Summarizing the effects of different exercise types in chronic low back pain – a systematic review of systematic reviews

Abstract

Background

In chronic LBP (CLBP), guideline-endorsed treatment is to stay active, return to normal activity, and to exercise. Several reviews on various exercise types used in CLBP have been published. We aimed to identify systematic reviews of common exercise types used in CLBP, to appraise their quality, and to summarize and compare their effect on pain and disability.

Methods

We searched the databases OVID MEDLINE, EMBASE, COCHRANE LIBRARY, and WEB OF SCIENCE (Core collection) for systematic reviews and meta-analyses on adults between 18 and 70 years of age suffering from chronic or recurrent LBP for a period of at least 12 weeks, which investigated the effects of exercises on pain and disability. All searches were conducted without language restriction. The search was performed up until 2022–01-26. The included reviews were grouped into nine exercise types: aerobic training, aquatic exercises, motor control exercises (MCE), resistance training, Pilates, sling exercises, traditional Chinese exercises (TCE), walking, and yoga. The study quality was assessed with AMSTAR-2. For each exercise type, a narrative analysis was performed, and the level of evidence for the effects of exercise was assessed through GRADE.

Results

Our database search resulted in 3,475 systematic reviews. Out of the 253 full texts that were screened, we included 45 systematic reviews and meta-analyses. The quality of the included reviews ranged from high to critically low. Due to large heterogeneity, no meta-analyses were performed. We found low-to-moderate evidence of mainly short-term and small beneficial effects on pain and disability for MCE, Pilates, resistance training, TCE, and yoga compared to no or minimal intervention.

Conclusions

Our findings show that the effect of various exercise types used in CLBP on pain and disability varies with no major difference between exercise types. Many of the included systematic reviews were of low-to-moderate quality and based on randomized controlled trials with high risk of bias. The conflicting results seen, undermine the certainty of the results leading to very-low-to-moderate quality of evidence for our results. Future systematic reviews should be of higher quality to minimize waste of resources.

Trial registration

PROSPERO: Reg no 190409 Registration date 01AUG 2020.

Introduction

Low back pain (LBP) continues to be the number one disorder, with most years lived with disability, meaning huge personal suffering and high socioeconomic costs [1,2,3]. For many, the pain follows a trajectory, recurring several times during their lifespan [4]. In chronic LBP (CLBP), guideline-endorsed treatment is to stay active, return to normal activity, and to exercise [5]. The effect of exercise therapy is reportedly moderately effective in reducing pain and disability [6,7,8] and is moreover cost-effective [9].

Exercise therapy is defined as “a regimen or plan of physical activities designed and prescribed for specific therapeutic goals, with the purpose to restore normal musculoskeletal function or to reduce pain caused by diseases or injuries” [10]. People with CLBP seeking primary care are often prescribed training or exercises by, for example, physiotherapists [11, 12]. Different exercise types are used in the treatment of CLBP such as strength and aerobic training on land or in water [13,14,15], as well as specific exercises such as motor control exercises (MCE) [16], sling exercises [17], Pilates [18], yoga [19], and traditional Chinese exercises (TCE) [20], such as Tai Chi and Qi Gong [20]. It is, however, not clear why a specific exercise is chosen for the individual patient suffering from CLBP. A recent review, summarized and suggested rationales on which various exercise types used in LBP are based; neuromuscular and psychosocial mechanisms were proposed most often, whereas neurophysiological, cardiometabolic, and tissue healing mechanisms were proposed less often [21]. The prescription of specific exercises might, in addition, be based on the therapists’ knowledge and the preconceived conviction of the effectiveness of certain exercises [22]. To date, there is no solid evidence that one exercise type is more effective in improving pain and disability in CLBP than another [23].

Since 2005, several systematic reviews (SR) and meta-analyses (MA) on the effectiveness of various exercise types used in LBP have been published and presented with various levels of risk of bias [7, 8, 16, 18,19,20,21, 23,24,25,26,27]. These systematic reviews report overall low effect sizes comparing exercises to various nonpharmacological interventions. A recent and updated Cochrane review on the effect of exercises in CLBP included > 200 original trials and the results seemingly did not change the evidence on the effectiveness of exercises in CLBP [8]. In addition, two recent reviews, a comprehensive review [28] and an individual participant data (IPD) meta-analysis [29] presenting data from 27 randomized controlled trials on the effect of exercises in CLBP, concluded that exercise therapy is minimally effective for nonspecific CLBP outcomes. Nonetheless, exercising in CLBP is recommended, and guideline endorsed treatment [5, 30].

Identifying and appraising published SRs and MAs on a specific topic enables a description of study quality and can give a comprehensive overview of the results, which allows a comparison and discussion of the strength of the conclusions [31]. Recently, a systematic review of systematic reviews of exercise therapies used in acute LBP concluded that there is very low-to-moderate evidence that exercise therapy of any type results in any important differences in pain or disability in people with acute LBP [32]. Whether different exercise types used in CLBP vary in effect has, to the best of our knowledge, not been summarized and appraised in a systematic review of systematic reviews. We therefore aimed to identify published SRs and MAs of common exercise types used in CLBP, to appraise their quality, and to summarize and compare their effect on pain and disability.

