Altered postural control in anticipation of postural instability in persons with recurrent low back pain
Introduction
Alterations in postural control have been reported in persons with low back pain (LBP). Compared with healthy controls, persons with LBP demonstrated a greater postural sway, an increased difficulty with adapting to changing conditions, and a decreased recovery of postural balance after perturbation [1], [2], [3]. Furthermore, persons with LBP seem to have altered proprioceptive sensitivity. Due to a less refined position sense of the lower back [4], [5], reweighting of the proprioceptive input by increasing the gain at the ankle level seems to have taken place in persons with LBP [6], [7], so that people with LBP tend to use less hip and low back movement for balance control in quiet standing [6], [7], [8], [9].
The vertical orientation of the body in the upright standing position is maintained by a dynamic interplay of vision, proprioception, haptic contact cues, efferent control and internal models [10], [11]. Sensory inflow is compared to the subjective vertical, the internal representation of verticality. On the basis of the comparison, compensatory reorientations of the body are performed with the aim of keeping the body longitudinal axis aligned with the vertical. The subjective vertical is also updated every time sensory inflow changes [12].
In patients with musculoskeletal impairments such as spasmodic torticollis (i.e., a pathological condition whereby torsion of the cervical spine due to neck muscle spasm occurs) changes in the reference system used in the control of body orientation have been demonstrated [13]. A small number of studies have reported alterations in the body inclination in relation to postural control in persons with LBP [1], [2], [6] despite the clinical relevance. However, the results have been inconsistent, with evidence supporting both a more anterior [6] and a more posterior [1], [2] located center of mass (COM) compared to the neutral upright posture of healthy controls.
The control of upright stance can change during conditions of increased postural anxiety. Adoption of postural strategies to maintain a tighter control of the COM by use of a stiffening strategy in response to increase perceived postural threat has been observed [14]. Although evidence for changes in balance control due to postural anxiety has been provided [15], all this evidence was obtained from a healthy young population. Since patients with LBP seem also to adopt a body stiffening strategy [7], [9], the question arises whether the mechanism of anticipation of postural instability plays an important role in the altered postural control observed in persons with LBP.
If so, adoption of an altered body inclination combined with a stiffening strategy (e.g., co-contraction of trunk muscles) when postural instability is anticipated could lead to more compressive forces on the spine [16]. Moreover, this postural strategy could reduce preparatory spinal movement [9], [17], which may adversely affect spinal control. Consequently, the reduction in spinal movement counterproductively exposes the spine to greater resultant displacement, as recently observed in patients with recurrent LBP [9].
When the mechanisms of postural control in persons with and without LBP are better known, results could provide new insight into the choice of assessment and rehabilitation for postural control impairments in this patient population, e.g., postural tasks including anticipation of postural instability, learning differential control versus stiffening in demanding postural conditions. Accordingly, addressing these postural control mechanisms could be effective in the prevention of recurrence of LBP.
The aim of this study was to examine whether possible changes in body inclination in persons with LBP could be identified as an aspect of postural control impairment and as a possible underlying mechanism of the high recurrence rate observed in patients with LBP. In addition, this study aims at investigating whether anticipation of postural instability further alters body inclination. Therefore, we evaluated body inclination in upright posture with and without vision in patients with recurrent LBP and healthy controls combined with anticipation of different degrees of postural perturbations such as ballistic arm movements or triceps surae muscle vibration.
Section snippets
Subjects
Eighty-nine university students and personnel members participated in this study. A medical screening by a physician of the Sports Medical Advice Center, UZ Leuven was performed to include and exclude subjects in the study. Individuals with a history of vestibular disorder, neurological or respiratory disease, previous spinal surgery, acute radicular pathology or serious neck problems were excluded. Subjects were included in the group with LBP when they had experienced non-specific LBP for more
Results
No differences in body inclination were observed when visual information was available between the LBP groups and healthy persons (P > 0.05). However, a tendency was seen that persons with LBP had already a more anterior positioned mean COP compared to the healthy individuals in the vision condition (P = 0.06) (e.g., Fig. 2).
A significant interaction between group and the trials (trials 1–5) was demonstrated (F(3, 87) = 26.13, P < 0.001). Further post hoc analysis revealed a significant difference
Discussion
The main finding of the current work is that young persons with LBP showed an altered body inclination during conditions that challenge postural control compared to healthy young persons by leaning more forward when postural instability was anticipated.
An offset in the reference frame in the anterior direction might explain the findings that individuals with LBP have changed their trunk inclination when vision was occluded. This is in agreement with results in patients with spasmodic
Acknowledgements
This work was supported by grants from the Research Council K.U. Leuven (PDM/99/121) and Fund for Scientific Research-Flanders (1.5.104.03).
Conflict of interest statement
The authors disclose any financial and personal relationships with other people or organizations that could inappropriately influence their work.
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