Non-specific chronic low back pain: differences in spinal kinematics in subgroups during functional tasks

Non-specific chronic low back pain (NSCLBP) is a complex heterogeneous biopsychosocial disorder with multiple manifestations [1]. Despite considerable NSCLBP research, there is little reported change in long-term prognosis [2] with intervention outcomes reported to be short term and moderate at best [3]. Difficulties in establishing effective interventions are likely to be due to an inability to define clear homogeneous NSCLBP subgroups; therefore, identifying specific NSCLBP subgroups using validated subclassification approaches is paramount [4].

For some NSCLBP individuals, the main driver for pain may be movement and posture behaviour indicating a mechanical basis for the disorder. A multidimensional classification system (MDCS), which considers such mechanisms, for example, maladaptive motor control impairment (MCI) (characterised by pain provocative behaviours), has been proposed [5].

This MDCS [5] outlines five NSCLBP MCI subgroups, where individuals are proposed to display full range of movement (ROM) in the direction of pain provocation and are clinically observed to habitually adopt end-range postures. These MCI subgroups are based upon subjective reporting of direction-specific aggravating and easing factors (described elsewhere [6]). MCI patients have been shown to display high levels of fear avoidance to adopt maladaptive end-range postures and movement strategies that may promote increased pain [6, 7, 8]. Treatment for these individuals focuses on reducing fear avoidance and optimising spinal control during functional activity to avoid end-range repetitive strain, reduce spinal loading, and subsequently reduce peripheral nociceptor sensitivity [5]. Previous work has established distinct physical characteristics between two proposed MCI subgroups (Active Extension Pattern and Flexion Pattern) and healthy individuals in adolescent and adult populations, with alterations in spinal position sense, spinal kinematics and trunk muscle activity observed during static postures [7, 8, 9, 10].

Interventions focussed on targeted intervention for these MCI subgroups, such as Classification-based cognitive functional therapy (CB-CFT), have also been proposed to be effective when compared with usual care [11]. Despite CB-CFT incorporating functional re-education approaches, to date, no published work has evaluated how NSCLBP MCI subgroups operate during dynamic functional activities.

The purpose of this study is to better understand differences in spinal kinematics between NSCLBP subgroups and healthy individuals during functional tasks. This knowledge may assist in informing specific movement re-education in CB-CFT and aid development of novel sensor and biofeedback technologies to promote back pain self-management.

The hypothesis for this observational, cross-sectional study is that a difference in regional sagittal spinal curvatures between NSCLBP subgroups and healthy controls will be observed during a series of functional tasks.

NSCLBP patients were recruited from routine physiotherapy waiting lists in Cardiff and Vale University Health Board (Cardiff, UK). Fifty NSCLBP and 28 healthy individuals volunteered. Ethical approval was obtained from The Research Ethics Committee 3 Wales (10/MRE09/28) within the Arthritis Research UK Biomechanics and Bioengineering Centre, Cardiff University. Informed consent was obtained from all participants. Sample size was based on lower lumbar sagittal spinal angle, which has been shown to discriminate between AEP and FP subgroups in sitting [7]. Power set at priori at 0.7, alpha level of 0.05, a sample of 24 subjects per group was calculated.

Data collection

Data were captured using an eight-camera Vicon^® motion analysis system (Vicon 512 Motion Systems Ltd, Oxford, UK) at a sampling rate of 100 samples per second. Spherical retro-reflective markers (10 mm) (Vicon Motion Systems Ltd, Oxford, UK) were attached using double-sided tape over the following anatomical positions: spinous processes of the 7th cervical, 2nd, 4th, 6th, 8th, 10th and 12th thoracic and 2nd and 4th lumbar vertebrae, manubrium sterni (superior border) and bilaterally on the anterior superior iliac spine (ASIS), posterior superior iliac spine (PSIS), iliac crest (mid-crest, vertically aligned with the greater trochanter) acromioclavicular joint, ulna styloid process, 10 cm lateral of the 12th thoracic spinous process, lateral knee joint line, and the lateral malleoli creating a full body model (Fig. 1). The marker set used was custom-designed (Cardiff University, UK). To minimise, error cameras were calibrated prior to data collection and a chartered physiotherapist (RH) performed all marker placement [17].

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Nine standardised functional tasks [reach up, sitting-to-standing, standing-to-sitting, step up, step down, box lift, box replace, bending to retrieve (and returning from retrieving) a pen from the floor] were evaluated. Tasks were chosen to reflect a range of usual functional activities and elicit a variation of flexion-related and extension-related tasks. Each task was repeated and recorded three times. Following each trial data was visualised in Vicon Nexus (Nexus 1.8.2 Vicon Motion Systems, Oxford, UK) to ensure that markers were consistently present (Fig. 2).