Material and methods

Design

We conducted this systematic review of systematic reviews (SRs) according to a protocol registered in PROSPERO (190,409) using the methods proposed by the Cochrane Collaboration’s recommendations for conducting an overview of systematic reviews [33], and the PRISMA checklist (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) is presented in Additional file 1 [34].

Eligibility criteria

We included systematic reviews (SRs) and meta-analyses (MAs), in which a majority (> 75%) of the included original studies were randomized controlled trials (RCTs). The inclusion was based on PICO (patients, intervention, comparator, outcome) (Additional file 4). We did not exclude any SRs or MAs on language, treatment duration, frequency or intensity, comparator intervention, follow-up time, or year of publication. All systematic reviews (with or without meta-analyses) will be referred to here as systematic reviews (SRs).

Patients

We included SRs based mainly on (> 75%) a working population aged 18 to 70 years, who suffered from nonspecific CLBP or were defined as having recurring LBP. Chronic LBP was defined as having LBP for a period of at least 12 weeks or more than 3 months, while recurring LBP was defined as having a pain period that was preceded by a pain-free period.

Intervention

We included SRs in which the effect of any exercise therapy or training was studied as the main (single) intervention. Exercise was defined as “a regimen or plan of physical activities designed and prescribed for specific therapeutic goals, with the purpose to restore normal musculoskeletal function or to reduce pain caused by diseases or injuries” [10].

Comparator

No limitations were set for comparator interventions.

Outcome

We included SRs that investigated pain and disability as primary outcomes in short-, intermediate- and long-term follow-up. We did not specify short-, intermediate, or long-term follow-up.

Search

Together with a medical librarian, we (authors WG and ERB) developed a comprehensive search strategy based on earlier published search strategies in Cochrane Reviews regarding exercise therapy and chronic low back pain in the following databases: OVID MEDLINE, EMBASE, COCHRANE LIBRARY (the Cochrane Database of Systematic Reviews) and WEB OF SCIENCE (Core collection). We combined search terms and MESH terms in a search strategy developed for OVID MEDLINE and adapted this strategy for the other databases. Only SRs were considered in the database searches. Search strategies are presented in Additional file 2. The search was performed up until 2022–01-26. After removing all duplicates, the papers were imported into RAYYAN QCRI [35]. All papers were alphabetically divided into five teams with two reviewers each. The reviewer pairs screened titles and abstracts retrieved from the searches, independent from each other, and assessed these for eligibility against the predetermined inclusion criteria (PICOS). At this stage of the process, regular reviewer meetings were held to reach consensus. All titles and abstracts meeting the inclusion criteria were retrieved in full text. In each pair, both reviewers independently checked the full-text articles to assess eligibility for the final inclusion in this review. Reasons for exclusion were noted in this stage, and if more than one reason for exclusion was available, the publication was excluded in PICO-order, that is, a publication with wrong intervention, wrong publication type, and wrong population was classified only as excluded based on population. We scrutinized the reference lists of the included SRs for additional potentially relevant publications.

Overlap

Overlap was defined when the same trial was included in more than one of the included SRs [36]. We calculated the total overlap (original studies in our included SRs) for each type of exercise type independent of the outcome, following the formula proposed by Pieper et al. [36]. We present the overlap with the percentage of corrected covered area (CCA). Interpretation of CCA: 0–5% = slight overlap, 6–10% = moderate overlap, 11–15% = high overlap, and > 15% = very high overlap.

Assessment of methodological quality of included reviews

The updated valid and reliable tool, AMSTAR-2 (A MeaSurement Tool to Assess systematic Reviews), is recommended to assess the methodological quality of SRs [37]. The study quality was categorized into four levels based on all 16 AMSTAR-2 items: critically low (1–4), low (5–8), moderate (9–12), and high (13–16), depending on the number of fulfilled criteria. Before the actual assessment started, a pilot test was carried out on one specific paper in which each reviewer learned how to use AMSTAR-2. The two reviewers from each of the five pairs performed their assessments independently and compared them with each other. Disagreements in the assessments were handled in a consensus dialogue after comparing discrepancies between assessors and discussed in the total group, guided by WG and ERB.

Data extraction and synthesis

One reviewer per pair extracted data from the included SRs, and the other reviewer from the same pair checked the extraction for accuracy. We extracted the data into a data extraction form adapted from a Cochrane form [33]. We extracted data primarily from the included SRs. If the data presented in the included systematic review were in doubt, the original included RCTs were checked for accuracy. The results of each included SR were separated on the outcomes pain and disability and on the follow-up in the short, intermediate, and long term. We did not perform a meta-analysis since clinical homogeneity was not present due to the large variation in exercise dosages, combinations of interventions, differences between the studies in control groups as well as outcome measures and follow-up times.

Assessment of certainty of evidence

We used the GRADE approach [38] to evaluate the certainty of the level of evidence for each exercise type and each separate outcome. In this systematic review of systematic reviews, we used the conclusions by the authors of the included SRs as the main source, but we also checked if the results were statistically significant compared to a control intervention. When possible, we also used the established minimal important difference (MID) as a specified threshold in our evaluation of the level of evidence.

