Maximum neck extension strength and relative neck muscular load in different cervical spine positions.

The aim of this project was to study how the isometrical neck extensor strength varies at different joint angles in the sagittal plane in order to calculate what fractions of the maximum strength are used in sitting postures to counteract the load moment induced by the weight of the head and neck segments. Ten female subjects sat with the torso fixed in a special device. The resistive force during maximum neck extension was recorded with a strain gauge in four different positions of the lower cervical spine; extended, vertical, slightly flexed and much flexed. For each of these four positions the upper cervical spine was kept in three positions-flexed, neutral and extended. Moments of force about the bilateral motion axes of the atlanto-occipital (Occ-C1) joint and the C7-T1 motion segment were calculated. Moment arms were measured from video images. The maximum muscular moment for Occ-C1 did not vary systematically, either with different positions of the lower or upper cervical spine. The mean neck extensor strength for C7-T, was lowest in the extended lower cervical spine position. In the vertical, slightly flexed or much flexed position little or no variation in strength was seen. However, the fraction of the strength (% MUR) utilized to counteract the load moment induced by the weight of the head and neck in the different postures showed higher utilization ratios when the head-and-neck was flexed (about 10 and 17% MUR, respectively) than when it was kept in a vertical position (about 2% MUR). The results strongly indicate that the flexed cervical spine position produces higher muscular load than vertical, even when taking muscular strength into account, and should thus be avoided during prolonged sitting.