Neck and Shoulder Muscle Activity During Work-Related Postural Tasks

Objective: The aim of the study was to investigate the neck and shoulder muscle contractions during a series of postural tasks mimicking dental work. Because of the high prevalence of musculoskeletal pain in females, we also hypothesized that the exerted activity differs between genders. Method: Electromyographic (EMG) activity was recorded unilaterally from the sternocleidomastoid and upper trapezius muscle in 17 participants (8 men and 9 women; mean age = 22.8, sd 1.7 years), who were instructed to perform 44 different postural tasks. EMG amplitude was measured by the root mean square values of the raw signals and normalized to peak maximum contractile values for each muscle (EMG%MVC). The intensity of muscle activity was ranked as light (<3%MVC), moderate (3%-8%MVC)and substantial (>8%MVC). Muscle co-activation was assessed by a co-activity index of both the investigated muscles. The effect of gender on muscle activity was tested by a linear mixed statistical model. Results: During most tasks the two muscles contracted light to moderately and were mildly co-active. However, a few tasks were associated with marked co-activation of the sternocleidomastoid and upper trapezius muscles, such as contralateral trunk and neck rotation (9.9%), head-leaning (6.3%) and shoulder shrugging (5.5%). Head-leaning yielded moderate to substantial muscle activity of both muscles. The sternocleidomastoid muscle was more active during postural tasks involving head movements (0-20%). During trunk and arm movements, the sternocleidomastoid muscle was minimally active compared to the trapezius muscle, which had up to 40% more activity than the sternocleidomastoid muscle. No significant interaction was found between muscle activity and gender (F=3.1; p=0.078), as well as between postural task and gender (F=2.0; p=0.153). Conclusion: The findings provided further insight into EMG muscle activity during tasks that mimicked dental work thus providing a basis for further investigation into the behaviour of these muscles in dentists during clinical work.