In short, the first step of GRADE is to choose a starting point for the level of evidence. Since our included SRs mainly comprised RCTs, we decided to start at the highest level. Thereafter, we lowered the level of evidence by appraising the potential limitations due to study limitations (high risk of bias/AMSTAR points), inconsistency (in results), imprecision (large confidence intervals, heterogeneity), indirectness (poor measurement quality), and publication bias. The level was increased if large effects or a “dose–response” were seen based on the reports of the SRs. In this way, we express our findings together with the confidence in the results using four levels of evidence: “high” (+ +  + +), “moderate” (+ + +), “low” (+ +), or “very low ( +) [38].

Results

Search results

The search results are summarized in Fig. 1. The literature search returned a total of 3,475 systematic reviews. Following removal based on duplicates, a review of the titles and abstracts (n = 2, 139) was performed, and 253 full texts were screened. After checking against our inclusion and exclusion criteria (Additional file 4), we included 45 SRs in the final review. In the 45 SRs, a total of 499 randomized controlled trials (RCTs) (overlap not accounted for) were included. While one publication did not present the number of patients included [39], all in all, 38 893 participants were included (overlap not accounted for). All of our included SRs were in English except for one that was in Spanish [40] and one in German [39]. A list of excluded SRs and reasons for exclusion is included in Additional file 3.

Fig. 1

PRISMA chart for eligible study selection process

Table 1 Description of the exercise types

Study characteristics

Our included SRs were published from 2001 to 2021. The majority (69%; 31 out of 45) were MAs, and most of the included patients were defined as having CLBP > 12 weeks or > 3 months (Tables 2, 3, 4, 5, 6, 7, 8, 9 and 10). Some of the included SRs [16, 43,44,45,46,47,48,49,50] included participants suffering from CLBP and, in addition, participants suffering from recurrent LBP. Recurrent LBP was defined as having LBP preceded by a pain-free period.

Table 2 Description of the included systematic reviews; number of original studies included, population, intervention and control. Aerobic exercises

Table 3 Description of the included systematic reviews; number of original studies included, population, intervention and control. Aquatic exercises

Table 4 Description of the included systematic reviews; number of original studies included, population, intervention and control. Motor Control Exercises (MCE)

Table 5 Description of the included systematic reviews; number of original studies included, population, intervention and control. Pilates

Table 6 Description of the included systematic reviews; number of original studies included, population, intervention and control. Resistance exercises

Table 7 Description of the included systematic reviews; number of original studies included, population, intervention and control. Sling exercises

Table 8 Description of the included systematic reviews; number of original studies included, population, intervention and control. Traditional Chinese Exercise (TCE)

Table 9 Description of the included systematic reviews; number of original studies included, population, intervention and control. Walking

Table 10 Description of the included systematic reviews; number of original studies included, population, intervention and control. Yoga

The included 45 publications were grouped based on exercise types: a) aerobic training [15], b) aquatic exercises [14], c) motor control exercises (MCE) [16, 40, 48, 49, 51,52,53,54,55,56,57,58], d) Pilates [18, 43, 47, 50, 59,60,61,62,63], e) resistance training [13, 39, 64], f) sling exercises [17, 65, 66], g) traditional Chinese exercises (TCE) [20, 67], h) walking [45, 68, 69], and i) yoga [19, 46, 70,71,72,73,74,75,76,77,78]. A short description of the exercise types is presented in Table 1.

Although several of the included SRs did not report all details, we summarized the population as being patients with CLPB, with pain lasting between 3 and 6 months, and in one study, even up to 8.6 years [14]. Moreover, most of the publications reported that the majority of the included participants were female, with a mean age span ranging from 38 to 50 years. There was a large variation in the dosage of the exercise interventions. Treatment duration ranged from 6 to 36 weeks [52], with a frequency from 1 to 5 times per week [55]. Concerning the comparator interventions, our included SRs compared the investigated exercise interventions with comparators such as other exercises, manual therapy, and usual care. Usual care was mostly defined as regular physiotherapy or visits by a general practitioner. Moreover, comparisons were made with minimal interventions such as pamphlets [78], educational booklets [56], placebo treatment [48], or waiting list controls [15, 19, 71]. Descriptions of the intervention and comparator treatments used in the included SRs are presented in Tables 2, 3, 4, 5, 6, 7, 8, 9, and 10.

All of our included SRs reported on the outcomes pain and disability. They used several patient-reported outcome measures to measure pain and disability. Most reported data for pain were on the Numeric Pain Rating Scale (NPRS) or Visual Analogue Scale (VAS) and for disability on the Oswestry Disability Index (ODI), Roland Morris Disability Questionnaire (RMDQ), or Patient Specific Functional Scale (PSFS). For MAs, data were often presented with MD or SMD, with 95% CI or standard deviation (SD).

The included publications presented diverse time frames for follow-up. The majority of the included SRs presented posttreatment data, and most presented follow-up data in the short term (up until 12 weeks), intermediate-term (12–52 weeks), and long term (≥ 12 months).

