Physial disability is one of the major consequences of osteoarthritis (OA) of the knee (1, 2). The way in which patients cope with their condition, especially the passive pain-coping style of avoiding physical activity, has been found to be associated with a higher level of disability (3, 4). When using this coping style, a patient tends to avoid physical activity in order to prevent an increase in pain or onset of a new painful period. In different groups of patients with chronic pain, most notably patients with rheumatoid arthritis (5, 6) and low back pain (7), the role of avoidance of activity as a determinant of disability has been established. Recently, this relationship has been established also among patients with OA of the knee (8).
It has been proposed that the effect of avoidance of activity on disability can be explained using the conceptual framework of the avoidance model (9). As shown in Figure 1, the theory states that a patient tends to avoid physical activity because of a (feared or actual) increase in pain. In the short term, pain can be reduced by avoiding physical activity. In the long term, however, low activity levels will result in a deterioration of physical condition, especially in muscle weakness. Due to this muscle weakness, joints become less stable and their ability to carry a load is reduced. This results in increased disability. Consequently, the patient avoids activity even more, thus entering a downward spiral toward increasing physical disability.
A vital pathway in the avoidance model is the relationship between avoidance of activity and disability. According to the model, this relationship is mediated by muscle strength. Baron and Kenny (10) define a mediator as a factor that “represents the generative mechanism through which the focal independent variable is able to influence the dependent variable of interest.” In this case, the muscle weakness resulting from inactivity is the mechanism through which the avoidance of activity exerts its influence on the level of physical disability. Establishing the role of muscle strength as a mediator in the relationship between avoidance of activity and disability would therefore be an important step in the validation of the avoidance model.
The general aim of the present study was to assess the validity of the avoidance model in patients with OA of the knee. More specifically, the aim was to establish the mediating role of muscle strength in the relationship between avoidance of activity and physical disability. It was hypothesized that avoidance of activity affects disability through the pathway identified by the avoidance model, i.e., avoidance of activity leads to reduced muscle strength, which in turn causes physical disability.
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- PATIENTS AND METHODS
Patient characteristics and mean scores on avoidance of activity, on muscle strength, and on disability are shown in Table 1. The radiologic status of the patients was determined based on the system of grading scales for individual radiographic features described by Altman et al (22). The disability measurements presented are Z scores, meaning that they represent the level of disability of a patient relative to the other patients in the study population. Additional measurements were obtained to gain insight into the functional status of these patients relative to other patient groups. Disability was measured using a Dutch adaptation of the Arthritis Impact Measurement Scales (23). On a scale of 7–28, with 7 representing maximum disability, the mean ± SD score was 19.7 ± 5.9, indicating clear but not excessive disability. Pain was measured with a 100-mm visual analog scale; the mean ± SD score was 48.3 ± 27.8 mm (for a more in-depth description of these patients' functional status, see ref. 13).
Table 1. Characteristics of the 107 patients with knee osteoarthritis
|No. (%) male/female||17 (16)/90 (84)|
|Age, mean ± SD years||68.7 ± 8.6|
|Radiologic status, median (interquartile value)*|| |
| Joint space narrowing (scored 0–18)||1.9 (2.2)|
| Osteophytes (scored 0–18)||2.3 (2.5)|
| Sclerosis (scored 0–18)||0.1 (0.5)|
| Cyst formation (scored 0–18)||0.0 (0.0)|
| Total status (scored 0–72)||4.3 (4.6)|
|Avoidance of activity, mean ± SD PCI, resting subscale†||10.7 ± 2.9|
|Muscle strength, mean ± SD|| |
| Flexion||1.2 ± 0.4|
| Extension||2.0 ± 0.7|
|Observed disability, mean ± SD Z score||0.0 ± 1.0|
The results of the analyses aimed at establishing mediation are presented in Table 2. The criteria for establishing mediation were met. In analysis 1, avoidance of activity was associated with higher disability levels. In analysis 2, avoidance of activity was associated with reduced muscle strength. In analysis 3, reduced muscle strength was associated with more disability, and the impact of avoidance of activity on disability was reduced compared with analysis 1. The regression coefficient (β) for avoidance of activity in analysis 3 was 0.404, compared with 0.463 in analysis 1. Similarly, in analysis 1, avoidance of activity accounted for 21.5% of variance in disability levels. In analysis 3, the fraction of variance in disability accounted for by avoidance of activity decreased to 15.7%. This constitutes a relative reduction of 27% in variance in disability accounted for by avoidance of activity. The mediation effect was statistically significant (P = 0.014).
