Effects of Stretching or Strengthening Exercise on Spinal and Lumbopelvic Posture: A Systematic Review with Meta-Analysis

Background Abnormal posture (e.g. loss of lordosis) has been associated with the occurrence of musculoskeletal pain. Stretching tight muscles while strengthening the antagonists represents the most common method to treat the assumed muscle imbalance. However, despite its high popularity, there is no quantitative synthesis of the available evidence examining the effectiveness of the stretch-and-strengthen approach. Methods A systematic review with meta-analysis was conducted, searching PubMed, Web of Science and Google Scholar. We included controlled clinical trials investigating the effects of stretching or strengthening on spinal and lumbopelvic posture (e.g., pelvic tilt, lumbar lordosis, thoracic kyphosis, head tilt) in healthy individuals. Effect sizes were pooled using robust variance estimation. To rate the certainty about the evidence, the GRADE approach was applied. Results A total of 23 studies with 969 participants were identified. Neither acute (d = 0.01, p = 0.97) nor chronic stretching (d=-0.19, p = 0.16) had an impact on posture. Chronic strengthening was associated with large improvements (d=-0.83, p = 0.01), but no study examined acute effects. Strengthening was superior (d = 0.81, p = 0.004) to stretching. Sub-analyses found strengthening to be effective in the thoracic and cervical spine (d=-1.04, p = 0.005) but not in the lumbar and lumbopelvic region (d=-0.23, p = 0.25). Stretching was ineffective in all locations (p > 0.05). Conclusion Moderate-certainty evidence does not support the use of stretching as a treatment of muscle imbalance. In contrast, therapists should focus on strengthening programs targeting weakened muscles. Supplementary Information The online version contains supplementary material available at 10.1186/s40798-024-00733-5.


Background
Spinal alignment and posture have been investigated for about 250 years [1,2].Evidence syntheses from recent decades suggest that deviations from the assumed physiological norm may be associated with the occurrence of musculoskeletal pain.Chun et al. [3] found a strong cross-sectional relationship of reduced lumbar lordosis and low back pain.In a meta-analysis of prospective cohort studies, limited lordosis predicted the development of low back pain with an odds ratio of 1.27 [4].With regard to the neck, patients with pain displayed a forward head posture (FHP) when compared to asymptomatic individuals.Interestingly, the magnitude of FHP correlated with neck pain intensity and subjective disability [5], which is frequently associated with, for instance, early fatigue, neck and shoulder pain, decreased respiratory capacity, as well as reduced aerobic endurance [6,7].Barrett et al. [5] focused on thoracic kyphosis.The authors found that persons with excessive spinal curvature exhibited reductions in shoulder range of motion.This is of relevance because restricted shoulder mobility has been shown to increase the risk for upper extremity pain and injury [8,9].
Changes of lumbopelvic or spinal posture are commonly related to muscle imbalance [10].Such imbalance is suggested to originate from extended periods of biomechanical, psychological and social stresses as well as repetitive activities [11,12] While some muscles respond with tightness or shortening, their antagonists may become too weak to maintain the normal joint position [13][14][15][16][17][18].As an example for muscle imbalance, Janda [13,14] hypothesized that shortening of the pectoralis major, upper trapezius and levator scapulae muscles in conjunction with weakness of the deep neck flexors, lower trapezius and rhomboids causes excessive kyphosis and FHP.
Besides various other methods including mobilization [19,20], yoga [21], Pilates [22,23], manual therapy [24], or taping [25], stretching of tight muscles and strengthening of weak muscles has gained high popularity in the treatment of muscle imbalance.A survey by Perriman and colleagues from 2012 [26] revealed that 71% and 64% of the physiotherapists use stretching and strengthening, respectively, to treat excessive kyphosis, while in 2024, 60% of the physiotherapists and sport scientists attending an Austrian training convention assumed stretching to be effective in treating muscular imbalance [27].Despite the frequent use of the stretch-and-strengthen approach, the effectiveness of corrective exercise routines on posture is questionable [15,16].A systematic review with meta-analysis by Gonzalez-Galvez et al. [18] reported a positive influence of exercise programs in general, mostly when combining stretch and strengthening exercise.Interestingly, they concluded that strengthening may be superior to stretching.Yet, this assumption was based on the analysis of only 10 studies and, more importantly, no investigation of the isolated effects of stretching and stretching was performed.Withers et al. [28] included different training approaches.Among these, they examined stretching as a stand-alone treatment for hyperkyphosis.Since only one isolated static stretching was found, further research seems necessary.In view of the lack of evidence on the individual components of the stretch-and-strengthen approach, the present systematic review with meta-analysis was conducted to summarize the evidence on isolated stretch and strengthening treatments aiming to modify spinal or lumbopelvic posture.