Methodological quality of included reviews

Based on the AMSTAR-2 ratings, we found 13 SRs with high quality [15, 16, 18,19,20, 47, 56, 58, 65, 69,70,71, 77], 16 with moderate quality [14, 17, 43, 45, 46, 48, 52,53,54, 59, 60, 62, 67, 68, 78], 14 with low quality [39, 40, 49,50,51, 57, 61, 63, 64, 66, 73,74,75,76], and two with critically low quality [13, 72]. The AMSTAR-2 ratings for all included publications are presented in Table 11. Most of the included publications answer “no” or “nearly yes” on the items “having established a protocol before the review”, “including a list of excluded studies”, and “lacked reporting on funding of the included studies”.

Table 11 Summary of methodological quality assessment of included systematic reviews using AMSTAR-2

For some exercise types that were investigated in more than one systematic review, there was a high or very high overlap (e.g. walking: CCA 38%) of the included original studies, except for resistance training (CCA 4%).

Summary results for exercises in chronic low back pain

The narrative analyses of the included SRs showed large effects when comparing the exercise interventions with minimal or no intervention. For most exercise types, there were no differences when different exercise types were compared with each other. Mostly small or nonsignificant effects on pain and disability were found in favor of the various exercise types compared with other control interventions, such as usual care. We found very low-to-moderate quality evidence that any exercise type is effective for reducing pain and disability compared to no or minimal intervention but that no exercise type seems to be more effective than another (Tables 12, 13, 14, 15, 16, 17, 18, 19 and 20).

Table 12 Results of different exercise types compared to control interventions for pain and disability. Aerobic exercises

Table 13 Results of different exercise types compared to control interventions for pain and disability. Aquatic exercises

Table 14 Results of different exercise types compared to control interventions for pain and disability. Motor Control Exercises (MCE)

Table 15 Results of different exercise types compared to control interventions for pain and disability. Pilates

Table 16 Results of different exercise types compared to control interventions for pain and disability. Resistance exercises

Table 17 Results of different exercise types compared to control interventions for pain and disability. Sling exercises

Table 18 Results of different exercise types compared to control interventions for pain and disability. Traditional Chinese Exersises (TCE)

Table 19 Results of different exercise types compared to control interventions for pain and disability. Walking

Table 20 Results of different exercise types compared to control interventions for pain and disability. Yoga

Table 21 shows that there is very low-to-moderate quality evidence (measured with GRADE) that various exercise types investigated are as effective for reducing pain and disability compared to no or minimal interventions in chronic LBP.

Table 21 Summary of findings and overall quality as assessed with GRADE

Aerobic exercise

Our search resulted in one MA on the effects of aerobic exercise, which covered a literature search up to March 2016 and included six publications with 333 subjects [15]. The review was rated as having high quality (Table 11). Aerobic exercise was compared with resistance training, or combined aerobic and resistance training versus exercise advice, to maintain normal activity, or waiting list not getting any intervention (Table 2).

The results showed that aerobic exercise reduced pain, although neither aerobic nor resistance training proved to be superior to the other (Table 12). No significant differences were reported for disability. The GRADE analysis showed moderate-quality evidence that aerobic exercise is as effective for the reduction of pain and disability compared to resistance training (Table 21). We downgraded due to possible publication bias since only one review was identified.

Aquatic exercise

Our literature search identified only one MA with moderate quality (Table 11) published in 2018 on the effectiveness of aquatic exercises compared to land-based or no exercises [14]. The MA included eight RCTs with a total of 311 participants (Table 3).

The MA found a statistically significant reduction in pain and disability in patients treated with aquatic therapy compared to patients treated with land-based therapy (Table 13). No information about the time point of outcome reporting was provided (Table 3). The GRADE analysis showed that there is low-quality evidence that aerobic exercise is superior to land-based exercise in the reduction of pain and disability (Table 21). The evidence was downgraded due to study limitations and possible publication bias since only one systematic review (MA) was identified.

Motor control exercises

We included 12 SRs [16, 40, 48, 49, 51,52,53,54,55,56,57,58] investigating motor control exercises (MCE). All but one of our included SRs [40] conducted a meta-analysis. The publication year ranged from 2006 to 2021, and the last updated search was August 2020 [58]. The SRs investigating MCE included between four and 34 low-to-high-quality RCTs and included between 209 and 2514 participants (Table 11). In total, 195 RCTs were included but, there was a high overlap of the included original studies (CCA 13%) since only 78 original trials were included. Only three of the included SRs were rated with an overall high quality [16, 56, 58].

The included publications reported outcomes of pain and disability in the short, intermediate, and long term (Table 14). Control interventions were general exercises (GE), spinal manual therapy (MT), multimodal treatment (MMT), or information/minimal intervention/usual care (Table 4). The narrative synthesis on pain showed a nonsignificant effect for MCE over general/other exercises mainly in the short and intermediate term [16, 48, 49, 51, 52, 56,57,58]. Compared to manual therapy, none of the ten publications presented any results on differences to MCE for pain [16, 48, 49, 51,52,53,54,55,56, 58]. Five SRs reported significant results showing that MCE was more effective in the short [58], intermediate, and long term than minimal intervention for pain [16, 48, 51, 54].

