EMG spectral characteristics of spinal muscles during isometric axial rotation.

The objective of this study was to determine the frequency profile, median frequency (MF) and mean power frequency (MPF) of trunk muscles in an isometric graded maximal voluntary contraction (MVC) in isometric axial trunk rotation from a neutral upright seated posture. Twelve young healthy subjects (seven males, five females) were instrumented with surface electrodes on their external obliques, internal obliques, rectus abdominis, pectoralis, latissimus dorsi and erector spinae at T10 and L3 levels bilaterally. These subjects were stabilized in seated posture in an axial rotation tester (AROT) and asked to perform a graded isometric contraction of their maximal value to both right and left directions from a neutral posture within a period of 10s. EMG from all 14 channels were sampled at 1 kHz at 10% intervals of MVC from 10% to MVC. These samples were subjected to fast Fourier transform analysis. The frequency profile plots demonstrated the power of muscles involved in agonistic and antagonistic activity. However, the frequency composition showed little difference between them. The MF was higher in agonists of the same muscle. The MPF was always higher than MF. Both values were generally insignificantly different between different levels of contraction. However, with increasing level of contraction there was increase in power.