Torque and EMG in isometric graded flexion-rotation and extension-rotation

The main objective of the study was to measure the magnitude and pattern of electromyographic (EMG) activity of the trunk muscles in combined motions of flexion-rotation and extension-rotation. Another objective of the study was to determine the torque-EMG Root Mean Square (RMS) relationship in these activities for predictive purposes. Nineteen normal young adult subjects who met the inclusion criteria were fitted with 12 pairs of surface electrodes on their external and internal oblique, rectus abdominis, latissimus dorsi, and erector spinae muscles at T10 and L3 levels bilaterally. Using a Posture Stabilizing Platform (PSP) they were fixed in 40 degrees flexed and 40 degrees right rotated trunk postures. From this posture subjects attempted an isometric extension-rotation and flexion-rotation in the plane defined by the asymmetrical postural axis in the plane of rotation of the assumed posture. Contractions of 25, 50 and 75% of the previously measured maximal voluntary contraction (MVC) were attempted using a Static Dynamic Strength Tester (SDST) and force monitor attached by a steel cable to a rigid thoracic harness worn by the subjects. The data were acquired at 1 kHz. Descriptive statistics were calculated and ANOVA, correlation and regression analyses were carried out. With linear increase in flexion-rotation and extension-rotation torque, the magnitude of total EMG output increased exponentially. In flexion-rotation the EMG magnitude of all muscles increased with increasing grades of contraction (% of MVC). However, in proportional terms, the magnitude of erector spinae EMG declined. In extension-rotation there was a reversal of roles between the ventral and dorsal muscles compared to the flexion-rotation. For the two genders, all muscles and grades of contractions were significantly different from each other (p<0.01). There was a significant but modest correlation between EMG and torque (r = 0.25 to 0.54; p<0.01). The regressions were significant (p<0.01) and explained up to 74% of the variance in torque. The flexion-rotation and extension-rotation torques can be predicted with only a moderate accuracy.