Biomechanical aspects of occupational neck postures during dental work

A typical occupational risk factor for developing neck symptoms is prolonge d flexion of the cervical spine. The present aim was to determine joint mom ents and muscle activity of the neck during forward flexion of the cervical spine to evaluate the load in the neck region. Three dimensional video (3- D) and surface electromyography (EMG) from the splenius muscles were record ed in two common work postures. Using a 3-D static link segment model, mome nts at the atlanto-occipital (A-O) joint and the seventh cervical-first tho racal (C7-T1) joint were estimated. Maximal extension moments were estimate d from maximal neck extension strength. Extension moments at the C7-T1 join t were significantly higher for a highly flexed position (45% of max) compa red to a moderately flexed position (32% of max), but remained unchanged at the A-O joint (40% of max). The mean RMS amplitude was 9% of maximal EMG i n both positions (no bilateral differences). This difference between mechan ical load and muscle load indicates that EMG may seriously underestimate th e total loads of the tissue. Lateral flexion influenced the lateral flexion moment while rotation did not influence the rotation moment. The study dem onstrates the importance of quantification of joint loads in occupational r isk assessment of the neck.