Efficacy of Core Stability in Non-Specific Chronic Low Back Pain

(1) Background: Management of chronic low back pain (cLBP) is often multidisciplinary, involving a combination of treatments, including therapeutic exercises. Core stability exercises aim to improve pain and disability in cLBP increasing spinal stability, neuromuscular control, and preventing shear force that causes injury to the lumbar spine. The purpose of this study was to review the available evidence about the effectiveness in reducing pain and improving disability of core stability exercises for non-specific cLBP. (2) Methods: We perform a systematic research on common Medline databases: PubMed, Pedro, and Cochrane Library. Search results were limited to articles written in English and published between January 2005 and November 2020.The search provided a total of 420 articles. Forty-nine articles met the inclusion criteria and 371 articles were excluded. (3) Results: Core stability provides great therapeutic effects in patients with non-specific chronic low back pain reducing pain intensity, functional disability, and improving quality of life, core muscle activation, and thickness. Evidences suggest that core stability is more effective than rest or no/minimal intervention and combination with other types of exercise for cLBP have shown grater efficacy. (4) Conclusion: Core stability could be proposed in a comprehensive approach in cLBP, the combination with other modalities of therapeutic exercise should be promoted. Patient compliance is crucial to determine the efficacy of the intervention.


Introduction
Low back pain (LBP) is an extremely common condition with a lifetime prevalence reported to be as high as about 80% [1]. About 5-10% of patients develop chronic pain (cLBP), responsible for high direct (health care) and indirect (lost production and lost household productivity) costs and high individual suffering and disability [2][3][4].
In particular LBP, globally, was ranked as the greatest contributor to global disability (measured in years lived with disability-YLD), and the sixth in terms of overall burden (measured in Disability-Adjusted Life Year-DALY) [5].
Low back pain is commonly classified as non-specific (90%) or specific (10%) as to reported cause and as acute (<6 weeks), subacute (6-12 weeks) or chronic (more than 12 weeks) according to duration of symptoms [1].
LBP is a multifactorial condition that develops as result of the interaction of several risk factors: constitutional risk factors (genetic predisposition, persons ages 40-80 years, and female sex), occupational risk factors (excessive static or dynamic loading, high number of lifts at work, vibrations, repeated torsion and bending movements, incorrect postures), behavioral and environmental factors (smoking, obesity, sedentary lifestyle), and psychosocial factors (stress, anxiety, depression, and work dissatisfaction) [4,6].
Management of cLBP is often multidisciplinary, involving a combination of treatments like self-care (including remaining active), pharmacotherapy (NSAID, opioids, muscle relaxants, glucocorticoid, etc.), physiotherapy, cognitive behavioral therapy, massage, physical therapy (laser therapy, TENS, T.E.CA.R. ® therapy), spinal manipulation, acupuncture, and in some cases, invasive interventions such as glucocorticoid injections and surgical procedures [4]. The use of kinesiotaping could be proposed beside manipulation and exercise considering its possible positive effects on posture and pain [7,8].
Core stability has reached a wide spread in recent years, considering that several studies have observed in cLBP delayed or decreased activation of lumbar multifidi and transversus abdominus and loss of physiologic tonic activation of transversus abdominus during gait and extremity movement. Dysfunction of these muscles may determine loss of lumbar spine support, increased stress and load on the joints and ligaments of lumbar spine [9][10][11][12][13][14].
The purpose of core stability exercises is to recreate normal muscle function in order to increase spinal stability, neuromuscular control within the lumbopelvic region, induce intersegmental stiffness and prevent shear force that causes injury to the lumbar spine [15][16][17].
The aim of the present study was to review the available evidence about the effectiveness of core stability for non-specific cLBP.

Materials and Methods
We perform a systematic research on common Medline databases: PubMed, Pedro, and Cochrane Library.
Studies were selected using the following keywords: "core stability" or "core stabilization" or "lumbar stabilization" or "core strengthening" combined with the term "low back pain", searching.
Search results were limited to studies written in English and published between January 2005 and November 2020.
Selection criteria: We included studies that met the following inclusion criteria: -Articles published in English, -Study population aged between 18 years and 80 years, -Randomized controlled trial, meta-analysis, and systematic review on core stability in the management of non-specific chronic low back pain.
We excluded: -Acute and subacute LBP; -LBP with specific etiologies; -LBP in pregnant women, athletes, and military personnel.
Articles were screened by title and abstract. Studies that resulted unclear from their title or abstract were reviewed according the selection criteria through full-text. Furthermore, we searched reference lists of all included studies to identify other potentially relevant studies.
After the initial research on the different electronic databases, we excluded duplicate articles. The search provided a total of 420 articles. Forty-nine articles met the inclusion criteria and 371 articles were excluded (Table 1).       Myofascial relaxation technique was performed with a roller massager, but many other methods also exist. Absence of Myofascial Release Technique alone group.