Table 2. Establishing mediation*
|Analysis||Dependent variable||Independent variable(s)||Regression coefficient, β||r2||Significance of r2, P|
|1||Disability||Avoidance of activity||0.463||0.215||<0.001|
|2||Muscle strength||Avoidance of activity||−0.197||0.039||0.005|
|3||Disability||1) Muscle strength||−0.366||0.148||<0.001|
| || ||2) Avoidance of activity||0.404||0.157||<0.001|
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- PATIENTS AND METHODS
The present study was undertaken to seek evidence that muscle strength acts as a mediator in the relationship between avoidance of activity and physical disability in patients with OA of the knee. The mediator function of muscle strength was predicted from the avoidance model, which offers a biobehavioral explanation for disability experienced by patients with chronic pain, such as those with knee OA (9). The model states that patients tend to avoid activity, fearing that activity will lead to an increase in pain. Due to this inactivity, their physical condition worsens; most notably, it will lead to muscle weakness. This in turn results in instability of joints and decreased ability of the joints to carry load. The end result is increased disability. According to the model, the influence of avoidance on disability is exerted through a reduction of muscle strength, i.e., reduced muscle strength is the mediator in the relationship between avoidance of activity and disability. This mediator hypothesis was tested in the manner proposed by Baron and Kenny (10).
Our findings provided evidence in favor of the mediation hypothesis. Avoidance of activity was found to be a predictor of both muscle weakness and disability. When muscle strength was taken into account, the effect of avoidance of activity on disability decreased. This is an indication of the mediating role of muscle strength in the relationship between avoidance of activity and disability (10), which was underscored by the statistical significance of the mediation effect. These results provide support for the avoidance model (9).
Although evidence in favor of the mediating role of muscle strength was obtained, it could not completely account for the effect of avoidance of activity on disability. After controlling for muscle strength, avoidance of activity still accounted for 15.7% of variance in disability levels. This may indicate that there is a second pathway via which avoidance of activity has an influence on disability. It is possible that inactive patients develop low self-beliefs about their capabilities (low self-efficacy), which is expressed in the inability to perform certain everyday tasks (functional disability), without there being clear physical reasons for this inability. Several studies, in different groups of patients, have provided evidence that inactivity is associated with low self-efficacy (24), and that low self-efficacy leads to disability (25–29).
We have attempted to replicate the findings presented here in a longitudinal-design analysis, using data from the patients participating in the present study, but we were unable to establish the mediating role of muscle strength in that analysis. However, this does not invalidate the results presented here, because there were some inadequacies in the longitudinal design. First, the patients participating in the present study did not have newly diagnosed OA, but had, on average, been experiencing OA-related symptoms (as identified by their general practitioners) for well over a year. The processes depicted in the avoidance model will be most pronounced in the early symptomatic stages of OA, as patients are first confronted with chronic pain and disability. In later stages, it may therefore be more difficult to identify the relationships responsible for the process of disablement.
Second, the period spanned by the longitudinal analyses was 24 weeks. This may not be sufficient time to register enough change in a patient's muscle strength and disability to identify the relationships in the avoidance model. Indeed, in this 24-week period, there was no significant decline in muscle strength in this group of patients with knee OA, even though the development of muscle weakness is a well-documented characteristic of the disease (30–33). Therefore, the cross-sectional results presented here still need to be replicated in a longitudinal study, preferably using patients with newly diagnosed OA, who are assessed over a longer period of time.
Other factors could be considered for inclusion in the avoidance model. Mainly, these would be factors that may moderate, rather than mediate, the relationship between avoidance of activity and disability. Given the muscle weakness that naturally occurs in old age, it could be argued that in the oldest age groups, the impact of avoidance of activity on muscle strength, and consequently disability, will be less pronounced. The same could apply to other subgroups, such as patients with specific comorbidities. In the present study, these potential moderators were not taken into account, because the number of patients enrolled provided insufficient statistical power to assess this issue in depth. Rather, the present study aimed at establishing the relationship between avoidance of activity and disability, which is mediated by muscle strength, thought to exist in general in patients with knee OA.
An important issue is the role of pain in the avoidance model. The mechanism through which avoidance of activity leads to disability is initially triggered by pain: patients start avoiding activity because they experience pain. In later stages, fear of pain, rather than pain itself, keeps fueling the mechanism. In a study of patients with chronic back pain, Crombez et al found that fear of pain was more disabling than actually experienced pain (34). Thus, although pain is not in itself represented in the circular part of the avoidance model (as shown in Figure 1), pain does initially play a key role in the avoidance model. Further research should elaborate the roles of pain and fear of pain.
In conclusion, the results of the present study indicate that the effect of avoidance of activity on disability can be (partially) explained by the avoidance model. In this model, avoidance leads to muscle weakness, which in turn causes disability.