Methods:

Twenty one healthy subjects (10M:11F) participated. Electromyographic activity was recorded from back and abdominal muscles and flexor torque was measured using a dynamometer. Subjects performed a series of isometric voluntary contractions (10%–100% MVC) of rectus abdominis during which TMS was applied to the motor cortex. The resulting superimposed twitches (SIT) were measured and VA was derived.

Methods and Results

Fifteen patients with chronic mechanical low back pain (LBP) and 10 healthy adults were recruited. Patients completed the Roland-Morris Disability Questionnaire (RMDQ) and bilateral EMG activity was obtained from erector spinae at vertebral level L4 and T12. Subjects performed 3 brief maximal voluntary contractions (MVCs) of the back extensors and the torque was measured using a dynamometer. CWT was applied to the EMG signals of each muscle in a 200ms window centred around the peak torque obtained during the MVCs. The ratio (low/high frequencies) of the energy, the peak energy, and the frequency of the peak energy were calculated for each muscle and then averaged and correlated with the individual's RMDQ score.

Patients had significantly lower peak power than the controls (p=0.04). Additionally, RMDQ positively correlated to the average ratio of energy (rho=0.71; p=0.01), meaning higher disability corresponded to a dominant distribution of energy in the lower-frequencies; but negatively correlated to the average frequency of peak energy (rho=-0.61; p=0.035), meaning lower frequency of peak energy corresponded to higher levels of disability.