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Abstract
Sustained joint load in extreme positions (namely maximally flexed or extended positions) has been described as causing pain. The aim of the present study is to analyse eight different sitting work postures with respect to extreme positions, and to assess the mechanical load and the levels of muscular activity arising in defined extreme positions of the cervical spine. Ten healthy female workers from an electronics plant took part in laboratory experiments. For seven of these, levels of neck and shoulder muscular activity in sitting postures with the cervical spine in different manually-adjusted extreme positions were recorded using surface electrodes. Loading moments of force about the bilateral motion axis of the atlanto-occipital joint (Occ-C1) and the spinal cervico-thoractc motion segments (C7-T1) were calculated. Extreme or almost extreme positions occurred in sitting postures with the thoracolumbar back inclined slightly backwards or with the whole spine flexed. Electromyographic (EMG) activity levels were very low in the manually-adjusted extreme positions. The load moment for the Occ-Cl joint when the whole neck was flexed was only 1·2 times the value for the neutral position of the head, but for C7-T1 it increased to 3·6 times. It is concluded that extreme positions of the cervical spine do occur in sitting work postures, and that the levels of muscular activity in such positions are low. Thus, recordings of muscle activity and calculations of load moment alone are not a sufficient basis for evaluating work postures: thorough recordings of spine positions should be included.