Effect of Core Stability on Short Duration (3-12 Months) and Long Duration (More Than 1 Year) cLBP
Kumar et al. evaluated the effect of core stability on cLBP patients with short pain duration (3-12 months) and long pain duration (more than 1 year) and concluded that core stability is an effective rehabilitation strategy for all cLBP patients regardless the duration of symptoms, improving pain, disability, and activation of Transversus Abdominis and gluteus maximus strength [18].

Progressive vs. Conventional Core Stability
A recent RCT, compared the short-term effects of two core stability interventions (progressive vs. conventional) and confirmed that core stability improves pain severity, functional disability, trunk endurance, lumbopelvic control, and body balance with no significant differences between the two interventions [19].
In each study, core stability was effective in improving the outcomes evaluated. In three articles, the reduction of pain was statistically greater in the core stability group, compared with minimal or no intervention [20,21,24]. In four articles, core stability was more effective in reducing disability [21][22][23]26].
Two studies evaluate the circulatory (induced tissue blood flow) and biochemical effects (Plasma β-endorphin level) of core stability, always compared with minimal or no intervention. The findings indicate that the mechanism of action of the pain-relieving effect of core stability might be related to a plasma β-endorphin elevation mechanism and tissue blood flow improvement in the pathological area as part of its effects [29,30].

Core Stability vs. General Typical Strengthening and Stretching Training
One review and fourteen articles compared the effect of core stability with general strengthening superficial muscle training, 1 compared core stability with stretching exercises, while 1 explored the effectiveness of lumbar stabilization exercise program in addition to general strengthening and stretching exercises.
Wang et al. reviewed the effects of core stability exercise or general exercise for patients with non-specific cLBP. They selected a total of 5 trials involving 414 participants. The results of their meta-analysis indicated that core stability exercise was superior to general exercise in pain improvement (VAS/NRS) and back functional status (Oswestry Disability Index) in the short term, with no significant differences in pain relief in the intermediate-(6 months) and long-term (12 months) follow-up periods [31].
Although the above-mentioned studies have shown greater effectiveness of core stability than routine exercises, particularly in the short term, other studies indicate that both types of exercises have similar effects [40][41][42][43][44][45].
França et al. compared the efficacy of core stability and stretching exercises in patients with cLBP. Both treatments were effective in relieving pain and in decreasing functional disability, while core stability had more significant improvements with in addition to an improved Transversus Abdominis muscle activation [46].
Stankovic et al. evaluated the effectiveness of a lumbar stabilization exercise program in combination with general strengthening and stretching exercises and showed that a combined exercise program is more effective in reducing pain, improving disability, and quality of life than the traditional exercises alone [47].

Core Stability vs. Pilates, Cognitive Functional Treatment, or Proprioceptive Neuromuscular Facilitation
Four articles compared core stability with Pilates, cognitive functional treatment, or proprioceptive neuromuscular facilitation.
Bhadauria et al. concluded that significant improvements are achieved in core stability and Pilates group, while the improvement was significantly greater with the lumbar stabilization program for all the outcome measures (pain, ROM, disability, and core strength) [41]. Akodu et al. found both approaches effective without significant differences in terms of outcome measures [48].
Two studies concluded that cognitive functional treatment, proprioceptive neuromuscular facilitation, and lumbar stabilization treatment improve lumbar movement control and pain compared with the control group, with no significant difference between the experimental groups [49,50].

Core Stability vs. Treadmill Walking Exercise
Lumbar stabilization exercises showed superiority to treadmill walking exercises in activating the multifidus muscle, reducing pain intensity and disability in cLBP patients [51].

Core Stability Using Training Device vs. on a Mat
Five studies evaluated the effects of core stability exercises using equipment such as ball, flexi-bar, unstable surface, sling, or device for assisted sit-up, compared to general core stability exercises on a mat.
In particular, performing core stability exercises on an unstable surface [52] or using flexi-bar [53] should be beneficial for improving lumbar pain, muscle strength, stability, disability, and depression. In addition, performing stabilization exercises with a ball allows a greater increase in the multifidus cross-sectional areas [54], while no statistically significant differences were found performing sling core stability exercises or assisted sit-up exercises using a new training device (HubEX-LEX ® ) and mat exercise program [55,56].

