Relationship between Electromyographic Signal Amplitude and Thickness Change of the Trunk Muscles in Patients with and Without Low Back Pain

Objectives: To compare the relative thickness change of the transversal abdominal (TrA) and lumbar multifidus (LM) muscles during activation in individuals with and without low back pain (LBP), and to establish a relationship between surface electromyography (sEMG) signal amplitude and the relative thickness change of the corresponding muscle during clinically relevant activity, with preferential activation of TrA/LM. Materials and Methods: Thirty-seven pain-free participants and 36 LBP patients were assessed by ultrasound for thickness changes of TrA and LM and by sEMG for changes of electrical activity of the same muscles. sEMG is done with wireless LUMBIA system. The position of the sEMG sensors and activation maneuvers were chosen carefully. Results: Significant group effect was found for relative thickness change of TrA (F 1,142 =60.69, P<0.0001) and LM (F 1,142 =36.01, P<0.0001). We found significant correlations between relative thickness change of TrA and sEMG signal amplitude on both sides for LBP (r=0.46 to 0.63, P<0.05) and pain-free patients (r=0.43-0.47, P<0.05). The correlation between LM thickness change and sEMG was significant in pain-free participants for both sides (r=0.36 to 0.38 P<0.05), and right LM in LBP participants (r=0.43, P<0.05), but not for LM in LBP group (r=0.16, P=0.351). Discussion: US and sEMG measurements can be used for objective TrA/LM assessment. Correlation results suggest that the relative change of the muscle thickness could be used as the indicator of the muscle activity. Insight into the activity of TrA/LM in pain-free individuals and LBP patients during and after painful episodes may clarify the role of functional abnormalities of these muscles in LBP.