      20.5    31.3      1.5-92.5
  (N-m) (g)

(a) Pain, Stiffness, and Physical Functioning subscales of the Western
Ontario and McMaster Universities Osteoarthritis Index.

(b) Gait speed was recorded during the Six-Minute Walk Test.

(c) Comorbidities diagnosed by a physician and requiring ongoing
treatment (>3 mo) (excluding knee osteoarthritis).

(d) Scores range between 0 (poor) and 100 (excellent) on these
subscales of the Arthritis Self-Efficacy Scale.

(e) Scores range between 0 (no depressive symptoms) and 60 (many
depressive symptoms) on the Center for Epidemiologic
Studies--Depression Scale.

(f) Scores range between 40 (no anxiety symptoms) and 160 (many anxiety
symptoms) on the State-Trait Anxiety Inventory.

(g) Mean peak concentric strength at 60 [degrees]/s. One subject did
not complete the strength testing because of a medical event related to
asthma (n=53).

Table 2.

Pearson Correlation Coefficients for Physical Performance Measures and
Independent Variables (n=54) (a)

                      PSE        FSE        OSE
                      (%) (b)    (%) (b)    (%) (b)     CES-D (c)

PSE (%) (b)           1          0.30       0.41 (f)    -0.04
FSE (%) (b)                      1          0.41 (f)    -0.37
OSE (%) (b)                                 1           -0.59 (f)
CES-D (c)                                                1
STAI (d)
BMI (kg/[m.sup.2])
QUAD (N*m) (e)
HAM (N*m) (e)
SMW (m)
TUG (s)
STR (s)

                                  BMI               QUAD
                      STAI (d)    (kg/[m.sup.2])    (N*m) (e)

PSE (%) (b)           -0.12        0.00              0.04
FSE (%) (b)           -0.22       -0.26              0.35
OSE (%) (b)           -0.48       -0.13              0.25
CES-D (c)              0.68        0.27             -0.33
STAI (d)               1           0.13             -0.22
BMI (kg/[m.sup.2])                 1                -0.10
QUAD (N*m) (e)                                       1
HAM (N*m) (e)
SMW (m)
TUG (s)
STR (s)

                      HAM          SMW          TUG          STR
                      (N*m) (e)    (m)          (S)          (S)

PSE (%) (b)           -0.06         0.35        -0.26        -0.15
FSE (%) (b)            0.39         0.72        -0.72 (f)    -0.68 (f)
OSE (%) (b)            0.34         0.34        -0.34        -0.38 (f)
CES-D (c)             -0.38        -0.33         0.33         0.37 (f)
STAI (d)              -0.15        -0.13         0.12         0.28
BMI (kg/[m.sup.2])    -0.14        -0.39 (f)     0.43 (f)     0.35
QUAD (N*m) (e)         0.79         0.47 (f)    -0.49 (f)    -0.50 (f)
HAM (N*m) (e)          1            0.47 (f)    -0.51 (f)    -0.52 (f)
SMW (m)                             1           -0.86 (f)    -0.76 (f)
TUG (s)                                          1            0.88 (f)
STR (s)                                                       1

(a) PSE=Pain Self-Efficacy subscale of the Arthritis Self-Efficacy
Scale, FSE=Functional Self-Efficacy subscale of the Arthritis
Self-Efficacy Scale, OSE=Other Self-Efficacy subscale of the Arthritis
Self-Efficacy Scale, CES-D=Center for Epidemiologic Studies-Depression
Scale, STAI=State-Trait Anxiety Inventory, BMI=body mass index,
QUAD=isokinetic quadriceps femoris muscle strength at 60[degrees]/s,
HAM=isokinetic hamstring muscle strength at 60 [degrees]/s,
SMW=Six-Minute Walk Test, TUG=Timed "Up & Go" Test, STR=stair-climbing
task.

(b) Scores range between 0 (poor) and 100 (excellent) on these
subscales of the Arthritis Self-Efficacy Scale.

(c) Scores range between 0 (no depressive symptoms) and 60 (many
depressive symptoms) on the CES-D.

(c) Scores range between 40 (no anxiety symptoms) and 160 (many anxiety
symptoms) on the STAI.

(d) Mean peak concentric strength at 60 [degrees]/s. One subject did
not complete the strength testing because of a medical event related
to asthma (n=53).

(e) Correlation is significant at a P value of <.001 (2-tailed,
Bonferroni correction for 55 comparisons).

Table 3.

Models of Physical Performance Measures (n=54) (a)

Dependent Variable   Model                             [R.sup.2]   F

Six-Minute Walk
  Test (m)           Functional Self-Efficacy
                       subscale (%)                    .506        54.2
                     Quadriceps femoris muscle
                       strength (N-m) (b)              .555        33.5
                     Body mass index (kg/[m.sup.2])    .591        26.0
                     Pain Self-Efficacy subscale (%)   .620        22.2
Timed "Up & Go"
  Test (s)           Functional Self-Efficacy
                       subscale (%)                    .517        40.2
                     Quadriceps femoris muscle
                       strength (N-m) (b)              .575        27.6
                     Body mass index (kg/[m.sup.2])    .632        25.2
Stair-climbing
  task (s)           Functional Self-Efficacy
                       subscale (%)                    .457        68.4
                     Hamstring muscle strength (N-m)
                       (b)                             .527        42.4

(a) For all comparisons, the P value was <.001.

(b) Mean peak concentric strength at 60 [degrees]/s. One subject did
not complete the strength testing because of a medical event related to
asthma (n=53).