Effect of whole-body vibration on neuromuscular performance: A literature review

Abstract

BACKGROUND:

Whole-body vibration (WBV) is a neuromuscular training method that has recently received popularity in health and fitness centers, as an additional or substitute method to conventional training and therapy, in order to improve muscle strength and power.

OBJECTIVE:

The purpose of this review is to critically observe the effect of WBV training on neuromuscular performance in view of its ability to enhance the muscles strength, power, and flexibility; and also to investigate the influence of the different vibration characteristics (viz., method of application of vibration, frequency, and amplitude) and exercise protocols on the effect of this training.

METHOD:

For this review 24 studies or articles were examined, and based on exclusion and inclusion criteria, 5 studies were finally selected; and an attempt was made to uncover the factors influencing the improvement in neuromuscular performance as a result of WBV intervention. During the review, it was considered to include and discuss as many characteristics as possible, such as, knee extension, knee flexion, counter movement jump (CMJ), squat exercise, and jumping height (JH).

RESULT:

Whole-body vibration, along with additional exercise training, has a potential to induce substantial improvement in neuromuscular performance.

CONCLUSION:

Whole-body vibration can bring about improvement in muscles strength, power, and flexibility. The main factors associated with the improvement in muscles performance are range of amplitude and frequency, type of vibration and its method of application, training intensity, exercise protocol, and the characteristics of the participants.

Keywords

Muscle strength jumping height counter movement jump isometric squat electromyography

1 Introduction

In various day-to-day activities, human body is exposed to whole-body vibration (WBV) from different sources, for example, while driving automobiles, piloting aircrafts, operating industrial vehicles, etc. Earlier, ergonomists generally discussed the adverse effects of whole-body vibration [1]; however, from the past few decades, vibratory massagers are being used to train and provide massage therapy to enhance muscles performance [2 –6]. WBV exposure is a neuromuscular training method that has recently received popularity in the health sector, local gyms, and fitness and rehabilitation centers, as an additional or substitute method to conventional training and therapy. It has been so because WBV intervention improves the muscle strength and power [7, 8], adds flexibility [9, 10], besides other health benefits [11 –13]. In the literature, it has been reported that the training programs that use WBV exposure [14 –18] resulted in the improved performance of muscles, in comparison to those that did not use it. During WBV, the body of an individual is exposed to a low frequency-low amplitude mechanical stimuli, via a vibrating platform. The vibration stimulates the muscle spindles and sends nerve impulse to initiate muscle contractions according to the tonic vibration reflex (TVR) [19].

Vibration training may be performed using either a hand gripping vibrating device (Hand Arm Vibration) or a vibrating platform on which the subject is made to sit/stand (Whole-Body Vibration). There are mainly two types of vibrating platforms: platforms that vibrate in the vertical direction (vertical platform) and the platforms that vibrate through rotation about the horizontal direction (oscillating platform) [20]. Vibration can be applied to the targeted muscles mainly by two methods: either directly or indirectly. In the direct method of application, vibration can be applied directly on the muscle’s belly [21 –23] or on the tendon [24] of the participant by a vibrating unit, such as, vibrating dumbbell [25], vibrating pulley-like device [26], vibrating barbell or a massage therapy vibrator [26 –28]. Moreover, the vibratory unit may either be held in hand [22, 24] or fixed to an external support [21, 23]. Whereas in the indirect method of application, vibration is applied over the muscles of the subject from a vibrating source situated away from the targeted muscles [14, 26].

Whole-body vibration (WBV) treatment, in recent times, has rekindled the attention of the researchers as a method for treating patients with nervous and/or musculoskeletal disorders. There is ample literature available that report the effectiveness of short term WBV training in the enhancement of lower-limb muscle strength, power, bone density, and functional mobility, as well as in prevention of falls in such patients [4 , 30]. WBV training, in combination with additional exercise protocol, has been proven to be more effective than balance training alone for enhancing balance and strength, and preventing falls [8, 30].

For the evaluation of muscles activities associated with forceful exertions, surface electromyography (sEMG) method is used. The sEMG signal is a low amplitude signal that emanates from contracting muscles and can be used directly to measure muscle activity. A typical sEMG frequency spectrum ranges from 0 to 450 Hz. There are numerous studies which have demonstrated, with the help of sEMG recordings, that the application of whole-body vibration (WBV) intervention acutely increases muscles activity during the exposure [4 , 16–19].

The purpose of this review is to critically observe the effect of WBV training on neuromuscular performance, in view of its ability to enhance the muscles strength, power, and flexibility; and also to investigate the influence of the different vibration characteristics (viz., method of application of vibration, frequency, and amplitude) and exercise protocols on the effect of this training.

