Ankle injuries are common in physically active individuals, with an incidence rate as high as 11.55 injuries per 1,000 exposures.1 These injuries incur chronic symptoms such as pain, weakness, and chronic ankle instability (CAI) in more than 70% of individuals within 2 years of injury2,3 and in more than 30% of individuals 7 years following injury.4 CAI results from a combination of deficits in strength, proprioception, and neuromuscular control,5,6 including increased peroneal reaction time,7,8 reduced activity of the tibialis anterior and peroneus longus,9 impaired joint position sense,10 and decreased postural stability.11
Whole body vibration (WBV) is a modality that has the potential to influence multiple factors associated with CAI. WBV excites a variety of sensory receptors, particularly muscle spindles,12 and has been demonstrated to enhance muscle activity (electromyography amplitude) and improve strength, balance, and reflexive responses to joint perturbation in healthy individuals.13–15 The potential for WBV to enhance rehabilitation of musculoskeletal injury and disease has also been demonstrated via improvements in balance, strength, and joint position sense following anterior cruciate ligament reconstruction.16–18 Additionally, WBV enhances muscle activity in stroke victims19 and individuals with simulated knee pathology (experimental knee effusion).20 These findings suggest that WBV has the potential to enhance neuromuscular function in individuals with CAI, potentially decreasing the risk of recurrent injury. Incidentally, Cloak et al.21 reported improvements in static balance in individuals with CAI following chronic (6 weeks) WBV exposure. Ross et al.22,23 demonstrated that time to stabilization (TTS), a measure of dynamic postural control, is impaired in individuals with CAI compared to healthy individuals. The TTS assessment, which involves landing on a single limb, is likely a better reflection of the demands placed on the neuromuscular restraint system during physical activity compared to static postural control assessments. However, no previous investigations have evaluated the effects of WBV on dynamic postural control in individuals with CAI.
The purpose of this investigation was to evaluate the acute effects of WBV on dynamic postural control and lower extremity muscle activity in individuals with CAI. We hypothesized that WBV would enhance the activity of muscles that are essential for dynamic ankle joint stability (tibialis anterior and peroneus longus muscles) and improve dynamic postural control (TTS).
This study used a crossover design in which each participant completed two counterbalanced testing sessions (control and WBV) separated by 1 week. TTS and electromyography activity of the tibialis anterior and peroneus longus muscles were evaluated at baseline and immediately, 15 minutes, and 30 minutes following the respective intervention.
A priori power analysis of unpublished data from Ross et al.22,23 using a similar modality (stochastic resonance electrical stimulation) in individuals with CAI indicated that 13 participants would provide power of 0.80 for alpha = 0.05 to evaluate the effects of WBV on TTS. Using a WBV intervention similar to ours, Tihanyi et al.19 reported increases in quadriceps electromyography activity following an acute WBV in 16 individuals with stroke, and Blackburn et al.20 reported increases in quadriceps function in 14 individuals with simulated knee pathology. Because the effects of WBV on neuromuscular function in individuals with CAI and the specific muscles we targeted are unknown, we recruited 26 individuals with CAI (12 females, 14 males; age = 20 ± 1 years; height = 1.7 ± 0.1 m; mass = 72.8 ± 11.4 kg) in an effort to ensure adequate statistical power.
Participants were required to be 18 to 30 years of age, recreationally active (ie, participation in physical activity at least 30 minutes three times per week), and display unilateral or bilateral CAI based on the following criteria: suffered at least two ankle sprains in the past year that resulted in swelling, ecchymosis, and/or decreased range of motion and experienced at least two sensations of “giving way” or feelings of instability after injury in the 12 months prior to participation.5,11,24,25 Participants were excluded if they completed ankle rehabilitation within 3 months prior to participation; displayed acute signs of ankle sprain at the time of testing (eg, pain, swelling, or ecchymosis); or reported a history of lower extremity fracture or surgery, visual or vestibular impairments, or neurologic disease or dysfunction (eg, stroke). Prior to participation, participants signed an informed consent document that was approved by the university's institutional review board.