The narrative synthesis on disability in the included SRs showed a nonsignificant effect for MCE over general exercises [16, 48, 51, 52, 54, 56,57,58], while Niederer & Mueller (2020) presented results of no difference at any time points [55]. MCE showed small effects compared to manual therapy in two reviews [48, 54]. Compared to minimal intervention, MCE showed significant differences in the short- [16, 48, 51, 58], intermediate- [16, 48, 51, 54], and long-term [16, 48] on the outcome disability. The GRADE analyses showed that there is a moderate level of evidence on the effect of MCE on pain compared to minimal intervention and a low level of evidence that there is such an effect on disability (Table 21). Downgrading was based on the inconsistency of the results and, for disability, on imprecision due to significant heterogeneity.

Pilates

The literature search resulted in nine systematic reviews [18, 43, 47, 50, 59,60,61,62,63], of which five had performed an MA on the effect of Pilates [18, 47, 50, 60, 62]. Our included publications included between four and 14 original trials published between 2011 [60, 63] and 2018 [59] with an updated search in April 2016 [59] and included between 134 [50] and 708 participants [59]. In total, 79 RCTs were included, but there was a very high overlap (CCA 32%), since only 25 original trials were included. The study quality for the included SRs investigating Pilates ranged from low [63] to high [18] quality (Table 11). The intervention dosage varied greatly between publications, and due to poor reporting, it was impossible to summarize a typical exercise duration, frequency, or intensity. The control interventions varied greatly and contained treatments such as other exercise types, McKenzie, massage, back school programs, or information/minimal intervention/usual care (Table 5).

The narrative synthesis on the outcome pain showed significant effects for Pilates over no or minimal intervention in eight of the SRs [18, 43, 47, 59,60,61,62,63]. In all of the included SRs, there were no differences compared to other types of exercises, except for one that found superiority for Pilates exercises compared to physical activity [43]. Similar results were found in the narrative synthesis on disability. Six of the included SRs reported nonsignificant effects for Pilates over minimal intervention [18, 43, 59, 61,62,63]. Most of the included publications pointed out that Pilates exercises were as effective as other types of exercises, mainly with short-term effects (Table 15). The GRADE analyses showed a moderate level of evidence on the short-term effects of Pilates compared to minimal intervention and no effect compared with other types of exercise concerning pain (Table 21). For disability, the level of evidence was low for this comparison. For both pain and disability, the evidence was downgraded due to the low and moderate quality of most of the included publications. Moreover, an additional downgrading for disability was added since the results were conflicting concerning the conclusion on the effectiveness of Pilates over minimal interventions.

Resistance training

We included three SRs on the effect of resistance training [13, 39, 64]. None of these conducted a meta-analysis. The publication year ranged from 2001 to 2012, and the last updated search was performed in April 2010. In one of the SRs, only two RCTs were included [64], while the other two SRs included 12 and seven RCTs, respectively, with a small overlap (CCA 3%) [13, 39]. AMSTAR-2 scores indicated critically low-quality and low-quality reviews (Table 11). The interventions in the SRs included resistance training, back muscle training, and medical training therapy. Resistance training was compared with passive treatments, fitness training, no treatment placebo, or cognitive interventions (Table 6).

All included SRs reported decreased pain scores compared to passive or no intervention [39, 64], but the effect was unclear or what period was used for follow-up. One SR found no difference compared to a cognitive-behavioral intervention, and the effect disappeared at the long-term follow-up [64], while another reported no difference in pain scores when compared to fitness training [39] (Table 16). Resistance training was found to be effective for the reduction of disability in all included SRs compared to passive or no intervention in one review [39], but it was unclear what comparison groups were used in the other two [13, 64] (Table 6). The GRADE analyses showed that there is a very low level of evidence that resistance training has positive effects on pain and disability but not compared to fitness training and cognitive-behavioral intervention (Table 21). The level of evidence was downgraded due to low study quality, inconsistency, imprecision, and an increased risk of publication bias.

Sling exercises

We found three SRs on the effect of sling exercises [17, 65, 66], including two meta-analyses [17, 65]. In total, 25 RCTs were included whereof 21 were original trials, resulting in an overlap between the three publications (CCA 9.5%).. The last updated search was conducted in April 2021 [17]. The AMSTAR-2 ratings showed moderate [17, 66] and high study quality [65] (Table 11). The interventions were primarily sling-exercise-based; however, the sling exercises were also combined with, for example, passive modalities and with other kinds of exercises, such as back school, contemporary treatment, and drugs. The control groups received other forms of exercise, passive modalities, manipulation, contemporary treatment, and drugs (Table 7).

