International Journal of Physical Medicine & Rehabilitation
Open Access

Adaptations Associated with Non-Specific Chronic Low Back Pain: A Narrative Review

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Introduction

Non-specific Chronic Low Back Pain (NSCLBP) is the leading cause of disability worldwide [1]. At least 80% of the population will experience Low Back Pain (LBP) in their lifetime [2] and approximately 5%-10% of those will go on to develop chronic symptoms [3]. The incidence of NSCLBP has increased by 19.6% since 2006 [4] and the enormity of this burden is expected to continue to grow with the global increase in the ageing population number [5]. The growth in NSCLBP is also in line with a more general rise in chronic disease such as diabetes [6] and dementia [7], but any association with these conditions remains unclear. Sequelae observed in people with NSCLBP occur across multiple domains, including psychological, behavioural and neuromuscular [8]. These changes may be adaptative (i.e., compensatory changes that decrease one’s symptoms, allowing better coping in their environment) or maladaptive (i.e., compensatory changes that increase one’s symptoms, inhibiting their ability to cope in their environment) [9]. Importantly, maladaptive changes commonly observed in people with NSCLBP usually result from negative beliefs and a poor understanding of the relationship between pain and harm (e.g., sitting upright despite an increase in one’s symptoms by doing so) [9]. Initial responses to LBP can be adaptive, but persistent adaptive behaviours can become maladaptive and contribute to the transition from acute to persistent pain [10].

Interventions for NSCLBP are primarily focused on reducing disability and improving function [11]. This has been a mainstay focus for almost a century [12]; however, the treatment approach has changed dramatically over the past three decades, with the introduction of the bio psychosocial model [12,13]. Clinical practice guidelines throughout the world support the use of exercise therapy and psychological interventions for the management of NSCLBP [14]. However, guidelines lack detail regarding delivery mode and specific exercises. This likely stems from the fact that numerous published interventional studies have reported minimal between-group effect sizes [15-19], ultimately leading to conclusions that each modality is equally effective. The clinical guidelines assert that the NSCLBP population is heterogenous, however, they provide no current recommendations as to how to distinguish or manage subgroups within the broader NSCLBP population [14]. As a result of these issues, it is difficult for clinicians to apply these guidelines to people with NSCLBP [20]. Furthermore, there is an absence of recommendations pertaining to behavioural and neuromuscular assessments for people with NSCLBP, despite a large portion of NSCLBP-related literature devoted to these domains. Finally, the methods used to quantify NSCLBP-related adaptations are diverse and, in many instances, the clinical relevance of these adaptations has not been investigated.

Therefore, it is timely to consider the adaptive and maladaptive changes that occur in people with NSCLBP, with the anticipation that a better understanding of the clinical significance of these changes will lead to the development of better treatment strategies. As such, the aims of this paper are to i) evaluate the appropriateness and ecological validity of methods used to identify neuromuscular NSCLBP-related maladaptations; ii) discuss the nature and inter-relationships of psychological, behavioural and neuromuscular NSCLBP-related maladaptations; iii) evaluate the clinical significance of these maladaptations in relation to meaningful NSCLBP-related outcomes (i.e., disability, function and quality of life); and iv) propose a new model for multi-domain adaptations associated with NSCLBP (Figure 1). In addressing this knowledge gap, this review provides a narrative overview of the research investigating NSCLBP-related adaptations.

Figure 1: Placement holder. Model for inter-related multi- domain maladaptations associated with meaningful outcomes for individuals with non-specific chronic low back pain.

Methods and Findings

A series of literature searches were conducted on MEDLINE (PubMed) between August 2020 and March 2021. Studies were included that evaluated the outcomes of fear-avoidance beliefs, pain self-efficacy, physical activity, lifting and sitting behaviour, motor weakness, lumbar muscle morphology and motor control in people with NSCLBP. Peer-review studies involving humans and available in English were included in the review. Randomised controlled trials, observational studies, systematic reviews and meta-analyses were also included as they denote higher levels of evidence [21]. All other study designs were excluded from this review. Three separate searches were conducted. Medical subject heading (MeSH) terms used to perform the first search were: (CLBP OR Chronic Low Back Pain OR LBP or Low Back Pain OR Non-Specific Chronic Low Back Pain OR NSCLBP) AND (kinesiophobia OR fearavoidance OR self-efficacy OR pain self-efficacy), yielding 1318 articles. MeSH terms used in the second search included: (CLBP OR chronic low back pain OR LBP or low back pain OR nonspecific chronic low back pain OR NSCLBP) AND (lifting OR sitting OR physical activity), yielding 7640 articles. The third search used MeSH terms including: (CLBP OR chronic low back pain OR LBP or low back pain OR non-specific chronic low back pain OR NSCLBP) AND (weakness OR strength OR lumbar multifidus OR erector spinae OR motor control OR neuromuscular control), yielding 4014 articles.

