The effect of level of muscle tension on the perception
of painful stimuli was assessed in 13 chronic back pain
patients, 14 subjects at high risk for chronic back pain,
and 14 matched healthy controls. Subjects received painful
intracutaneous electric stimuli to the forearm or the lower
back while they produced either high or low muscle tension
levels. Visual analog scale (VAS) ratings of acute pain
were obtained after each trial. Electroencephalograms,
electromyograms, skin conductance levels, and blood pressure
were measured during the trials. Although subjective pain
ratings were not significantly affected by muscle tension
levels, the chronic pain patients displayed elevated N150
and N150/P260 amplitudes of the somatosensory-evoked potentials
in the low as compared to the high muscle tension condition.
The high risk group showed a trend toward higher N150 amplitudes
in the low as compared to the high tension condition. The
results of this study partially support the hypothesis
that increases in muscle tension might serve as a pain-reducing
mechanism in chronic pain patients and those at risk for
chronicity, thus leading to a vicious pain-tension cycle.