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Abstract

Balance impairment is a major motor symptom of Parkinson’s disease (PD), leading to falls, reduced mobility, and lower quality of life. Despite advances in pharmacological and rehabilitation approaches, postural instability often persists, requiring effective adjunctive interventions. Vibration therapy (VT) is a non-invasive, drug-free method that may enhance neuromuscular and balance control. However, evidence remains inconsistent due to methodological heterogeneity and small samples. This protocol describes a systematic review and meta-analysis designed to evaluate the effects of VT on balance in individuals with PD. Following PRISMA-P 2015 and Cochrane Handbook guidelines, the protocol is registered in PROSPERO (CRD420251124173). Searches will include PubMed, EMBASE, MEDLINE, Web of Science, and Cochrane Library for English-language, peer-reviewed randomized and non-randomized controlled studies comparing VT (WBV, FMV, or SRT) with control interventions. Risk of bias will be assessed using RoB 2 and ROBINS-I tools. A 3-level random-effects meta-analysis will address within-study dependencies, followed by subgroup and meta-regression analyses. Certainty of evidence will be rated using GRADEpro. This review will synthesize available evidence on the effects of vibration therapy on balance and postural outcomes in individuals with Parkinson’s disease. Pooled effect sizes will be estimated, and potential sources of heterogeneity will be examined through predefined subgroup analyses and multilevel meta-regression based on clinical and intervention-related factors. This review will clarify the therapeutic potential of VT for balance in PD and guide evidence-based rehabilitation strategies.

Systematic review registration: PROSPERO CRD420251124173

Keywords

Parkinson’s disease vibration therapy balance function postural stability systematic review meta-analysis rehabilitation

Introduction

Parkinson’s disease (PD) is a progressive neurodegenerative disorder affecting approximately 11.8 million individuals worldwide, and its prevalence is projected to increase to about 25.2 million by 2050, representing more than a twofold increase in its global burden.¹ It is characterized by tremor, rigidity, bradykinesia, and postural instability.² Among these, balance impairment represents one of the most disabling motor symptoms, contributing substantially to falls, functional decline, and loss of independence.³ Epidemiological studies indicate that approximately 60% of individuals with PD experience at least 1 fall each year, and nearly 40% experience recurrent falls, leading to increased morbidity, fear of falling, reduced physical activity, and a poorer quality of life.⁴ This high incidence of falls is primarily attributed to the profound balance dysfunction observed in PD patients.⁴

Balance dysfunction in PD is not merely a consequence of muscle weakness but rather the result of a complex interplay involving basal ganglia dysfunction, impaired postural reflexes, and disrupted integration of proprioceptive, vestibular, and visual sensory inputs.^5
-7 Despite notable advances in pharmacological management and conventional rehabilitation, postural instability often persists because dopaminergic therapy, while effective in alleviating tremor and bradykinesia associated with limb movement, exerts only limited influence on the neural mechanisms underlying balance control.^8,9 Conventional rehabilitation programs such as resistance training, gait training, and task-specific balance exercises have demonstrated partial improvements in postural stability and gait performance; however, these benefits are generally modest and short-lived, with balance impairments frequently recurring once supervised intervention ends.^10,11 Moreover, older adults with PD often face difficulties in maintaining high-intensity exercise regimens due to fatigue, musculoskeletal comorbidities, and medication-related side effects, which further limit the sustainability of conventional exercise approaches.^12
-14 Collectively, these limitations highlight the urgent need for rehabilitation strategies that are neurophysiologically grounded yet practically feasible, enabling sustained enhancement of sensorimotor integration and postural control in individuals with PD. In this context, vibration therapy (VT), encompassing both whole-body vibration (WBV) and focal muscle vibration (FMV), has emerged as a promising non-pharmacological intervention for improving neuromuscular performance and postural stability. During VT, mechanical oscillations stimulate sensory receptors in the muscles, thereby activating Ia afferent fibers and enhance the excitability of α-motor neurons.^15,16 This process increases motor unit recruitment and firing frequency, thereby improving neuromuscular activation and coordination.¹⁷ Moreover, repeated vibratory stimulation facilitates proprioceptive feedback and augments the integration of sensory inputs from the proprioceptive, vestibular, and visual systems, which are essential for maintaining postural stability.^18
-20 These neurophysiological effects are believed to induce adaptive changes within the sensorimotor cortex and basal ganglia circuits, ultimately contributing to improved balance control and motor performance.^21,22 VT is a non-invasive, time-efficient, and easily administered intervention with several practical advantages. Owing to these advantages, VT may hold considerable potential for integration into both clinical and home-based rehabilitation programs targeting individuals with neurological disorders, particularly those with PD.

