On neck muscle activity and load reduction in sitting postures. An electromyographic and biomechanical study with applications in ergonomics and rehabilitation.

In this study of the biomechanics and muscular function of the cervical spine, skilled women workers simulated standardized electromechanical assembly work in eight sitting postures. Normalized electromyography was used to quantify activity in neck-and-shoulder muscles. With the whole spine flexed, muscle activity in the cervical erector spinae, trapezius and thoracic erector spinae muscles was higher than when the whole spine was straight and vertical. The posture with the trunk slightly inclined backward and neck vertical gave the lowest activity levels. Flexed neck compared to vertical neck gave higher activity in the cervical erector spinae. Work with abducted arm gave high neck muscle activity. Work postures can thus be optimized to diminish neck muscle load. Two ergonomic acids were studied during the work cycle. Elbow support reduced the activity in the trapezius and thoracic erector spinae/rhomboids muscles in the posture with the whole spine flexed and in the posture with the whole spine vertical. Arm suspension gave mainly similar reduction in these postures, and also a reduction in the cervical erector spinae. In the position with the trunk slightly inclined backward, arm suspension gave a reduction in the trapezius. These findings indicate that arm support or arm suspension can be used to reduce neck muscle load. Three methodological studies related to neck muscle load and normalization were included. 1) Examination of the effect of different isometric maximum test contractions on neck muscles showed that all contractions activated all muscles studied, including those on the contralateral side, to some extent and at various levels. The highest frequency of attained maximum levels was: for neck extension, in cervical erector spinae; for cervical spinae lateral flexion, in splenius and levator scapulae; for arm abduction, in trapezius, and, for shoulder elevation and scapular retraction/elevation, in thoracic erector spinae/rhomboids. Proximal resistance gave higher activity than distal. 2) The relationship between EMG activity and muscular moment was studied in women during submaximal and maximum isometric neck extension. The relationship found was non-linear, with greater increase in activity at high moments in the posterior neck muscles studied. The slightly flexed cervical spine position induced a higher level of activity in erector spinae cervicalis than did the neutral position for a given relative muscular moment. 3) Muscular activity was related to cervical spine position during maximum isometric neck extension. Peak activity in the cervical erector spinae was found in the slightly flexed lower-cervical spine position.(ABSTRACT TRUNCATED AT 400 WORDS)