The Cumberland Ankle Instability Tool (CAIT) was administered to quantify the level of CAI.26 All testing procedures were conducted barefoot and involved only the limb with CAI in individuals with unilateral CAI or the most impaired limb/lower CAIT score in individuals with bilateral CAI. Upon reporting to the laboratory, participants completed a 5-minute warm up on a stationary cycle ergometer at a self-selected pace. Surface electromyography electrodes (Delsys Inc., Boston, MA) were then secured over the tibialis anterior and peroneus longus muscles of the test limb. Prior to electrode placement, the skin was shaved, lightly abraded, and cleaned with isopropyl alcohol to enhance signal quality and skin adherence. Recording electrodes were placed over the area of greatest muscle bulk parallel to the direction of action potential propagation, and a reference electrode was positioned over the proximal anteromedial tibia. Manual muscle tests were performed to ensure appropriate electrode placement and rule out crosstalk.
Dynamic postural stability was assessed via TTS with participants barefoot. Maximal vertical jump height was first measured three times with the participant standing 70 cm away from a Vertec device (Sports Imports, Columbus, OH), with the highest value representing maximal vertical jump height. The Vertec vanes representing 50% maximal vertical jump height ± 0.5 inches were then exposed, and participants jumped off both legs while reaching to this height and landed in the center of a force plate (model #NC4060; Bertec Corporation, Columbus, OH) with the test limb. Participants were instructed to jump and touch the vanes, then land on the test limb and stabilize as quickly as possible for 20 seconds while positioning the hands on the hips. Participants were allowed three to five practice trials to familiarize themselves with the procedure, followed by five trials that were recorded for data analysis. Trials were discarded and repeated if the participant touched down with the contralateral leg, or stumbled or slipped on the test limb while trying to stabilize. Three-dimensional ground reaction forces and lower extremity electromyography data were sampled simultaneously at 1,000 Hz.
Immediately following baseline TTS measurements, participants moved to the WBV device (Power Plate pro5; Performance Health Systems, Northbrook, IL). For both the control and WBV conditions, participants maintained an isometric squat position (approximately 40° knee flexion; Figure 1) for 1 minute followed by a 2-minute rest period, which was repeated six times. During the WBV condition, a vibratory stimulus (30 Hz, 2 g) was provided while participants maintained this position.19,20 During the control condition, participants performed the same procedures on the WBV device but the vibratory stimulus was not provided. The aforementioned TTS procedures were then repeated immediately, 15 minutes, and 30 minutes following the respective intervention. Participants rested in a seated position between testing intervals.
Participant positioning for whole body vibration and control interventions.
Data Reduction and Analysis
Ground reaction forces were lowpass filtered (second order recursive Butterworth) at an estimated optimum cut-off frequency of 12.53 Hz.22 During the final 10 seconds of each trial, the minimum range of the resultant ground reaction force was calculated (range-variation) and averaged across trials to provide the mean variation. This value represents the average variability of the resultant ground reaction force during static stance when the participant has regained postural stability following the initial ground impact. The resultant ground reaction force was then fit with an unbounded third order polynomial, and TTS was defined as the point at which this polynomial curve fell below the mean + 3 standard deviations of the range-variation (ie, when the resultant ground reaction force resembled static stance).22
Electromyography data were bandpass (20 to 350 Hz) and notch (59.5 to 60.5 Hz) filtered (fourth order Butterworth), and smoothed using a 25 ms root mean square sliding window function. Mean electromyography amplitudes were calculated prior to (preparatory) and following (loading) initial ground contact (initial ground contact = vertical ground reaction force > 10 N). The preparatory phase was defined as the 150 ms interval prior to initial ground contact, and the loading phase was defined as the 150 ms interval following initial ground contact.27,28 Three 5-second maximal voluntary isometric contractions (MVICs) were recorded for each muscle for electromyography normalization (manual muscle tests), and MVIC was defined as the largest value derived from a 100 ms moving average. Electromyography data during the TTS procedure were normalized as %MVIC.
The effects of WBV on peroneus longus and tibialis anterior muscle electromyography amplitudes and TTS were evaluated via 2 (Condition) × (4 Time) repeated measures analysis of variance (alpha = 0.05). Electromyography data were evaluated separately for the preparatory and loading phases of the landing. All statistical analyses were conducted using SPSS statistical software (IBM Corp., Armonk, NY).
