Corset hypothesis rebutted — Transversus abdominis does not co-contract in unison prior to rapid arm movements

BackgroundThe aim of this paper is to test “corset” model of spinal stability, specifically the hypothesis that feed forward transversus abdominis activity is bilaterally symmetrical and independent of the direction of perturbation to posture due to arm. This study will assess transversus abdominis electromyographical activity bilaterally.MethodsFeed forward intramuscular transversus abdominis electromyographical data and reaction forces on the thorax due to the arm movement were collected and processed for 6 healthy subjects during 6 trials of 8 types of arm movements (randomised order). Reciprocal transversus abdominis indices were calculated as the difference between the normalised integrated feed forward transversus abdominis electromyographical data from each side of the trunk.FindingsThe main finding of the study was that the reciprocal transversus abdominis index was significantly related to the axial rotational forces on the thorax due to arm movement (F = 109.991, p < 0.001). Right arm movements produced clockwise axial rotation forces on the thorax and dominant left transversus abdominis muscle activity.InterpretationThe consequence of this finding is that feed forward transversus abdominis activity is not bilaterally symmetrical and is not independent of the direction of perturbation to posture due to arm movement. Transversus abdominis forms part of a synergy of muscles contributing to the generation of axial rotation forces in the core that oppose the forces due to arm movement. These findings indicate that training bilateral pre-activation of the transversus abdominis prior to rapid movement is not justified and may potentially be problematic for the production of normal movement patterns.