Muscle Pain Induces a Shift of the Spatial Distribution of Upper Trapezius Muscle Activity During a Repetitive Task: A Mechanism for Perpetuation of Pain With Repetitive Activity?

OBJECTIVE: An association exists between repetitive movements and neck-shoulder muscle pain. The mechanisms underlying this association remain unclear. This observational study investigated the effect of upper trapezius muscle pain on the distribution of upper trapezius activity during repetitive lifting. It was hypothesized that nociception would change the distribution of activity resulting in activation of muscle regions which would not normally be active during the task.

MATERIALS AND METHODS: Healthy men repeatedly lifted a box with a cycle time of 3 seconds for 50 cycles, at baseline, following injection of isotonic and hypertonic saline into the upper trapezius muscle and 15 minutes after the last injection. High-density surface electromyography (EMG) was recorded from the upper trapezius using a grid of 64 electrodes. The EMG amplitude was computed for each location to form a map of the EMG amplitude distribution.

RESULTS: During the painful condition, the overall EMG amplitude was lower compared with all other conditions (P<0.05) and in addition, the center of activity of upper trapezius was shifted toward the caudal region of the muscle (P<0.01), a region not normally active during the task. The described alterations of muscle activity likely play an important role in the perpetuation of pain during repetitive activity.

DISCUSSION: Novel mapping of the spatial distribution of upper trapezius muscle activity showed that nociception induced a redistribution of activity during repetitive lifting. This knowledge provides new insights into the mechanisms underlying the perpetuation of pain with repetitive activity.