EFFECTS OF 6‐WEEK WHOLE BODY VIBRATION TRAINING ON THE REFLEX RESPONSE OF THE ANKLE MUSCLES: A RANDOMIZED CONTROLLED TRIAL

Abstract

Background:

The ligament sprain of the lateral ankle is the most frequent injury that occurs when participating in sports. Whole body vibration (WBV) is a training method that has been recently introduced as a rehabilitative tool for treatment of athletes. It has been hypothesized that the transmission of mechanical oscillations from the vibrating platform may lead to physiological changes in muscle spindles, joint mechanoreceptors, as well as improve balance.

Propose:

The aim of this study was to assess the effects of a 6‐week WBV training program on the reflex response mechanism of the peroneus longus (PL), peroneus brevis (PB) and anterior tibialis (AT) muscles in ankle inversion at 30º from horizontal, in a static position.

Methods:

This study was a single‐blinded and randomized controlled trial. Forty‐four healthy, physically active participants were randomly split into two groups: the experimental group (n = 26) (the WBV training) and control group (n = 18). Reaction time (RT), maximum electromyographic (EMG) peak (peak EMG), time to the maximum peak EMG (peak EMG time) and reflex electrical activity of all the muscles were assessed before and after the WBV training through surface EMG.

Results:

After 6‐weeks WBV training, there were no significant changes in the variables analysed for all the muscles involved.

Conclusion:

A 6‐week WBV training does not improve the reflex response mechanism of the lateral stabilizing muscles of the ankle.

Level of evidence:

1b

INTRODUCTION

Injuries that occur during sport performance constitute an important economic burden to public health in several countries,¹ representing the reason for 17% of medical check‐ups.² One of the most frequent injuries that occurs during sport performance is the sprain of the lateral ankle.³ Ferran and Mafulli⁴ calculated that in the West Midlands, United Kingdom, there were 60.9 ankle sprains per year for every 10,000 people, 14% of which were considered severe. Once this injury has occurred, there is a 70%⁵ to 80%⁶ probability that it will reoccur. This high injury incidence is associated with major economic costs, which would indicate that prevention strategies could be important. The most common prevention strategies currently described within the literature include neuromuscular training and the use of orthosis or bandages.⁷

Ankle sprains can cause a deficit in the neuromuscular and proprioceptive control of the joint. Neuromuscular control is defined as the unconscious activation produced when preparing for and executing joint movement. Neuromuscular control also plays an important role in keeping and restoring functional stability.⁸ The deficit in the proprioceptive control has been assessed in surveys that highlight an increase in the reaction time (RT) of the stabilizing muscles⁹ and a decrease in balance capacity.¹⁰ In this regard, a prospective study by McHugh et al established that balance improvement significantly decreases the number of ankle sprains in different sport disciplines.¹¹

The oscillating nature of whole body vibration (WBV) training can have advantageous effects on the proprioceptive mechanisms of the ankle joint, because with this intervention, it is possible to impact the muscle stretch‐shortening cycle, EMG responses, as well as muscular strength, and power.¹² This type of training, which was created as a therapeutic method to produce cardiovascular and musculoskeletal treatment effects, was applied in sport performance through the research of Nazarov and Spivak.¹³ During recent years, WBV training is being utilized more as a neuromuscular training technique^14,15 and as an injury rehabilitation and prevention method.¹⁶

Some of the long‐term effects of this type of training include increases in muscular strength,¹⁷ power,¹⁸ vertical jump height,¹⁹ and balance.²⁰ There are different theories that may explain the reason for these improvements. Some authors state that they may be due to muscle adaptations produced by the gravitational load²¹ while others relate it to possible hormonal changes²² or an increase of the EMG signal which occurs due to the exercise itself without applying vibration.^14,17

Acute physiological adaptations to WBV training are related to an increase in the sensitivity of muscle spindles, of the gamma motor neurons, and their effects on the stretch‐shortening reflex.^21,23 These adaptations may develop the active protection mechanism of the joints, by decreasing the RT as well as the stimuli threshold necessary to produce an action potential. Therefore, this type of training may have positive consequences for the prevention and rehabilitation of joint injuries.²⁴

For this reason, the aim of this research was to assess the effects of a 6‐week WBV training program on the RT and the reflex electrical activity of the peroneus longus (PL), peroneus brevis (PB) and anterior tibialis (AT) muscles of both legs in healthy and physically active people. For this study, participants were selected without previous injuries in order to assess WBV as a potential ankle sprain prevention strategy. Since Rittweger,²⁵ showed that WBV training improved strength and power, the EMG response, and improved the stretch‐shortening cycle as well as body balance, it was hypothesized that the present intervention protocol would improve measures of neuromuscular response mechanisms within the ankle joint.

