Pain, Fatigue, and Physical Activity in Osteoarthritis: The Moderating Effects of Pain- and Fatigue-Related Activity Interference.

Objective: To examine how self-reported pain- and fatigue-related activity interference relates to symptoms and physical activity (PA) in daily life among people with knee or hip osteoarthritis.

Design: Cross-sectional study with a 7-day repeated-measures assessment period.

Setting: General community.

Participants: Participants (N=154; mean age, 65y; 60% women [n=92]) with knee or hip osteoarthritis and pain lasting ≥3 months.

Interventions: Not applicable.

Main Outcome Measures: Pain- or fatigue-related activity interference items on the Brief Pain Inventory or Brief Fatigue Inventory, respectively, from baseline survey, momentary pain and fatigue severity (measured 5times/d for 7d), and PA measured with a wrist-worn accelerometer over 7 days. We hypothesized that perception of pain- and fatigue-related activity interference would moderate the association between symptoms (pain or fatigue) and PA. People with higher pain- or fatigue-related activity interference were thought to have stronger negative associations between momentary ratings of pain and fatigue and PA than did those with lower activity interference.

Results: Pain-related activity interference moderated the association between momentary pain and PA, but only in the first part of the day. Contrary to expectation, during early to midday (from wake-up time through 3 pm), low pain-related interference was associated with stronger positive associations between pain and PA but high pain-related interference was associated with a small negative association between pain and PA. Fatigue-related activity interference did not moderate the relation between fatigue and activity over the course of a day.

Conclusions: Depending on a person's reported level of pain-related activity interference, associations between pain and PA were different earlier in the day. Only those with high pain-related activity interference had lower levels of PA as pain increased and only in the morning. High pain-related activity interference may be important to address, particularly to maintain PA early in the day despite pain.