Control of positioning the cervical spine and its application to measuring extensor strength.

The great variability of the flexion of the cervical spine renders an exact description of the control of various positions difficult. A method was developed enabling a precise control of positioning the cervical spine and head in the sagittal plane. In three repeated measurements the mean values of the position of external anatomical landmarks and distances between them exhibited a good reproducibility. Any variable effect of gravity on the activity of the neck muscles at different positions of the cervical spine was eliminated by the passive compensation of gravity. The significance of methodical details is illustrated by the results of an applied study. The maximum strength of neck extensors was examined in 12 male subjects in a supine position at four different flexion angles from 0 to 30 degrees of the cervical spine. The vertical force component was measured. The maximum voluntary moments of forces about the bilateral motion axis of the C7T1 motion segment exhibited a tendency to decrease with increasing flexion.