In total, the searches returned 12972 articles. After removing duplicates, 10711 remained. Of the 10711 studies, 538 were not available in English and 8605 were excluded based on study design, leaving 2106 for title and abstract review. The first author reviewed the titles and abstracts for relevance, resulting in 238 studies remaining for full-text review. As a result of the full-text review, 93 articles were included in the final review.

Psychological Adaptations

Psychological maladaptations are widely reported in people with NSCLBP [22-24]. Importantly, maladaptive psychological changes are strongly associated with functional disability [25-27] and development of chronic LBP [23].

Fear-avoidance beliefs

Pain is one of the most aversive stimuli and, as such, has a close relationship with fear [28]. Therefore, it is not surprising that the tendency to avoid painful tasks is a common adaptive mechanism seen in people with LBP [29,30]. Fear-avoidance beliefs have been described as both adaptive and maladaptive in that behaviours adopted in the short-term may be protective but when sustained in the longer-term, may be deleterious [28]. Fearavoidance beliefs are generally sustained as a result of ‘catastrophising’ beliefs and negative interpretations of painful stimuli and their association with harm [31]. In those with LBP, there is limited evidence to suggest the commonality of these negative beliefs; however, it is well understood that they are associated with the persistence of LBP [32-35]. This could explain why fear-avoidance beliefs are sustained into the chronic phase of NSCLBP.

The longevity of behavioural adaptations resulting from fearavoidance beliefs may be responsible for changes in the neuromuscular system [36]. Whilst it has long been hypothesised that people with NSCLBP who demonstrate high levels of fear-avoidance beliefs would demonstrate more protective behaviours [37], the evidence pertaining to this is unclear. Specifically, the uncertainty regarding the relationship between fear-avoidance beliefs and behavioural changes is partly due to conflicting results [37-44]. Furthermore, a proportion of the uncertainty is due to varied assessment methods of fearavoidance beliefs. For example, there is recent evidence suggesting that data derived from task-specific fear-avoidance questionnaires [45] are more strongly associated with behavioural changes than data from general fear-avoidance questionnaires [46].

Whilst a body of literature has investigated the relationship between fear-avoidance and behavioural changes [37,39-41], there is a paucity of evidence investigating the relationship with neuromuscular-related deficits in NSCLBP. Of the muscular properties investigated, high fear-avoidance beliefs are associated with reduced lumbar extension strength [47]. The evidence is conflicting with respect to the relationship between fearavoidance beliefs and lower limb muscle strength [48,49]. Importantly, studies investigating relationships between fearavoidance beliefs and NSCLBP-related muscle deficits are in short supply with considerable measurement variability [48,49] and small sample sizes [50]. Thus, there is uncertainty about the relationship between fear-avoidance beliefs and neuromuscular maladaptations. Furthermore, there is evidence to suggest neuromuscular maladaptations and fear-avoidance beliefs contribute to disability independently [51].