Combination of Core Stability and Other Exercises
Greater prevalence of diaphragm fatigue was found in cLBP individuals compared with healthy controls, which results in lack of active spinal control [57]. Four articles investigated the effect of core stability in combination with respiratory training. In particular, the association of lumbar stabilization exercises with respiratory training is more effective than lumbar stabilization alone, improving respiratory variables (maximum inspiratory pressure and maximum voluntary ventilation) by enhancing posture and stability [58,59], disability, and stabilizer muscles thickness such as diaphragm, transversus abdominis, and multifidus [59,60].
Additionally, inspiratory training (using respirometer) with lumbar stabilization exercise proved to be superior in terms of improving pain compared to expiratory training (using ball and balloon) with lumbar stabilization exercises [61].
Core stability increases contracted thickness and activity of gluteus maximus [18,26], which plays an important role in stabilizing the pelvis and is involved in patients with LBP [62]. Performing lumbar stabilization exercises with specific exercises for the gluteus muscle is more effective to increase the lumbar low back pain disability index, isometric muscle strength of lumbar flexion and extension, and stability [63]. You et al. showed that adding ankle dorsiflexion to "drawing in" the abdominal wall results in improved benefit on physical disability, pain, and core instability [64].

Core Stability Plus Other Classic Chronic Non-Specific LBP Treatments
Core stability exercises combined with other chronic non-specific LBP treatments have shown greater effectiveness.
In particular, the association with Neuromuscular electrical stimulation (NMES) resulted in greater analgesia, improved function, disability, and lumbopelvic stability in patients with non-specific cLBP [65], combined with thoracic spine mobilization resulted in a significant increases in the strengths of the trunk flexor and extensor muscles [66], while combined with myofascial release technique was more effective in terms of a greater increase in core stability endurance and spinal mobility (in the sagittal plane) [67].

Discussion
LBP is one of the most common pathology worldwide. Non-specific chronic low back pain is defined as low back pain without underlying specific cause and symptoms lasting over 12 weeks.
cLBP should be approached with a comprehensive treatment strategy, considering pharmacological, psychological (cognitive behavioral therapy (CBT), progressive relaxation, and biofeedback), physical, and rehabilitation strategies, and eventually minimally invasive or invasive approaches [68].
Rehabilitation treatments can involve different techniques ranging from spinal manipulations, mobilization, advice, general exercises, and specifically tailored exercise.
Lack of stability of the spine seems to have a key role in the development of LBP and it is arguable that therapeutic exercise aimed to retrain motor skills and the activation of local spinal stabilization muscles should be proposed in a multidisciplinary approach. Currently, the use of core stability in clinical practice is growing, accompanied by a growing number of level I and level II studies conducted in the last years.
Increasing evidence supports core stability exercises in comparison to no intervention, shame intervention, or rest in improving pain and disability, strengthening that rest in LBP should be avoided and exercise should be promoted.
When core stability is compared to general exercise protocols, most of the studies observed the superiority of core stability, while only four studies evidenced similar effects.
However, the combination of core stability with other exercise modalities seems to lead to a greater improvement in pain and disability compared to both treatments alone.
Furthermore, the combination of core stability with respiratory training (in particular inspiratory training), specific exercises for the gluteus muscle, NMES, thoracic spine mobilization or myofascial release technique allows a greater improvement. Respiratory resistance determines strong contractions of the abdominal muscles and diaphragm with an increase in the intra-abdominal pressure, contributing to a decrease in lordotic curve, promoting postural adjustment.
Hip extensor (gluteus maximus) and abductor (gluteus medium) should also be evaluated, considering that lack of strength in gluteal muscles has been linked to LBP. Gluteal muscles are crucial to modulate forces between lower limbs to the spine and impaired hip extensor function may cause increased L5-S1 and sacroiliac joint pressure, which lead to functional failure and low back pain. Different studies have revealed hip muscle impairment especially in female population, that could be targeted with combined programs.
The use of unstable surface, flexi-bar, and balls could be proposed, considering that these approaches have been linked with improvement on pain and disability. Balance strategies (as unstable surfaces and ball exercises) lead to improvements in motor control, activation of multifidus, and muscle strength.
The success of core stability exercise interventions is dependent on the high adherence of the patients and the correct dosage. Even non-conclusive considerations should be achieved, as a recent review examined the dose-response-relationship of stabilization exercise interventions in non-specific cLBP patients recommending 20-30 min time per session (Grade A), three to five times a week (Grade C), while no impact of the duration of intervention in weeks on the pain intensity was demonstrated [69].

Conclusions
Core stability may provide great therapeutic effects in patients with non-specific chronic low back pain, reducing pain intensity, functional disability, and improving quality of life, core muscle activation, and thickness.
Our review aimed to better elucidate the current evidences on the role of corestability considering only high quality studies and grouping the studies on the basis of intervention modalities.
Several studies indicate that core stability exercises are certainly more effective than rest or minimal intervention while conflicting evidences are present about the superiority of core stability exercise in comparison other types of exercise for chronic LBP.
However, the combination of core stability with other exercise modalities seems to lead to a greater improvement in pain and disability compared to treatments alone.
The success of the core stability program depends on the patient's compliance and the correct dosage of the exercise program, which should be customized for each patient.