Methods
A systematic review with meta-analysis was performed adhering to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines.We considered ethical publishing standards [29] and registered the study in the PROSPERO database (CRD42023412854).

Literature Search
Two authors (KW & LHL) conducted a systematic literature search using MEDLINE/PubMed and Web of Science (inception to April, 2023) and assessed all records independently.Disagreements at each screening level (title, abstract + full-text) were resolved by discussion (see Fig. 1).Database queries were supplemented by a hand search using Google Scholar as well as citation searching in eligible studies.The following criteria were applied for study inclusion: (1) randomized or non-randomized controlled intervention study design, (2) assessment of acute (post-testing immediately following the intervention) or chronic (intervention period of at least one week) effects, (3) comparison of stretching vs. strengthening, stretching vs. non-intervention control, or strengthening vs. non-intervention control, (4) measurement of pelvic tilt, lumbar lordosis, kyphosis, and/or forward head/forward shoulder posture using objective and quantifiable measurements (e.g., radiographs or camera systems), (5) inclusion of healthy adults.Patients with a history of musculoskeletal, neurologic, or cardiopulmonary disorders, joint replacements, osteoporosis, specific back pain or other pathologies were excluded from this analysis to improve homogeneity.Trials combining different interventions (i.e., stretching plus strengthening) were excluded as well.
Stretching interventions eligible for inclusion were static, dynamic and ballistic stretching and proprioceptive neuromuscular facilitation in accordance with Warneke & Lohmann [30] and Behm [31].Static stretching was defined as muscle lengthening until onset of a stretch sensation or to the point of discomfort.By definition, this position is to be held and can be performed passively via partner, external weight or a tool, or actively via movement.Proprioceptive neuromuscular facilitation includes a (sub-) maximal voluntary contraction to a stretching bout with or without antagonist contraction.Dynamic stretching was defined as controlled back-andforth movement in the end range of motion with ballistic stretching as a sub-category and less controlled, bouncing movements [32].Strengthening interventions were considered eligible if the authors stated the application of dynamic or isometric muscle actions sufficient to increase strength capacity, while the control group was considered to be inactive if no structured intervention was performed within the study.
The search terms were created based on the requirements of each database (see Appendix S1).In addition to the database searches, the reference lists of all included studies were screened for further eligible articles [33].

Methodological Study Quality and Risk of Bias
We used the PEDro scale for the assessment of methodological study quality [34,35].Scoring was performed by two independent investigators (KW & LHL).If both did not reach consensus, a third examiner provided the decisive vote (JW) [28].To estimate the risk of publication bias, funnel plots, created using the modification of Fernandez-Castilla et al. [36] for multiple study outcomes, were visually inspected.In addition, we performed Egger's regression test with the extension for dependent effect sizes [36].
To rate the certainty about the evidence, we applied the GRADE working group criteria [37].Briefly, the quality of evidence of randomized, controlled trials was initially classified as high and adjusted afterwards, considering the GRADE framework.In detail, in case of limitations in study design or execution, inconsistency of results, indirectness of evidence, imprecision or publication bias, one point was subtracted for each weakness.On the contrary, large magnitude effects or a dose-response gradient led to improvements of the quality of evidence by one point each.This resulted in a final rating of the certainty about the evidence as very low, low, moderate, or high.

Data Processing and Statistics
The means (M) and standard deviations (SD) from preand post-tests were extracted for all parameters (e.g.lordotic angle).In case of missing data, the authors of the primary studies were contacted.KW and LHL extracted data from eligible studies cooperatively, meaning that one read the values aloud and checked the shared screen while the other entered the numbers in a Microsoft Excel sheet.Additionally, KW double-checked the entered values for accuracy at the end of the extraction process.Changes from pre-to post-test were calculated as M (posttest) -M (pretest) and standard deviations were pooled as .
A meta-analysis with robust variance estimation, accounting for the dependency of effect sizes (e.g. in case of multiple outcomes in the same study), was performed to pool the standardized mean differences (SMD) and 95% confidence intervals (CI) between the intervention (stretching or strengthening) and control groups [38].