Pain self-efficacy

Pain self-efficacy is defined as the belief in one’s ability to perform painful or perceived painful tasks or movements in order to achieve a desirable outcome [52]. Self-efficacy beliefs are modifiable through an individual’s experiences [53]. Indeed, longstanding improvements in pain self-efficacy have been identified in people with NSCLBP as a result of education and exercise [54]. By contrast, it is also possible that experiencing pain can negatively impact one’s perception of their ability to perform painful tasks, thereby resulting in lower pain selfefficacy. Low pain self-efficacy during the acute phase of LBP could also lead to avoidance of provocative tasks, not because of fear-avoidance itself, but due to the lack of confidence in their ability to cope with pain. The continued avoidance of painful tasks may perpetuate the existence of low pain self-efficacy and lead to the persistence of LBP. In those with NSCLBP, higher levels of pain self-efficacy are associated with decreased pain intensity, reduced disability scores [55, 56] and greater functional capacity [57-59]. Moreover, low pain self-efficacy and high fear-avoidance beliefs typically coexist in people with NSCLBP [60]. Despite greater research emphasis on fearavoidance beliefs, there is evidence to suggest that pain selfefficacy is a more important psychological mediator in the relationship between pain and disability in the NSCLBP population [61]. Furthermore, pain self-efficacy appears to be a critical mediator in the relationship between fear-avoidance and NSCLBP-related outcomes, such as pain and disability [62]. However, to date, the impact of pain-self efficacy on physical measures such as muscle weakness, muscle morphology and neuromuscular control is currently unknown.

Behavioural Adaptations

Functional behaviours are impacted by pain or the threat of pain in people with NSCLBP. Physical activity, lifting and sitting are examples of behaviours that are commonly reported as problematic in this population [63-65]. Importantly, changes in these behaviours in response to pain or fear of pain vary amongst individuals with NSCLBP [66].

Physical activity

It is a commonly held belief by clinicians that people with NSCLBP are less physically active than healthy individuals. Arguably, this belief is attributed to the theoretical fearavoidance model which indicates that decreases in physical activity are the result of fear of pain or harm [23]. Indeed, theoretical models have identified physical inactivity following the onset of LBP as a perpetuating factor for chronicity [65]. However, these theoretical models are only partially supported as there is conflicting evidence to suggest that physical activity deficits are commonplace in those with NSCLBP [67,68]. One limitation to our understanding of physical activity behaviours in people with NSCLBP stems from the outcome measures used to quantify physical activity that are unable to differentiate between different types of activities (e.g., walking or lifting). Therefore, whilst some people with NSCLBP demonstrate levels of physical activity similar to healthy individuals, they may actually modify or avoid certain provocative tasks (e.g., lifting). Indeed, no previous studies have performed time and motion analyses of people with NSCLBP. Importantly, time and motion analyses has led to a paradigm shift in the early rehabilitation phase following stroke [69,70]. The nature and extent of adaptations to physical activity in NSCLBP need to be established before appropriate interventions and education can be developed.

The heterogenic nature of the NSCLBP population has likely contributed to findings presented in previous studies that indicate negligible to no differences in physical activity-related behaviours compared with healthy control participants. The avoidance-endurance theoretical model suggests that a proportion of people with NSCLBP will decrease their physical activity-related behaviours whilst others will persevere despite their LBP [71]. Recent evidence supports this model by demonstrating that people with NSCLBP exhibit avoidance, persistence or a combination of both behaviours, towards tasks that reproduce pain [72]. Thus, people with NSCLBP display considerable diversity in their behaviours relating to physical activity, thereby supporting the notion that this population is heterogenous.

Lifting

Lifting is a risk factor for the development of LBP [73]. A popular belief held by people with NSCLBP, and clinicians is that a safe lifting technique should be characterised by a ‘straight’ back with movement and force produced from the lower limbs [74,75]. This squat-lift technique is also commonly advocated for the prevention of LBP in the workplace [64]; however, from a biomechanical and scientific perspective, ‘best’ lifting technique is equivocal [76-78] with spinal load (i.e., compression and shear) comparisons between ‘squat’ (i.e., bent knee and straight back) and ‘stoop’ (i.e., minimal knee bend and bent back) lifting techniques demonstrating mixed results [79-82]. Moreover, biomechanical comparisons of lifting techniques between those with and without NSCLBP have shown i) considerable variability, and ii) conflicting results with increased [83], and reduced to no differences [84,85] in lumbar range of motion. These inconsistent findings also provide support to the notion that people with NSCLBP are not homogenous with respect to their lifting behaviour [86]. Subclassification, based on observed motor control impairments (i.e., flexion pattern or active extension pattern), has demonstrated significant differences in spinal kinematics between healthy controls and people with NSCLBP, as well as significant differences between NSCLBP sub-groups [87]. Therefore, the commonly held community perceptions together with general clinical recommendations suggesting that there is a singular ‘safe’ way of lifting are not evidence-based.