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Data processing and analysis

Data processing was conducted in Vicon Nexus. The sacral marker S1 was calculated as the average point between PSIS markers. Processed trials were converted to a c3d file and run through a custom developed analysis programme in MATLAB (version R2013a, The Mathworks Inc., Natick, MA, USA). Prior to the calculation of spine curvatures, the coordinates of the motion capture markers on the spine were transformed from the global (lab) coordinate system to the local (pelvis) coordinate system. The spine was modeled as a curve in the sagittal plane defined by the marker placement over the spinous processes. The change in orientation between the lines interconnecting the adjacent markers was used to define each spinal region curvature in degrees. This was calculated by summing all angular changes within each region: lower lumbar (S1–L3); upper lumbar (L3–T12); lower thoracic (T12–T6); and upper thoracic (T6–C7) (Fig. 3). The mid-point spinal curvature of the subjects’ total range of movement was determined for each task. This was calculated as follows: (maximum flexion sagittal spinal angle + maximum extension sagittal spinal angle)/2.

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This is the first study to identify distinct subgroup differences in lower thoracic spinal kinematics during functional activity which can aid in informing personalised rehabilitation strategies for CB-CFT interventions [11].

In-line with recent observations of lumbar movement in LBP subgroups [21], consistent patterns of different spinal movement in the thoraco-lumbar spine were noted between the NSCLBP subgroups throughout the functional tasks. In addition, this region is able to discriminate between the FP and healthy groups, highlighting that distinct increases in thoraco-lumbar flexion in the FP group. These findings support those of Dankaerts et al. [7], where differences were observed in usual sitting upper lumbar posture between AEP and FP groups. However, in contrast to the results reported here, Dankaerts et al. [7] observed differences between AEP and healthy individuals in the upper lumbar region, whilst no differences were observed between FP and healthy individuals. However, similarities are noted between the current study and adolescent cohort data, both identifying significant differences in usual sitting in the upper but not lower lumbar spine [9]. Both Dankaerts et al. [7] and Astfalck et al. [9] demonstrate significant differences between all three groups (FP, AEP, and healthy) in upper lumbar posture which were not replicated in this study. Differences in instrumentation (electromagnetic 3Space Fastrak^®), subject numbers, and mean age (41.1 compared to 36.0 [7] and 15.6 [9] years) may account for some variation in results as well as observation of static postures rather than functional activities.

Overall, AEP individuals operated in significantly greater extension compared to FP during almost all functional tasks in upper lumbar and lower thoracic regions (p < 0.05). This may reflect the more extended nature of the task, where ROM in this region appears to be similar across all groups. The between group differences observed in the lower thoracic region are novel findings in this study. A non-significant trend (following adjustment for gender differences) for a reduction in overall trunk curvature in AEP compared to FP individuals has been observed previously in adolescent cohorts [22], suggesting that the AEP group may adopt a less kyphotic trunk posture compared to the FP group. However the methods utilised to calculate gross trunk angle do not allow for regional analysis.

Interestingly, no significant between group differences was identified between the AEP and healthy groups, contradicting previous work evaluating static postures [7, 10] and intimates that AEP and healthy individuals may utilise similar spinal movement patterns during functional tasks. There are multiple hypotheses for this observation. AEP individuals subjectively report extension-related activities as pain provocative due to a reduced capacity to control extension movement [5]. Therefore, the functional tasks may not have challenged the spine into substantial extension to elicit pain and maladaptive movement control. Patients often report prolonged exposure to static postures such as sitting and standing as provoking pain; therefore, extension may be more apparent in static postures compared to dynamic functional activity. There are also reported difficulties in accurately classifying AEP individuals, with approximately only 50–62% of AEP subjects correctly classified by experienced clinicians [23], since the AEP group displayed an overall higher risk of distress compared to the FP group other factors, such as maladaptive trunk muscle activity, or psychosocial factors [24], may be greater contributory mechanisms to pain than spinal movement.

Regional spinal curvatures appear to distinctly differ in MCI subgroups, with the thoraco-lumbar region discriminating between FP and AEP, and FP and healthy groups during functional tasks. The FP group demonstrated more kyphotic postures in these spinal regions, whereas no significant differences between the AEP and healthy groups were observed, suggesting that these groups adopt more similar functional movement strategies. Alternatively, postures or activities may have needed to be more involved or sustained to elicit a response in these groups. The differences observed in the thoracic spine also highlight the importance of incorporating thoracic spine evaluation into NSCLBP assessment. Gender should also be considered as a confounding factor in future studies.