Stability of the head in pitch (neck flexion-extension): studies in normal subjects and patients with axial rigidity.

The dynamic stability of the head in pitch during normal upright posture has been studied in normal subjects and patients with neurological disease affecting neck muscle tone by examining angular head acceleration responses to unpredictable linear motion of the trunk in the direction of surge. Within the frequency range of natural head movements the transfer function between head and trunk for both normal subjects and patients approximated a second-order linear differential equation involving inertia and coefficients of viscosity and elasticity. The degree of neck rigidity was determined by the damping ratio (viscosity:elasticity), which averaged .35 for normal subjects and ranged from 0.6 to 0.96 for patients with rigid syndromes. A patient with absent labyrinthine function and a "floppy" head had a damping ratio 0.18. The technique gives a numerical measurement of neck rigidity, which could be of value in characterising severity of disorder and response to therapy.