Coordination deficits between the trunk and lower limbs during lifting has also been associated with NSCLBP [88,89]. Individuals with NSCLBP display differences in trunk and lower limb coordination and reduced movement variability compared with healthy controls [90]. Coordination deficits extend beyond the musculoskeletal system as dysfunction of the respiratory system during lifting has been identified in individuals with NSCLBP. Specifically, individuals with NSCLBP perform lifts with greater inhaled lung volume than those without LBP [91]. The increase in inspired lung volume is associated with increased spinal stability and requires further thoracic spine extension [92,93]. Furthermore, altered breathing patterns may be associated with psychological factors commonly seen in people with NSCLBP, such as fear and apprehension [94].

Sitting

People with NSCLBP often identify sitting as a task that exacerbates their symptoms [63]. There are two key components regarding sitting: i) time and ii) posture. Similar to lifting, clinicians and people with NSCLBP commonly share the misconception that an upright sitting posture is optimal [95-97]. However, like lifting, there is a lack of evidence to support the notion that there is a singular ‘best’ universal sitting posture. In reality, people with NSCLBP tend to adopt sitting positions at either extreme of range (i.e., lordotic or kyphotic positions) compared to healthy individuals [98,99]. Adopting end of range sitting postures is a NSCLBP-related maladaptive trait as evidence demonstrates that when patients are positioned at the opposing end of range position or in a more neutral sitting position, they report reductions in pain intensity during sitting [98]. When it comes to sitting posture, it is apparent that a ‘one size fits all’ approach fails to address the complex and highly specific needs of people with NSCLBP. The evidence relating to the impact of sitting time on NSCLBP, however, is unclear due to inconsistencies in the literature [100-104], which likely result from differences in outcome measures used (e.g., patient reported measures vs. accelerometry).

Neuromuscular Adaptations

Maladaptations within the neuromuscular system including muscle weakness, muscle atrophy and motor control impairments, have long been described in those with NSCLBP [105,106]. Exercise interventions aimed at reducing disability in people with NSCLBP have targeted these neuromuscular deficits [15,16].

Morphological adaptations

Adaptations in NSCLBP-related muscle morphology have been investigated for almost 30 years [107]. These studies have primarily focussed on the morphology of the lumbar extensors and trunk muscles given their involvement in lumbar spine stability [107,108]. Indeed, numerous imaging studies have reported altered morphology and decreased size of the lumbar multifidus in NSCLBP people compared to healthy controls [109-112]. People with unilateral NSCLBP display significantly reduced multifidus size on the symptomatic side with a moderate positive relationship with pain duration. However, no associations with weakness or self-reported disability have been identified [113,114]. Importantly, these studies have focused on people with minimal disability and may not be representative of a more disabled NSCLBP population.

In addition to muscle size, numerous studies have investigated NSCLBP-related changes in muscle composition. Specifically, magnetic resonance imaging studies have demonstrated that people with NSCLBP exhibit increased fat infiltration in their erector spinae and lumbar multifidus muscles [115-120]. Surprisingly, the severity of fat infiltration in the lumbar muscles is only weakly associated with the severity of NSCLBPFarragher related disability [117,118]. Whilst similar to the abovementioned evidence pertaining to lumbar muscle size, no studies have investigated relationships between lumbar muscle morphology, weakness and performance of functional tasks. Considering the weak associations between lumbar muscle morphology and disability, it is unlikely that interventions aimed at augmenting muscle size and composition would yield clinically relevant improvements in people with NSCLBP

Motor weakness

The impact of motor weakness on function in people with NSCLBP is contentious [121]. Even the existence of NSCLBPrelated motor weakness is debated in the literature. In this respect, there are inconsistent findings regarding the presence of motor weakness in the lumbar extensors of those with NSCLBP when compared with healthy controls [122-130].

The disparity of findings between studies may be partly due to the testing methods employed as some protocols limit pelvic rotation in a seated position [129,130] in order to isolate lumbar extension [131], whilst others permit posterior pelvic rotation [122-128]. Studies that restricted pelvic rotation reported no difference in lumbar extensor force-output between people with and without NSCLBP [129,130]. In contrast, six of the seven trials that did not restrict pelvic rotation found significant evidence of motor weakness during lumbar extension between participants with NSCLBP and healthy controls [122-127]. Restriction of pelvic rotation may lack ecological validity. Some studies have attempted to address this issue by testing lumbar extensor muscle strength in standing [122,123,126]. However, this test is ‘static’ in nature and, as such, does not test the types of isotonic muscle contractions utilised during functional tasks, such as lifting.

