Studies using surface electromyography have demonstrated a reorganization of muscle activation patterns of the neck and shoulder muscles in patients with posttraumatic neck pain disability. The neurophysiologically oriented "pain adaptation" model explains this reorganization as a useful adaptation to prevent further pain and injury. The cognitive-behavioral-oriented "fear avoidance" model suggests that fear of movement, in addition to the effects of pain, modulates the muscle activation level. We analyzed the extent to which pain and fear of movement influenced the activation patterns of the upper trapezius muscle during the transition from acute to chronic posttraumatic neck pain. Ninety-two people with an acute traumatic neck injury after a motor vehicle accident were followed up for 24 weeks. Visual analog scale ratings of pain intensity, response on the Tampa Scale of Kinesophobia--fear of movement, and surface electromyography of the upper trapezius muscles during a submaximal isometric physical task were obtained at 1, 4, 8, 12, and 24 weeks after the motor vehicle accident. Multilevel analysis revealed that an increased level of both fear of movement (t value=-2.19, P=0.030) and pain intensity (t value=-2.94, P=0.004) were independently associated with a decreased level of muscle activation. Moreover, the results suggest that the association between fear of movement and lower muscle activity level is stronger in patients reporting high pain intensity (t value=2.15, P=0.033). The contribution of pain intensity to the muscle activation level appeared to decrease over time after the trauma (t value=2.58, P=0.011). The results support both the "pain adaptation" and the "fear avoidance" models. It is likely that the decrease in muscle activation level is aimed at "avoiding" the use of painful muscles.