Investigating differences in trunk muscle activity in non-specific chronic low back pain subgroups and no-low back pain controls during functional tasks: a case-control study

Non-specific chronic low back pain (NSCLBP) is highly complex with a multitude of physical, cognitive and lifestyle factors contributing to the disorder. Often the dominant driver for pain may be linked to movement and posture behaviour indicating a mechanical basis for the disorder [1]. People with NSCLBP have been found to demonstrate differences in temporal and spatial parameters of trunk muscle activity, although the nature of these differences is poorly understood with substantial variability reported [2, 3].

This variability may be due to lack of definition of homogeneous NSCLBP subgroups; therefore, identifying specific NSCLBP subgroups using validated subclassification approaches is paramount [4]. One such multidimensional classification system (MDCS) which categorises the disorder into movement impairment and/or motor control impairment (MCI) subgroups has been proposed [1]. Patients within these subgroups display high levels of fear-avoidance, and adopt subgroup-specific maladaptive postures and movement strategies, potentially exacerbating the disorder [5‐7]. It is proposed that NSCLBP individuals with flexion pattern MCI (FP) demonstrate an inability to activate lumbar multifidus (LM), reporting pain during flexion biased activities, whereas extension pattern MCI (AEP) individuals may present with hyperextended lower lumbar postures with dominant erector spinae and LM activity to actively ‘hold’ themselves in lumbar hyperextension and report pain during more extended/upright) activitiesO’Sullivan [1].

Previous work evaluating trunk muscle activity in MDCS subgroups in sitting demonstrated AEP individuals having spinal musculature hyperactivity compared to healthy and FP groups [6, 7]. Similarly, increased abdominal muscle activity (EO and TrIO) in FP and AEP subgroups, compared with healthy individuals, during repositioning tasks in sitting and standing is reported [8].

This suggests that muscle hyperactivity is prevalent in the presence of pain, but differences in muscle activity between NSCLBP subgroups may not be sufficient to provide a rationale for differential diagnosis. Reasons for this may be due to static postures not being sufficiently challenging for the trunk muscles. We hypothesise that different subgroups of NSCLBP would accomplish more challenging functional tasks differently. To date this hypothesis has not been tested in functional tasks which could be considered more relevant than static postures.

This study investigated trunk muscle activity in subgroups of NSCLBP patients and healthy individuals during functional tasks, to identify whether differences in muscle activity are task dependent or whether a consistent pattern of muscle activity is identified irrespective of task performed.

This will inform an understanding of subgroup trunk muscle dysfunction, which may influence spinal loading and pain [6, 7], thus enabling refinement of motor control approaches for specific subgroups of NSCLBP patients [9].

The hypothesis for this case-control study is that differences in trunk muscle activity between NSCLBP subgroups and healthy individuals will be observed during a series of functional tasks reflecting a range of flexion-dominant and extension-dominant trunk motions.

NSCLBP patients were recruited from physiotherapy waiting lists in Cardiff and Vale University Health Board, Cardiff, UK. Fifty NSCLBP and 28 healthy individuals volunteered. Sample size calculation is reported elsewhere [10]. Power set at priori at 0.7, alpha level of 0.05, a sample of 24 subjects per group was calculated.

Inclusion and exclusion criteria for NSCLBP and healthy individuals are outlined in Tables 1 and 2. NSCLBP subjects were classified independently by two physiotherapists (RH, LS) based on MDCS criteria [1]. Only subjects classified as FP or AEP (both clinicians in agreement) were included. To establish NSCLBP classification a comprehensive subjective and objective assessment was conducted. Full assessment procedures are published elsewhere [1, 11]. Gender, age, anthropometric data (weight, height) were collected. Patient reported measures for pain (Visual Analogue Scale (VAS)) [12], disability (Oswestry Disability Questionnaire (ODQ)) [13], psychological distress (Distress and Risk Assessment Method (DRAM)) [14] and fear of movement (Tampa Scale of Kinesiophobia (TSK)) [15] evaluated baseline characteristics. Data collection was conducted at the Research Centre for Clinical Kinesiology, Cardiff University.

Fifty NSCLBP subjects (23 AEP, 27 FP) and 28 healthy individuals data were analysed. One FP participant failed to complete the patient reported measures. Demographic data is in Table 3. Of note significant differences were observed between groups for gender (AEP 82.6% female, FP 17.4% female). The FP group were significantly heavier (compared to AEP) and taller (compared to AEP and healthy), although BMI was comparable across groups. Participants were matched across groups for age. Back pain location was similar between NSCLBP groups with most subjects reporting central or right-sided symptoms at the time of testing.

No significant differences between AEP and FP groups in ODQ, VAS, and TSK were revealed. DRAM scores revealed the AEP group displayed significantly greater psychological distress (depressive and somatic distress combined) scores compared to FP (Table 3).

