Effects of Whole-Body Vibration Exercise on Athletes with Ankle Instability: A Systematic Review

Objective: Chronic Ankle Instability (CAI) or Functional Ankle Instability (FAI) is a condition characterized by laxity and mechanical instability in the ankle joint. This instability interferes with the activities and physical-functional parameters of athletes, which leads to repetitive ankle sprains. The current systematic review was carried out to identify the effects of whole-body vibration exercise (WBVE) in athletes with CAI. Methods: We conducted electronic searches in Pubmed, the Cochrane Library, Embase, Web of Science, Scopus, Science Direct, Allied Health Literature (CINAHL) and Academic Search Premier (ASP) (EBSCO) databases on 26 February 2022. Registers were identified, and studies were selected for inclusion according to the eligibility criteria. The methodological quality was assessed by the Physiotherapy Evidence Database (PEDro) scale. Results: Seven studies were included with a mean methodological quality score of 5.85, considered ‘regular’ quality on the PEDro scale. WBVE interventions in athletes with CAI showed that this exercise contributes to a better response on parameters of neuromuscular performance, muscle strength and consequently in balance and postural control, variables that are for the management of CAI. Conclusion: WBVE interventions in sports modalities promote physiological responses that may lead to positive effects in several parameters. The protocols proposed in each modality can be carried out in practice and are considered effective additional exercise and training methods beyond traditional types of training for athletes. However, more studies are needed on athletes with this condition, with specific protocols, to highlight the possible physiological and physical-functional responses. Protocol study registration: PROSPERO (CRD42020204434).


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Programa de Pós-Graduação em Fisiopatologia Clínica e Experimental, Universidade do Estado do Rio de Janeiro, Rio de Janeiro 20551-030, RJ, Brazil there is only one systematic review about WBVE and sensorimotor effects that supports the use of WBVE to improve sensorimotor deficits involving strength, balance, muscle activity, and joint position sense, but it was in individuals with CAI, not athletes [43]. Considering this rationale, the idea is to present the possibility of using an efficient exercise program to assist in the dysfunctions promoted by CAI, which is useful to stimulate the physical-functional performance of the athlete, as well as the return to sports practice. So, the aim of this study was to present a systematic review to identify the effects of WBVE in athletes with CAI.

Materials and Methods
The review was realized according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA, Supplementary Materials) guidelines [44] and the Synthesis without meta-analysis (SWiM) in systematic reviews [45]. The protocol for the review was registered with the International Prospective Register of Systematic Reviews (PROSPERO-CRD42020204434).

Search Strategy
An electronic search in databases was realized in Pubmed, Cochrane Library, Embase, Scopus, Science Direct, Web of Science, Allied Health Literature (CINAHL) and Academic Search Premier (ASP) (EBSCO) on 3 September 2021 and updated on 26 February 2023, using the following search strings: ("whole body vibration" OR "vibration therapy" OR "vibration training") AND ("chronic ankle instability" OR "ankle instability" OR "functional ankle instability" OR "mechanical ankle instability" OR "recurrent ankle instability"). Using the PICOS strategy, the keywords used in the search were defined, such as athletes with CAI (Participants) receiving WBVE intervention (Intervention), allowing comparison to control/no intervention, placebo or usual care (Comparison). All reported outcomes related to balance, muscle strength, muscle activity and body composition (Outcomes) were allowed if they were considered relevant to the population studied and were "randomized clinical trials" (RCT) (Study design) [46]. Additionally, a hand search was performed in gray literature and in studies references.

Eligibility Criteria
Inclusion criterion: RCT, studies with the use of WBVE in athletes with ankle instability, independently of the year of the publication, and with no restrictions on language.
Exclusion criteria: Congress abstracts, incomplete articles, abstracts, pilot studies, protocols, postintervention results not reported, and findings not related to CAI and athletes and WBVE.

Level of Evidence of the Selected Articles
The level of evidence of the included studies was evaluated using the National Health and Medical Research Council hierarchy of evidence [47] and defined according to below Table 1.

Methodological Quality and Risk of Bias
The Physiotherapy Evidence Database (PEDro)scale (http://www.pedro.org.au/english/ downloads/pedro-scale/ (accessed on 5 April 2022)), with eleven items, was used to the methodological quality of the randomized controlled trials included. The first item of the PEDro scale is related to external validity and is not used to calculate the scale score. Therefore, the scores range from 0 to 10. Articles with a score greater than or equal to 7 were of 'high' methodological quality, those with a score of 5 to 6 were of 'regular' quality, and those with a score of 4 or less were of 'poor' quality [48,49].
The Cochrane Collaboration tool, with 7 domains, was used to assess the risk of bias in the articles included. The domains encompass selection, performance, detection, attrition, reporting, and other sources of bias. The domains are classified into 1 of 3 categories: low, high or uncertain risk of bias [50].

