Measuring postural control in back pain patients – Center of Pressure vs. Star Excursion Balance Test

Purpose: To compare two different measurements of postural control and the effects on the outcomes through a training intervention including sensory motor approach. Design: Randomized controlled trial, Re-Test design. Methods: In a re-test design (3 weeks intervention), five different centers (of the MiSpEx consortium) investigated changes in balance ability in subjects with back pain. Primary outcomes included Single-Leg Balance-Stance for 15s (center of pressure, COP) for assessment of static postural control and the Star-Excursion-Balance-Test (SEBT) for dynamic (functional) balance ability. Changes in pain intensity were registered due to a Visual Analogue Scale (VAS) and Korff Pain Questionnaire (Korff). Results: Summarizing retest measurements of 331 patients, COP outcomes had a low (r=-0.17) but significant correlation to SEBT-values, but were not significantly influenced by pain intensity (p>0.05) in contrast to functional balance (p<0.05, r=-0.19). The intervention group seemed to be more resistant to a loss of static and dynamic postural control at more intense pain after 12 weeks of regular training (p=0.023). Conclusion: Although both tests were able to detect effects of regular sensory motor training, only the functional balance task could be connected to pain intensity and disability. These findings moot a closer look at functional tests for prognosis and diagnostics of back pain and challenge the current standard.

postural control, or if functional balance tasks (e.g. SEBT) may be more suitable. Thus, the aim of this investigation was to identify whether a functional test (like the star-excursion-balance-test) is more likely to detect psychosocial distress and changes in postural control than the COP assessment, or not.

Trial Design
This study is, as part of MiSpEx, designed as a multi-center randomized controlled trial, investigating an intervention for prevention and therapy of back pain (2010-2018) over a period of 12 weeks of center/homebased training (M1-M3, Figure 1). This paper is focusing the effects on static and/or dynamic postural control of a sensory motor training intervention (neuronal adaptation) and may identify, whether back pain intensity can be moderated by outcomes of COP/SEBT, or not. The main study is investigating effects of a sensory motor training for a period up to 12 weeks with a follow-up of 3 months.
Regarding drop-outs in the follow-up, data analysis within this paper has been reduced to 80% completion of the study (M4/5; 12 weeks of training). Nevertheless, current literature is confirming changes of neuromuscular activity already within six weeks of sensory motor training. Thus, it can be assumed that our study design is adequate to cause identifiable adaptations.

Intervention
The intervention applied can be addressed via www. mispex-network.de and consists of four different sensory-motor based exercises, each divided into 12 levels of difficulty ( Figure 2).
Training on unstable surface was obligate three times a week, the first three weeks within the research center and further 1.5 months at home on one's own responsibility. All participants started at the lowest difficulty regardless of their current physical fitness but were able to individually add further weights (e.g. barbells). The training process (pace, breaks) was standardized with an auditory control mechanism (signals via DVD), introduced and explained at the center and easily applicable for the use at home. Therapists confirmed the regular completion of training, while a diary needs to be filled out after every session of the home-based phase.

Outcomes
As a primary outcome, COP was defined as 'total path of the point of force application, during a single leg balance stance for 15s' on a modified Wii-board (USB-port, test frequency: 100Hz). Means of two trials for each leg (randomized; hands on hips, barefoot) were calculated. The equipment has been validated against AMTI and Kistler force plates, before. Additionally, the SEBT was assessed in accordance with frequently used protocols (BELEG Gribble), defined as the maximum reach distance in a single leg stance, normalized by leg length (so called: Composite-Score (CS); max.
of three trials, hands on hips, barefoot). As a secondary outcome, a Visual-Analogue-Scale was consulted to interpret pain intensity during test intervals. Furthermore, Korff questionnaire (von Korff BELEG) should evaluate disability (KDis) through symptoms, pain intensity (KPain) and pain grade (KGrade) of the subjects within three parameters. Both scales were used to identify whether the static and dynamic balance tests are able to address changes in pain intensity, or not. The staff responsible for the measurements was instructed centralized, two months before study kick-off. All tests were absolved in order as they appear in the text, once each time of measurement.