The narrative analyses of the included SRs showed that sling exercises are no more effective in reducing pain or improving disability compared with other types of exercise (Table 17). Sling exercise combined with modalities had a better effect on pain than modalities alone; for disability, there was no difference between the groups [17]. In comparison to passive modalities or the combination of physical agents and drug therapy, sling exercises were more effective in decreasing pain and improving disability. Sling exercise vs thermomagnetic therapy [65], vs no treatment, or vs MCE [17] showed differences between the groups in favour of sling exercise. In addition, sling exercises were found to be not more effective than traditional Chinese medical therapies [65]. Sling exercises in addition to acupuncture therapy were as effective as acupuncture therapy alone for reduction of pain and improvement of disability [65]. The GRADE analyses showed that there is a low-to-moderate level of evidence for short-term and long-term effects on pain and disability for sling exercises over passive therapies (Table 21). We downgraded due to study limitations and imprecision.

Traditional Chinese Exercises (Tai chi/Qigong)

Two MAs were identified that evaluated the efficacy of Tai Chi and Qigong [20, 67]. Included were 10 and 11 RCTs published between 2008 and 2019, respectively, and the total sample size ranged between 886 and 959 participants. With 18 orginal studies, there was a high overlap of the original studies investigating traditional Chinese exercises (CCA 13%). The risk of bias indicated moderate-to-high quality (Table 11). Both MAs compared the effect of either Tai Chi or various types of Qigong (Wuqinxi, Baduanjin, Liuzijue) to either no treatment, active treatment (strength exercise, backward walking, or other physiotherapy), or usual care, with or without the experimental component (Table 8).

The narrative synthesis on pain showed small to moderate effects for TCE over no treatment, active treatment, or usual care only. Subgroup analyses revealed a larger effect when Tai Chi was compared to no treatment than to active control interventions or to routine care (without an added Tai Chi component) (Table 18). Only short-term effects seem to have been evaluated, but the exact follow-up time was not reported. The synthesis on disability showed a variability in effect, from small to large effect for TCE over no treatment, active treatment, or usual care only. In both MAs, the effects differed depending on the outcome measure used [20, 67]. The GRADE analyses showed a moderate level of evidence concerning pain and a low level of evidence for disability on the short-term effects of TCE compared to no intervention (passive control), various active treatments, or usual care in CLBP patients concerning pain (Table 21). The evidence was downgraded for imprecision due to heterogeneity (pain, disability) and due to large confidence intervals of the effects (disability).

Walking

We identified three SRs [45, 68, 69] of the effectiveness of walking interventions, two of which performed a meta-analysis [45, 69]. The included publications included in total 20 RCTs with 329 to 869 participants with a very high overlap of the original studies (CAA 37.5%) since only 12 original studies were included. The SRs were published between 2016 and 2019, with the last updated search up in October 2017 [69]. Two of the SRs were of moderate quality [45, 68], and one was a high-quality study [69]. Two of the included SRs [45, 68] did not report excluded studies or the source of funding and did not investigate the impact of study quality on summary estimates (Table 11). All compared the effectiveness of walking interventions (overland and/or treadmill and/or Nordic walking) with other types of exercise, physical therapy, and education, while two compared walking and exercise to exercise alone [45, 69] (Table 9).

Both the MAs [45, 69] for either pain or disability and the SR [68] for disability found no significant differences between walking and the comparison groups that received other interventions (Table 19). The addition of walking to the comparison groups did not induce a significant improvement. The GRADE analysis showed that there is a low quality of evidence that walking as an exercise intervention is as effective as other nonpharmacological interventions for pain and disability improvement in chronic LBP patients and that adding walking to exercise does not increase effectiveness (Table 21). The evidence was downgraded due to study limitations and for imprecision due to large confidence intervals of the effect and a large overlap of the reviews.

Yoga

Seven out of the nine publications conducted an MA [19, 46, 70, 71, 75, 77, 78]. The SRs and MAs included in total 85 RCTs, but with a high overlap (CCA 13.6%) since only 23 original trials were included. The publication year ranged from 2011 [76] to 2021 [70], and the last updated search was in 2018 [78]. The study quality of the included publications ranged from 3 [72] to 16 points [19] on AMSTAR-2 (Table 11). Four of the publications were rated as having high quality [19, 70, 71, 77], and only one presented a list of the excluded studies [19]. The yoga interventions were highly heterogeneous, not only in terms of which kind of yoga was used but also in the length, frequency, and intensity of the sessions. Some interventions were combined with other physical therapy modalities, with book readings or usual treatments. There were no clear manuals or protocols that described the yoga interventions. The control interventions were treatment such as physical therapy, waitlist control, stabilizing exercise and physical therapy, conventional exercise therapy, usual care, educational control group, and self-directed medical care (Table 10).

The narrative synthesis on both pain intensity and disability in the included SRs showed a short-term effect for yoga, especially compared to no or minimal intervention, but also compared to general exercises. Three MAs showed medium and medium-to-large effects, indicating that the effects of yoga may be of clinical importance [46, 71, 75]. However, the long-term effects did not seem to demonstrate better effects than usual care (Table 20). There is a low level of evidence for a short-term effect in pain and disability for yoga over general exercises; however, the long-term effects did not seem to demonstrate better effects than usual care or compared to usual care or compared to other types of exercises (Table 21). We downgraded due to large heterogeneity between the publications and inconsistent results. Although the risk of bias was high in most of the reviews, two reviews had a low risk of bias (16 points); hence, we decided not to downgrade due to study limitations.

