Cervical spine position versus e.m.g. activity in neck muscles during maximum isometric neck extension.

The aim of the study was to describe the relationships between cervical spine positions and neck muscle activity during maximum isometric neck extension. Ten healthy women participated (mean age 25·6 years). Maximum isometric neck extensions were performed in 12 different neck positions with resistance applied againstthe back of the head. Surface e.m.g. was recorded from posterior neck muscles at six locations. The e.m.g. signals were rectified, low-pass filtered and normalized. The position of the upper cervical spine did not influence the e.m.g. levels, whereas the lower cervical spine positions did. The e.m.g. levels, in the cervical erector spinae/trapezius were higher in the slightly flexed lower cervical spine position than in the neutral (despite an approximately similar magnitude of muscular moment developed), when the upper cervical spine was in the neutral position. However, when the lower cervical spine was much flexed the e.m.g. level was about the same as that of the neutral lower cervical spine position (with upper cervical spine neutral). The e.m.g. levels of the splenius and the thoracic erector spinae/rhomboids varied in a similar way, but less pronouncedly. These findings are of interest for the method of normalization of e.m.g,. e.g. in studies of work postures and/or movements.