METHODS

Design

This study was a single‐blinded and randomized controlled trial. The independent variables were group (experimental and control) and WBV training. The dependent variables were reaction time (RT), maximum electromyographic (EMG) peak (peak EMG), time to maximum EMG peak (peak EMG time) and reflex electrical activity of the PL, PB and AT muscles of both legs (measured by surface EMG).

Participants

Forty‐four healthy, physically active volunteers participated in this study. Subjects were randomly allocated into the experimental group (EG) (n = 26, 12 male and 14 female; age = 21.3 ± 2.4 years; height = 168.1 ± 7.5 cm; weight = 65.2 ± 2.5 kg), who participated in the 6‐week WBV training program, and the control group (CG) (n = 18, 10 male and 8 female; age = 24.5 ± 5.8 years, height = 172.1 ± 62.5 cm; weight = 68.1 ± 9.0 kg), who did not participate in the training. Before participation all subjects signed an informed consent document that was approved by the Ethics Committee of the Hospital Universitario de Albacete.

Inclusion criteria included being healthy and physically active according to the International Physical Activity Questionnaire (IPAQ).²⁶ The exclusion criteria for both groups included any type of injury within the six months prior to the study, participating in any competitive sport, being sedentary, or having medical history of neuromuscular or cardio respiratory disease. Each subject was instructed to maintain constant physical activity during the study.

Pre‐test procedure

The pre‐assessment of both groups was performed one week before the start of the WBV training protocol. In this pre‐assessment, the subjects signed the informed consent documents and anthropometric data was collected.

Ankle inversion test

Once the anthropometric data were taken, the ankle inversion test was carried out. In order to achieve this, the surface electrodes were placed on the peroneus longus (PL), peroneus brevis (PB) and anterior tibialis (AT) muscles of both legs following the surface EMG for non‐invasive assessment of muscles (SENIAM) guidelines.²⁷ For correct conductivity of the myoelectrical pulse, participants' skin was previously prepared. For that purpose, areas where electrodes would be placed were shaved and cleaned with denatured 70% alcohol. Two electrodes (Ag/AgC1 sensor, Ambu Blue Sensor N‐00‐S/25, Ambu A/S, Ballerup, Denmark), with a separation of 1 cm between their active parts and in the direction of the muscle fibres, were placed for each muscle under study. A third reference electrode was placed on the tibia. In order to ensure that the electrodes were positioned in the same place in the pre‐ and post‐training evaluations, participants' skin was marked in the exact place with a dermographic pen and this mark was checked daily.

Once electrodes were placed, the inversion test was carried out (Figure 1). For the purpose of this assessment, the authors designed and manufactured a platform with similar characteristics to the one used by Clark and Burden.²⁸ A double axis goniometer (SG110/A, Biometrics Ltd, Gwent, UK) was placed at the platform doors. This device detected any change in the angle of the doors. Anti‐slip material and adhesive bands, which allowed the steady position of the feet, were placed on the surface of the doors. The participant was placed barefoot on the platform with their feet parallel and at a distance of 5 cm from the rotation shaft of the platform to the medial part of the feet. Additionally, he/she was given visual occlusion glasses and earphones with music for the purpose of increasing the surprise factor and eliminating an anticipatory response to the ankle inversion from the subject. The leg to be studied remained with the knee extended supporting the body weight and the other was slightly bent and resting on the ball of the foot. Once the participant was in the initial test position, electromyographic data was collected and the platform was only opened (to induce inversion) when an EMG base line indicated the absence of an anticipatory response to the ankle inversion of the muscles studied. Three tests were carried out for each ankle with undefined time between them.

