TRUNK MUSCLE ACTIVATION AND COCONTRACTION WHILE RESISTING APPLIED MOMENTS IN A TWISTED POSTURE

Previous studies of twisting have revealed substantial cocontraction o f agonist and antagonist muscles within the torso when torsional momen ts are generated. The objective of the current study was to quantify t he activations and cocontraction of eight trunk muscles as subjects ma intained an axially rotated trunk posture and resisted external applie d bending moments. Ten subjects twisted their torsos 25-degrees to the right (clockwise) and resisted 20 and 40 Nm bending moments from 12 d irections. The moment directions were in a transverse plane and labell ed clockwise as viewed from above, ranging from 0-degrees (mid-saggita l, anterior) to 330-degrees, in 30-degrees increments. RMS EMG amplitu de data were collected using surface electrodes and normalized to maxi mal voluntary contractions. Significant changes were observed in the m uscle responses due to the interaction of the moment direction and mom ent magnitude for six of the eight muscles tested. Comparison of the p resent data with that collected previously in neutral postures indicat ed: (1) a large increase in the activation levels of the right erector spinae and the left external oblique muscles; and (2) a counter-clock wise shift in the moment direction at which the peak activation of the se same muscles occurs. Analysis of the relative activation levels (RA Ls), constructed from the NEMG data to quantify the cocontraction, ind icated that the changes in cocontraction were more robust in response to changes in the bending moment's direction as opposed to changes in bending moment's magnitude.