Preliminary evidence in individuals with PD suggests that VT may improve balance, gait, and postural stability.^23
-28 For example, an acute whole-body vibration intervention (30 Hz for 1 min) was reported to significantly improve walking time and pelvic tilt compared with control conditions in individuals with mild PD (Hoehn–Yahr stages 1 and 2), supporting the potential of VT to influence gait kinematics.²⁹ In addition, earlier clinical trials comparing whole-body vibration with conventional physiotherapy have reported improvements not only in balance and gait outcomes but also in quality of life among individuals with PD.^24,30 More recently, a systematic review and meta-analysis reported that VT may exert beneficial effects on gait-related parameters in individuals with PD. However, substantial heterogeneity across studies was observed, limiting the interpretability and generalizability of the pooled estimates.³¹

Such heterogeneity is likely attributable to variations in intervention protocols, participant characteristics, and study methodologies.^31,32 Despite numerous reviews examining the effects of VT, the optimal intervention guidelines for individuals with PD have yet to be established due to inconsistent findings and methodological limitations across existing studies.^23,33 In particular, previous research has been constrained by small sample sizes and a lack of statistical precision.²⁷ Furthermore, the absence of systematic analyses on clinical factors that may moderate treatment efficacy makes it difficult to ascertain the true magnitude and key determinants of VT effects.³²

To address these limitations, the present study aims to conduct a systematic review and meta-analysis to comprehensively evaluate the effects of vibration therapy on balance-related outcomes in individuals with PD. In particular, this review will examine how variations in clinical characteristics (ie, disease duration, severity, and medication use) and intervention parameters (ie, vibration frequency, amplitude, session duration, and total exposure) influence therapeutic efficacy. A multilevel analytical framework will be employed to account for dependencies among multiple balance outcomes within studies and to enable a more accurate and nuanced estimation of the pooled effects. The overall certainty of the evidence will also be assessed. By synthesizing high-quality data and exploring moderating factors through this multilevel perspective, this study seeks to clarify the therapeutic value of vibration therapy and provide evidence-based recommendations for its application as a targeted rehabilitation strategy to improve balance and postural stability in individuals with PD.

Methods

Protocol and Registration

This study will be conducted in accordance with the Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols (PRISMA-P 2015) guidelines. The protocol has been prospectively registered with the International Prospective Register of Systematic Reviews (PROSPERO), registration number CRD420251124173.

Inclusion and Exclusion Criteria

Eligibility criteria will be defined a priori based on the PICO framework:

Population: Adults diagnosed with idiopathic Parkinson’s disease according to established clinical diagnostic criteria, corresponding to Hoehn & Yahr stages I to IV. Studies involving atypical Parkinsonism (eg, multiple system atrophy, progressive supranuclear palsy, corticobasal degeneration) or other neurological disorders will be excluded.

Intervention: Any mechanically delivered vibration therapy, including whole-body vibration (WBV), focal muscle vibration (FMV), and stochastic resonance therapy (SRT), applied either alone or in combination with conventional physiotherapy. Studies that target non-motor outcomes only (eg, pain, sleep, mood) or employ non-mechanical stimulation (eg, electrical, auditory, or visual cues) will be excluded.

Comparator: Sham vibration (placebo-equivalent control), usual care, no intervention, or non-vibration physical therapy (eg, treadmill training, conventional rehabilitation). Studies lacking a clearly defined comparator group will be excluded.

Outcomes: Quantitative measures of balance and postural stability will be included. The primary outcome of this review is balance function in individuals with Parkinson’s disease. Outcome measures will be synthesized and classified a priori into 3 domains: (1) balance and gait performance, (2) postural stability and sway, and (3) foot and plantar pressure measures.