Discussion

We aimed to summarize and synthesize systematic reviews (SRs) investigating the effects of common exercise types prescribed and used in CLBP on pain and disability. We found low-to-moderate quality evidence that participating in any of the exercise types that we included in this systematic review of systematic reviews is effective for reducing pain and disability compared to no or minimal intervention but that no exercise type seems to be more effective than another (very low-to-moderate evidence). Our findings are mainly in keeping with several previously published SRs on the effects of exercises in CLBP, summarizing the existing evidence that no exercise type seems to have a better effect over another [7, 23, 25, 26, 28].

A recent and newly updated Cochrane review summarized 249 original studies on various exercises in non-specific CLBP and concluded that “exercise probably reduces pain compared to no treatment, usual care or placebo in people with long-lasting (chronic) low back pain” [8]. The preceding Cochrane review (2005) on exercises and CLBP, also by Hayden et al., included 61 original studies and concluded that exercise therapy seems to be slightly effective at decreasing pain and improving disability in adults with CLBP [7]. Even if the most recent Cochrane review included four times more studies than the preceding one, the evidence does not seem to have changed over the last 15 years [8]. The finding from our systematic review of systematic reviews was therefore not surprising, as we included SRs published up until 2021, including the original studies from previously published SRs and MAs. In addition, we included results on less studied exercise types, such as aquatic training, walking, TCE, and sling exercises, which shows consistency in that no exercise type seems to be more beneficial than another. We chose to include TCE and analyze these exercises separately while other reviews compiled them into wider categories [25]. Recently, other designs than SRs and MAs to summarize and appraise the evidence of interventions have been proposed and published. Two studies used network analyses [25, 27]. Owen et al. suggested that there is low-quality evidence that some exercise types, Pilates, MCE, resistance training, and aerobic exercises, are the most effective exercises [25]. The authors compared various types of common exercises in CLBP with no or minimal interventions, and their results are partly in line with ours even if we conclude that no exercise type seems to be more effective than the other. Hayden et al. (2021) also conducted a network analysis somewhat aligning with the results of Owen et al. [25, 27]. Hayden et al. (2021) concluded that Pilates, McKenzie therapy, and functional restoration interventions are more effective than other types of exercises for reducing pain and disability in CLBP [27]. These two systematic reviews using network analyses add to the current knowledge that indeed some exercise types seem to be more effective compared with others [25, 27].

Our systematic review of systematic reviews including 45 SRs shows that some exercise types used in CLBP are seemingly more studied than others. There has obviously been a research focus on MCE, Pilates, and yoga interventions based on the number of systematic reviews we found in our database searches (MCE n = 12; Pilates n = 9; yoga n = 11) compared to the other exercise types. The SRs included different numbers of original studies and showed varying quality. In addition, a high overlap was detected meaning that the original studies were included in several of the SRs. This raises the question of how many systematic reviews are needed and whethter SRs should be of higher quality to be published. Twelve SRs showed mainly consistent results that MCE are as effective as manual therapy regarding pain and disability, and showed no relevant findings that MCE are more effective than general exercises. Based on these findings we suggest that no further original studies will change the current evidence albeit, Saragiotti et al. (2016) proposed that there might be subgroups of CLBP that could benefit from MCE [16]. For Pilates interventions, we found low-to-moderate evidence that Pilates is no better than other exercises but better than minimal interventions; only small effect sizes were found. Pilates and MCE might be considered comparable as exercise type but differ in that MCE seems to be more often supervised, individualized, and performed as a graded program, starting with low load and specific exercises. Hayden et al. [79] concluded that exercise therapy that consists of individually designed programs and is delivered with supervision may improve pain and disability in non-specific CLBP. Moreover, adherence to exercise programs has been shown to be highly correlated with positive outcomes [79, 80]. We did not, however, summarize adherence to exercises or whether a program was performed individually or in a group. This might, however, be of value for future systematic reviews and might show a difference in effects.

For some exercise types, such as resistance training, we found and included a few and mainly older SRs [13, 39, 64]. This in addition, mirror a trend on what exercise interventions are popular in CLBP and thus affects the number of publications. However, there seems to be a new interest in studying loaded exercise in LBP [81, 82], which might be a reaction to two decades of study interest in low load exercises such as MCE.

Strengths and limitations

A strength of our study is that, to our knowledge, this is the first systematic review of systematic reviews including SRs and MAs on the effect of various exercise types used in CLBP, in addition to including exercise types that previously have not been summarized in this way. We did not limit our search but included systematic reviews in all languages and without any restrictions on publishing year or comparators. Furthermore, we followed and complied with the PRISMA guidelines, graded the quality of the included publications using the recommended instrument AMSTAR-2 [37], and summarized the graded evidence of the different exercise types.

A bias that needs to be discussed is that two-thirds of the included systematic reviews were of moderate to high quality, while 12 were assessed as having a critically low or low quality. The evidence for the included systematic reviews was mostly downgraded due to study limitations, and further high-quality systematic reviews are therefore warranted also recommended by the authors from the most recent Cochrane review on exercises in CLBP [8]. The included publications with low quality might have contributed to the certainty of evidence. Thus, our results might have been different if we had included only those with high or moderate quality. Our study aim, however, was to include all studies with no restrictions.

