Shoulder and neck pain during office work : The significance of muscle activity and microcirculation

Abstract

Pain in the neck and shoulder region is common in office work involving very low‐level muscle activities. The pathogenetic mechanisms underlying the pain development are poorly understood. Evidence for an association between muscle activity and pain is conflicting. Hypotheses focusing on factors regulating blood vessels and/or factors from the microcirculation that activate nociceptors have been proposed.<br> The overall objective of this thesis was to elucidate the significance of muscle activity and microcirculation in shoulder muscles as possible mechanisms underlying development of pain in the shoulders and neck during office work. We established in the laboratory a computer‐based work task which models office work (work processing). The task comprised a 90 min work without pauses, and with high hand precision demands and time pressure. Twenty‐four subjects, 14 women and 10 men, with chronic shoulder and neck pain, and 28 healthy pain‐free reference subjects, 16 women and 12 men, were recruited. The measurements included ratings of pain intensity, intramuscular trapezius blood flux by laser‐Doppler flowmetry (LDF), trapezius muscle activity by surface electromyography (EMG), pressure pain thresholds (PPT), and salivary cortisol.<br> The subjects, both with and without shoulder and neck pain, developed substantial pain during the work task, but with different time courses. The level of muscle activity was very low during the task (< 4% of EMG during maximal voluntary contraction; MVC). The LDF revealed local vasodilation throughout the work task in both groups. Shoulder PPT decreased from pre‐ to post‐task similarly in both groups.<br> Salivary cortisol levels remained stable during the work task. We found no correlations between pain and muscle activity. There were positive correlations with pain and blood flux in the pain‐afflicted subjects and negative correlations in the reference group. For both groups this correlation was stronger after 30‐45 min of the time spent on the work task.<br> The findings from the present studies suggest that office work involving time pressure and precise hand movements may induce substantial shoulder and neck pain and deep tissue hyperalgesia both in healthy pain‐free subjects and in people with chronic shoulder and neck pain. This pain development seems to be related to the microcirculation of the upper trapezius muscles and not to the muscle activity.