Study design: Randomized controlled trials (RCTs), crossover RCTs, non-randomized controlled trials (non-RCTs), controlled clinical trials, and quasi-experimental designs.

Exclusion criteria

Animal or in vitro studies; uncontrolled pre–post designs; case reports or case series; conference abstracts without full data; narrative or systematic reviews; editorials or opinion papers; and studies in which the independent effect of vibration therapy cannot be isolated.

Information Sources and Search Strategy

A comprehensive literature search will be conducted in PubMed, Embase, MEDLINE, Web of Science, and the Cochrane Library (CENTRAL). The search strategy will be developed a priori based on 4 core concepts: Parkinson’s disease, vibration therapy (including whole-body vibration, focal muscle vibration, and stochastic resonance therapy), balance or postural stability, and clinical trials.

To account for differences in indexing systems and search functionalities across databases, database-specific search strategies will be developed and applied. Controlled vocabulary terms (eg, Medical Subject Headings [MeSH] in PubMed and MEDLINE, and Emtree terms in Embase) will be used where applicable, in combination with free-text keywords. In databases without controlled vocabulary, such as Web of Science, topic-based keyword searches will be employed. Boolean operators (AND, OR) will be used to combine search terms consistently across databases while adapting the syntax to each platform.

A representative PubMed search strategy is provided below, and full database-specific search strategies will be reported in the Supplemental Materials to ensure transparency and reproducibility. Only peer-reviewed articles published in English will be considered eligible. In addition, the reference lists of included studies and relevant systematic reviews will be manually screened to identify any additional eligible articles.

All records retrieved from the different databases will be merged, and duplicate entries will be removed prior to title and abstract screening.

Study Selection and Data Management

All retrieved citations will be imported into EndNote 20 for deduplication. Title and abstract screening will be performed independently by 2 reviewers. Potentially eligible articles will undergo full-text review according to the predefined inclusion/exclusion criteria. Disagreements will be resolved through discussion or arbitration by a third reviewer. The screening process will be summarized in a PRISMA 2020 flow diagram.

Data Extraction

Data extraction will be performed independently by 2 reviewers using a standardized data extraction form developed in Microsoft Excel. The extraction form will be pilot-tested on a subset of studies to ensure consistency and clarity before full implementation. Discrepancies between reviewers will be resolved through discussion or consultation with a third reviewer.

For each included study, the following information will be extracted:

General study characteristics: first author, year of publication, country, study design (eg, randomized controlled trial, quasi-experimental design), and sample size;

Participant characteristics: mean age, sex distribution, disease duration, disease severity (Hoehn & Yahr stage), and average dopaminergic medication dosage;

Intervention characteristics: vibration modality (whole-body vibration, focal muscle vibration, stochastic resonance therapy), device type, vibration frequency (Hz), amplitude (mm), duration of each bout and session, number of repetitions per session, total number of sessions, total intervention period, and participant posture or stimulation site (eg, standing, semi-squat, Achilles tendon, gastrocnemius, cervical spine);

Comparator characteristics: sham vibration, usual care, conventional rehabilitation, or no intervention;

Outcome measures: all quantitative indices related to balance, gait, and postural stability, such as the Berg Balance Scale, Timed Up and Go test, Functional Reach Test, posturography measures, and sway parameters; and

Statistical information: means, standard deviations (SDs), and sample sizes for intervention and control groups at baseline and post-intervention. When change scores are reported without sufficient variance data, standard deviations will be imputed following Cochrane Handbook recommendations.

When outcome data are presented graphically rather than numerically, values will be extracted using WebPlotDigitizer 5.0. If essential statistical information is missing or unclear, the corresponding authors will be contacted via email to obtain additional data. In cases where multiple balance-related outcomes are reported within the same study, all eligible outcomes will be extracted to allow for inclusion in the planned 3-level random-effects meta-analysis, which accounts for within-study dependency. All extracted data will be cross-checked for accuracy and completeness prior to statistical synthesis.

