Electromyographic response of the trunk muscles to postural perturbation in sitting subjects.

Electromyographic (EMG) patterns of the trunk muscles were investigated during balance perturbations in a sitting position. Five healthy subjects (two females and three males, mean age 24.4 yr) were seated on a platform allowing rotational perturbations in the frontal and sagittal planes. Each of the forward, backward, right and left perturbations were delivered with and without expectation at the velocities 8 degrees s-1 (SD +/- 4 degrees s-1) and 26 degrees s-1 (SD +/- 6 degrees s-1). The fast expected and unexpected perturbations were compared in blindfolded subjects. In the slow perturbations the effect of a blindfold on the balance reactions was tested. The root mean square (RMS) EMG signals from 12 trunk muscles were recorded and analysed to determine the EMG magnitudes. During the forward and backward rotations there was always a symmetrical EMG pattern between corresponding muscles on the right and left sides. A reciprocal phasic EMG activity between the left and right back muscles was found during frontal rotations. No reciprocal phasic activity was found in the abdominal muscles. Neither verbal instruction about the upcoming perturbation nor the blindfold changed the activation patterns.