Attenuation of human neck muscle activity following repeated imposed trunk-forward linear acceleration

Abstract

It has been suggested that, after a passive linear acceleration of a seated subject which resembles a small, rear-end car impact, sensory information from proprioceptive, vestibular, and visual systems elicit stabilizing neck muscular responses. These neck muscular responses are presumably reflex based and are modified with the magnitude of the perturbation. A key issue that remains is to determine whether the neck and head postural responses can be modulated by a previous experience of the acceleration and not only by the magnitude of the acceleration. This question is of interest because, contrary to cadaver studies, one could expect that humans apprehending a rapid trunk acceleration would adopt a bracing behavior to minimize head movements. The aim of the present experiment was to verify whether neck-muscle activities can be modulated when prior knowledge about whole-body acceleration onset, direction, and magnitude are unknown compared with when only acceleration onset is unknown. Nine seated subjects were submitted to 11 imposed, forward linear accelerations (1.1 g). For the first trial, subjects were completely unaware of the platform acceleration characteristics (onset, direction, amplitude, and acceleration magnitude). For the subsequent ten trials, subjects knew they would be submitted to a forward linear acceleration, but the onset of the acceleration was unknown. Head kinematics and EMG responses of the neck muscles to the first perturbation were similar for all subjects (6.2 degrees head extension, EMG activity starting from 55 to 72 ms after platform onset). Following the first trial, however, all subjects showed a decreased neck EMG activity. Moreover, subjects responded in one of two ways across trials: one group of subjects ( n=5) maintained a constant head angular position and velocity, whereas the other group ( n=4) showed an increased head angular position (up to 12.6 degrees ) and velocity. This suggests that the first perturbation trial revealed a completely reactive response. After this initial trial, the responses observed may present a mixture of feedforward and feedback control. It is likely that whiplash injuries occur under conditions resembling those observed for the first trial only. If this is the case, the behavior for the following trials cannot be representative of injury mechanisms occurring in whiplash-like motion. Altogether, our results strongly suggest that, following repeated trunk linear accelerations of a constant magnitude, the nervous system prefers to minimize muscle stress instead of adopting a bracing strategy.

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

“…The order of repetitions continued to have an effect during different head postures. These results conform to the results of Blouin et al, and Siegmund et al, who found decreased EMG amplitudes in the neck muscles (sternocleidomastoid, cervical paraspinal, scalenes and trapezius) following repeated forward perturbations [19,29]. The kinematics showed that a significantly decreased peak angle displacement in the neck occurred after the first perturbation.…”

“…The kinematics showed that a significantly decreased peak angle displacement in the neck occurred after the first perturbation. Previous studies of forward perturbations have reported both increased head angles and no change in head angles [18,19]. An increased segment angle should be possible as the EMG amplitudes was reduced.…”

“…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.…”

Interaction between acoustic startle and habituated neck postural responses in seated subjects

“…Axial and appendicular muscles respond to acoustic startle (16), but the strongest and most consistent response occurs in the neck muscles, particularly the sternocleidomastoid (SCM) muscle (8). Since consistent neck muscle responses are also evoked by forward accelerations in seated subjects (3,4,6,32,34), the neck muscles are well suited for study of the potential interaction between the acoustic startle response and the postural responses elicited by whole body accelerations.Reflex activation of the neck muscles can be evoked via one or more of the visual, auditory, vestibular, proprioceptive, and cutaneous sensory systems. In some real-world conditions, such as rear-end automobile collisions, all these sensory systems are stimulated, and some form of cross-modal summation likely occurs (45).…”

“…posture; neck muscle reflexes; habituation POSTURAL RESPONSES RAPIDLY adapt with repeated exposures to the same perturbation in standing (2,19,24), seated (3,34), and supine (2) individuals. This process of adaptation, called habituation (18), typically consists of attenuated muscle activity and related kinematic changes.…”