Psychological influences on repetition-induced summation of activity-related pain in patients with chronic low back pain.

This study examined the role of pain catastrophizing, fear of movement and depression as determinants of repetition-induced summation of activity-related pain. The sample consisted of 90 (44 women and 46 men) work-disabled individuals with chronic low back pain. Participants were asked to lift a series of 18 canisters that varied according to weight (2.9kg, 3.4kg, 3.9kg) and distance from the body. The canisters were arranged in a 3x6 matrix and the weights were distributed such that each 'column' of three canisters was equated in terms of physical demands. Participants rated their pain after each lift, and in a separate trial, estimated the weight of each canister. Mean activity-related pain ratings were computed for each Column of the task. An index of repetition-induced summation of pain was derived as the change in pain ratings across the six 'columns' of the task. Pain catastrophizing, fear of movement and depression were significantly correlated with condition-related pain (e.g., MPQ) and activity-related pain ratings. Women rated their pain as more intense than men, and estimated weights to be greater than men. A repetition-induced summation of pain effect was observed where pain ratings increased as participants lifted successive canisters. Fear of movement, but not pain catastrophizing or depression, was associated with greater repetition-induced summation of pain. The findings point to possible neurophysiological mechanisms that could help explain why fear of pain is a robust predictor of pain-related disability. Mechanisms of repetition-induced summation of activity-related pain are discussed.