Changes in muscle response as a result of perceived pain

Abstract

Chronic musculoskeletal pain is a leading cause of functional limitation, resulting in increased use of the health care system and lost work days. Surface electromyography (EMG) is used as a monitoring test for skeletal muscle activity for individuals with chronic pain. The Baltimore Pain Model is an EMG-based mathematical model developed for use as an objective correlate to levels of perceived pain in skeletal muscle. The purpose of this study was to assess the validity of the EMG model as a physiologic correlate to perceived pain in skeletal muscle. Sixty-one subjects with chronic pain completed the study. Subjects with low back pain (LBP) or facial pain (FP) were assigned to 1 of 3 pain groups, according to their respective EMG recording sites. Pain-free controls were used for comparison. Physical signs and symptoms of pain, and psychological and functional status were assessed. This double-blind study utilized a single subject-subject replication design. All subjects were randomly assigned to 1 of 6 treatment sequences via a Latin square design. Treatment conditions included Lodine, Placebo and Rest (no treatment). EMG from the bilateral masseters was recorded during 4 modes of activity. EMG from the bilateral erector spinae was recorded during 8 modes of activity. All subjects received a pre-treatment EMC, and in addition, subjects with pain recorded their perceived pain levels on a visual analog scale (VAS). The EMG and VAS were repeated 3 times. Subjects were then given 1 of 3 treatments. The EMC and VAS were repeated 3 times at both 30 and 60 minutes following treatment. Subjects returned for 2 more identical study sessions, with the exception of a different treatment intervention. VAS scores were compared with 6 EMG parameters to determine whether there was a relationship between change in pain level and change in EMG activity. Changes in the EMG and pain measures were assessed with both single case and multivariate analyses. Results indicated that changes in the majority of the EMG parameters were reflective of changes in level of pain intensity, regardless of type of treatment intervention. EMG predicted change in pain most consistently when recordings were made over the masseters. During specific movements, CF intercept was the best predictor of change in LBP. Placebo was more effective in reducing LBP and FP than either Lodine or Rest. Results of this study support the use of surface EMG as a physiologic correlate to perceived chronic pain.