We were unable to enroll a sufficient number of NSLBP participants for all pain intensity groups to allow analysis of all 10 NRS scores. This may be explained by the fact that patients with NRS scores of 9 and higher are not commonly encountered in a chiropractic practice as the potential severity of the condition warrants medical attention instead.
We were able to demonstrate a linear relationship between pain intensity and postural sway velocities in both AP and ML direction as well as for the parameter 90% circle diameter. This is in agreement with a general observation by Lihavainen et al.  who conducted a similar study in a geriatric population. They did not, however, investigate postural sway related to the individual pain scores but reached their conclusions based on a subdivision into mild or moderate/severe pain only.
Even though an increased sway velocity started at a lower pain score in the AP direction, the overall difference compared to healthy controls was similar to that in the ML direction. On the other hand, the ML sway velocity increased at a faster rate. In addition, this study confirms the altered postural sway characteristics previously reported in a systematic review of NSLBP sufferers . The review noted higher COP mVel values (particularly the AP direction) and a larger sway area compared to healthy controls was described.
The non-overlapping 95% CIs associated with NRS scores at higher pain intensities, particularly with mVel AP/ML, are surprising and may be attributable to our standardized experimental setup and selection of participants. Such a clear subdivision appears unlikely at first sight due to the inherently varying pain perception between individuals.
As the 90% circle diameter is exclusively used with the Metitur system, it is not possible to put the respective results into context. However, it corresponds to the various parameters applied in the literature to describe COP sway area and may therefore offer at least limited comparability.
Our data, however, does not allow for an explanation of the underlying mechanism of the observed pain associated alterations in COP sway velocity. However, as previous pain duration did not exhibit a significant effect on postural sway as pain intensity has, this may suggest that pain interference  may be the determining factor. Neuro-physiological changes, on the other hand, are rather dependent on pain duration and therefore a significant time effect would have been expected. Future studies assessing postural sway before and after acute pain stimulation or using analgesics in chronic and acute NSLBP participants may add valuable information in this respect.
Furthermore, as no other studies have looked into the relationship between a broader range of pain intensities and COP measures it is not possible to compare our results.
At lower and medium pain intensities there was no apparent change in the COP parameters. This may be due to participants finding it difficult to decide on their "true" score, NRS-5 for example shows the widest standard deviations across all parameters. This may therefore explain why no statistically significant differences were observed between lower pain scores (NRS 2-4) for most parameters and may account at least partially for the variability in the associated COP measurements. However, as the confidence intervals across all pain scores remain fairly consistent, the variability of the postural sway measurements most likely reflects individual variations within the COP excursions. The results also suggest that the neurological alteration previously described [4–8] may only have an impact on COP measures at medium to high intensities (i.e. NRS ≥ 5).
In contrast to other studies [9–13], we could not demonstrate any significant effect of age, height, weight or gender on COP excursions in the patient group. This may be attributed to the demographics and physical characteristics of the participants as well as our COP measurement protocol. Our results were derived using a protocol based on best evidence , nevertheless future studies are needed to confirm these findings using the same protocol.
The COP measurement protocol used in this study may in future be suitable as an objective outcome measure for clinical monitoring purposes. However, the results are unidirectional in that increasing pain was associated with increasing postural sway. We have not established that decreasing pain leads to a decreasing postural sway.
Secondly, given the linear relationship between pain intensity and, for example, mVel, a clinically significant decrease of two points on a pain NRS  is equivalent to a reduction in mean sway velocity of 3.6 mm/s in ML and of 3.0 mm/s in AP direction. These changes lie between 1 and 2 standard deviations from the mean. It remains to be seen if such a reduction is also clinically significant.
In addition, this study indicates that any future sample size calculations for COP measurements involving pain sufferers should be considered in the light of the respective perceived intensity. Depending on the research purpose, the inclusion criteria may focus on those with NRS-scores of 5 or higher to reach significance compared to controls more readily.
The results may also cast a new light on the interpretation of studies that reported no significant differences in postural sway between symptomatic individuals and healthy controls. In those instances (e.g. Brumange et al.  and Mok et al. ), these observations may be attributable to the low perceived pain intensities of the NSLBP participants enrolled.
There is evidence that higher COP sway is associated with a higher risk of falling in the elderly  and sustaining injuries as a consequence, although this is subject to debate [25, 26]. Our results did not include geriatric participants and therefore cannot be generalized to that population, however our data may nevertheless underline the importance of suitable pain control in elderly pain sufferers to avoid falls.
In addition, as pain interference appears a likely underlying mechanism, the focus of a rehabilitative approach in pain sufferers with increased COP excursions should be on pain reduction rather than proprioceptive training.
Future studies may also show a role for COP measurements as part of a suite of other procedures to identify malingerers. Even if the individual is aware that pain is associated with greater COP excursions, a study with pseudo-malingerers showed that imitating pain related sway pattern is difficult at best and the average results for sway velocity and sway area greatly exceeded those expected from a real pain sufferer .
Strengths and Limitations
The major strength of this study is in its best practice experimental setup which ensured reliable data collection. Our inclusion and exclusion criteria further prohibited our overall results from being affected by demographic or anthropometric factors.
In this cross-sectional study the subjective nature of pain perception and therefore pain rating may have influenced the results. In addition, pain perception between younger and older NSLBP participants varies and a decrease in pain perception in geriatric individuals has been described . Although this does not affect our sample groups with a cut-off age of 50 yrs, it nevertheless prohibits our results to be generalized to elderly patients.
While significant differences in postural sway compared to healthy controls could be demonstrated in our patient population, the overall number of participants per NRS score was still comparably small. Our results are therefore prone to be affected by extreme COP measures. Other sample groups with identical NRS scores may therefore show varying results. However, we expect the linear trend to be preserved. Similar studies with an identical experimental setup and larger sample sizes should be conducted to confirm our results.