Study One:This study aimed to quantitatively describe neck and shoulder muscle activity during a series of postural tasks, and determine any muscle activity difference between genders.
Electromyographic (EMG) activity was recorded unilaterally from the sternocleidomastoid and trapezius muscle of 17 university students (eight males and nine females; 20-26 years) whilst they were directed to perform a series of postural tasks related to dental work. 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%MVC ≤ EMG ≤ 8%MVC), and substantial (>8%MVC).
During the majority of postural tasks, both muscles elicited only light to moderate contraction intensities. Very few postures tested yielded substantial contraction activity in either muscle. Those postures that did were, to a large degree, anatomically predictable.
Muscle activity did not differ significantly between male and female participants (F = 3.1; P = 0.078). Our findings provide normative values to assist future studies of muscle activity of neck and shoulder muscles.
Study Two: Work related musculoskeletal disorders (WMSDs) in dental workers, including orthodontists, are common. This study aimed to assess the contraction pattern of neck and shoulder muscles of orthodontists in terms of intensity and duration, during natural vocational and non-vocational conditions.
Electromyographic (EMG) activity of the right sternocleidomastoid and trapezius muscles was recorded by means of portable recorders in eight postgraduate orthodontic students (5 males, 3 females; 27-39 years) during working conditions, and both active and resting non-working conditions. Recordings were analysed in terms of contraction episode count, amplitude and duration.
The sternocleidomastoid and trapezius muscles contract approximately 40-60 times per hour in the natural environment. Their EMG activity patterns generally consist of short duration, low-amplitude contraction episodes. For both muscles investigated, the median duration of contraction episodes ranged from 4.6 to 11.8 seconds. The amplitude of the large majority (95%) of contraction episodes was less than 25% of maximum voluntary activity (MVC). There were no significant differences (P > 0.05) in overall counts, or intensity of contraction episodes across the experimental conditions. There were, however, strongly significant (P < 0.001) differences in duration of contraction episodes across experimental conditions, with a two to threefold increase in average duration of contraction of the trapezius muscle found in the vocational setting.
During orthodontic work, operators commonly sustain muscular contractions for significantly longer periods than are encountered in non-vocational settings. These findings provided evidence that orthodontic work was accompanied with long-lasting low-level muscle contractions, the occurrence of which have frequently been considered a risk factor for muscle pain and WMSD. Our findings may help strengthen associations between epidemiological risk assessments and proposed pathophysiological mechanisms occurring at the motor unit level.
