Experimentally induced pain alters the EMG activity of the lumbar multifidus in asymptomatic subjects.

Lumbar musculature plays an important role in stabilization during functional movements such as walking, bending and overhead activities. Current research has focused on the lumbar multifidus (LM) muscle to determine if altered activity of the LM may contribute to low back pain (LBP). Therefore, the purpose of this study was to determine whether experimentally induced LBP affects deep LM muscle activity during functionally oriented tasks, weight shifting and upper extremity lifting. Intramuscular electromyography (EMG) activity of the LM at L4 was measured in 17 healthy adults during a dominant side shoulder flexion and extension task and during a staggered-stance weight shift task. Data were collected at baseline, after pain was induced with hypertonic saline and after the pain had subsided. Comparisons of within-subjects effects were performed with the use of a two-way repeated-measure ANOVA. A significantly higher magnitude of activity was found in the induced pain condition as compared to the baseline condition for the shoulder extension phase of the upper extremity task (p = 0.04). During forward weight shifting, lower values in both the induced pain (p = 0.02) and the recovery conditions (p = 0.01) were measured. During backward weight shifting, lower values were measured during the recovery condition as compared to baseline (p = 0.03). Across trials and subjects the EMG amplitudes were increased during the extension phase of the upper extremity task and decreased during the weight shifting task. These results suggest that the LM does not respond to induced pain in the same manner during different phases of functionally oriented tasks.