Electromyographic mapping of the erector spinae muscle with varying load and during sustained contraction

The purpose of the study was to explore changes in the spatial distribution of erector spinae electromyography amplitude during static, sustained contractions and during contractions of increasing load. Surface electromyographic (EMG) signals were detected from nine healthy subjects using a grid of 13 x 5 electrodes placed unilaterally over the lumbar erector spinae musculature. Subjects stood in a 20 degrees forward flexed position and performed: (1) six 20-s long contractions with loads ranging from 2.5 kg to 12.5 kg (2.5 kg increments) and (2) a 6 min sustained contraction with 7.5 kg load. Root mean square (RMS) and mean power spectral frequency (MNF) were computed from the recorded EMG signals. EMG RMS increased (P<0.0001) and MNF remained constant during contractions of increased load. During the sustained contraction, MNF decreased (P<0.0001) and RMS did not change over time. The centroid (center of activity) of the RMS map shifted caudally (P<0.0001) with time during the sustained contraction but did not change with varying load. These results suggest a change in the distribution of erector spinae muscle activity with fatigue and a uniform distribution of muscle activation across loads.