Methodological Quality, Risk of Bias and Certainty About the Evidence
For stretching studies, the average risk of bias was rated as fair with a PEDro score of 4.1 ± 1.3 (range: 3 to 8 points).The same applied to strengthening studies, which averaged 4.3 ± 1.4 points (range: 2 to 7).Almost all studies used random group allocation, reported statistical between-group comparisons and provided both, point measures and measures of variability.In contrast, blinding of the participants was only reported in one study, and not at all for therapist blinding.Also, very few studies (n = 2) declared application of the intention-to-treat principle (see Table 2).
With regard to the stretching studies, the certainty about the evidence was downgraded by 1 level (high to moderate) due to (1) risk of bias classified as fair via the PEDro score.For the strengthening studies, due to (1) risk of bias and (2) heterogeneity, certainty was downgraded by 2 levels (high to low) but upgraded one level due to the large effect size.Therefore, in sum, for both stretching and strengthening, the certainty about the evidence was moderate.

Stretching vs. Strengthening
No study comparing acute stretch and strengthening interventions was found.For chronic interventions, a large effect in favour of strengthening exercise (d = 0.81, 0.4, 1.22 95% CI, p = 0.004, τ²=0.02,6 studies, 9 ES) was detected.Since all studies but one focused on the thoracic/cervical spine region, no sub-analysis of body locations was possible.

Discussion
Stretching of tight or shortened skeletal muscles represents one of the most popular strategies used to tackle muscle imbalance and postural impairments [26].As early as 1997, Spring et al. [64] recommended it as the gold standard of posture treatment and twenty years  later, the application of stretch was still described a viable method preventing hypertonia-induced muscular imbalance [65].While recent reviews did not consider stretching as a stand-alone intervention [18,66], Withers et al. [28] were only able to include one stretching study in their meta-analysis.Summarizing the effects of 12 chronic stretching studies, our systematic review is the first to extensively examine the foundation of this approach.Of note, in contrast to popular beliefs in practice, moderate-certainty evidence does not support the use of stretching when aiming to tackle imbalancerelated posture deficits (e.g.hyperkyphosis or forward head posture).However, our analysis revealed a large effect of strengthening which also was superior in direct comparison to stretching.This finding confirms earlier speculations by Gonzalez-Galvez et al. [18] who reported combined stretching and strengthening to improve spinal posture, but suggested that only strengthening may be effective.As a consequence, exercise therapy for posture can be substantially economized by forgoing stretching From a physiological point of view, it has been argued that chronic stretching of a tight or shortened muscle would lower its stiffness or tone.While stretching of two to eight minutes acutely reduced muscle stiffness [67][68][69][70][71], a rapid return to baseline occurred after a short recovery of only up to 20 min.This is highly plausible considering the mechanical role of the titin filament.The protein, which is attached to the myosin filament and the z-disk, has substantial elastic properties and after being lengthened (e.g., during a stretch), it helps to restore the original passive resting length.Acting as a molecular spring [72][73][74], it hence regulates the mechanical behavior of the muscle fiber [75].Data collected in rabbits revealed that titin contributes up to 60% of the total passive stiffness of a skeletal muscle [76].Experimentally disrupting the filament decreased passive tension by 50 to 100% [77].Considering the elastic properties of titin and its role in passive muscle tension, the acute reductions in stiffness after stretching as well as the fast restoration of baseline values seem logical.Interestingly, the evidence of potential stiffness changes following chronic stretching treatments seems controversial.While in 2018, Freitas and colleagues [78] found stretch-mediated stiffness reduction in response to weekly volumes of up to 20 min over up to eight weeks unlikely, more recent literature found opposing results [79].Yet, even if long-term stretching could reduce muscle stiffness, the causal relationship between decreasing stiffness of shortened muscles and improvements in posture remains speculative, calling for further exploration.While there is currently no evidence for positive chronic effects of stretching on posture, this might potentially be due to a lack of investigations that use sufficient stretching volumes meaning further research is necessary.Irrespective, it needs to be acknowledged that only two studies were available on acute stretch application.Additional research evaluating the immediate impact on posture is therefore warranted as well.
Besides reduced stiffness, another suggested effect of chronic stretching is an increase in muscle length.As such, one might expect the formation of new serial sarcomeres within the muscle-tendon-unit [80,81].Indeed, Williams and Goldspink et al. [82] observed a higher sarcomere number following long-term immobilization of animal limbs.However, on the one hand, immobilization cannot be readily compared to stretching and, on the other hand, the applicability of animal findings to humans is disputed [80].Interestingly, titin does not only regulate the resting tension of the skeletal muscle but also appears to play an important role in structural adaptations.Van der Pjil et al. [83] described the importance of titin unfolding at high muscle lengths for sarcomerogenesis and with this, longitudinal (and parallel) hypertrophy.Even though viable, observations indicating a possible influence of chronic stretch training on structural properties were, to the best of our knowledge, exclusively made in animals [84,85].However, again, no transfer of longitudinal hypertrophy effects to humans was found [86].Before 2020, stretch-induced chronic structural stretching adaptations were classified unlikely [78,86], but within the past 5 years, evidence emerged that large stretching volumes (≥ 15 min per day, ≥6 weeks intervention period) have the potential to induce muscle hypertrophy, and with this, changes in tissue morphology [87,88].As, to date, no evidence could be found for longitudinal hypertrophy, it could be speculated that the studies matching the inclusion criteria of this systematic review did not perform stretching with the required stretching duration and/or intensity [87][88][89].
Contrarily to stretching, we found a large beneficial influence of strengthening on posture.However, the underlying mechanisms are a matter of debate.Surprisingly, there is a lack of conclusive research on resistance training-induced changes of the muscle's passive mechanical properties [90].In 1998, the hypothesis of increases in passive muscle stiffness as an adaptation to resistance training arose [91], leading to the recommendation to strengthen lengthened or weak muscle groups in muscle imbalance.The authors argued that hypertrophy would be associated with a larger number of parallel titin-myosin filaments, which, in agreement with the above-described evidence, would lead to a higher resting tension [91].Indeed, in a ten-week strength training study, the authors reported a 30%-increase in passive tension without decreases in extensibility of the muscle.In another study, isometric resistance training led to an increase in core stiffness [92].However, a recent systematic review found no stiffness changes in the long-term as a response to resistance training [93].Of note, the review only included measurements with ultrasound elastography which allows assumptions on compressive tissue stiffness.Assuming specific resistance training adaptations occur following induction of tensile/shortening stress to the muscle, it seems necessary to distinguish between compressive and tensile or strain stiffness.Research on foam rolling effects revealed that decreases in compressive stiffness could be detected using elastography and indentometric methods, while this was not the case for tensile stiffness using passive resistive torque during stretch [94,95].As a consequence, it may be assumed that stiffness changes are specific to the applied stimulus (compression in foam rolling, but stretch-shortening in resistance exercise).Following this theory, it would still be possible that resistance training does only modify tensile stiffness, which would also align with the role of titin as a serial agent for passive tension regulation.In sum, more research is warranted in order to gain further insight into the mechanisms of strengthening-induced improvements of posture.

