The role of prerotation of the trunk in axial twisting efforts.

The myoelectric activity of selected trunk muscles was recorded during the development of controlled isometric axial torques. Muscle activity was measured bilaterally over the erector spinae, the rectus abdominus, the oblique external and the oblique internal abdominal muscles at the L3 level. Subjects first applied graded isometric torque efforts over a 10 second ramp up to maximum voluntary contraction with the trunk in neutral rotation. They then repeated the effort with the trunk twisted to the left and right. The largest electromyographic activities were found in the agonistic oblique muscles, but considerable antagonistic activity was present also. While the activity of the internal oblique and rectus were bilaterally similar in symmetric standing a difference occurred between the two sides when the trunk was twisted to the right or left. Axial prerotation of the trunk by 30 degrees in the direction of torque development marginally decreased the maximal developed torque, whereas prerotation in the opposite direction increased the developed torque.