**1. Introduction**

Lateral ankle sprain is one of the most common sports injuries and often causes a decrease in neuromuscular control and loss of proprioception [1]. Neuromuscular control helps maintain the functional stability of the ankle, whereas proprioception influences ankle joint position and sense of movement. Impairments in these functions cause reduced dynamic balance and reaction time of the ankle joint during exercise [2]. An ankle sprain resulting in damage to the lateral ankle capsuloligamentous complex can cause sequelae, such as recurring ankle sprains. It decreases ankle function as well as causing a loose feeling, and this postinjury symptom is classified as chronic ankle instability (CAI) [3]. Athletes with CAI may experience limited functionality of their lower extremities, which may affect their sports performance [4]. Therefore, specific balance exercises for athletes with CAI are critical in rehabilitation and training programs and could effectively reduce the risk of ankle sprain during sports activities.

**Citation:** Chang, W.-D.; Chen, S.; Tsou, Y.-A. Effects of Whole-Body Vibration and Balance Training on Female Athletes with Chronic Ankle Instability. *J. Clin. Med.* **2021**, *10*, 2380. https://doi.org/10.3390/jcm10112380

Academic Editors: David Rodríguez-Sanz and Yasuhito Tanaka

Received: 9 March 2021 Accepted: 24 May 2021 Published: 28 May 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Balance training is a progressive type of exercise performed on an unstable surface, and the resultant efferent output causes changes in α motor neuron excitability [5]. Balance training was used to improve muscle excitability in the ankle joint and increase motor control for CAI [6]. Whole-body vibration (WBV) training is another popular method used in CAI rehabilitation [7]. WBV training was performed on an oscillating vibration platform, which activates muscle spindles to facilitate tonic vibration reflex [8]. This training also enhances α motor neuron excitability and the synchronization of motor units to increase motor control in the ankle [9]. To the best of our knowledge, there have only been a few studies comparing the effects between WBC and traditional balance training on CAI. Therefore, WBV and balance training programs were designed and used for athletes with CAI in the current study.

For basketball and volleyball players, ankle stability and motor control are crucial for ground impact during jumping and landing. The dominant leg is a commonly discussed factor in these types of actions by injured athletes because it plays a critical role in object manipulation and lead-out movements [10]. However, the nondominant leg performs a stabilizing and supporting role in sports activities [10]. Therefore, the dominant leg is highly susceptible to sports injuries such as ankle sprain and may suddenly become injured during jumping and landing on an unstable surface. A study by van Melick et al. revealed that female athletes were more likely to jump using the dominant leg than male athletes [11]. A systematic review revealed that female athletes sustained ankle sprains more often than male athletes [12]. Sex-related differences in the epidemiology of ankle injuries were noted in sports injury protection [12]. Ristolainen et al. indicated female athletes tend to have a higher risk of sport-related ankle injury than male athletes [13]. Therefore, importance needs to be placed on an effective strategy of rehabilitation for female athletes. Previous studies have also reported a high incidence of ankle injuries in basketball and volleyball players [14,15]. Therefore, investigating the effects of specific interventions for CAI in female basketball and volleyball athletes is essential. Our study aim was to compare the effects of 6-week WBV and balance training on dominant-leg CAI in female athletes.

## **2. Methods**

This study was a randomized controlled trial approved by the Institutional Review Board of China Medical University Hospital (CRREC-106-063). Participants with CAI were recruited from women's basketball and volleyball teams at neighboring colleges. For inclusion criteria, female athletes had to have a history of at least one ankle sprain, lateral ankle instability of the dominant leg with a severity score ≤24 measured using the Cumberland Ankle Instability Tool, and a continual feeling of the ankle "giving way" after one year [16]. The dominant leg was used for lead-out movements and was determined as the foot used to kick a ball [10]. Exclusion criteria were acute ankle sprain, a history of surgery in both legs, and any musculoskeletal diseases of the lower extremities. The sample size calculation using G\*Power software reported by Sefton et al. [17] was used, which resulted in a total of 21 participants. We also tried to use the statistical power of 80%, α level of 0.05, and effect size (f = 0.25) to calculate via G\*Power software (version 3.1.9.2; Heinrich-Heine-Universität, Düsseldorf, Germany). A total sample size of 46 was calculated and required. In the current study, the estimated sample size was set to at least 48 participants (16 participants per group), which was a statistically adequate sample size.

### *2.1. Study Procedures*

This experimental trial was conducted at the end of the semester, and there was no practice or competition during this study period. The participants were randomly divided into three groups: Group A, who completed a 6-week WVB training program; Group B, who completed a 6-week balance training program; and Group C, who did not participate in a training program (Figure 1). All participants continued their normal daily activity and were instructed not to receive any treatments or therapy for CAI. The participants were

assessed before and after the study. The participants underwent three assessments, the Star Excursion Balance Test (SEBT), a joint position sense test, and an isokinetic strength test, consecutively, and their performance was evaluated by the same physiotherapist. The participants and researchers were not blinded to the study process.

**Figure 1.** Flowchart of the current study. WBV: whole-body vibration.

#### *2.2. Exercise Training Program*

Groups A and B performed the same exercises during the 6-week training programs, but Group A used a vibration platform (AIBI Power Shaper, AIBI Fitness, Singapore), and Group B used a balance ball (BOSU Balance Trainer, Fitness Quest, Ashland, OH, USA). Group A performed the exercises while standing on the vibration platform, which operated at a frequency and amplitude of 5 Hz and 3 mm, respectively. Group B participants performed the exercises on the balance ball. Participants in both groups were asked to maintain balance on either leg or an affected leg while having eyes closed. Both training programs were conducted respectively three times per week for 6 weeks and consisted of a 5-min warm-up exercise, a 20-min main exercise, and a 5-min cool-down exercise. They were designed with standard exercise prescription and clinical experience by one physical therapist. The main exercise comprised three exercise movements: a double-leg stance, a one-legged stance, and a tandem stance (Figure 2). Weeks 1–3 consisted of four sets of 45-s exercises with a 40-s rest interval between exercises, and weeks 4–6 consisted of five sets of 45-s exercises with a 30-s rest interval between exercises. The training programs in Group A and B were identical in the current study, except standing on different training devices. Group C participants were encouraged to continue their normal daily activity and avoid additional training programs or therapeutic exercise.

**Figure 2.** Demonstration of exercises on the vibration platform (**A**–**C**) and balance ball (**D**–**F**). (**A**,**D**): double-leg stance; (**B**,**E**): one-legged stance; (**C**,**F**): tandem stance.

#### *2.3. Assessments*

2.3.1. Star Excursion Balance Test

The SEBT was used to measure dynamic balance. The SEBT exhibited moderate-tofavorable interrater reliability (intraclass correlation coefficients [ICC] = 0.67–0.97) in an assessment of ankle instability [18]. An asterisk comprising eight tape segments joined at the center was placed on the floor. The tape segments were extended in eight directions (i.e., anterior, anterolateral, anteromedial, posteromedial, posterior, posterolateral, medial, and lateral) from the center at 45◦ angles [19]. The participants were asked to stand at the center of the asterisk using the leg with the involved ankle and then lightly touch the asterisk with the contralateral leg as far as possible in a direction chosen by the physiotherapist. The participants were instructed to maintain their balance in the one-legged stance by using the leg with the involved ankle and then return the contralateral leg to its initial position. Reach distances in the eight directions were recorded following three consecutive tests. The length of the involved leg was measured from the anterior superior iliac spine to the medial malleolus. The average reach distances in each direction were normalized according to the length of the involved leg and represented as a percentage.
