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

Abstract

1. The aim of this study was to determine how context and on-line sensory information are combined to control posture in seated subjects submitted to high-jerk, passive linear accelerations. Subjects were seated with eyes closed on a servo-controlled linear sled. They were asked to relax and received brief accelerations either sideways or in the fore-aft direction. The stimuli had an abrupt onset, comparable to the jerk experienced during a minor car collision.2. Rotation and translation of the head and body were measured using an Optotrak system. In some of the subjects, surface electromyographic (EMG) responses of selected neck and/or back muscles were recorded simultaneously. For 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 accelerations. During each impulse, the head lagged behind the trunk for several tens of milliseconds. The subjects' head movement responses were distributed as a continuum in between two extreme categories. The 'stiff' subjects showed little rotation or translation of the head relative to the trunk for the whole duration of the impulse. In contrast, the 'floppy' subjects showed a large roll or pitch of the head relative to the trunk in the direction opposite 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 contracting their superficial neck or back muscles. We think they relied on bilateral contractions of their deep, axial musculature to keep the head-neck ensemble in line with the trunk during the movement.5. About half of the floppy subjects displayed reflex activation of the neck muscles on the side opposite to the direction of acceleration, which occurred before or during the head movement and tended to exaggerate it. The other floppy subjects seemed to rely on only the passive biomechanical properties 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 responses and head movements we observed were mainly triggered by the activation of stretch receptors in the hips, trunk and/or neck.7. The visualisation of an imaginary reference in space during sideways impulses significantly reduced the head roll exhibited by floppy subjects. This 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. Evolution of postural control may not have taken into account the implications of passive, high-acceleration perturbations affecting seated subjects.

Adaptation of Postural Reactions in Seated Positions and Influence of Head Posture when Exposed to a Single Sideway Perturbation: Relevance for Driving on Irregular Terrain

“…A strict stiff strategy might cause muscle fatigue or myalgia while a relaxed strategy, depending on passive structures, might increase the risk for injury in joint structures and tissues. Contrary to Vibert et al (2001), who found no or little adaptation in sideways reactions, seated postural reactions in the forward direction have been found to adapt after the first perturbation with decreased EMG amplitude over time [18,19]. If seated postural reactions in sideways directions adapts or not is still an unanswered question.…”

“…An increased segment angle should be possible as the EMG amplitudes was reduced. However, deeper muscles might be more active [8,30].…”

“…However, sideways perturbations have been suggested to cause two types of reaction strategies, stiff and sloppy. Both reactions were found to be stereotypical, using either muscle co-contractions (stiff strategy) or a reciprocal more relaxed muscle activity (sloppy strategy) [8]. A strict stiff strategy might cause muscle fatigue or myalgia while a relaxed strategy, depending on passive structures, might increase the risk for injury in joint structures and tissues.…”

“…reaction to a sideways perturbation has been suggested to have a reciprocal activation pattern in the neck, starting in the contralateral muscle that stretches first [8,14]. The EMG amplitude in the neck region has been reported to be high, especially in the contralateral splenius capitis and is therefore most likely to be injured [14].…”

“…Perturbations are suggested to be hazardous to the spine where muscles may overcompensate in their reactions [31] or be passive relying on intrinsic mechanism [8]. In this study the UT was excluded from further analyses since the level was considered too low (< 2% of an MVC), reducing the possibility to detect clear muscle onset.…”