Risk of Bias Assessment

The risk of bias will be independently assessed by 2 reviewers using standardized tools appropriate for each study design, and any disagreements will be resolved through discussion or consultation with a third reviewer.

For randomized controlled trials (RCTs), including randomized crossover designs, the Cochrane Risk of Bias 2 (RoB 2) tool will be applied. This tool evaluates 5 domains: randomization, deviations from intended interventions, missing data, outcome measurement, and selective reporting. For crossover trials, the adapted RoB 2 version will be used to account for carry-over effects, washout adequacy, and period effects. Studies lacking clear randomization of intervention sequence or appropriate washout will be considered at high risk for bias in the randomization domain.

For non-randomized studies, the ROBINS-I tool will be used to evaluate 7 domains, including confounding, participant selection, and outcome measurement. Each domain and overall judgment will be categorized as low, moderate, serious, or critical risk of bias.

Given the perceptible nature of vibration therapy, participant or therapist blinding may be difficult to achieve. Therefore, emphasis will be placed on assessor blinding and the validity of sham or placebo control conditions. Risk-of-bias assessments will focus on balance and postural stability outcomes included in the quantitative synthesis.

Results will be presented in summary tables and visual diagrams (eg, traffic-light plots). Sensitivity analyses will be performed excluding studies with high or serious/critical risk to confirm the robustness of the results. Risk-of-bias findings will also inform the GRADE evaluation of evidence certainty.

Data Synthesis and Statistical Analysis

All quantitative analyses will be conducted using R software (version 4.3.0; R Foundation for Statistical Computing, Vienna, Austria) with the metafor package. For continuous outcomes, standardized mean differences (SMDs) with 95% confidence intervals (CIs) will be calculated. When both pre- and post-intervention data are available, change scores will be used, and pooled standard deviations will be estimated assuming a correlation coefficient of r = .5. Hedges’ g correction will be applied to adjust for small-sample bias.

A random-effects meta-analysis model will be used to account for variation across studies. When multiple effect sizes are reported within a single study (eg, different balance measures), a 3-level model will be applied to address statistical dependency between outcomes. Between-study variance (τ²) will be estimated using the restricted maximum likelihood (REML) method, and heterogeneity will be assessed using the Q statistic and I² index. The Knapp–Hartung method will be used for robust estimation of CI.

Because both randomized and non-randomized controlled studies will be included, study design will be explicitly considered in the quantitative synthesis. When sufficient data are available, randomized and non-randomized studies will first be synthesized separately. In addition, study design (randomized vs non-randomized) will be examined as a moderator in subgroup analyses or meta-regression to explore its potential influence on the estimated effects. Sensitivity analyses excluding non-randomized studies will also be conducted to assess the robustness of the pooled results.

If sufficient data are available, subgroup analyses and meta-regression will be performed to explore potential sources of heterogeneity. Planned moderators include clinical variables (disease duration, severity), intervention characteristics (vibration type, frequency, amplitude, exposure time, and total sessions), and outcome type, which will be classified a priori into 3 domains: (1) balance and gait performance measures (eg, clinical balance performance tests), (2) postural stability and sway measures (eg, instrumented sway or center-of-pressure parameters), and (3) foot and plantar pressure measures.

Where follow-up data permit, outcomes will be analyzed separately to distinguish acute effects (immediate or short-term post-intervention) from chronic effects (longer-term follow-up after repeated intervention).

For clinical interpretation, balance scales with established minimal clinically important differences (MCIDs) in Parkinson’s disease (eg, the Berg Balance Scale and Timed Up and Go test) will be identified. Because the primary meta-analysis synthesizes multiple balance-related outcomes using SMDs, pooled effect sizes will not be directly translated into MCIDs. Instead, scale-specific meta-analyses will be conducted for individual balance measures where sufficient data are available, and the resulting SMDs will be translated into scale-specific units using established MCID thresholds. These scale-based conversions will be reported in the Supplemental Materials and used to support clinical interpretation in the Discussion.

Potential publication bias will be evaluated visually using funnel plots and statistically through Egger’s regression and Begg–Mazumdar tests. The trim-and-fill method will be applied if asymmetry is detected, and sensitivity analyses will be conducted by sequentially omitting individual studies to test the robustness of results.