Figure 1.

Ankle inversion test.

The platform caused 30º of inversion in the ankle and it was only opened when an EMG baseline indicated the absence of an anticipatory response of the studied muscles.

WBV Training

For WBV training, the Fitbive ExcelPro (Bilzen, Belgium) platform was used. The program lasted six weeks with a frequency of three sessions per week, with a necessary rest period of 24 hours between each session. Each session lasted 30 minutes, 15 minutes of which were vibration stimuli and 15 minutes to rest between repetitions and exercises, as according to previous investigations instructions,³¹ because it has been proposed that stimuli for less than 20 minutes cause higher neuromuscular adaptations. Frequency and amplitude parameters were 30 Hz and 2 mm in the first week, and 45 Hz and 4 mm in the sixth week, respectively. Hezell et al³² showed that frequencies of 35‐45 Hz combined with 4 mm of amplitude increase the muscle EMG activity in static and dynamic contractions. Load progressions of the vibratory stimuli are shown in Table 1.

Table 1.

6‐week WBV training protocol carried out by the Experimental Group (EG).

WEEK	FRECUENCY (Hz)	AMPLITUDE (mm)
Week l	30−45−60	2
Week 2	30	4
Week 3	35	4
Week 4	40	4
Week 5	45	4
Week 6	45	4

Three positions were adopted on the platform (Figure 3), which included a plantar flexion of the ankle joint by an active contraction of the posterior muscles of the leg, since the muscle contraction added to the vibratory stimuli increases the effects of this type of intervention.³³ In the first one, the subject remained with one of the legs in front of the other, at a distance of one meter between the support points of the feet (toes) and with the knees semi‐flexed at about 100º. In the second one, the subject remained squatting, stood on the toes, the feet were separated at about 50 centimetres and the knees flexed at 100º approximately. Finally, the third position was similar to the second one. However, in this case the subject was stood in just one foot.

Figure 3.

Protocol of exercises used in WBV training.

First, the left exercise was carried out (3 repetitions of 60 s with each leg in alternate way, resting 30 s between each one), after that, the center exercise (3 repetitions of 60 seconds with, 30 seconds rest between each repetition) and the right exercise (3 repetitions of 60 seconds with each leg in alternate way, with 30 seconds rest between each) resting 60 seconds between each exercise. All the exercises included plantar flexion through isometric contraction of the triceps surae.

In all these positions the subject remained static on the platform. Each exercise was repeated three times per session, with a break of 30 seconds between repetitions and 60 seconds between exercises without vibratory stimulus outside the platform.

Post‐Training Testing

The week subsequent to the WBV training, the post‐training testing was carried out. The electrodes were placed in the marks drawn on the skin and all testing was performed using the same procedures previously described (in the ankle inversion testing section) in order to assess the effects of the WBV training program.

Data analysis

The EMG activity of muscles studied was obtained using the ME6000‐T8 electromyograph (Mega Electronics, Kupio, Finland) and was analysed using MegaWin 3.1‐b10 software (Mega Electronics, Kupio, Finland) through a visual examination on the computer screen, in which two cursors identify the beginning and end of the period to be analysed. EMG data, obtained in raw at a sampling frequency of 1000 Hz, were integrated at 1000 Hz for each test. The RT of the muscles under study was determined as the time between the opening of the platform and the increase to more than double the electrical activity (150 ms, maximum time allowed for assessment) to the ankle inversion.²⁹ Electrical activity of the muscles analysed (average level) was determined by a smoothing the EMG at a frequency of 50 Hz. Following the methodology of previous studies, this electrical activity was analysed in four fractions of 25 ms (0‐25 ms; 26‐50 ms; 51‐75 ms; 76‐100 ms) (Figure 2) after the beginning of the activation of each muscle and standardised according to the values obtained in the initial tests.³⁰

Figure 2.

Graphic representation of variables analysed.