The statistical analysis for the functional tasks are shown in Tables 4 and 5. No significant between group (AEP, FP, Healthy) differences in all muscles were observed during step down, bending to pick up a pen, returning from picking up a pen or sit-to-stand functional tasks. No significant differences were observed between the groups (AEP, FP, Healthy) in the LT muscles during any functional task. Compared to healthy, the AEP group showed significantly greater right-sided sLM activation during step up (p = 0.015), reach up (p = 0.013) and box replace (p = 0.007) tasks. Right EO activity was significantly greater in the AEP group compared to the healthy group during the box lift (p = 0.016) task. Left-sided sLM (p = 0.009) and TrA/IO (p = 0.009) were found to be significantly greater during stand-to-sit in FP compared to the healthy group.

There was a statistically significant difference overall between the groups (AEP, FP, Healthy) during sit-to-stand in the left TrA/IO (p = 0.044, p < 0.05) however post-hoc testing revealed no significant pairwise between group differences (AEP vs. healthy p = 0.056, FP vs. healthy p = 0.023).

The purpose of this study was to identify whether differences in muscle activity are task dependent or whether phenotypes of muscle activity can be identified for NSCLBP subgroups irrespective of task performed. Overall, different patterns of muscle activity between tasks and subgroups were observed suggesting potentially individualised trunk muscle responses.

The tasks eliciting significant results for the AEP group, compared to healthy, were step up, reach up and box replace tasks. It could be suggested that these tasks require substantial spinal extension, leading to the AEP group demonstrating greater co-activation of abdominal and extensor musculature during these tasks. Further it could be hypothesised that AEP patients who report pain during extension activities may adopt motor strategies to protect the spine in these movement directions. Trunk musculature hyperactivity in AEP individuals has been previously proposed [1]. In support of clinical observations [1], in nearly all instances the AEP group in this study demonstrated greater overall muscle activation compared to the healthy group (Tables 3 and 4), indicating increased co-activation of the trunk musculature throughout all functional tasks. This was irrespective of the extension or flexion bias to the task. Of note, the AEP group were significantly more psychologically distressed compared to FP (DRAM score: 29.8 vs 22.7 respectively, p = 0.027), a factor known to influence NSCLBP [24, 25]. Although this is an interesting observation, the rationale for increased distress in the AEP group is not fully understood and further work evaluating psychological profiles in NSCLBP subgroups is required.

The increased muscle activity demonstrated in the FP compared to the healthy group in the left TrA/IO and sLM during the stand-to-sit activity is less clearly understood. Since no other tasks demonstrated increased TrA/IO activity it is unclear as to whether TrA/IO activity may actually differ between these subgroups. With no consistency in muscle activation between groups noted bilaterally, this further supports the reported inherent wide individual variation in muscle activity [26].

No significant differences were observed between the AEP and FP groups for any muscle group or functional task. Generally the NSCLBP subgroups demonstrated greater muscle activation in each muscle group (compared to the healthy subjects), indicating that co-contraction may be a factor for individuals in pain. Net trunk muscle activity has been previously shown to be increased during the presence of acute pain [26]. This suggests that persistent muscle activation may restrict intervertebral motion as a protective mechanism of the neuromuscular system to increase local spinal stability and thus protect dysfunctional passive spinal structures from pain provocative movement.

Consideration must be made that the tasks were not sufficiently flexion biased to challenge the FP individuals, and nor sufficiently extension biased to challenge the AEP individuals. Additionally, some functional tasks were asymmetrical, (e.g. the box replace task (see Additional file 1)) where the lifting component utilised right trunk rotation). Previous work has demonstrated regional differences in TrA, IO and EO activation during trunk rotation [27] which may have contributed to the unilateral significant differences in muscle activity observed. Also functional tasks can be performed utilising different movement strategies, thus movement variability will be greater than in sitting [6].

Overall, we report differences in muscle activity in MDCS subgroups concurring with previous literature [6‐8]. Increased trunk muscle activity is a key feature in the presence of pain [2, 3], however, the study suggests that muscle activity reflects individual patterns of muscle behaviour in NSCLBP cohorts, as well as the healthy subjects. Identifying potential subgroups based on muscle activation responses remains challenging.

This study demonstrates differing patterns in trunk muscle activity between subgroups during functional tasks, suggesting potentially individualised trunk muscle responses in the presence of pain. The findings show inconsistencies in muscle activity previously observed in MDCS subgroups [6, 8]. When considered alongside work evaluating MDCS subgroups during functional activity [10] it appears that spinal kinematics may be better at differentiating between clinical (NSCLBP) subgroups, however further work into EMG in NSCLBP subgroups during more demanding functional tasks is warranted to inform targeted interventions for these individuals.