Another limitation is that our findings are based on systematic reviews with a high or very high overlap of included original studies. Overlap can be a problem if one original study is included in several SRs and thus drives the results in one direction. This could have been the case in our findings of the systematic reviews of the yoga and Pilates interventions, where one high-quality original study with beneficial results was included in all of the included SRs and thus might have affected the overall results [83, 84]. To cope with problems such as overlap, it might thus be more relevant to include all original studies in one large systematic review instead of summarizing the results in a systematic review of systematic reviews as in ours. However, the aim of a systematic review of systematic reviews such as ours is also to identify and appraise all published reviews in one area of interest and to describe their quality, summarize and compare their conclusions and discuss the strength of these conclusions [31]. We find that this aspect is important, especially since many clinical guidelines base their recommendations on published systematic reviews. On the other hand, a systematic review of systematic reviews also mirrors the methodological flaws of the included publications, which might be discussed as a limitation and a challenge in summarizing the findings. In addition, the results of a systematic review of systematic reviews, such as ours, are also based on a variety of definitions and patient-reported outcome measures, which might be difficult to merge into one overall conclusion. Thus, the results from our study must be understood from the perspective that the included publications used various measurement points and various comparator treatments, ranging from other types of exercises to minimal interventions and usual care, all with different definitions. Hence, one should take this into consideration when interpreting our findings. Then again, our study shows that some exercise types are studied more often than other exercise types, and questioning if some SRs with a (too) low quality should be published at all as this could mean a waste of research and unnecessary publications [85].

Our study is a collaboration of 10 researchers, which brings both advantages and disadvantages. We worked in pairs to include the systematic reviews, to draw the data from the SRs to the tables, and to assess the risk of bias of these. A third arbitrary party was always used when no consensus could be reached. This procedure was necessary to manage the enormous number of systematic reviews included. Moreover, we changed the reviewing pairs during the process so that four researchers were involved in the reviewing and extracting of data for each exercise type.

There are other limitations that need to be discussed, such as inclusion bias, since we might have missed including some important systematic reviews in our database search. However, the database searches were conducted using relevant search strategies in several databases by a librarian from the Karolinska Institutet. Moreover, the reference lists of the included reviews were studied for additional reviews to include. A publication bias might be that we did not perform any further search of the grey literature on, for example, web pages. Another issue could be that the nine exercise types included in our systematic review of systematic reviews emerged from our database searches, and there could thus be other types of exercises important in CLBP that we did not include. We, however, still consider that we found the most relevant literature in our searches. As the results from our systematic review of systematic reviews are consistent with a recent Cochrane review on exercises in CLBP [8], we consider that additional reviews would probably not have changed our main findings to a large extent.

Clinical and future perspectives

Exercises are suggested as first-line treatment by most clinical guidelines in CLBP [5, 30] and are commonly used and prescribed by health care professionals, but with no clear recommendation for one type of exercise over another [5, 86,87,88]. The current broad recommendation on exercises by health care professionals probably reflects uncertainty about the mechanism(s) through which exercises yield positive effects on pain and disability presented in CLBP [21]. It could also be that those prescribing exercise programs have different preferences for specific exercise types based on education and interest or nonawareness of clinical guidelines [89]. In addition, to decide on what exercise type should be chosen for an individual patient suffering from CLBP, the patient should always be included in the decision-making process [90]. The results from our systematic reviews of systematic reviews might add to the current knowledge of the effect of various exercises presenting similar effectiveness of various exercise types.Yet, the results from the present systematic review of systematic reviews must be interpreted with caution given the variation in quality and conclusions between the SRs.

In our systematic review of systematic reviews, the outcomes studied were pain and disability. Future studies should incorporate other outcomes that reflect additional effects of exercise, such as fear avoidance, quality of life, and pain catastrophizing, as these are seemingly important factors in the transition from acute to chronic LBP. Moreover, the choice of outcome should also be more specifically related to the goals of the exercises.

For some exercise types, such as aquatic and sling exercises, more high-quality research is warranted, but for others, such as MCE, there is a need for specifically analyzing different subgroups. Perhaps the use of other analyses, such as Bayesian network meta-analysis, Markov chain methods, and/or meta-regression analysis, could make it possible to adjust for the large heterogeneity of covariates in these studies and could be a suggestion for future research. Overall, we are concerned that the evidence on exercises in CLBP has not changed over the last decades and that the quality of the included systematic reviews varies. There is a need for larger RCTs with a low risk of biased, and summarized in high quality systematic reviews adding to the overall evidence on the effect of exercises in CLBP.

Conclusion

Our findings show that the effect of various exercise types used in CLBP on pain and disability varies with no major difference between exercise types. Many of the included systematic reviews were of low-to-moderate quality and based on randomized controlled trials with high risk of bias. The conflicting results seen, undermine the certainty of the results leading to very-low-to-moderate quality of evidence for our results. Future systematic reviews should be of higher quality to minimize waste of resources.