The upper graphic represents the inversion of the ankle in degrees. The lower graphics represents the reflex EMG response of the peroneus longus (PL) muscle at ankle inversion. The reaction time (c) was determined as the time between the opening of the platform (a) and the beginning of the muscle activation (b). Average EMG level was calculated in four time fractions after the beginning of muscular activity, of 0-25 ms (d), of 26-50 (e), of 51-75 (f) and of 76-100 (g). The maximum EMG peak (h) and the time to the maximum EMG peak (i) after the beginning of muscular activation were also calculated.

The maximum EMG peak (peak EMG) and the time to obtaining the maximum EMG peak (peak EMG time) of all the muscles in a maximum period allowed of 100 ms after starting the muscle activity were analysed. All the variables were obtained by using the average of results from the three inversion tests.

Statistical analysis

SPSS 17.0 was used for the statistical analysis. The mean and standard deviations of the variables were calculated for both groups in pre‐ and post‐evaluations and the Shapiro‐Wilk test was carried out to determine whether parametric and non‐parametric tests should be used. To compare the electrical activity data obtained before and after the training, the student's t‐test, for samples related to parametric variables, and Wilcoxon test, for ‐non‐parametric ‐variables were used. To compare the groups pre‐ and post‐ training reaction times, the student's t‐test (for samples related to parametric variables) and the Mann‐Whitney U test (for non‐parametric variables) were used. The level of significance of p ≤ 0.05 was chosen.

RESULTS

Reaction time (RT)

Average values of the RT of both groups before and after the training are shown in Table 2. In the initial evaluation there were no significant differences between the EG and the CG in the RT of the muscles of the right leg (PL, p = 0.227; PB, p = 0.081; AT, p= 0.582) and of the left leg (PL, p = 0.211; PB, p = 0.058; AT, p = 0.267). In the post‐training evaluation, there were also no significant differences between groups. The EG showed a non‐significant increase in muscle RT of the right leg (PL, p = 0.192; PB, p = 0.092; AT, p = 0.087) and of the left leg (PL, p = 0.573; PB, p = 0.621; AT, p = 0.141).

Table 2.

Results in the Reaction Time (ms) of the muscles studied of both groups before and after training

		Control Group				Experimental Group
		Pre‐Training		Post‐Training		Pre‐Training		Post‐Training
LEG	MUSCLES	Mean	SD	Mean	SD	Mean	SD	Mean	SD
RIGHT	Peroneus Longus	36.31	11.60	42.13	13.99	41.56	14.36	46.43	13.00
	Peroneus Brevis	36.75	6.22	40.60	13.46	42.40	13.68	47.48	14.95
	Anterior Tibialis	44.19	4.52	49.27	15.31	46.12	16.40	51.81	13.33
LEFT	Peroneus Longus	40.00	5.12	41.50	4.03	36.25	13.05	40.00	12.45
	PeroneusBrevis	43.35	5.01	42.88	7.27	38.00	11.95	44.64	10.97
	Anterior Tibialis	46.94	8.15	47.81	9.12	42.33	9.96	47.86	13.22

Legend: Means and Standard Deviations

Maximum EMG peak (peak EMG) and time to maximum EMG peak (peak EMG time)

Average values obtained from the peak EMG and the peak EMG time of both groups before and after training are shown in Table 3. There were no significant differences between the EG and CG groups in the peak EMG before starting the training. After 6‐weeks of WBV training, there was a non‐significant decrease in the peak EMG of the PL (p = 0.067) and PB (p = 0.150) muscles of the right leg and of the PB (p = 0.222) muscle of the left leg of the EG. The peak EMG of the AT (p = 0.660) of the right leg and PL (p = 0.941) and AT (p = 0.058) of the left leg increased but not significantly.

Table 3.

Results in the Maximum EMG Peak (Peak EMG) and in the up to the maximum EMG peak (peak EMG time) of the experimental and control groups before and after the WBV training.

