The purpose of this study was to examine the influences of chronic shoulder pain on the muscle tone in trunk muscles. The study's subjects were 40 men and women in their 30 to 50s, which were divided into two groups. A chronic shoulder pain group consisted of 20 subjects who had been diagnosed with chronic shoulder pain by doctors, and a painless group consisted of 20 subjects who had experienced no such pain. An analysis was performed using electromyography on the muscle tone in the rectus abdominalis, external oblique, internal oblique, and erector spinae muscles under the same conditions between the two groups. The analysis results were as follows. The chronic shoulder pain group exhibited an overall high level of trunk muscle tone than the painless group, along with a statistically significant difference in the rectus abdominalis(p<.05). Moreover, the chronic shoulder pain group showed differences in the trunk muscle tone depending on the affected side. The chronic left shoulder pain group yielded higher levels of muscle tone in the right-side trunk muscles. In particular, the group revealed statistically significant differences in the rectus abdominalis and internal oblique(p<.05). The chronic right shoulder pain group exhibited higher levels of muscle tone in the left-side trunk muscles with a statistically significant difference in the internal oblique(p<.05). The above results suggested that chronic shoulder pain influences increases in the muscle tone in the trunk muscles on the opposite side to the affected shoulder.