Sample Size
To achieve clinical relevance for our primary outcomes within an analysis of variance with repeated measurements, an effect size of f=0.14 (cohen, 1988) with α=0.05 and 1-β=0.8 -regarding our pilot investigation -was determined. Assuming a drop-out rate of 30%, calculations via g-power lead to a total sample size of n=580 for all research centers. Based on current literature, secondary outcomes were evaluated as clinical relevant within a middle effect size of f=0.25.

Randomization
Each center assessed 145 participants for eligibility and assigned them randomly (the total of N=725 got block randomized by Potsdam, 5 Blocks, 145 each, proportion 1:2) to one of two groups: control group (CG, no intervention) or intervention group (IG, 3 weeks sensory motor training). The study coordinators of each center enrolled the participants and assigned them to the interventions, based on sequentially numbered blocks given by Potsdam. 532 participants were randomized to either CG or IG, as the rest declined to participate or did not meet the inclusion criteria.

Statistical Methods
Anthropometric data was assessed during the clinical anamnesis within a standardized protocol. Normal distribution was evaluated graphically including box-, distribution-and qq-plots as well as a numerical test (shapiro-wilk-test). Correlations between variables were calculated via Pearson product moment correlations Additionally, 75 records were excluded due to missing data, resulting in a sample of n=331 participants (108/223 CG/IG) entering the analysis. Baseline characteristics are shown in Table   1 No significant differences could be identified between the two intervention groups, but gender specific characteristics within each. Therefore, our study design seems qualified to detect possible associations between the groups within the outcomes, if any occur.
Descriptive statistics are shown in Table 1  KPain) did improve highly significant in the intervention group (p<2.9*e-9) but also still significant in the CG (p=0.007) Table 2.

Discussion
Reviewing our results, the assessment of functional parameters seems to be more appropriate for diagnosis and preventive management of back pain patients than measuring the static postural control. Although it is mentioned that primarily daily/sport-specific movement patterns need to be considered in functional tasks (approving single and dual balance stance tests), the COP could not distinguish between a change in functionality or pain grade. In contrast to, the functional test (SEBT) showed superior effects from a diagnostic point of view. This hypothesis is also supported by psycho-social investigations, which identified limitations in motion and within the social/work context as essential factors for the evolution of back pain. Therefore, the content validity seems higher than in static balance tasks.
Furthermore, the so-called "gold-standard" seems questionable, because the main stabilization in one-leg stance tests (without Changes in the outcome (e.g. in SEBT) must not be referred to a single cause but can be the result of several uncertainties of measurement. Thus, especially the objective evaluation of the outcomes or abortion criteria remains the main weakness.
Considering that the assessment of the postural sway can be seen as a trivial task without any physical demands, it might be more advising a total of 60s for each measurement. This is supposed to avoid interference of singular frequencies, as the main spectrum lies beneath 1Hz (human body oscillation <0.1Hz). Nevertheless, as most COP data is based on measurements of 15 to 30s, we decided to create comparable data. Future Research should expand measurement time to at least 60s. Summing up all arguments, functional balance control should be considered in back pain diagnostics, as it is significantly prone to pain. Especially, the SEBT, which is able to predict injuries for the lower extremities, may be used as a prognostic tool. With its' normalization within a total score and feasibility, there is less to be said against using the SEBT as an additional screening method for functional/neuronal imbalances, which may lead to back pain.

Practical Applications
This investigation recommends adding functional balance tasks to the diagnostics of back pain patients, especially for less intense pain grades. The results of our study do not support the concept of COP as a 'gold standard' for the assessment of postural control in chronic back pain patients with minor but constant pain (VAS 0-1, Korff 1

Conclusion
This study was conducted to identify, whether current methods for the assessment of postural control may detect sensory motor training-induced changes in neuronal adaptation, or not.
Furthermore, this investigation evaluated, if the existence of pain moderates the outcomes of functional or static balance control.
Although both tests were able to detect effects of regular sensory motor training, only the functional balance task could be connected to pain intensity and disability (Korff). These findings moot a closer look at functional tests for prognosis and diagnostics of back pain and challenge the current 'gold standard' (COP). Nevertheless, it is advised to investigate the connection between the center of pressure and severe pain in back pain patients. company support, for the authorship or promotion of contribution.
2) Contributors, either individually or collectively, have no commercial or financial involvements that might present an appearance of a conflict of interest related to the contribution.