			Peak EMG (μV)				Peak EMG time (ms)
			Control Group		Experimental Group		Control Group		Experimental Group
LEG	MUSCLES	Mean	SD	Mean	SD	Mean	SD	Mean	SD
Pre‐training	RIGHT	Peroneus Longus	421.50	309.06	378.68	208.27	5763	21.89	59.48	21.70
		Peroneus Brevis	368.81	206.38	374.08	124.11	56.88	23.61	60.56	21.28
		Anterior Tibialis	272.31	170.40	247.60	150.66	63.56	22.54	59.08	21.72
	LEFT	Peroneus Longus	360.23	247.06	327.87	114.93	58.88	22.53	62.12	23.50
		Peroneus Brevis	402.52	228.81	366.88	120.51	58.71	23.16	64.79	23.10
		Anterior Tibialis	172.41	91.78	183.46	99.84	67.00	19.96	77.17	14.65

LEG	MUSCLES	Mean	SD	Mean	SD	Mean	SD	Mean	SD
Post‐training	RIGHT	Peroneus Longus	396.85	298.91	283.90	140.73	61.53	25.55	57.45	24.02
		Peroneus Brevis	290.31	150.31	313.85	152.87	67.00	18.34	64.15	22.73
		Anterior Tibialis	201.00	183.07	254.40	172.57	56.31	19.10	70.05	23.88
	LEFT	Peroneus Longus	362.87	171.47	342.75	192.81	64.53	20.66	64.95	17.61
		Peroneus Brevis	350.67	206.63	359.42	155.07	64.13	21.47	53.32	16.18
		Anterior Tibialis	109.67	63.40	260.05	177.48	72.93	15.11	71.95	18.05

Legend: Means and Standard Deviations

Similarly, before starting training there were no significant differences between the groups in the peak EMG time of the muscles analysed of the right leg (PL, p = 0.792; PB, p = 0.607; AT p = 0.529) and of the left leg (PL, p = 0.660; PB, p = 0.412; AT, p = 0.067). After the training program, the peak EMG time of the EMG for the PB (p = 0.274) and AT (p = 0.069) muscles of the right leg and the PL (p = 0.207) muscle of the left leg increased, but not significantly. The peak EMG time for the PL (p = 0.858) and the PB (p = 0.207) muscles of the right leg, as well as the AT (p = 0.180) of the left leg decreased, but not significantly.

Reflex electrical activity

The student's t‐test for independent samples did not show significant differences between the EG and CG groups in the reflex motor activity of the muscles analysed before and after the WBV training. Likewise, no significant differences existed in this variable in any of the muscles of both groups at the end of the training. Regardless of the calculation in the determined time bands (0‐25 ms, 26‐50 ms, 51‐75 ms and 76‐100 ms), the normalized muscle electrical activity (percentage of muscle activation relative to the pre‐evaluation) of the EG was of 72.21% for the muscles of the right leg and of 104.58% for the muscles of the left leg (Figure 4). In the CG, these values were of 87.88% for the muscles of the right leg and of 84.51% for the muscles of the left leg (Figure 4).

Figure 4.

Results of average EMG level in both groups.

Average values and standard deviations of the reflex electrical activity of the peroneus longus (PL), peroneus brevis (PB) and tibialis anterior (AT) muscles of both legs of the experimental group (EG) (a) and of the control group (CG) (b). The results are standardized according to the values obtained in the initial evaluation.

DISCUSSION

The aim of this research was to investigate whether 6‐weeks of WBV training affected the reflex response mechanisms of the stabilizer muscles of the ankle joint in healthy and physically active people. The results of the Cardinale and Lim³⁴ supported the hypothesis that WBV training causes acute alterations in sensitivity of the muscle spindles. At the beginning of the current research, the authors thought that these alterations would also cause long‐term adaptations that would result in improvements in the reflex patterns of muscle contractions. However, the results obtained did not confirm this hypothesis.

Neuromuscular training is considered one of the most efficient measures used to prevent ankle sprain.⁷ Gardner and Gray³⁵ showed that the terminal sensory branches of the tibial, sural, and deep peroneal nerves are present in the ligaments and joint capsule of the ankle. These nerve endings serve to collect and afferently send proprioceptive information to the spinal centers of motor control, which in turn develops the efferent response pattern.³⁶ The main way to maintain total body balance is the reflex cycle that originates in the receptors of the muscles that surround the ankle joint.³⁷ These sensory receptors are parallel to the muscle fibers and have different afferent and efferent fibers that detect changes in the fiber length. These proprioceptors (mainly the neuromuscular spindles) may be stimulated with WBV training.³⁸ However the effects of vibration training are not believed to be limited to improvements in spinal reflexes that direct movements. The increase of the motor potentials,³⁹ together with the increase in the EMG signal frequency,¹⁵ suggest an important excitability of the motor cortex that along with muscle adaptations produce greater neuromuscular efficiency.^40,41 These adaptations produced at a neuromuscular level could improve the reflex response mechanism of the stabilising muscles of the ankle, however, the results of the current research did not support this mechanism.

