No abstract

Predicting neck kinematics and tissue level response is essential to evaluate the potential for occupant injury in rear impact. A detailed 50th percentile male finite element model, previously validated for frontal impact, was validated for rear impact scenarios with material properties based on actual tissue properties from the literature. The model was validated for kinematic response using 4 g volunteer and 7 g cadaver rear impacts, and at the tissue level with 8 g isolated full spine rear impact data. The model was then used to predict capsular ligament (CL) strain for increasing rear impact severity, since CL strain has been implicated as a source of prolonged pain resulting from whiplash injury. The model predicted the onset of CL injury for a 14 g rear impact, in agreement with motor vehicle crash epidemiology. More extensive and severe injuries were predicted with increasing impact severity. The importance of muscle activation was demonstrated for a 7 g rear impact where the CL strain was reduced from 28 to 13% with active muscles. These aspects have not previously been demonstrated experimentally, since injurious load levels cannot be applied to live human subjects. This study bridges the gap between low intensity volunteer impacts and high intensity cadaver impacts, and predicts tissue level response to assess the potential for occupant injury.

No abstract

Neck muscle activity evoked by vestibular stimuli is a clinical measure for evaluating the function of the vestibular apparatus. Cervical vestibular-evoked myogenic potentials (cVEMP) are most commonly measured in the sternocleidomastoid muscle (and more recently the splenius capitis muscle) in response to air-conducted sound, bone-conducted vibration or electrical vestibular stimuli. It is currently unknown, however, whether and how other neck muscles respond to vestibular stimuli. Here we measured activity bilaterally in the sternocleidomastoid, splenius capitis, sternohyoid, semispinalis capitis, multifidus, rectus capitis posterior, and obliquus capitis inferior using indwelling electrodes in two subjects exposed to binaural bipolar electrical vestibular stimuli. All recorded neck muscles responded to the electrical vestibular stimuli (0–100 Hz) provided they were active. Furthermore, the evoked responses were inverted on either side of the neck, consistent with a coordinated contribution of all left-right muscle pairs acting as antagonists in response to the electrically-evoked vestibular error of head motion. Overall, our results suggest that, as previously observed in cat neck muscles, broad connections exist between the human vestibular system and neck motoneurons and highlight the need for future investigations to establish their neural connections.

We measured maximum isometric neck strength under combinations of flexion/extension, lateral bending and axial rotation to determine whether neck strength in three dimensions (3D) can be predicted from principal axes strength. This would allow biomechanical modelers to validate their neck models across many directions using only principal axis strength data. Maximum isometric neck moments were measured in 9 male volunteers (29±9 years) for 17 directions. The 3D moments were normalized by the principal axis moments, and compared to unity for all directions tested. Finally, each subject's maximum principal axis moments were used to predict their resultant moment in the off-axis directions. Maximum moments were 30±6 N m in flexion, 32±9 N m in lateral bending, 51±11 N m in extension, and 13±5 N m in axial rotation. The normalized 3D moments were not significantly different from unity (95% confidence interval contained one), except for three directions that combined ipsilateral axial rotation and lateral bending; in these directions the normalized moments exceeded one. Predicted resultant moments compared well to the actual measured values (r2=0.88). Despite exceeding unity, the normalized moments were consistent across subjects to allow prediction of maximum 3D neck strength using principal axes neck strength.