Assessment of Certainty of Evidence

We will rate the certainty of evidence by outcome using the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) framework, considering 5 key domains: risk of bias, inconsistency, indirectness, imprecision, and publication bias.

Studies employing randomized designs, including both parallel-group randomized controlled trials (RCTs) and randomized crossover trials, will start at a high level of certainty. Randomized crossover trials will be evaluated under the same GRADE principles as parallel RCTs but with additional consideration for potential carryover effects, adequacy of washout periods, and period effects, which may result in downgrading under the risk of bias domain. In contrast, non-randomized studies will begin at a low certainty level.

Certainty will be downgraded when methodological limitations, unexplained heterogeneity, indirect evidence, wide confidence intervals, or suspected publication bias are identified. Conversely, upgrading may occur in non-randomized studies if there is a large and consistent effect, a dose–response gradient, or evidence that residual confounding would likely reduce the observed effect.

Two reviewers will independently conduct the GRADE assessments, resolving discrepancies through discussion or consultation with a third reviewer. Final ratings (high, moderate, low, or very low) and the rationale for each decision will be summarized in Summary of Findings and evidence profile tables generated using GRADEpro GDT.

Discussion

This systematic review and meta-analysis protocol was developed to comprehensively synthesize and quantify the effects of VT on balance function in individuals with PD. Despite increasing clinical interest in the use of VT as an adjunctive rehabilitation strategy, existing evidence remains fragmented and inconsistent due to small sample sizes, methodological heterogeneity, and variability in intervention parameters and outcome measures. By applying a multilevel modeling framework and standardized methodological procedures, the present review aims to overcome these limitations and provide an updated and statistically robust evaluation of VT’s therapeutic potential.^34,35

A key methodological strength of this protocol lies in its use of a 3-level random-effects meta-analytic approach, which allows the modeling of statistical dependence among multiple correlated balance outcomes within the same study. Conventional meta-analytic techniques often treat such outcomes as independent, which may lead to biased estimates and inflated type I error rates. In contrast, the proposed model accounts for both within- and between-study variance, improving the precision and interpretability of pooled estimates. Additionally, the planned multilevel meta-regression and subgroup analyses will enable the exploration of clinically relevant moderators, including disease severity, medication status, vibration frequency, amplitude, session duration, and total exposure. This analytical structure will not only clarify the consistency of VT effects across heterogeneous populations but also help identify optimal intervention parameters for clinical application. The methodological rigor of this review is further reinforced through adherence to PRISMA 2020 and Cochrane Handbook recommendations, as well as prospective registration in the PROSPERO database.^36,37 Independent dual screening, standardized data extraction, and risk of bias assessment using the RoB 2 and ROBINS-I tools ensure transparency and reproducibility.^38,39 The GRADE framework will be used to evaluate the certainty of evidence across all outcomes, facilitating the translation of findings into clinical recommendations with defined confidence levels.⁴⁰ Collectively, these methodological features represent a substantial improvement over previous reviews, which often lacked consistent control conditions, failed to account for statistical dependencies, or provided limited exploration of moderating factors.

The planned review also incorporates a comprehensive search strategy encompassing 5 major databases, including PubMed, Embase, MEDLINE, Web of Science, and the Cochrane Library, without temporal restrictions. This approach minimizes the risk of publication bias and maximizes the inclusivity of relevant studies, thereby increasing the reliability of the synthesis. By systematically extracting and analyzing vibration parameters based on the “Big Five variables” framework, the review will enable detailed characterization of how specific mechanical properties of VT (eg, frequency, amplitude, and exposure time) influence postural stability and balance outcomes in PD.⁴¹

In addition to evaluating statistical significance, the planned review will also consider the clinical meaningfulness of the observed effects by comparing outcome improvements with established minimally clinically important difference (MCID) thresholds where available. This will help clarify whether the expected benefits of VT translate into clinically perceptible improvements in functional balance.