Implications
Our findings have implications for clinical practice.As indicated, stretching is highly popular among therapists aiming to treat muscle imbalance and frequently recommended in the scientific literature [26,64,65].Yet, the available evidence speaks strongly against this approach.In line with earlier speculations of Gonzalez-Galvez et al. [18], beneficial exercise effects seem rather attributable to strengthening, while stretching programs are ineffective.Consequently, when aiming to counteract muscular imbalances and to improve spinal and lumbopelvic posture, no evidence-based recommendation for the implementation of stretching can be given.Interestingly, we found a beneficial influence of strengthening for the thoracic and cervical spine region, while no changes were detected in the lumbar and pelvic region.On the one hand, effect sizes were in fact trivial to small for the lumbar spine and pelvis.On the other hand, with a total of only 5 ES from two studies, this region is underresearched.Future investigations, besides aiming to better understand the physiological adaptions of stretching and strengthening with regard to passive tissue properties (muscle, tendons, fascia) [90] and neuromuscular aspects [10], should be geared to provide more data on exercise treatments in the lumbar spine region.

Conclusion
The common recommendation of stretching tight or shortened skeletal muscle to improve muscle imbalance and posture lacks scientific evidence (moderate certainty).In contrast, our review reinforces the role of strengthening weak antagonists which, however, was only effective in the thoracic and cervical but not in the lumbar spine (moderate certainty).Further well-designed RCTs, e.g.applying high stretch durations and experimental studies elaborating the underlying physiological mechanisms, are required to conclusively judge the role of treatments aiming to modify postural abnormalities.

Fig. 1
Fig. 1 Flow-chart of literature search for studies assessing the influence of stretching or strengthening on posture

Fig. 2 Fig. 3
Fig. 2 Forest plot for chronic stretching interventions on posture.Negative values illustrate effects favoring stretching compared to control.The effect size includes the 95% confidence interval

Table 2
Quality assessment using the PEDro scale

Table 3
Meta-analytic results providing effect size, 95% CI, significance and heterogeneity negative values indicate beneficial impact of stretching on posture compared to the comparison group/control condition, b negative values indicate beneficial impact of strengthening on posture compared to the control condition, 95% CI = 95% confidence interval a