Low back pain (LBP) represents a significant epidemiologic burden in modern society, standing as the single leading cause of disability worldwide and showing an upward trend. It also accounts for the largest proportion of patients who could benefit from rehabilitation with an increasing focus on reducing low value care and high risk, costly and invasive interventions [1] . The majority of LBP is termed non-specific, owing to the absence of a clearly identified pathoanatomical origin, however, the primary source of pain in chronic LBP is typically attributed to nociceptive mechanisms [2] [3] . Nociceptive pain sources may derive from the active or passive stabilising systems of the spine [4] . Of the active structures, the function and morphology of the paraspinal muscles is established as being closely associated with the aetiology, management and prognosis of patients with LBP [5] . Among these, the lumbar multifidus (LM) muscles are the greatest contributors to the stability of the lumbar neutral zone, a range of intervertebral motion where passive support offers the least resistance, which significantly influences LBP intensity and also has a key proprioceptive role. It is thought that where there is impairment of stability, restraint and proprioception through the neutral zone, pain can be elicited through increased shear or torsional stress on the bone and ligamentous elements [6] - [8] .

The appearance of the LM muscle on advanced imaging has clinical relevance and is suggested as a biomarker of its function and competence. Computer tomography derived cross sectional area of the LM is shown to be statistically smaller when compared to surrounding paraspinal muscles, in LBP populations [9] . These atrophic changes may be attributed to either denervation and/or arthrogenic inhibition following low back injury, therefore creating potential for acute LBP recurrence through cyclical inhibition with irregular movements through the spinal segments. This is thought to lead to chronic inflammation, allowing for differentiation of fibroblasts and pre-adipocytes, creating fat infiltration and fibrotic changes to the LM muscle [9] [10] . Fat infiltration of the LM on imaging is strongly associated with LBP in adults [11] [12] . Additionally, the degree of fat infiltration in the LM muscle is directly associated with the risk of high intensity pain and dysfunction [13] .

LM dysfunction is a clinical diagnosis and subclassification of LBP which is hypothesised to be a likely mechanism behind the development and maintenance of pain in chronic LBP populations [14] . LM dysfunction is proposed to be derived from the arthrogenic inhibitory effect on the role of the multifidus in stabilising the spine in the neutral zone [10] [14] . Clinical diagnosis of LM dysfunction has been reported by Chakravarthy and colleagues to be mainly based on physical examination through the performance of clinical tests [14] . Isolated lumbar extension (ILE) resistance training has been shown to be effective at significantly improving perceived pain, disability and multifidus functional cross sectional area in patients with chronic non-specific LBP [15] [16] . However, current literature outlines the functional benefits of ILE on non-specific LBP patients without a sub-diagnosis of LM dysfunction. The objective of this study is to investigate the efficacy of ILE on lumbar extension power and disability in patients with diagnosed LM dysfunction.

Forty-five subjects completed the twelve-week intervention, with demographics shown in Table 1 .

Outcome measures pre- and post-intervention are shown in Table 2 . The classification terms used in the ODI are outlined in Table 3 . The median scores of the ODI improved from 24, indicating “Moderate disability”, to 15, representing “Minimal disability” (p ≤ 0.05). Additionally, a 52% reduction was seen in subjects classified as “Severely Disabled”, with a 74% decrease in “Crippled” subjects. Median maximal lumbar extension power improved post-intervention by 52% (p ≤ 0.05). Out of the 45 subjects, only 2 had a worse post-intervention ODI score, and 33 had a ≥50% predicted ILE maximal strength post-intervention. Although ODI score was worse in these 2 subjects, ILE strength was improved in both. With reference to absolute strength, the bottom 33% of subjects improved their strength ≥ 50% except for 3 individuals. No significance between gender for presence of strength improvement (X², p > 0.05).

IQR: Interquartile range.

ODI: Oswestry Disability Index; *Median scores p ≤ 0.05 for both.