Selections and Data Extraction
All references found by the database searches were exported to a data management software (EndNote X9 (Clarivate Analytics, London, UK)), and duplicates were removed. Studies were independently reviewed by 2 authors (A.C.C.-O. and J.P.-F.) according to the eligibility criteria at all stages of the review. If disagreements could not be resolved through discussion, a third reviewer (DC) was consulted. For data extraction, both reviewers extracted the following information: study information (author and year), study design, subjects (sample size), demographics characteristics (age, sex, body mass index), modality of athletes, instruments, tests/measured variables, WBVE intervention, WBVE protocols, and outcomes.

Results
A total of 135 records were identified through database searches (Pubmed = 14, Cochrane Library = 22, Embase = 16, Scopus = 18, Science Direct = 22, Web of Science = 24, CINAHL = 8 and ASP (EBSCO) = 11) and, after the removal of 76 duplicates, 59 studies were identified. During the screening process, the titles and abstracts of 59 publications were read, and 47 were excluded because they were abstracts, reviews (systematic, scoping or narrative), and meta-analyses, they did not meet the topic criteria, were with individuals or patients with CAI and did not use vibratory therapy. The complete text of 12 publications was reviewed in detail. After careful analysis, five studies were excluded because they were not athletes with CAI and because they did not contain available results. Finally, seven articles were included in the current systematic review. The selection process is shown in Figure 1.
The publications included had a mean methodological quality score of 5.85 as assessed, with the PEDro scale ( Figure 2), with a minimum of 3 points and a maximum of 7, reflecting 'regular' methodological quality. Only in one study [51] was the allocation of groups performed blindly, and no included study was able to blind all subjects or therapists who administered the therapy; two studies [40,51] applied the blinding of all assessors who measured at least one key outcome, three studies [10,23,36] did not provide at least one key result; only one study [23] failed to provide measurements of all outcomes.
For the risk of bias (Figure 3), among the studies included in this review, only one study [51] guaranteed that it performed allocation concealment, five studies [10,23,26,40,51] did not involve blinding of participants and personnel, two studies [24,36] presented insufficient information to judge this domain. Two studies [40,51] blinded the evaluation of the results, all studies included had no missing data, and all reported the results of the variables analyzed. However, according to the Cochrane classification tool, all the studies included had the potential for other biases due to methodological flaws. The articles included in this review were finally classified as high-risk (two studies [10,23]), low-risk (two studies [40,51]), and uncertain risk (three studies [24,26,36]).  (2) Subjects were randomly allocated to groups; (3) allocation concealed; (4) the groups were similar at baseline regarding the prognostic indicators; (5) was blinding of all subjects; (6) was blinding of all therapists who administered the intervention; (7) was blinding of all assessors who measured at   (2) Subjects were randomly allocated to groups; (3) allocation concealed; (4) the groups were similar at baseline regarding the prognostic indicators; (5) was blinding of all subjects; (6) was blinding of all therapists who administered the intervention; (7) was blinding of all assessors who measured at (2) Subjects were randomly allocated to groups; (3) allocation concealed; (4) the groups were similar at baseline regarding the prognostic indicators; (5) was blinding of all subjects; (6) was blinding of all therapists who administered the intervention; (7) was blinding of all assessors who measured at minimal one key outcome; (8) measures of at least one key outcome were obtained from more than 85%; of the subjects firstly allocated to groups; (9) all subjects for whom outcome measures were disposable received the treatment or control condition as allocated or, where this was not possible, data for at least one key outcome was analyzed by "intention to treat"; (10) the results of betweengroup statistical comparisons are reported for at minimal one key outcome; (11) the study give both point measures and measures of variability for at minimal one key outcome [10,23,24,26,36,40,51]. minimal one key outcome; (8) measures of at least one key outcome were