Experimental muscle pain induces a shift of the spatial distribution of upper trapezius muscle activity during a repetitive task

Abstract

Background. Myalgia frequently affects the upper trapezius muscle, and patients typically complain of dull pain and stiffness. Evidence suggests a correlation between repetitive movements, awkward posture, and the development of neck-shoulder muscle pain. However the mechanisms underlying these associations remain unclear. Purpose. To investigate the effect of experimentally induced upper trapezius muscle pain on the distribution of upper trapezius muscle activity during a repetitive task. It was hypothesized that pain would change the distribution of muscle activity resulting in activation of upper trapezius muscle regions which would not normally be active during the task. Methods. Ten healthy male (age: 26.2 ± 3.1 years) volunteers participated. Subjects lifted a 1 kg box between shelves positioned at hip and shoulder height with a cycle time of 3 s for 50 cycles. The task was repeated: 1. baseline, 2. following injection of 0.4 ml isotonic saline (0.9%) into the right upper trapezius muscle, 3. following injection of 0.4 ml hypertonic saline (5.8%) into the right upper trapezius muscle and 4. 15 mins after the last injection. Pain intensity ratings were obtained immediately following the injection and every 30 s until pain was no longer reported. Surface electromyography (EMG) was recorded from the upper trapezius muscle using a grid of 64 electrodes and 4 uniaxial accelerometers were mounted on the box to obtain the start and end points of the cyclic movement. The EMG root mean square (RMS) was computed for each location of the electrode grid to form a map of the EMG amplitude distribution.The following EMG variables were extracted from the RMS maps: mean RMS, entropy, and y- (cranial-caudal) and x-axis (medio-lateral) coordinates of the centroid of the RMS map. The EMG variables were then averaged across the 50 cycles for each epoch of the cycle. Results. Peak pain intensity was greater following the injection of hypertonic (4.7 ± 1.9/10) compared to isotonic saline (0.8 ± 1.1, P 0.05). Conclusions. The results indicate that nociceptive stimulation of the upper trapezius during a repetitive task changes the distribution of upper trapezius muscle activity and its amplitude. Implications. The described alterations of muscle activity may have an important role in the perpetuation of pain during repetitive activity.