Variability in the control of head movements in seated humans: a link withwhiplash injuries?

1. The aim of this study was to determine how context and on-line sensory i nformation are combined to control posture in seated subjects submitted to high-jerk, passive linear accelerations. Subjects were seated with eyes clo sed on a servo-controlled linear sled. They were asked to relax and receive d brief accelerations either sideways or in the fore-aft direction. The sti muli had an abrupt onset, comparable to the jerk experienced during a minor car collision. 2. notation and translation of the head and body were measured using an Opt otrak system. In some of the subjects, surface electromyographic (EMG) resp onses of selected neck and/or back muscles were recorded simultaneously. Fo r each subject, responses were highly stereotyped from the first trial, and showed little sign of habituation or sensitisation. 3. Comparable results were obtained with sideways and fore-aft acceleration s. During each impulse, the head lagged behind the trunk for several tens o f milliseconds. The subjects' head movement responses were distributed as a continuum in between two extreme categories. The 'stiff' subjects showed l ittle rotation or translation of the head relative to the trunk for the who le duration of the impulse. In contrast, the 'floppy' subjects showed a lar ge roll or pitch of the head relative to the trunk in the direction opposit e to the sled movement. This response appeared as an exaggerated 'inertial' response to the impulse. 4. Surface EMG recordings showed that most of the stiff subjects were not c ontracting their superficial neck or back muscles. We think they relied on bilateral contractions of their deep, axial musculature to keep the head-ne ck ensemble in line with the trunk during the movement. 5. About half of the floppy subjects displayed reflex activation of the nec k muscles on the side opposite to the direction of acceleration, which occu rred before or during the head movement and tended to exaggerate it. The ot her floppy subjects seemed to rely on only the passive biomechanical proper ties of their head-neck ensemble to compensate for the perturbation. 6. In our study, proprioception was the sole source of sensory information as long as the head did not move. We therefore presume that the EMG respons es and head movements we observed were mainly triggered by the activation o f stretch receptors in the hips, trunk and/or neck. 7. The visualisation of an imaginary reference in space during sideways imp ulses significantly reduced the head roll exhibited by floppy subjects. Thi s suggests that the adoption by the central nervous system of an extrinsic, 'allocentric' frame of reference instead of an intrinsic, 'egocentric' one may be instrumental for the selection of the stiff strategy. 8. The response of floppy subjects appeared to be maladaptive and likely to increase the risk of whiplash injury during motor vehicle accidents. Evolu tion of postural control may not have taken into account the implications o f passive, high-acceleration perturbations affecting seated subjects.