Despite the known involvement of the posterior pelvic rotators (i.e., gluteal and hamstring muscles) in functional tasks [132], there is a paucity of evidence evaluating them in people with NSCLBP. Of the single study identified, no difference between a NSCLBP group and healthy controls was identified [133]. However, obvious methodological flaws create uncertainty, as muscle strength testing has been performed i) using manual muscle testing, which is inherently inaccurate [134,135]; ii) using maximal effort contractions which are rarely required in day-today activities, iii) are conducted in non-functional positions (i.e., side-lying), or iv) are quantified using isometric contractions that are not representative of the types of isotonic (i.e., concentric and eccentric) muscle contractions involved in functional, closed kinetic chain movements. Therefore, it is not surprising that neuromuscular impairments including motor weakness exhibit limited associations with clinical and functional outcome measures (i.e., disability and functional performance) given that the testing methods lack ecological validity. Assessment protocols designed to quantify muscle force output though coordinated functionally relevant movements are likely to be more relatable to functional deficits in this population.

Motor control deficits

Motor control is defined as the ability of the central nervous systems to produce purposeful and coordinated movements [136]. In people with LBP, motor control deficits have been described in the literature for decades. A landmark study by Hodges and Richardson [105] indicated that spinal stability and control are altered with LBP. Early studies focussed on the deep trunk muscles (i.e., transversus abdominus and multifidus muscles) [137-139]. However, there are conflicting results regarding differences in recruitment patterns of the transversus abdominus between those with and without NSCLBP [105, 140-143]. Potential reasons for the disparity between studies may be due to low participant numbers and differing assessment methods (e.g., lower-limb tasks in supine [143], bilateral upper limb tasks whilst standing [105] or performing abdominal drawing-in in a hook-lying position and in supine [140-142]). Only one study has reported an association, albeit weak, between improvements in the ability to contract the transversus abdominus muscle and LBP-related disability [144].

Motor control deficits have also been explored via other methods. One prospective cohort study assessed LBP-related motor control impairments via three different methods. Firstly, visual observation of thoracolumbar dissociation was used to assess motor control, but visual observation is inherently inaccurate. Secondly, passive extension of the lumbar spine was assessed, but this measures range of movement rather than motor control. Thirdly, deep muscle contraction, which is similar to the abovementioned methods, lacks ecological validity [145]. Overall, none of these clinical tests were able to predict changes in NSCLBP-related disability or pain over an 8-week period [145]. In an attempt to implement a more ecologically valid assessment of motor control, Pranata, Perraton [129] found isometric assessment of force control of the lumbar extensor muscles was predictive of disability level. Whilst this protocol has some functional relevance in that it examined i) a provocative task (sitting) and ii) a sub-maximal force output of a muscle group associated with lumbar spine stability and movement, data collected in sitting may not translate to other functional tasks such and walking and lifting. It is apparent that testing muscle strength and motor control in static, isometric or non-functional tasks limits the strength of associations with NSCLBP-related disability.

Conclusion

Maladaptations in multiple domains can lead to disability, impaired function and health related quality of life for people with NSCLBP. However, current evidence suggests that maladaptations in the psychological and behavioural domains have a greater impact than those in the neuromuscular domain. Furthermore, the interrelationship between behavioural and psychological factors is stronger than any interrelationships with neuromuscular factors. Based on the available evidence, it is clear that the neuromuscular system is impacted by NSCLBP; however, studies evaluating the measures of these impairments have failed to identify meaningful relationships with clinically important outcomes. This lack of association is arguably attributed to variability in methods. Future investigation could consider: i) developing novel ecologically valid tests for neuromuscular impairments in relation to functional deficits commonly experienced in NSCLBP; ii) identifying and/or validating already proposed heterogenous sub-groups within the NSCLBP population; and iii) if NSCLBP sub-groups exist, identify discriminating features between them in order to tailor specific interventions towards these sub-groups. This will more importantly inform targeted and specific interventions for NSCLBP.

Funding

There was no funding source for this review.

Competing Interests

The authors declare they have no competing interests.

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