Full length articleLow back pain development differentially influences centre of pressure regularity following prolonged standing
Introduction
An interesting subgroup of individuals without pre-existing low back pain (LBP) consists of individuals who develop a transient acute episode of LBP during 2 continuous hours of standing [1], [2], [3], [4]. These individuals identified as pain developers (PDs) have a reported 3× the likelihood to seek clinical care for LBP in the future [5]. Since balance control has been shown to be affected in those who have clinical LBP [6], [7], [8], [9] it is possible that this subgroup may also demonstrate differences during a constrained balance task. As a result, determining if changes in standing dynamic balance control occur following a 2-h bout of upright standing may provide additional insight to the acute development of LBP during standing, subsequent development of clinical LBP, and potential intervention strategies.
Individuals identified as PDs have been shown to adopt a pattern of coactivitation between the right and left gluteus medius muscles while standing [1], [2], [4]. Bilateral coactivity of the gluteus medius muscles may be a predisposing factor for the development of transient acute LBP in PDs during prolonged standing [10]. The strategy of muscular co-activation is theoretically adopted to increase joint stiffness and enhance robustness [11]; however, co-activation has been associated with an increased average velocity for the centre of pressure (i.e. diminished performance) during an unstable seated balance task [11]. This suggests that the gluteal co-activation strategy adopted by PDs may diminish performance during balance assessment.
Traditional measures of balance control derived from the COP time-series use the principle of centrality to describe the magnitudes of movement and variability [12], [13] Under the principle of centrality the mean is the desired outcome, and deviation away from the mean is considered undesirable noise or error. Nonlinear analysis techniques attempt to characterize the structure of variability in the COP time-series, which is not necessarily correlated with the magnitude of variability [13], [14]. Several recent investigations have employed nonlinear analysis techniques to the COP time-series to assess differences between those with varying degrees of LBP [7], [9], [15], [16], [17]. These investigations have primarily focused on quantifying regularity/complexity in the COP time-series by using various techniques to estimate signal entropy [18], [19]. Findings from these investigations present conflicting evidence that individuals with increased LBP intensity exhibited either increased [7], [16] or decreased [9], [17] regularity with varying sensory and support surface conditions. Nonetheless, a consistent finding across these studies was that regularity of the COP time-series was differentially influenced by the presence of LBP. Employing similar analysis techniques to standing balance data obtained before and after a 2-h standing protocol may provide additional insight to differences in postural control between PDs and non-PDs.
The purpose of this study was to determine if regularity, quantified using sample entropy, derived from the COP time-series during standing was altered after 2-h of standing. Furthermore, it was our goal to determine if PDs and non-PDs were differentially influenced by the 2-h of standing. In addition, linear measures of postural sway were also computed to provide a reference for comparison with COP regularity. It was hypothesized that regularity would be affected by the prolonged standing protocol, and that PDs would be influenced to a greater extent than non-PDs.
Section snippets
Participants
Thirty-one volunteer participants (18 male, 14 female) were recruited from a university population. Exclusion criteria included any previous history of low back pain that was significant enough to seek medical intervention or that resulted in greater than three days off work or school, previous lumbar or hip surgery, employment in a task that required prolonged static standing during the past 12 months, and the inability to stand for at least two hours. Ethics approval for research involving
Participants
Of the 31 participants 42% were identified as reporting LBP during the 2-h standing protocol. Baseline characteristics of the participants within each PD and non-PD group were statistically similar. There were no significant differences between pain groups for age, body mass index, and baseline visual analog scale score.
Post 2-h change in sample entropy
Statistical results from the 2-way ANOVA did not reveal a significant interaction between vision and pain group (p = 0.105; F(1,29) = 6.249), or main effect of vision (p = 0.520;
Discussion
The current investigation used a nonlinear dynamics analysis of the COP time-series to quantify changes in neuromuscular control of upright standing following a 2-h standing protocol in people either identified as PDs or non-PDs. Consistent with the hypothesis, regularity of the COP increased (i.e. decreased sample entropy) after 2-h of standing for both PDs and non-PDs; and, PDs had a larger decrease in sample entropy after 2-h of standing.
Increased regularity in the COP has been attributed to
Conclusion
Changes in neuromuscular control of upright standing pre and post 2-h of standing did occur based on an increase in COP regularity after 2-h of standing for both PDs and non-PDs. PDs had a larger change in COP regularity and this finding supports the theory that increased COP regularity occurs with pain/pathology. Using the proposed approach, sample entropy could be a good dynamic analysis technique to characterize and differentiate the postural effects of standing induced LBP and form the
Conflict of interest statement
The authors have no conflicts of interests to disclose.
Acknowledgements
This research was supported by a Natural Sciences and Engineering Research Council (NSERC) Discovery grant and the Ontario Graduate Scholarship (OGS).
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Present Address: Health, Human Performance and Recreation, University of Arkansas, Fayetteville, AR, USA.