Several potential limitations must be acknowledged. The inclusion of heterogeneous study designs, varying sample characteristics, and differences in intervention protocols may introduce residual heterogeneity that cannot be fully addressed statistically. Restricting the search to English-language publications could lead to language bias, and publication bias may persist despite the use of funnel plots, Egger’s regression, and Trim-and-Fill adjustments.^42
-44 Additionally, the number of high-quality RCTs available may be insufficient to conduct extensive moderator analyses or dose–response evaluations with high precision. Given that responsiveness to VT may vary according to disease progression, particular attention will also be given to potential differential effects across PD stages (eg, early, mid, and advanced), as disease duration and severity may moderate treatment efficacy.

Future research should therefore focus on large-scale, rigorously controlled clinical trials that systematically compare vibration modalities and parameters, assess longer-term outcomes, and evaluate clinically meaningful endpoints such as fall frequency, gait stability, and quality of life.

Ultimately, this protocol aims to establish a methodologically sound and reproducible framework for evaluating the effects of VT on balance in PD. The planned systematic review and meta-analysis will seek to synthesize the available evidence and to examine the consistency and robustness of reported effects across different study designs, populations, and intervention parameters. Rather than presuming clinical benefit, this review is designed to critically appraise the current evidence base and to identify the extent to which VT may be associated with improvements in balance and postural outcomes. The findings are expected to inform future research directions and to support evidence-based decision-making regarding the potential role of vibration therapy as an adjunctive intervention in PD rehabilitation.

Conclusion

This protocol outlines the methodology for a systematic review and meta-analysis that will comprehensively evaluate the effects of vibration therapy on balance function in individuals with PD. By applying a multilevel random-effects modeling framework, this study will address key methodological limitations in previous reviews, particularly the inappropriate handling of correlated outcomes and the lack of moderator exploration. Through rigorous statistical synthesis and moderator analyses under a 3-level model, the planned review will aim to identify the clinical and intervention-specific factors that influence the efficacy of vibration therapy, thereby clarifying its therapeutic potential as an adjunctive intervention in PD rehabilitation.

The findings of this review are expected to generate high-quality, reproducible evidence that will inform clinical decision-making and guide the design of future trials. By incorporating both statistical and clinical interpretability (eg, MCID-based assessment of meaningful change), the review will bridge the gap between quantitative synthesis and practical application. Ultimately, this work seeks to contribute to the development of evidence-based rehabilitation strategies and to establish vibration therapy as a feasible, low-cost, and non-invasive approach to mitigate balance dysfunction and reduce fall risk in individuals with PD.

Supplemental Material

sj-docx-1-inq-10.1177_00469580261433160 – Supplemental material for Balance and Postural Outcomes of Vibration Therapies in Parkinson’s Disease: Protocol for a Systematic Review and Meta-Analysis

Supplemental material, sj-docx-1-inq-10.1177_00469580261433160 for Balance and Postural Outcomes of Vibration Therapies in Parkinson’s Disease: Protocol for a Systematic Review and Meta-Analysis by Ji-Woo Seok, Jung-Dae Kim, Jaeuk U. Kim and Se-Ra Park in INQUIRY: The Journal of Health Care Organization, Provision, and Financing

Footnotes

Acknowledgements

Not applicable.

ORCID iD

Se-Ra Park

Ethical Considerations

This study is a protocol for a systematic review and meta-analysis based exclusively on previously published literature. Therefore, ethical approval from an institutional review board and informed consent from participants were not required.

Consent to Participate

As this study used only previously published data and did not involve direct participation of human subjects, informed consent from participants was not required.

Consent for Publication

Not applicable.

Author Contributions

J.-W.S., J.-D.K. and S.-R.P. conceptualized the study, designed the review protocol, developed the methodological framework, conducted the preliminary literature search, and prepared the initial draft of the manuscript. J.U.K. supervised the protocol development, provided methodological and conceptual guidance, and critically reviewed the manuscript. All authors contributed to refining the study design, reviewed the final version, and approved the submission of the manuscript.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Korea Institute of Oriental Medicine (KSN2511012) and the National Research Council of Science & Technology (NST) grant by the Korea government (MIST) (GTL25071–000).

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data Availability Statement

Not applicable.

Supplemental Material

Supplemental material for this article is available online.

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