Muscular rest and gap frequency as EMG measures of physical exposure: the impact of work tasks and individual related factors

Owing to an orderly recruitment of motor units, low threshold type I fibres are presumed to be vulnerable in contractions of long duration. To study l oad on these fibres muscular rest was registered as the time fraction of el ectromyographic (EMG) activity below a threshold. Moreover, the frequency o f periods with muscular rest, EMG gaps, was derived, since a low gap freque ncy has been shown to be a risk factor for musculoskeletal disorders. Trape zius EMG was registered in 24 female hospital cleaners, 21 female office wo rkers and 13 male office workers during one working day. Cleaners have a hi gh risk of neck/shoulder pain and had much less muscular rest than office w orkers measured as a percentage of total registered time (median value= 1.5 %, range= 0.2-13% vs. median value= 12%, range= 0.0-32%, respectively). Gap frequency showed no difference between the two occupational groups. Both m easures displayed a wide inter-individual variation. For the cleaners, some of the variance was explained by body mass index (BMI) and age, with lower values of muscular rest for older subjects with a high BMI. Among the offi ce workers, low values of muscular rest and a high gap frequency were regis tered in subjects with a low subjective muscular tension tendency. Gender, strength, smoking, job strain, employment time and musculoskeletal symptoms had no impact on either EMG measure.