Association of pain with frequency and magnitude of knee loading in knee osteoarthritis

OBJECTIVE: Although the relationship between pain and the magnitude of medial knee loading has been previously studied, the contribution of frequency of loading has not. The objective of this study was to determine whether the addition of loading frequency (steps/day) to loading magnitude (knee adduction moment [KAM] impulse) helps explain variance in knee pain in people with knee osteoarthritis (OA). METHODS: Participants were adults with symptomatic knee OA with radiographic signs in the medial knee compartment (n = 38, 10 women). Pain was measured using the pain subscale of the Knee Injury and Osteoarthritis Outcome Score. Participants wore an accelerometer for 1 week to determine the average number of steps/day. The external KAM impulse was calculated from 3-dimensional gait analysis as participants ambulated at self-selected speeds. Knee extensor strength was measured with an isokinetic dynamometer. Linear regression was used to examine the relationship between pain and steps/day after controlling for the KAM impulse, knee extensor strength, and body mass index (BMI). RESULTS: After controlling for BMI (R(2) = 0.02), knee extensor strength (R2 change = 0.26, P < 0.05), and KAM impulse (R2 change = 0.11, P < 0.05), steps/day contributed an additional 9% of variance in pain (P < 0.05). This model accounted for a total of 49% of the variance in pain (F[4,33] = 7.77, P < 0.05). CONCLUSION: Increased knee loading frequency and magnitude were associated with increased pain. Objective measures of loading frequency should be considered when investigating the incidence and progression of knee OA.