Changes in coordination of postural control during dynamic stance in chronic low back pain patients
Introduction
The maintenance and control of balance, whether under static or dynamic conditions, is an essential requirement for physical and daily activities. In humans, the balance-controlling system is believed to be phylogenetically old, and to operate relatively autonomously through the spinal and brainstem reflex networks [1]. However, there is evidence that standing, rather than rely on mechano-reflex mechanisms [2], [3], may require activity of higher order structures [4], [5]. Therefore, the control of erect posture may be more integrated into the movement control scheme than has been previously considered.
The human postural system operates on the basis of the integrated information from three independent sensory sources: somatosensory, vestibular and visual inputs [6]. This information, which allows to assess the position and motion of the body in space, is constantly reweighted so as to generate the appropriate forces to control and maintain balance in a wide range of situations [6]. It is thus conceivable that a derangement to any of the three sensory systems will influence the overall output of the postural system.
The coordination of postural control may be affected in subjects with chronic low back pain (CLBP) [7], [8], [9], [10], [11], [12]. The cause of this disturbance is not known. Specifically, it is not clear whether changes in postural control are related to pain itself and to its stressful nature, so-called “pain interference” [9], [10]. In humans, discharge from high-threshold nociceptive afferents interacts with spinal motor pathways [13], [14], [15] as well as with primary somatosensory and motor cortex [16], [17]. These complex actions are likely to contribute to adaptive changes in postural control [18], [19]. In addition, proprioceptive impairment has been suspected as one of the possible causes for balance impairments in CLBP [20], [21], [22]. The majority of studies have focused on the lumbar spine, while little attention has been paid to the possibility that a reduced proprioception from the lower limbs may contribute to an altered control of balance in CLBP patients. In normal adults, postural adjustments during quiet standing are generally achieved using an “ankle strategy” [23], in which the ankle torque maintains the center of force over the base of support. In this strategy, the muscles groups acting at the ankle joint are considered as the main musculature for the control of stability during quiet standing [24]. Patients with CLBP due to spondylolisthesis have been found to have abnormalities of the soleus H-reflex [25], [26] which depends on the activation of large-diameter mechano-receptive afferents (group Ia fibres) in the muscle [27]. It is known that changes in Ia input may result in altered proprioception [28] and distortion of sensory maps [29]. In addition, altered processing of non-noxious afferent information from large-diameter afferent fibres has been suspected to contribute to some aspects of pain [30], [31].
The purpose of the present experiments was to investigate the possibility that patients with CLBP exhibited an altered postural control during quiet standing with respect to a control population, as assessed by dynamic posturography [32]. This method measures whole body performance during quiet standing and allows to differentiate the contribution of the vestibular, proprioceptive and visual system to balance control. The hypothesis was that changes in postural control associated with CLBP, rather than being caused by pain interference, probably reflect an altered postural strategy underlying a dysfunction of the peripheral proprioceptive system or of the central processing of proprioceptive information.
Section snippets
Subjects
Twelve CLBP patients participated in this study. Inclusion criteria were: (1) localized back pain, lasting more than 6 months and radiating no further than the buttock, (2) no previous history of sciatica or other radicular involvement and (3) normal neurological examination. Self-reported disability was assessed with the Oswestry Low Back Pain Disability Questionnaire [33]. None of the patients had any history of vestibular and neurological disease nor of hip/knee/ankle/foot problems. The
Results
Duration of CLBP ranged from 1 year to 10 years (5.2 ± 3.3 years). The level of discomfort and disability of CLBP patients prior to the performance of the postural tests was on average low (see Table 1). Sensory and motor testing was normal. Radiological findings mainly showed moderate degenerative changes of the lumbar discs with no sign of root compression.
Fig. 2 compares the measures of sway in AP (A and B) and ML (C and D) directions between the healthy group and the CLBP group. Overall ANOVA
Discussion
The main result of the present study was that subjects with CLBP exhibited an increased sway in the AP direction during quiet standing, as assessed by dynamic posturography. This procedure, which has been extensively used in clinical practice [39], evaluates utilization of visual, vestibular and proprioceptive sensors by manipulating the accuracy of visual and/or somatosensory inputs during quiet stance [36]. While the amount of sway was comparable between the CLBP group and the control group
Acknowledgements
Dr. Raimondo della Volpe was financed by a grant from University of Siena.
We wish to thank Dr. Pietro Piu for statistical counseling. This work was in part financed by a grant from the University of Siena (PAR 2003).
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