Associations of Coexisting Pain and Fatigue Severity with Physical Performance and Quality of Life Among Middle-Aged and Older Individuals with Chronic Knee Pain: Secondary Analysis of a Randomized Clinical Trial

Objective. To examine associations of combined pain and fatigue severity with physical performance and quality of life in people with chronic knee pain. Design. Cross-sectional. Setting. General community. Participants. Adults (N = 193) >50 years of age with chronic knee pain. Methods. Physical performance measures included the Six-Minute Walk test, Timed Up and Go test, and 20-foot timed walk test. Quality of life (QOL) was measured by the Medical Outcomes Study Short Form-12 Health Survey. The Brief Pain Inventory (BPI) and Brief Fatigue Inventory (BFI) were used to evaluate pain and fatigue. Multiple linear regression analyses were conducted to examine associations of coexisting pain and fatigue severity with physical performance and QOL. Results. Participants were categorized by BPI and BFI scores into four severity groups: mild pain / mild fatigue; moderate pain / mild fatigue; mild pain / moderate fatigue; and moderate pain / moderate fatigue (which included severe pain / severe fatigue). The moderate pain / moderate fatigue group had significantly worse physical performance on the Six-Minute Walk (standardized beta [beta] = -0.22, 95% confidence interval [CI] -0.38 to -0.06, P< 0.05) and poorer physical (beta= -0.41, 95% CI -0.61 to -0.20, P < 0.001) and mental QOL (beta= -0.26, 95% CI -0.46 to -0.07, P < 0.05) than the mild pain/mild fatigue group. Moreover, the mild pain / moderate fatigue group had significantly lower levels of mental QOL (beta= -0.27, 95% CI -0.44 to -0.10, P < 0.05) than those of the mild pain / mild fatigue group. The moderate pain / mild fatigue group did not differ statistically from the mild pain / mild fatigue group. Conclusions. Coexisting moderate pain and moderate fatigue were related to worse physical performance and QOL. Fatigue contributed to lower levels of mental QOL. Clinical assessment of fatigue in addition to pain should be included as a standard examination for chronic knee pain.