Associations between pain and neuromuscular activity in the human jaw and neck muscles.

The aim of this study was to test the effects of glutamate-evoked jaw or neck muscle pain on electromyographic (EMG) activity of jaw and neck muscles in humans. EMG recordings were made from left (MAL) and right (MAR) masseter muscles, and right sternocleidomastoid (SCM) and splenius (SP) muscles in three different head positions (head rest, head back, head right) or during maximal jaw clenching in 19 men. Glutamate (1 M) or isotonic saline was injected into MAR or SP, and induced pain was recorded on visual analogue scales. EMG activity in MAL and MAR was increased in the head back position compared to head rest and head right positions, whereas EMG activity in SCM and SP was progressively increased as the head was moved from rest position to head back to head right positions. Glutamate-evoked MAR pain was associated with increases in EMG activity in MAR, SCM and SP at rest but not in the head back or head right positions. Glutamate-evoked SP pain was associated with an increase in SP EMG activity at rest and a decrease in SCM EMG activity in the head right position. Decreases in jaw clench-related EMG activity were observed in MAL, MAR and SCM muscles only during glutamate-evoked MAR pain. Isotonic saline injections induced no pain or EMG changes. In conclusion, experimental neck pain is not associated with tonic increases in jaw EMG activity although jaw muscle pain can be linked to increases in neck EMG activity with the head and jaw at rest.