EMG power spectra of trunk muscles during graded maximal voluntary isometric contraction in flexion-rotation and extension-rotation.

The purpose of this study was to determine the electromyographic (EMG) power spectral characteristics of seven trunk muscles bilaterally during two complex isometric activities extension-rotation and flexion-rotation, in both genders to describe the frequency-domain parameters. Eighteen normal young subjects volunteered for the study. The subjects performed steadily increasing isometric extension-rotation and flexion-rotation contractions in a standard trunk posture (40 degrees flexed and 40 degrees rotated to the right). A surface EMG was recorded from the external and internal oblique, rectus abdominis, pectoralis, latissimus dorsi, and erector spinae muscles at the 10th thoracic and the 3rd lumbar vertebral levels, at 1 kHz and 25%, 50%, 75% and 100% of maximal voluntary contraction (MVC). The median frequency (MF), mean power frequency (MPF), frequency spread and peak power were obtained from fast Fourier transform analysis. The MF and MPF for both extension-rotation and flexion-rotation increased with the grade of contraction for both males and females. The EMG spectra in flexion-rotation were different from those of extension-rotation (P < 0.001). The left external and right internal oblique muscles played the role of antagonists in trunk extension-rotation. There was an increase in the MF of the trunk muscles with increasing magnitude of contraction. Frequency-domain parameters for both the male and female subjects were significantly different (P < 0.001).