Direction-dependent neck and trunk postural reactions during sitting.

Postural reactions in healthy individuals in the seated position have previously been described and have been shown to depend on the direction of the perturbation; however the neck response following forward and backward translations has not been compared. The overall objective of the present study was to compare neck and trunk kinematic, kinetic and electromyographic (EMG) stabilization patterns of seated healthy individuals to forward and backward translations. Ten healthy individuals, seated on a chair fixed onto a movable platform, were exposed to forward and backward translations (distance=0.15m, peak acceleration=1.2m/s(2)). The head and trunk kinematics as well as the EMG activity of 16 neck and trunk muscles were recorded. Neck and trunk angular displacements were computed in the sagittal plane. The centers of mass (COMs) of the head (HEAD), upper thorax (UPTX), lower thorax (LOWTX) and abdomen (ABDO) segments were also computed. Moments of force at the C7-T1 and L5-S1 levels were calculated using a top-down, inverse dynamics approach. Forward translations provoked greater overall COM peak displacements. The first peak of moment of force was also reached earlier following forward translations which may have played a role in preventing the trunk from leaning backwards. These responses can be explained by the higher postural threat imposed by a forward translation.

A paradigm to assess postural responses triggered by anteroposterior translations in healthy seated individuals.

Postural adjustments following mechanical perturbations have been studied in healthy seated humans. However, little is known on the minimal intensity that should be used to provoke a reaction. This knowledge could be essential to assess seated postural deficits in some pathological populations. The goal of the present study was to identify a low-intensity perturbation that could elicit postural reactions in healthy seated individuals. Six healthy participants sat on an adapted ergonomic chair fixed on a moveable support surface that was submitted to forward and backward translations. The head and trunk kinematics as well as the activity of sixteen neck and trunk muscles were recorded. The head, arm and trunk center of mass was computed using kinematics and standard anthropometric tables. We found that ramp displacements with an acceleration profile reaching a maximal value of 1.17 m/s(2) elicited reliable kinematic and electromyographic reactions across participants. Head and trunk segments initially responded opposite to the direction of translation, then reversed direction. Median peak-to-peak angular displacements in the neck, head and trunk, respectively, reached 3.6 degrees, 7.0 degrees and 7.1 degrees for forward translations, and 4.0 degrees, 8.2 degrees and 7.0 degrees for backward translations. For forward translations, neck and trunk flexor muscles were activated first, followed by the extensor muscles, whereas for backward translations, extensor muscles were activated first, followed by flexors. Although this perturbation is of low-intensity compared to those typically used previously to evoke postural reactions, this stimulus is sufficient to elicit a reliable response. We suggest that such a perturbation could be used to assess the physical condition of individuals with neck injuries.

Whiplash-associated disorders affect postural reactions to antero-posterior support surface translations during sitting.

Previous studies have shown that individuals with WAD display decreased postural stability during standing and walking tasks. However, their ability to maintain seated upright posture has never been investigated. The objective of this study was to characterize kinematic and electromyographic postural stabilization patterns in individuals with chronic WAD and to compare these patterns with those in an able-bodied control group. Ten individuals with WAD and an age- and gender-matched group of healthy individuals were exposed to sudden forward and backward support surface translations while they were seated. Neck and trunk muscle activity and angular displacements as well as centers of mass (COMs) linear displacements at four levels of the head and trunk were computed. The displacement onset of the combined head, arms and trunk COM was significantly delayed in persons with WAD. However, their peak trunk angles were smaller and were reached sooner. In the WAD group, the activation onset of the lumbar erector spinae was less affected by perturbation direction and the sternocleidomastoid muscle, a neck flexor, showed a trend towards being activated later, compared to the healthy group. These results suggest that individuals with WAD may alter stretch reflex threshold and/or elicit a learned response for pain avoidance that may be direction-specific. Such findings highlight the importance of assessing both spatial and temporal characteristics across different levels of the spinal musculoskeletal system to evaluate multidirectional postural responses in WAD individuals.