Study Design. In vivo human volunteer study of the intervertebral postural changes and muscle activity levels while tensing the neck muscles.
Objective. To determine if actively tensing the neck muscles changes the posture of the cervical spine and, because axial impact neck injury often occurs while inverted, whether these changes exist both upright and upside down.
Summary of Background Data. Rollover accidents are dynamic and complex events in which head contacts with the vehicle interior can cause catastrophic neck injuries. Computational modeling has suggested that active neck muscles may increase the risk of cervical spine fracture in a rollover crash. Cadaver testing has also demonstrated that overall neck alignment and curvature are key to understanding and preventing catastrophic neck injuries. Although muscle activity and neck posture affects the resulting injury, there are currently no in vivo data describing how tensing the neck muscles influences intervertebral posture.
Methods. Eleven human subjects (6 females, 5 males) actively tensed their neck muscles while seated upright and inverted. Vertebral alignment was measured using fluoroscopy and muscle activity was recorded using surface and indwelling electrodes in 8 neck muscles.
Results. On average, tensed muscles increased cervical spine curvature and anterior motion of the cervical vertebrae relative to the torso. These changes, which were magnified by inversion, indicate that cervical intervertebral posture differs considerably between the relaxed and tensed states.
Conclusion. Active muscle contraction can change the vertebral alignment in upright and inverted postures. This change in posture may alter the load path and injury mechanics during an axial head impact and may help explain the disparity between the neck injuries observed in real-world rollover accidents and ex vivo cadaver experiments.
Level of Evidence: N/A