Autonomic activation and muscle activity in relation to musculoskeletal pain

Background and objectives:\n\nMusculoskeletal pain is frequent and the most common cause of sick leave in Norway. Although chronic musculoskeletal pain is conceived to be related to social or mental stress, the mechanism for such a relation is not known. The overall objective of the present thesis was to elucidate whether stress-associated musculoskeletal pain and stress-induced muscle activity may be related to activation of the autonomic nervous system.\n\nMethods:\n\nThe present work is carried out in the laboratory where subjects performed a complex two-choice reaction time test designed to mimic mental load in a work place, resembling stressful and repetitive office work. Subjects were investigated while performing this stressful task for one hour, as well as during a baseline period immediately before and during a recovery period. We measured muscle activity and different parameters indicative of activity in the autonomic nervous system, as well as subjective variables as pain, tension and fatigue. We included patients with generalised (fibromyalgia) and patients with regionalised (shoulder/neck) musculoskeletal pain, as well as healthy controls. A group of patients with musculoskeletal pain were also subject to a block of peripheral sympathetic nerves with a unilaterally anaesthetic block of the lower cervical sympathetic ganglion before the stressful task.\n\nResults:\n\nWe found the vascular response to the stressful task to be more protracted than other bodily responses (paper I). We found the muscular responses to the stressful task to be an unlikely explanation for the simultaneously developing pain response for all subjects, irrespective of diagnostic group (paper II). We found delayed pain recovery in both patients groups, but not in the healthy controls (paper II). We found an attenuated cardiovascular response to the stressful task in the fibromyalgia group compared to the healthy controls, with the shoulder/neck pain patients in an intermediate position (paper III). We also found an inverse relation between the heart rate response and the pain response for the fibromyalgia group (paper III). Lastly, we found that a block of peripheral sympathetic fibres did not affect neither pain nor muscle responses to the stressful task for patients with musculoskeletal pain.\n\nConclusions:\n\nFrom the studies presented in the present thesis one may conclude that stress-associated musculoskeletal pain is probably not directly related to muscle activity or autonomic activation at a peripheral level. We found indications of central sensitisation of pain in both patient groups. In patients with generalised musculoskeletal pain (fibromyalgia), the pain may be related to a reduced physiological ability to respond adequately to stress.