Clark and Burden²⁸ and Akhbari⁴² concluded that balance training decreases the RT of the muscles that stabilize the ankle joint. The reliability of the measurement of the RT of the stabilizing muscles of the ankle has been demonstrated.²⁹ ‐Average values of the RT in the subjects in the current study before training are similar to the ones obtained in other previous investigations.^43,44 Before starting the training, there were no significant differences between the groups in the variables analysed, which means that the participants in the study began at the similar levels of the reflex motor activity. Since no significant differences were found in either group after WBV training, the intervention protocol used in this study did not improve the reflex response patterns of the studied muscles.

There are studies in literature that describe the acute effects of WBV training on the reflex response of the knee extensors⁴⁵ and the PL muscle in a sudden inversion of the ankle.²⁴ Only one study in literature assessed the effects of long‐term WBV training on the reflex activity of the ankle.³⁰ A 4‐week intervention program, 3 times a week with 2 minutes of exposure to vibration every session was performed. Like the current investigation, no significant differences in the RT and reflex activity of the analyzed muscles were found. One of the reasons for a lack of significant results in the study by Melnyk et al is the simulation of ankle sprain. Combined movement of 24º of inversion and 15º of plantar flexion was used, which may not have sufficiently stimulated the reflex mechanism of the muscles analysed. After analysing the results obtained by Melnyk et al, the authors cannot indirectly conclude that WBV training does not stimulate the stretch reflex of ankle stabilizers. Bearing in mind that the stimulus generated by the vibratory training may cause an increase in the activation of the neuromuscular spindles,^21,31,46 the following explanations may clarify the results obtained in the current study. One explanation may be the perturbation used to simulate the ankle sprain. The static position of the subject and the 30° inversion caused in the joint may have not adequately stimulated the neuromuscular spindles of the analysed muscles. It would be advisable to repeat this study in an alternate situation with the subject walking, running, or performing cutting movements and using a combined movement of plantar flexion, abduction and inversion, as suggested by Eechaute et al²⁹ in their study.

As far as the WBV training protocol is concerned, the authors of the current study employed the parameters of amplitude and frequency of the stimulus used in previous studies which have achieved the desired EMG responses,^31,32 muscular strength and power responses,¹⁷ and whole body balance improvement.²⁰ These parameters may not have been sufficient to induce a more long‐term effect. Another possible cause of not having found significant changes after intervention may be attributed to the subjects' position on the platform. The contraction of the posterior musculature of the lower leg, as well as the quadriceps, and the hamstring muscles could attenuate part of energy produced by the vibration and this circumstance may have contributed to the lack of significant results. It would be interesting to consider alternate exercises used to stimulate the proprioceptive and vestibular systems; such as the subject standing on unstable surfaces (wobble boards) or taking alternate positions on the platform with the eyes closed.

The authors do not consider that the size of the sample to be the cause of the non‐significant improvement in the results, but rather the subjects characteristics. The fact that the subjects in the current study were healthy, physically active, and without history of previous injuries in the ankle joint may be the reason that the vibratory stimulus used in this study was not enough to obtain adaptations in the EMG response of the analysed muscles. The authors intend to follow this line of research with subjects who have chronic instability in the ankle joint in order to determine if WBV training is effective in prevention of recurrence.

CONCLUSION

In conclusion, the 6‐week WBV training protocol used with the healthy active subjects in this study did not produce advantageous effects on the reflex response mechanism of the PL, PB and AT muscles after a sudden inversion of the ankle. Thus, the authors are unable to recommend using WBV training as an intervention in the prevention of ankle sprain in healthy active individuals. More studies are necessary to determine if WBV training helps to prevent reoccurance of this injury in people with previous sprains.