Elsevier

Manual Therapy

Volume 16, Issue 3, June 2011, Pages 273-278
Manual Therapy

Original article
The slow and fast components of postural sway in chronic neck pain

https://doi.org/10.1016/j.math.2010.11.008Get rights and content

Abstract

Background

Several studies have reported altered postural control in people with neck pain. The aim of this study was to increase the understanding of the nature of altered postural control in neck pain by studying the slow and fast components of body sway.

Methods

Subjects with whiplash associated disorders (WAD, n = 21) and chronic non-specific neck pain (NS, n = 24) were compared to healthy controls (CON, n = 21) in this cross-sectional study. The magnitudes of the slow and fast sway components were assessed in Rhomberg quiet stance for 30 s on a force plate with eyes closed. We also investigated associations between postural sway and symptoms, self-ratings of functioning and kinesiophobia.

Results

Increased magnitude of the slow sway component was found in WAD, but not in NS. Greater magnitude of the slow component in WAD was associated with poorer physical functioning, including balance disturbances, and more severe sensory symptoms.

Conclusions

Increased magnitude of the slow sway component implies an aberration in sensory feedback or processing of sensory information in WAD. The associations between postural sway and self-rated characteristics support the clinical validity of the test. Further investigation into NS, involving a longer test time is warranted.

Introduction

Several studies have reported altered postural control in people with neck pain (Karlberg et al., 1995, Michaelson et al., 2003, Madeleine et al., 2004, Treleaven et al., 2005, Field et al., 2008). Most of these studies include assessment of the center of pressure (CoP) migration in unperturbed quiet stance on a force plate (Michaelson et al., 2003, Madeleine et al., 2004, Treleaven et al., 2005, Field et al., 2008). From the CoP migration data a wide range of outcome variables can be calculated. Different studies report different variables, for example CoP migration area (Michaelson et al., 2003), CoP displacement amplitude and path length (Madeleine et al., 2004) and energy of the CoP signal in anterior–posterior and medial–lateral directions (Field et al., 2008). The understanding of the neurophysiologic mechanisms reflected by the different variables from the CoP trajectory is, however, still poor. This may explain the scarceness of theoretical motives for the choice of variables, and the lack of standardization of outcome variables.

Several researchers have concluded that postural sway in quiet stance includes a slow and a fast component (Zatsiorsky and Duarte, 2000, Bottaro et al., 2005, Kiemel et al., 2006). The slow component of the CoP reflects a trajectory that approximates the gravitational line of the body’s center of mass (CoM) (Zatsiorsky and Duarte, 1999, Zatsiorsky and Duarte, 2000). The magnitude of the slow component has been attributed to noise in sensory information transmission and central processing when estimating the location and movement of the CoM (Kiemel et al., 2006). The fast component has been ascribed to restoring forces, generated by mechanical stiffness and tonic and phasic neural commands of the ankle muscles, in order to control CoM location (Bottaro et al., 2005). A method for decomposing the CoP trajectory into its slow and fast component has been presented by Zatsiorsky and Duarte (1999). This allows for a more in depth analysis of the postural sway mechanisms.

Our primary aim was to shed further light on the mechanisms behind increased postural sway in people with chronic neck pain. Therefore, the magnitude of the slow and fast CoP components were calculated to obtain information about possible alterations in the size of the sway that can be related to noise in the sensory system, and the amplitude of the forces controlling CoM. One group with chronic neck pain with traumatic origin, and another with non-traumatic origin, were compared with a group of healthy controls. Group comparisons were limited to pre-planned contrasts between each of the neck-pain groups and the control group.

A second aim was to investigate the associations between postural sway performance and self-ratings of health, functioning, kinesiophobia and symptoms. Such analysis may provide important clues to the mechanisms behind impairments as well as an indication of the clinical validity of the outcome variables.

Section snippets

Methods

This single blinded controlled cross-sectional study was performed at a vocational rehabilitation center (Alfta Rehab Center, Alfta, Sweden). The study was approved by the Regional Ethical Review Board in Uppsala. All participants gave their written consent to participate.

Results

All subjects completed the 30 s quiet stance test.

Fig. 1 shows representative examples of the CoP trajectories and the slow Ra and fast Tr components from one CON and one WAD subject.

Descriptive statistics for the CoP-based variables Ra and Tr area are shown in Fig. 2. Group difference was revealed for the slow component, Ra area (univariate ANOVA: F(2, 63) = 3.95, p = 0.02). The difference was significant between the WAD group and CON (p = 0.01), but not between NS and CON (p = 0.17)

Discussion

In the present study we found that subjects with WAD had an increased magnitude of the slow component of postural sway (Ra area) in quiet standing without vision. Greater magnitude of the slow sway component in the WAD group was associated with poorer self-rated functioning and more severe symptoms.

Conclusions

Decomposing the CoP signal into the slow and fast components may contribute to an enhanced insight into the mechanism behind altered postural sway in chronic neck pain. The results revealed that alterations are present for the slow (Ra) but not for the fast (Tr) component in WAD. This implies an aberration in sensory feedback or central processing of sensory input. The clinical validity of the assessment of the slow component was supported by the associations found between the magnitude of the

Competing interests

None.

Author contribution

UR conceived of the study and design, carried out the data collection, participated in the data processing and statistical analysis and drafted the manuscript.

MB conceived of the study and design and helped to draft the manuscript.

MD conceived of the study and design, carried out the data processing and helped to draft the manuscript.

All authors read and approved the final manuscript.

Acknowledgements

The authors would like to thank Nisse Larson for excellent engineering support and Maria Frykman for excellent administrative work. The study was partly funded by Alfta Research Foundation.

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