Three participants did not return for the second testing session. Therefore, statistical analyses were completed on 23 participants. The mean CAIT score was 16.35 ± 4.82 (range: 5 to 25), thereby confirming that the participants possessed CAI.29,30 WBV did not influence TTS, as the condition × time interaction effect for TTS was non-significant (F3,66 = 1.133; P = .342; partial η2 = 0.054; Figure 2). Similarly, WBV did not influence lower extremity muscle activity during the preparatory phase, as the condition × time interaction effects for peroneus longus (F3,66 = 0.045; P = .987; partial η2 = 0.002) and tibialis anterior (F3,66 = 1.080; P = .363; partial η2 = 0.045) muscle electromyography amplitudes were non-significant. Finally, loading phase electromyography amplitudes for the peroneus longus (F3,66 = 0.811; P = .492; partial η2 = 0.034) and tibialis anterior (F3,66 = 1.326; P = 0.273; partial η2 = 0.054) muscles were also not influenced by WBV. Electromyography data for the tibialis anterior and peroneus longus muscles are presented in Figures 3–4, respectively. Mean differences between the WBV and Control conditions at each time point for each outcome measure and the associated effect sizes (Cohen's d) and 95% confidence intervals are presented in Table 1.
Effects of whole body vibration (WBV) on time to stabilization (TTS). No significant differences were identified between baseline (IL) and immediately (Imm) or 15 or 30 minutes after intervention. Similarly, no differences were noted between the WBV and control (CON) conditions.
Effects of whole body vibration (WBV) on tibialis anterior muscle (A) preparatory and (B) loading phase electromyography amplitudes. No significant differences were identified between measurement intervals or conditions. CON = control; % MVIC = maximal voluntary isometric contractions; BL = baseline; Imm = immediately
Effects of whole body vibration (WBV) on peroneus longus muscle (A) preparatory and (B) loading phase electromyography amplitudes. No significant differences were identified between measurement intervals or conditions. % MVIC = maximal voluntary isometric contractions; CON = control; BL = baseline; Imm = immediately
Descriptive Statistics for Comparisons Between WBV and Control
The purpose of this study was to investigate the acute effects of WBV on dynamic postural stability and lower extremity muscle activity in individuals with CAI. Based on previous research identifying acute enhancements in muscle spindle sensitivity, proprioception, and muscle activity with WBV,19,31,32 we hypothesized that TTS would decrease and electromyography amplitudes would increase in individuals with CAI following exposure to WBV. However, we did not observe any acute effects of WBV on these variables.
Our findings agree with previous literature regarding the effects of WBV on neuromuscular deficits associated with CAI. Both Martínez et al.33 and Melnyk et al.34 reported that chronic (4 to 6 weeks) exposure to WBV did not alter reflex amplitudes or latencies of the peroneus longus, peroneus brevis, or tibialis anterior muscles in response to sudden inversion perturbation in healthy individuals, and Hopkins et al.35 reported similar results with acute WBV exposure.
In contrast to our findings, however, several studies have reported improvements in neuromuscular factors linked to dynamic joint stability following WBV exposure in pathological populations. Tihanyi et al.19 reported acute improvements in knee extension strength in participants with stroke following WBV. Strength assessments were much less variable than the TTS task in our investigation; therefore, improvements in muscle activity may have been more readily identified from a statistical perspective. Chronic WBV exposure (8 to 12 weeks) has also been demonstrated to improve postural stability, strength, and joint position sense following anterior cruciate ligament reconstruction more effectively than traditional rehabilitation,16–18 all of which would presumably improve TTS in individuals with CAI. To our knowledge, only Cloak et al.21 have investigated the effects of WBV in individuals with CAI and demonstrated improvements in single-leg static balance after 6 weeks of vibration exposure, but there was no influence on peroneus longus muscle activity. These findings may suggest that chronic exposure to WBV is sufficient for improving static postural control, but the single, acute exposure in our study may not have been sufficient to produce meaningful improvements in neuromuscular function. Additionally, it is possible that WBV may produce neuromuscular facilitation sufficient to improve static postural control, but dynamic postural control as assessed by TTS may be too physically demanding to be affected by WBV. The stimulus provided by WBV devices is directed almost exclusively in the vertical dimension; thus, its effects are primarily exerted on sagittal plane muscles (eg, quadriceps or triceps surae) when applied in a standing position. Because dysfunction of muscles controlling frontal plane motion at the ankle is the primary contributor to CAI, WBV as currently applied may have minimal effects on these muscles, and therefore on CAI. This may explain the lack of an effect of WBV on peroneus longus muscle activity, as noted both in our investigation and by Cloak et al.21
Limitations of our study include the subjective self-reported history of ankle sprains. Similarly, the participants were at different points along the spectrum of CAI as is indicated by the wide range of CAIT scores. Additionally, the cumulative exposure to WBV was 6 minutes, whereas other studies have used longer treatment sessions ranging from 6 to 16 minutes of WBV exposure.16,19 Although prolonged exposure to vibration (ie, 20 minutes) has a fatiguing effect,36 an optimal exposure duration may exist that evokes greater neuromuscular enhancements. Participant positioning could have also influenced our results, as WBV applied during single-leg stance mimicking the TTS procedure may have enhanced the neuromuscular response. Our study also only examined the acute effects of WBV; thus, it is unclear if repeated exposure to WBV with the same parameters would produce similar results.