The findings from this study underscore the efficacy of ILE resistance training in improving extension power and reducing disability in individuals diagnosed with LM dysfunction. The notable enhancements in ODI scores and lumbar extension strength not only demonstrate ILE training’s ability to alleviate symptoms in this particular cohort, but also point to a connection between treating LM atrophy and reducing pain and weakness. It appears that LM dysfunction is more than a result of ‘disuse’, dominant theories discuss impaired motor control due to arthrogenic inhibition and interruption of neural input (e.g. by radiculopathy), suggesting the need for investigation into active interventions beyond basic conditioning [14] . Our results align with the hypothesis that targeted strengthening exercises may assist with overcoming corticospinal inhibition of the LM, improving recruitment of LM and mitigating the biomechanical and neuromuscular deficits associated with LM atrophy [14] . Although improvements in disability were observed in 95% of subjects, it should be acknowledged that the remaining 5% were observed to have a worse post-intervention ODI score. This may suggest that there is further nuance in the selection of effective intervention for this specific population and that there are multiple psychosocial and pathoanatomic factors that may not make this technique universally effective to all patients with LM atrophy. Nonetheless, this discrepancy accentuates the multifactorial nature of chronic LBP [2] .

LM dysfunction amongst subjects with LBP is almost universal and this study gives support to this non-invasive and low risk technique where there has been an increasing emphasis on avoidance of invasive and medicalised interventions [26] . This study adds to the growing body of literature supporting the use of ILE resistance training in LBP patients, and creates a novel perspective with the specific population inclusion. Although it is known that LM atrophy is seen in patients with chronic LBP, it has been suggested that the morphology of the LM is not a critical prognostic factor for short-term functional restorations [16] [27] [28] . Therefore, the mechanisms behind the efficacy of ILE on pain and disability in patients with LBP must not be exclusive to architectural change of the muscle, but rather multifactorial including correction of movement patterns and improved recruitment on remaining ‘normal’ musculature [27] .

In recent years, there have been randomised and cohort trials conducted into peripheral multifidus stimulation for subjects diagnosed with LM dysfunction which has been shown to have efficacy in improving pain and dysfunction [29] . The exact role of ILE training to delay, complement or prevent this intervention is unclear. To our knowledge, this study supports these training methods for patients being considered for peripheral multifidus stimulation as it is the first study of function and strength in patients with a specific LM dysfunction diagnosis.

Limitations of our study surround the convenience sampling method, leading to heterogenous pathologies and patient population. Additionally, there was only a single unblinded orthopaedic surgeon assessing the patients which could produce diagnostic bias and limit the external validity. Further, we assumed the driving factor of the disability index and strength was multifidus atrophy without accounting for denervation or erector spinae involvement or other patient factors such as personality and psychosocial characteristics. Equally, it is prudent to acknowledge the role of corticospinal stimulation, independent to the appearance of the LM muscle. Additionally, due to the LM segments surrounding the L5 vertebrae being involved in the inclusion criteria, the same results cannot be extrapolated for the upper lumbar spine. Similarly, due to the anatomical nature, we cannot isolate the LM in ILE and therefore could be involving other erector spinae tissue. Confounding variables related to education and lifestyle changes after patient visitation to the orthopaedic surgeon were not accounted for, therefore limiting internal validity. Finally, the lack of control and measurement of subject activity between the formal and measured training days must be recognised.

Future research should further explore the effects of resistance exercise on pain and disability in LM dysfunction patients through a standardised large cohort long-term study with successive imaging (and possibly electromyography) at set time points assessing the degree of dysfunction and extent of atrophy. Long-term functional outcomes and maintenance is yet to be discovered and should be investigated further. This is difficult to achieve as it is limited by healthcare resources and demands high patient compliance and low attrition. Optimum exercise selection should be evaluated to consider other methods that could be complementary or even superior to techniques presented here, including motor control training utilizing free weights, or functional movement training which emphasises correct movement and lifting techniques [14] [30] .

Conclusively, this study provides a foundational step on the efficacy of ILE resistance training on improving strength and disability status in patients diagnosed with LM dysfunction. The notable changes in disability scores and strength suggest that ILE resistance may mitigate the pathological effects of LM atrophy in this population. Although a small proportion of subjects did not benefit, the overall findings highlight ILE resistance training as a valuable, non-invasive intervention for managing LBP in the context of LM dysfunction. Future research should examine the long-term efficacy of ILE training with experimentation of adjunctive therapeutic modalities to optimise patient care.