Effects of sitting posture and jaw clenching on neck and trunk muscle activities during typing

Neck pain is one of the most common musculoskeletal complaints. Evidence suggests that increased activities of neck and trunk muscles are one of the mechanisms related to neck pain. Jaw clenching and sitting posture may modulate the muscle activity in neck and trunk muscles during typing. The present study aimed to assess the effects of different postural positions and clenching conditions on neck and trunk muscle activities. Thirteen healthy adults (39.8 +/- 5.0 years) performed computer typing tasks in four conditions (two postural positions [upright vs slouched] and two jaw clenching conditions [clenching vs non-clenching]). Integrated surface electromyography (iEMG) was measured in sternocleidomastoid (SCM), upper trapezius (uTP) and middle trapezius (mTP) muscles and compared between conditions. The Friedman and Wilcoxon signed-rank tests with Bonferroni's corrections were used to estimate the condition-specific differences in the iEMG data. The statistical significance level was set at 5%. In both postural positions, iEMG(SCM) was significantly greater under the jaw clenching than under the non-clenching condition (chi(2) = 21.700, P < .01). Under both jaw clenching conditions, iEMG(uTP) was significantly greater in the slouched than in the upright postural position (chi(2) = 23.182, P < .01). No significant differences in iEMG(mTP) were seen across conditions (chi(2) = 5.018, P = .10). Sitting posture and jaw clenching appear to influence activities of different muscles.