Neck muscle activation in response to eye movement depends on sitting posture and is modified in whiplash associated disorders: Cross-sectional study

BACKGROUND

Activity of specific neck muscles is modulated by eye movement. This activity modulation is exaggerated in people with whiplash associated disorders (WAD), but it is unknown whether it is impacted by sitting posture.

OBJECTIVE

This study investigated; (i) whether activity of cervical muscles differs with spinal posture; (ii) whether the effect of eye gaze direction (horizontal/vertical) on neck muscle activity differs between postures, and (iii) whether these effects differ between individuals with and without WAD.

METHODS

In three seated postures (normal relaxed, head forward, sit tall) electromyography (EMG) was recorded right obliquus capitis inferior (OI), multifidus (MF), splenius capitis (SP) and left sternocleidomastoid (SCM) with fine-wire and surface electrodes in ten healthy controls and nine with WAD. Electro-oculography recorded eye movements.

RESULTS

In controls, EMG was less for extensor muscles in Sit Tall than Head Forward, but higher in SCM. Only SC EMG modulated with eye movement. When WAD participants adopted similar sitting postures several responses were different; compared to Normal Relaxed posture OI EMG was less in Head Forward; MF EMG was less in Sit Tall; and SC was less in Head Forward and Sit Tall. Neck muscles in WAD were generally more sensitive to eye movement, except SC which did not modulate.

CONCLUSIONS

These finding support the hypothesis that neck muscle activity is influenced by spinal posture and eye movement. In WAD, this relationship is distorted and the response to eye movement is increased.

SIGNIFICANCE

These observations have potential implications for clinical management of individuals with WAD.

Cervical Rotator Muscle Activity With Eye Movement at Different Speeds is Distorted in Whiplash

BACKGROUND

People with whiplash-associated disorders (WADs) report difficulty with quick head movements and cervicoocular dysfunction. Changes in coordination between eye movement and neck muscle activity may be involved.

OBJECTIVE

To examine whether activity of superficial and deep neck muscles increases with eye movement when the head is held in a fixed position, whether this differs between directions and speed of eye movement, and whether this is modified in WAD.

DESIGN

Convenience case series with unmatched controls.

SETTING

Research laboratory.

PARTICIPANTS

Nine individuals with chronic WAD grade II and 11 pain-free controls.

METHODS

Electromyography (EMG) was recorded from muscles that could act to rotate the neck to the right (right obliquus capitis inferior [OI], multifidus [MF], splenius capitis [SC], and left sternocleidomastoid [SCM]) with intramuscular or surface electrodes in 9 WAD participants and 11 pain-free controls. Eyes were rotated without head movement to track slow and medium-speed targets to the right or left, and as fast as possible (FP).

MAIN OUTCOME MEASUREMENTS

Amplitude of EMG.

RESULTS

In controls, SCM and SC EMG increased with right gaze (all P's < .03). EMG of the deep OI muscle increased in both directions (P < .001). WAD involved counterintuitive greater activity of SCM with left rotation across speeds of eye movement (SC with slow movement, P < .036), decreased OI EMG with gaze left (P < .019), and no change in MF EMG (P < .6) in either gaze direction. For FP tasks, EMG of all muscles was greater than slower speeds in controls (all P's < .0001), but not WAD (all P's > .33).

CONCLUSIONS

Coordination between neck muscle activity and eye movements with increasing speed is modified in WAD. Contrasting changes are present in deep and superficial neck muscles with implications for neck function that may explain some common WAD symptoms.

LEVEL OF EVIDENCE

IV.

Cervico-ocular coordination during neck rotation is distorted in people with whiplash-associated disorders

People with whiplash-associated disorders (WAD) not only suffer from neck/head pain, but commonly report deficits in eye movement control. Recent work has highlighted a strong relationship between eye and neck muscle activation in pain-free subjects. It is possible that WAD may disrupt the intricate coordination between eye and neck movement. Electromyographic activity (EMG) of muscles that rotate the cervical spine to the right (left sternocleidomastoid, right obliquus capitis inferior (OI), right splenius capitis (SC) and right multifidus (MF)) was recorded in nine people with chronic WAD. Cervical rotation was performed with five gaze conditions involving different gaze directions relative to cervical rotation. The relationship between eye position/movement and neck muscle activity was contrasted with previous observations from pain-free controls. Three main differences were observed in WAD. First, the superficial muscle SC was active with both directions of cervical rotation in contrast to activity only with right rotation in pain-free controls. Second, activity of OI and MF varied between directions of cervical rotation, unlike the non-direction-specific activity in controls. Third, the effect of horizontal gaze direction on neck muscle EMG was augmented compared to controls. These observations provide evidence of redistribution of activity between neck muscles during cervical rotation and increased interaction between eye and neck muscle activity in people with WAD. These changes in cervico-ocular coordination may underlie clinical symptoms reported by people with WAD that involve visual deficits and changes in function during cervical rotation such as postural control.

Effect of gaze direction on neck muscle activity during cervical rotation

Control of the neck muscles is coordinated with the sensory organs of vision, hearing and balance. For instance, activity of splenius capitis (SC) is modified with gaze shift. This interaction between eye movement and neck muscle activity is likely to influence the control of neck movement. The aim of this study was to investigate the effect of eye position on neck muscle activity during cervical rotation. In eleven subjects we recorded electromyographic activity (EMG) of muscles that rotate the neck to the right [right obliquus capitis inferior (OI), multifides (MF), and SC, and left sternocleidomastoid (SCM)] with intramuscular or surface electrodes. In sitting, subjects rotated the neck in each direction to specific points in range that were held statically with gaze either fixed to a guide (at three different positions) that moved with the head to maintain a constant intra-orbit eye position or to a panel in front of the subject. Although right SC and left SCM EMG increased with rotation to the right, contrary to anatomical texts, OI EMG increased with both directions and MF EMG did not change from the activity recorded at rest. During neck rotation SCM and MF EMG was less when the eyes were maintained with a constant intra-orbit position that was opposite to the direction of rotation compared to trials in which the eyes were maintained in the same direction as the head movement. The inter-relationship between eye position and neck muscle activity may affect the control of neck posture and movement.