Lack of Endogenous Pain Inhibition During Exercise in People with Chronic Whiplash: An Experimental Study

Introduction Evidence for altered central pain processing and hypersensitivity of the central nervous system, i.e. central sensitization, in chronic whiplash associated disorders (CWAD) is accumulating. Dysfunctional descending inhibitory action is one of the major characteristics of central sensitization and has been studied during exercise in some chronic pain conditions, but no data regarding the response of people with CWAD to exercise are available. Therefore the purpose of this study was to examine the efficacy of the endogenous pain inhibitory systems during exercise, and exercise response in CWAD patients during two different types of acute exercise.

Methods Twenty-two women with CWAD and 22 healthy sedentary controls performed a submaximal and a self-paced, physiologically limited exercise test on a cycle ergometer on two separate occasions. The exercise tests were undertaken with cardiorespiratory monitoring. Before and after the exercise bouts, subjects filled out questionnaires to assess health status, and underwent pain pressure threshold (PPT) measurements. Throughout the study, subjects’ activity levels were assessed using accelerometry in order to monitor potential influences of daily activity levels. Possible changes in any of the outcome measures in response to exercise were compared between the two groups, and between the two types of exercise, using repeated measures ANOVA.

Results In CWAD PPTs decreased following submaximal exercise, whereas they increased in healthy subjects (between group difference p-value between 0.021 and 0.044). The same effect was established in response to the self-paced, physiologically limited exercise, with exception of the PPTs measured at the calf which increased (p=0.291). A worsening of the CWAD symptom complex was reported post-exercise (p<0.05). Fewer symptoms were reported in response to the self-paced, physiologically limited exercise. In addition, no differences in submaximal exercise capacity or daily physical activity were observed between people with CWAD and controls (p>0.05).

Discussion The present study is the first to examine and suggest an impaired descending endogenous pain inhibition during exercise in CWAD patients, and hence provides additional evidence for the presence of central sensitization in CWAD. Submaximal exercise triggers post-exertional malaise, while a self-paced and physiologically limited exercise will trigger less ‘severe’ symptoms, and therefore seems more appropriate for CWAD patients. These finding highlights the fact that one should be cautious when evaluating and recommending exercise in people with chronic WAD, and that the use of more individual, targeted exercise therapies is recommended.