The effect of vision, muscle fatigue and backrest inclination on the repositioning ability of the cervical spine

xiii, 94, 45 leaves : ill. ; 30 cm.PolyU Library Call No.: [THS] LG51 .H577M REC 2004 WongNeck pain is prevalent among workers engaged in sedentary work facing a video display unit (VDU) for long periods. Poor working posture has been claimed as one of the possible causes of the pain. Current treatment for postural neck pain is focused mainly on education to improve posture. However, previous studies have not revealed any particular cervical postures that are more highly associated with neck pain than others. Moreover, clinical postural assessment is typically based on a snapshot of the resting head-on-trunk relationship, and does not take dynamic neck postural control into account. The neck postural and movement control system relies on multiple sensory afferent inputs from visual, vestibular and cervical proprioceptive cues. The recent discovery of a region of the cortex which is active in response to incongruence between motor intention, awareness of movement and visual feedback has lead to the hypothesis that disorganized cortical representation of proprioception may falsely signal incongruence between motor intention and movement, resulting in pathological pain.In this study, the postural control of the cervical spine in terms of active repositioning ability was assessed in normal subjects. Reflective markers were attached to the subjects' head and trunk. Subjects were tested in a Hong Kong Government recommended standard office chair with adjustable backrest tilting angle (Labour Department, 2002). A motion analysis system (Vicon 370, Oxford Metrics, UK) was used to monitor the head position relative to the trunk and to the environment. Initially, subjects were instructed to memorize the starting normal working head posture. Then, this starting position was reproduced after a standardized series of movements. Repositioning ability was calculated as the discrepancy between the repeated position and the memorized starting position. This procedure was repeated for different inclinations of the chair back and with vision either occluded or not. The measurements were also repeated after a fatigue protocol, in which fatigue of both upper trapezius muscles were induced. Repositioning ability of cervical spine was quantified in terms of angular and translational repositioning accuracy (mean error) and precision (variability error) with respect to a local three-dimensional coordinate system defined with respect to the trunk and a global three-dimensional coordinate system defined with respect to the external environment.Vision was demonstrated to have significant effect on the repositioning precision of the cervical spine in nearly all directions. Interestingly, fatigue of both upper trapezius muscles was found to result in improvement of the angular repositioning precision of the cervical spine in axial rotation but deterioration of translational repositioning accuracy in sidegliding. Tilting of the chair was found to have little effect on angular repositioning accuracy and precision, but did have a significant effect on the translational repositioning accuracy along the vertical axis.Treatment of postural neck pain by education will be in vain if the clinical assessment of neck posture by obtaining a snapshot of the resting head-on-trunk is inadequate to document the normal function of the postural system. In order to address the deficit of each of the underlying components of postural control, it was concluded that the integrity of the head postural control would be better assessed by differential assessment of repositioning accuracy and precision of the cervical spine.Department of Health Technology and InformaticsM.Phil., Jockey Club Rehabilitation Engineering Centre, The Hong Kong Polytechnic University, 2004