Electromyographic study of trunk muscle activity during unresisted twisting posture in various twisting angles.

The study investigated muscular activities of trunk twisting during standing posture by observing the surface EMG signal to estimate which muscles were active in graded isometric twisting posture. A laser pointer was used to maintain the twisting angle, while external resistances, which were commonly used to maintain the posture, were avoided. This procedure approached actual working posture in real life. Grading postural effort into various twisting angles provided reliable and natural posture measurements. Erector spinae (ES) at levels of L2 and L5 were selected to represent low back muscles, while external oblique (EO) and internal oblique (IO) were represented as abdominal muscles. Ten male subjects were asked to maintain a trunk twisting posture in various twisting angles for each right and left twisting direction. The EMG of the observed muscles were analyzed by power spectral density, and the activity changes were normalized by procedures of relative presentation and standardization. A Tukey multiple comparison was also applied to compare the activity changes among postural stages. Higher activity in contralateral ES, contralateral EO and ipsilateral IO is shown in the results, however, a significant increase against the initial posture (i.e., no-twisting posture) was found when the twisting angle was more than 30 degrees. Consistency with most past studies was shown in the result for the muscular activity of abdominal muscles, but not in the results for the low back muscles. The result emphasized the consideration that trunk muscle activity under a natural twisting condition was not always the same as that with the twisting under external resistance.