Postural sway and integration of proprioceptive signals in subjects with LBP
Introduction
A greater understanding of possible causes and mechanisms underlying the development and the persistence of low back pain (LBP) is needed for the development of new and better treatment strategies (Costa et al., 2013). Changes in motor control have been established in subjects with LBP, and could be one of the mechanisms that could cause LBP or could result from LBP and then play a role in persistence or recurrence (Hodges & Tucker, 2011).
Postural control, the part of motor control involved in maintaining an upright position (Massion, 1992), is often studied by analyzing postural sway. Postural sway is usually quantified as the movement of the center of pressure (CoP), the point at which the resultant of the exerted forces is applied to the support surface. Recently, two reviews investigating standing postural sway in subjects with LBP were published. The majority of the included studies reported an increased postural sway in LBP, or no effect of LBP on postural sway. In a minority of studies, a decreased sway was found in patients with LBP (Mazaheri et al., 2013, Ruhe et al., 2011a). No systematic differences that could explain these differences were identified (Mazaheri et al., 2013). Only studies that used sway amplitude or velocity related variables were included. Nonlinear variables, that give insight into the dynamic structure of the sway pattern, have been used much less frequently in LBP research. This is surprising since CoP regularity has helped understanding the complexity of changes in postural control in many other pathologies. For example, increased regularity of postural sway has been interpreted as evidence of increased cognitive control over posture (Donker, Roerdink, Greven, & Beek, 2007), to compensate for impairments due to e.g., contusion (Cavanaugh et al., 2005), cerebral palsy (Donker, Ledebt, Roerdink, Savelsbergh, & Beek, 2008), Ehlers–Danlos syndrome (Rigoldi et al., 2013) and stroke (Roerdink et al., 2006).
Postural control depends, among other sources of information, on proprioception, which may be impaired in subjects with LBP (Brumagne et al., 1999, Gill and Callaghan, 1998, O’Sullivan et al., 2003, Willigenburg et al., 2013, Yilmaz et al., 2010). The relative weight assigned to proprioceptive signals from a specific body part can be quantified by means of muscle vibration. Muscle vibration is a potent stimulus for muscle spindles (Burke et al., 1976, Roll et al., 1989) and muscle spindles play the major role in the detection of movement (Proske & Gandevia, 2012). Under vibration, the muscle is usually perceived to be longer than it actually is (Cordo et al., 2005, Goodwin et al., 1972, Roll and Vedel, 1982), and consequently a corrective movement is made. For example, when Triceps Surae muscles (TSM) are vibrated, a backward shift in CoP occurs. The magnitude of the shift depends on the weight that the central nervous system assigns to these artificially induced signals compared to other sources of information (Brumagne, Cordo, & Verschueren, 2004). This weighting is influenced by the surface a person is standing on (Ivanenko et al., 1999, Kiers et al., 2011), but is also changed in subjects with LBP (Brumagne et al., 2004, Brumagne et al., 2008, Claeys et al., 2011).
Based on the above, we were interested in the relationship of LBP with the structure of the postural sway pattern in standing and the effects of muscle vibration. However, the pattern of CoP movement in quiet standing and in response to muscle vibration can be characterized by a large number of parameters. It is unknown which parameters represent unique properties of the sway pattern and which parameters covary. This makes an a priori choice of parameters not possible, while measuring all possible parameters results in an unacceptable increase in the probability of type I errors. A well-known method to reduce dimensionality in multi-dimensional data sets is factor analysis. Therefore, we tested our hypotheses on factor analysis scores.
Our primary research questions were: (1) is there a difference in the amount and/or structure of postural sway between people with LBP and healthy individuals? (2) Is proprioceptive weighting in subjects with LBP different from non-LBP subjects? (3) Is there an association between postural sway and effects of muscle vibration? We hypothesized that, compared to subjects without LBP, subjects with LBP would show a more regular sway pattern, a decrease in response to lumbar paraspinal musculature (LPM) vibration and an increase to TSM vibration, that these differences would increase when standing on foam, and that recovery after cessation of vibration would take more time in subjects with LBP.
Section snippets
Methods
We examined a cohort of 215 subjects (162 males, 53 females, age 39 years ± 11, weight 80 kg ± 13, height 179 cm ± 9) from The Utrecht Police Lifestyle Intervention Fitness and Training (UPLIFT) study. The UPLIFT study is a voluntary fitness and lifestyle test for police employees in Utrecht, The Netherlands. Data for the present study were collected between December 2007 and June 2008. All subjects provided written informed consent and the protocol had been approved by the Ethical Committee of
Results
No significant differences were detected between groups in age, height, weight, gender and physical activity level. The group with moderate to severe pain differed from the group with mild pain in that they showed higher levels of disability (ODI) and pain, but did not differ in fear avoidance and duration of complaints (Table 2). Recovery data were missing for two subjects; one in the LBP group, one in the no LBP group.
Four factors were identified for CoP sway, with a combination of variables
Discussion
We studied postural sway and proprioceptive weighting in a cohort of 215 subjects. In contrast with our initial hypothesis, we found subjects with LBP not to show increased sway compared to subjects without or with minor LBP pain, but more irregular and higher frequency sway and only when standing on foam. Subjects with LBP also showed less impact of TSM vibration, which was correlated with higher frequency and irregularity of sway, independent from the presence of LBP (Table 6).
Based on the
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