The results of this study have implications for future research evaluating the potential for WBV as a therapeutic intervention for CAI. Our study did not measure strength or proprioception, both of which contribute to dynamic joint stability and demonstrate improvements following WBV.16,17 Also, the parameters of the WBV stimulus (ie, frequency and amplitude) influence its efficacy,37 and the ideal parameters for enhancing dynamic neuromuscular function are unknown. Other devices for applying vibratory stimuli may also be warranted, as vibrating wobble boards38 and local muscle vibration39 have been demonstrated to improve characteristics linked to CAI. Conducting a longitudinal study evaluating the effects of prolonged WBV intervention on individuals with CAI is also warranted due to neuromuscular adaptations that could occur from a longer intervention, particularly when combined with traditional rehabilitation techniques.16,17,21
Implications for Clincal Practice
This study found no significant effect of acute WBV exposure on TTS and lower extremity electromyography during single-leg landing. Although there are many studies supporting WBV as a valuable adjunct for strength, proprioceptive, or balance training, the exact parameters of the study should be considered based on the desired therapeutic effect. As such, it is unclear if WBV is effective as an adjunct therapy for treating CAI.
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- Torvinen S, Kannus P, Sievänen H, et al. Effect of a vibration exposure on muscular performance and body balance: randomized cross-over study. Clin Physiol Funct Img. 2002;22:145–152. doi:10.1046/j.1365-2281.2002.00410.x [CrossRef]
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Descriptive Statistics for Comparisons Between WBV and Control
|PARAMETER||MEAN Δ (WBV - CONTROL) MEAN ± SD||EFFECT SIZE (COHEN'S D)||95% CI (LOWER LIMIT, UPPER LIMIT)|
| Baseline||−0.33 ± 2.64||0.19||−0.80, 1.48|
| Immediately after||0.40 ± 1.74||0.24||−1.16, 0.35|
| 15 min after||−0.25 ± 1.66||0.26||−0.47, 0.96|
| 30 min after||0.21 ± 1.73||0.12||−0.95, 0.54|
|Preparatory PL EMG (%MVIC)|
| Baseline||−4.75 ± 56.36||0.11||−19.05, 28.54|
| Immediately after||−2.83 ± 23.94||0.05||−7.28, 12.93|
| 15 min after||−3.58 ± 24.64||0.08||−8.83, 13.98|
| 30 min after||1.34 ± 24.81||0.03||−9.13, 11.82|
|Loading PL EMG (%MVIC)|
| Baseline||−24.91 ± 252.54||0.11||−81.73, 131.55|
| Immediately after||22.85 ± 102.81||0.08||−66.26, 20.56|
| 15 min after||−24.48 ± 115.27||0.10||−24.19, 73.15|
| 30 min after||8.02 ± 98.65||0.03||−46.68, 33.63|
|Preparatory TA EMG (%MVIC)|
| Baseline||14.90 ± 87.55||0.27||−51.87, 22.07|
| Immediately after||−1.52 ± 44.43||0.05||−17.24, 20.28|
| 15 min after||3.66 ± 41.76||0.12||−21.29, 13.98|
| 30 min after||1.91 ± 37.41||0.07||−17.71, 13.88|
|Loading TA EMG (%MVIC)|
| Baseline||43.75 ± 334.14||0.19||−184.86, 97.33|
| Immediately after||12.90 ± 243.79||0.08||−115.83, 90.02|
| 15 min after||−11.90 ± 235.79||0.08||−87.66, 111.47|
| 30 min after||10.72 ± 203.29||0.08||−96.56, 75.12|