obtained from more 85%; of the subjects firstly allocated to groups; (9) all subjects for whom outcome measures w disposable received the treatment or control condition as allocated or, where this was not poss data for at least one key outcome was analyzed by "intention to treat"; (10) the results of betw group statistical comparisons are reported for at minimal one key outcome; (11) the study give point measures and measures of variability for at minimal one key outcome [10,23,24,26,36,40,5 Table 2 displays the characteristics of the articles included, including the author year of the study, the participants involved, the objectives, the variables evaluated, results found and the level of evidence. The level of evidence (NHMRC, 2003(NHMRC, -2007 of all selected studies was Level of Evidence II.
Regarding the main findings (Table 2), WBVE: improves single-leg balance and S performance in dancers with unilateral functional ankle instability (FAI) [10]; Combi vibration and wobble board training improves COM distribution, modified SEBT sco and SLTHD among footballers suffering from FAI [23]and may have a beneficial effec the improvement of lower extremity muscle activity and balance ability in CAI foot players [36]. WBVE improved the RT of the PB, PL and TA muscles in CAI recreatio athletes [40]; resulted in different enhancements in balance ability on the BBS and S [51]; exhibited very small or small effect sizes for CAI in the SEBT, the joint position se test, and the isokinetic strength test [26]; and, it appears that the WBT is superior to conventional PPT program for improving dynamic balance in athletes with FAI w COM and ECC were enhanced at an ankle inversion of 30°/s [24].  Table 2 displays the characteristics of the articles included, including the author and year of the study, the participants involved, the objectives, the variables evaluated, the results found and the level of evidence. The level of evidence (NHMRC, 2003(NHMRC, -2007 [47] of all selected studies was Level of Evidence II. Regarding the main findings (Table 2), WBVE: improves single-leg balance and SEBT performance in dancers with unilateral functional ankle instability (FAI) [10]; Combined vibration and wobble board training improves COM distribution, modified SEBT scores, and SLTHD among footballers suffering from FAI [23] and may have a beneficial effect on the improvement of lower extremity muscle activity and balance ability in CAI football players [36]. WBVE improved the RT of the PB, PL and TA muscles in CAI recreational athletes [40]; resulted in different enhancements in balance ability on the BBS and SEBT [51]; exhibited very small or small effect sizes for CAI in the SEBT, the joint position sense test, and the isokinetic strength test [26]; and, it appears that the WBT is superior to the conventional PPT program for improving dynamic balance in athletes with FAI while COM and ECC were enhanced at an ankle inversion of 30 • /s [24].
The protocols and biomechanical parameters of the WBVE (frequency, peak-to-peak displacement, positioning of the individual at the base of the VP, the VP model and the type of stimulus) are reported in Table 3. Overall, the results support using balance training with or without WBVE to address balance impairments in participants with CAI.
It was also hypothesized that lean mass could be increased with the WBVE training program but observed no change in body-composition variables.  Vibrosphere (ProMedvi) 4 exercises: standing on 1 leg; heel raises on 1 leg; single-leg step-ups; single-leg straight-leg deadlift.
Considering the WBVE intervention protocols, in general, the included studies used the VP or other vibration devices with synchronous and alternating vertical stimuli. Four studies used another tool on top of the VP [26,36,40,51], one study performed the exercise directly on the VP [24], and two studies used another vibrating device [10,23]. In five studies, the participants were barefoot [10,23,26,40,51], and in two studies [24,36], they were wearing sneakers. Participants performed different positions and exercises on the platform, ranging from two-legged and one-legged exercises, with and without heel lift, involving balance and stability; the working time involved training lasting from 10 to 30 min, including protocols with progressive training as the exercise weeks progressed.
All studies included were homogeneous with regard to the time of application of the WBVE protocol lasting 6 weeks; however, five studies [24,26,36,40,51] used a frequency of three times a week, totaling 18 sessions, and two [10,23] twice a week, totaling 12 sessions.

