This study evaluated the differences in the right and left muscular activities of the trapezius and erector spinae when operating a computer mouse during various sitting postures (Backrest conditions (3) x Lower extremity conditions (3)). The effects of personal characteristics, asymmetric sitting posture, asymmetric task of upper extremity, and compensation mechanism on muscle activity will be discussed as follows. One important point in our discussions is that the lower muscle activity of trapezius should the better condition in musculoskeletal activity of neck/shoulders regions, and higher muscle activity of erectus spinae should the better condition in musculoskeletal activity of low back region. These considerations were based on the previous studies findings; the higher muscle activity of trapezius and the lower muscle activity of erector spinae in poor sitting postures were stated (15, 16).
Effect of personal characteristics on trapezius activity
In this study, we found that the muscle activity of the trapezius was lower in females than in males, and lower in the healthy and underweight groups than in the overweight group. The previous studies also found that increased contraction of neck/shoulder muscles in poor sitting posture (16). And, a significant trend for increased obesity risk and metabolic symptoms among office workers was found (4). In our study, personal characteristics, male gender and overweight BMI, are indicative of a need for more attention to sitting postures and their effects on work-related musculoskeletal discomfort in the neck/shoulder regions in sedentary people.
Effect of personal characteristics on erector spinae activity
In this study, regarding the BMI variable, we found that the muscle activity of the erector spinae was higher in the healthy group than in the overweight and underweight groups. Regarding the age variable, the muscle activity of the erector spinae was higher in the 20–29 and forty-and-over groups than in the 30–39 group. A previous study reported that participants without low back pain could control their lumbar-pelvic regions better while sitting than could people without pain (15, 17, 18). The previous studies also found that reduced contraction of trunk muscles in poor sitting posture (15). The personal characteristics, such as 30–39 in the Age variable, and the overweight and underweight groups in the BMI variable, should indicate a need to pay more attention to sitting postures and their effects on work-related musculoskeletal discomfort in the low back in sedentary people.
Effect of sitting postures on trapezius activity
Our finding that trapezius activity was lower in the left cross-legged posture (asymmetric sitting posture) than in symmetric sitting posture, but no difference was found between the right cross-legged posture (asymmetric sitting posture) and the symmetric sitting posture. The game task with a computer mouse (an asymmetric task) was adopted in our research. All of the subjects were coincidentally right handed, so in this study, the right side was the task side and the left side was the non-task side. A previous study reported that trapezius activity was lower on the non-task side than on the task side when sedentary people performed computer mouse task in symmetric sitting (5). The previous studies also found that the compensation mechanism in cross-legged sitting, such as weight shifting to the ipsilateral side (9), posterior pelvic tilt (7, 8), and reduced contraction of abdominal muscles (8, 15). Therefore, the compensation mechanism might be an explanation for our finding. It suggested that the compensation mechanism have been adopted in the trunk and pelvic, so that both right and left sides of trapezius muscle activity decreased in non-task side cross-legged sitting than in symmetric sitting during the operation of a computer mouse.
Another finding of this study was that the trapezius activities were no significant differences in the backrest conditions is consistent with the study of Caneiro, O'Sullivan (19). The backrest conditions, including full-backrest, non-backrest, and humpbacked-backrest, have a few minor effects on the trapezius activities.
Effect of sitting postures on erector spinae activity
Our research found greater muscle activity in the right erector spinae than in the left erector spinae in asymmetric sitting postures (both left and right cross-legged postures), but not in the symmetric sitting posture. Previous research showed that the muscle activity of the external oblique was significantly higher in asymmetrical sitting than in symmetric sitting (7). Another study found that continuous cross-legged sitting caused significant malalignment of the pelvis (9) and trunk (10). In our study, cross-legged sitting, in both right and left cross-legged sitting postures, was found to significantly increase right erector spinae activity. In our study, the right side was task side during computer mouse task. Therefore, it was suggested that the body using the right (task side) erector spinae muscles to compensate for the spine asymmetry caused by the cross-legged sitting postures.
The present study did not collect the information regarding the subjects’ perception of musculoskeletal discomfort. In the future, the relationship between musculoskeletal discomfort and compensation mechanism should be investigated for sedentary people performing symmetric and asymmetric tasks in cross-legged sitting posture for prolonged periods.