Identification of a two pivot human neck model using linear anterior-posterior perturbations

Eight healthy young adult males seated in a rigid chair and restrained by a five point harness belt underwent anterior-posterior random appearing multisine perturbations with a frequency range of 0.3-20 Hz. Six different conditions were tested differentiating in maximum acceleration level ([1;2;4;8] m/s²) and in task (mental arithmetic and blindfolded). The head and neck kinematics were captured by a Qualisys motion capture system and Xsens accelerometers. Muscle activity of the trapezius and sternocleidomastoid muscles was collected by a Delsys EMG system. A two pivot neck model was developed representing the head-neck kinematics separating upper and lower neck kinematics. The kinematics were described with an error margin of 2.5 % of the maximum range of motion of the head. The amount of neck deformation relative to the perturbation is expressed in gain and phase, the linearity is expressed in squared coherence. For the head-neck kinematics a significant (P < 0.05) increase in gain was found for decreasing acceleration levels, indicating non-linearity of the human reflexes and/or the passive neck mechanics. At lower frequencies, the mental arithmetic task resulted in a 9 % decrease of neck deformations (P < 0.01). At lower frequencies, blindfolding resulted in a 16 % increase of neck deformations (P < 0.05). The pivot rotations showed for low acceleration levels similar gain and phase characteristics for both the upper and lower pivot up to 3 Hz. Increasing acceleration levels resulted in a major decrease of relative upper neck deformations (P < 0.05) and an increase of relative lower neck deformations (P < 0.01), suggesting different control strategies for both pivots. For frequencies above approximately 5 Hz an increasing phase lag up to 180o for the upper pivot with respect to the lower pivot is found, indicating C-shaped neck bending for low frequencies and S-shaped neck bending for high frequencies. With the exception of the upper pivot response the squared coherence showed globally values above 0.5 between 1-12 Hz.