Discussion
The main goal of this systematic review was to identify the effects of WBVE in athletes with ankle instability. After analyzing the studies included and considering their limitations, the results suggest that WBVE may be a feasible and valid intervention for athletes with CAI. The methodological quality of the included studies was regular. However, according to the risk of bias, the included clinical trials have methodological errors and still lack detailed information on the methods used and which may compromise their internal validity.
As observed, among the consequences of CAI, there is a deficit in balance and stability. Five included studies [10,23,24,36,51] suggest that vibration provided additional benefits regarding improvements in unipedal balance, dynamic balance, center of mass distribution (COM), and SEBT, MSEBT, SLTHD, and BBS test performance and scores, even with the authors using different VP devices, different WBVE protocols and parameters, as well as different athlete modalities. Corroborating with these findings, Sofla et al. [52] and Tohidast et al. [53] also suggest that WBVE may induce some improvement in postural control in individuals with CAI using protocols of 4 and 6 weeks of vibration, respectively. On the other hand, Chang et al., 2021 [26] observed very small or small effect sizes for the vibration results in the balance and stability test in athletes. Moreover, Adelman et al., 2016 [54] and Rendos et al. [39] did not observe significant changes with WBVE in terms of balance/stability; however, these studies used an acute effect WBVE protocol (a single session), and the participants were not athletes, which suggests that one session would not be enough to promote changes and adaptations in these parameters regardless of physical activity level. The tests performed in the pre-and post-intervention studies with WBVE are considered indicative of postural control, and balance training improves the individual's ability to maintain the center of gravity and posture, stimulating the musculoskeletal and vestibular systems. Therefore, stability is the result of the activation of the proprioceptive system and is of fundamental importance for the performance of motor tasks, mainly in the ca of athletes with CAI [24,55].
Regarding muscle strength, Sierra-Guzman et al. [40] and Chang et al. [26] did not find significant results regarding muscle strength of the ankle muscles or found very small or small effects after the intervention, suggesting that the WBVE protocols and parameters applied in the studies (frequency of 30, 35 and 40 Hz and amplitude varying from 2 to 4 mm, and 5 Hz and 3 mm, respectively), may not have promoted sufficient stimuli to provide an increase in the isokinetic strength of the ankle invertor and evertor muscles, even with 6 weeks of training. Similarly, Sofla et al. [52] did not observe significant changes regarding ankle muscle strength in individuals with CAI with a 4-week protocol (frequency 30-40 Hz and 3 mm amplitude) of vibration exercises. In the studies by Sierra-Guzman et al. [40], Chang et al. [26] and Sofla et al. [52], although they used different protocols, exercises and populations, strength was assessed with an isokinetic dynamometer, considered the "gold standard" instrument to measure muscle strength and performance [56]. On the other hand, Cloak et al. [23] observed significant effects on strength with the combined vibration and oscillating board training; however, they used the triple jump test for distance (SLTHD) to assess muscle strength, which is a valid clinical tool to assess strength and power characteristics. The reported differences in ankle, knee, and hip kinematics, kinetics, and muscle activation patterns in individuals with CAI, could be due to those lower limb muscle strength deficits and imbalances, and the proposed WBVE protocols, although progressive and associated with exercises, were not sufficient to balance the musculatures involved to support the sports demand of athletes.
Considering that CAI is one of the commonly observed sports conditions, and given that it causes decreased neuromuscular control and loss of proprioception, with respect to the results for muscle activation, Jeong et al., 2017 [36] suggest that WBVE may have a beneficial effect in improving muscle activity of the lower limbs in soccer players, as well as Cloak et al., 2010 [10] found that WBVE can lead to long term effects in the recruitment of motor units of the peroneus longus in dancers. These findings suggest that effective exercise protocols, independent of the type of vibrating platform, stimulus intensity and exposure time, can be used to increase α motor neuron excitability and motor unit synchronization to increase ankle motor control. In contrast, Sierra-Guzman et al., 2017 [40] found no significant changes in EMG activity after a 6-week WBVE training program on a soft surface, although it could improve ankle muscle RT of the PB, PL and TA muscles in recreational athletes with CAI. Similarly, Otzel et al., 2019 [57] observed changes in motoneuron function with a single session of WBVE, suggesting that both acute and cumulative protocols did not represent a sufficient stimulus to the ankle muscles. Neuromuscular control is therefore important for athletes as it helps maintain functional ankle stability, while proprioception influences the sensation of movement and the ankle joint position.
The strength of the current systematic review lies in offering a possibility for an efficient exercise program to counter the dysfunctions promoted by CAI and useful to stimulate the physical-functional performance of the athlete, as well as the return to sports practice with greater safety and, consequently, less chance of recurrence of the lesion that promoted instability. However, this systematic review has some limitations, and the results should be interpreted with caution. Despite the positive results of interventions with WBVE, the diversity of WBVE training protocols makes it difficult to interpret the findings. On the other hand, there is also a lack of objective evidence to support the use of WBVE in athletes with CAI. In addition, different sports modalities were studied, each with different characteristics which may influence the athlete's performance. Therefore, it is not possible to draw convincing conclusions based on a small number of relevant studies (only seven publications) evaluating WBVE intervention in the athletic population. In addition, some studies used multiple instruments or tools together with the vibration stimulus, and it was not possible to evaluate the vibration stimulus alone.
In this context, as facts and perspectives, the WBVE is a systemic mechanical vibration intervention capable of stimulating the physical-functional performance of the athletes through efficient exercise to promote benefits in the dysfunctions promoted by CAI. Regarding its prospects, and in view of the limitations of the association of WBVE with other "interventions," it would be interesting that more studies on systemic vibration, applying WBVE alone, focusing on protocol parameters that would help determine the most efficient and effective protocol to promote beneficial effects in athletes with CAI.

Conclusions
WBVE interventions in athletes with CAI show that this exercise method improves the response in parameters of neuromuscular performance, muscle strength and, consequently, balance and postural control, which are all indispensable variables for the management of this clinical condition. However, more studies are needed on athletes with this condition, with specific protocols, to highlight the possible physiological and physical-functional responses. The implementation of WBVE interventions in this population, regardless of the sport modality, promotes positive effects in several parameters and is a viable intervention to be carried out in practice. However, more studies are indeed needed to investigate the effects of WBVE in athletes with CAI, as there is evidence that WBVE can be an additional exercise, promising and effective, as a training method beyond traditional types of training for athletes.