Chronic neck pain disability due to an acute whiplash injury.

Several theories about musculoskeletal pain syndromes such as whiplash-associated disorder (WAD) suggest that pain and muscle activity interact and may contribute to the chronicity of symptoms. Studies using surface electromyography (sEMG) have demonstrated abnormal muscle activation patterns of the upper trapezius muscles in the chronic stage of WAD (grade II). There are, however, no studies that confirm that these muscle reactions are initiated in the acute stage of WAD, nor that these muscle reactions persist in the transition from acute neck pain to chronic neck pain disability. We analyzed the muscle activation patterns of the upper trapezius muscles in a cohort of 92 subjects with acute neck pain due to a motor vehicle accident (MVA). This cohort was followed up in order to evaluate differences in muscular activation patterns between subjects who have recovered and those subjects who have not recovered following an acute WAD and developed chronic neck pain. sEMG parameters were obtained at 1, 4, 8, 12, and 24 weeks after an MVA. The level of muscle reactivity (the difference in pre- and post-exercise EMG levels) and the level of muscle activity during an isometric and a dynamic task were used as EMG parameters. The results revealed no elevated muscle reactivity either in the acute stage, or during the follow-up period. The results of both the isometric and dynamic task, showed statistically significant different EMG levels between four neck pain disability subgroups (analysis of variance reaching P-levels of 0.000), with an inverse relationship between the level of neck pain disability and EMG level. Furthermore, follow-up assessments of the EMG level during these two tasks, did not show a time related change. In conclusion, in subjects with future disability, the acute stage is characterized by a reorganization of the muscular activation of neck and shoulder muscles, possibly aimed at minimizing the use of painful muscles. This change of motor control, is in accordance with both the (neurophysiological) 'pain adaptation model' and (cognitive behavioral) 'fear avoidance model'.