TRUNK MUSCLE ACTIVITIES DURING ASYMMETRIC TWISTING MOTIONS

Axial twisting of the torso has been identified via epidemiologic stud ies as a significant risk factor for occupationally-related low back d isorders. However, only recently have biomechanical studies been able to describe how twisting is accomplished through the use of the trunk musculature. These studies have been performed on subjects whose torso twists were performed in an upright posture. In this study, the elect romyographic activity of ten trunk muscles was observed while 12 subje cts performed twisting exertions in three different trunk postures. Th ese postures included upright twisting, twisting while the trunk was f lexed in the sagittal plane, and twisting while the trunk was flexed a nd rotated asymmetrically. In addition, twisting velocity and directio n of motion were changed under the experimental conditions. Under upri ght twisting conditions, the twisting torque was generated easily and relatively efficiently through the employment of the oblique (internal and external) and latissimus dorsi muscles. When the trunk was flexed the activity of erector spinae muscles increased (about 10-15%) while the external oblique activity decreased (about 3-5%). Twisting while in asymmetric bent postures was accomplished with a reduction in obliq ue and latissimus dorsi muscle activities (approximately 5%) while the erector spinae muscle activity remained elevated. The change in muscl e activity needed to balance the torso during twisting while bending a lso increased the amount of lateral torque that was produced by the tr unk. These findings suggest that studies observing trunk muscle activi ties and trunk loading while subjects were in upright postures should be interpreted with caution when evaluating the activity of the trunk during occupational activities. Since many occupational twisting tasks are performed in awkward, asymmetric postures, application of results from upright twisting studies might underestimate the risk of these a ctivities. (C) 1998 Elsevier Science Ltd. All rights reserved.