2 Methods

For the present review, twenty four papers were studied and after critical examination only 16 were selected, according to their relevance to the purpose of this review. Thereafter, based on exclusion criteria, 6 studies were further excluded because 4 of them were non-RCTs and the remaining 2 didn’t report WBV. Eventually, 10 studies were finally chosen and thoroughly studied. It was then observed that only 5 of them met the required criteria for the review [16 , 31–34]. The various characteristics of the participants and vibration parameters of the selected studies are listed in Tables 1 and 2 respectively. A total of 102 participants, with an age of 23.08 ± 2.96 years (mean ± SD), were exposed to vibration frequencies ranging from 26 to 50 Hz. One of the 5 studies included [34] used a frequency of 26 Hz, 2 of them [31, 32] used frequency of 30 Hz, another [16] used a frequency range of 30 to 50 Hz, and the remaining [33] used frequency ranging between 35 to 40 Hz. The amplitude of the vibration frequencies were ranged from 2 mm to 12 mm. Additional exercises included static, dynamic, half squat, and hard squatting. The most common exercise was squat, which was used in all of the five studies. The training sessions consisted of 1–6 sets of 20–60 seconds duration WBV training, with a rest of 5–60 seconds between WBV interventions. The warm-ups included either bicycling ergometer, squats, or streching movements.

Table 1
Study details and participants characteristics

Author (Year) Group Age (yrs) (mean ± SD) Weight (kg) (mean ± SD) Height (cm) (mean ± SD) Sample size (N) Gender Category

Samah et al. [33] WBV (N = 20) PL (N = 20) 21.6 ± 1.5 64.8 ± 9.7 174.0 ± 6.4 40 M Untrained

Lienhard et al. [32] WBV 23.8 ± 3.2 67.8 ± 10.9 173.0 ± 8.0 10 M Physically active

8 F

Rittweger et al. [34] WBV 24.4 ± 2.8 75.3 ± 6.4 181.8 ± 5.51 9 M Untrained young adults

21.8 ± 2.7 63.2 ± 3.4 72.6 ± 6.1 10 F

Cormie et al. [31] WBV PL 19–23 Yrs 80.0 ± 11.2 176.4 ± 7.8 9 M Recreational sporting adults

Cardinale et al. [16] WBV 23.5 ± 4.6 75.1 ± 7.4 183.4 ± 8.4 16 F Professional volleyball players

Author (Year)	Group	Age (yrs) (mean ± SD)	Weight (kg) (mean ± SD)	Height (cm) (mean ± SD)	Sample size (N)	Gender	Category
Samah et al. [33]	WBV (N = 20) PL (N = 20)	21.6 ± 1.5	64.8 ± 9.7	174.0 ± 6.4	40	M	Untrained
Lienhard et al. [32]	WBV	23.8 ± 3.2	67.8 ± 10.9	173.0 ± 8.0	10	M	Physically active
					8	F
Rittweger et al. [34]	WBV	24.4 ± 2.8	75.3 ± 6.4	181.8 ± 5.51	9	M	Untrained young adults
		21.8 ± 2.7	63.2 ± 3.4	72.6 ± 6.1	10	F
Cormie et al. [31]	WBV PL	19–23 Yrs	80.0 ± 11.2	176.4 ± 7.8	9	M	Recreational sporting adults
Cardinale et al. [16]	WBV	23.5 ± 4.6	75.1 ± 7.4	183.4 ± 8.4	16	F	Professional volleyball players

Note: WBV-Whole-Body Vibration, PL-placebo, N-Sample size.

Table 2

Various characteristics and vibration parameters of included studies

Author Year	Purpose	Type of Study	Subjects	Exercise	Specification of vibration exercise	Muscles outcomes		Result on muscles performance
						Muscles Strength	Muscles Power
Samah et al. [33]	To investigate the effects of 12 weeks WBV on muscular performance	Randomized controlled trial four groups:1. WBV2. PL3. Resistance4. Control	40 male adults, Mean age 21.6 ± 1.5 yrs, Untrained	Knee strength at 100°, During CMJ Knee at 90°, 3 times in week, 35–40 min session, 5 Min warm up, 30–60 s exposure, 5–60 s break time b/w set, half squat exercise	A = 2.5–5 mm F = 35–40 Hz a = 0.4 g	Isometric muscles strength	Counter movement jump (CMJ)	Improvement in muscles performance.
Lienhard et al. [32]	To investigate that during WBV contain motion artifacts and/or reflex activity	Randomized controlled trial three groups:1. No Load2. WBV with no load3. WBV with 33 kg additional load	10 male, 8 Female, Mean age 23.8 ± 3.2 yrs Physically active	Knee flexed at 70°, 20 s exposure, 60 s break time, Static squat exercise	A = 2 mm F = 30 Hz	NM	NM	sEMG signals during WBV induced motion artifacts and reflex activity.
Rittweger et al. [34]	To investigate the effect of hard squatting exhaustive exercise with and without WBV on 19 healthy young adults	Randomized controlled trial three group1. WBV2. Without WBV3. WBV with 40% additional load	10 Female, mean age 21.8 ± 2.7 yrs 9 Male 24.4 ± 2.8 yrs, Untrained young adults	Isometric knee extension at knee angle set at 100° and 70% MVC, 30 s exposure, 10 min recovery, 6 trial, At least 3-day interval b/w Visit, 10 min warm up, hard squatting	A = 6–12 mm F = 26 Hz	Isometric knee extension.	Serial Isometric Jumping	Increase in neuromuscular excitability and fatigue in muscles.
Cormie et al. [31]	To investigate the effects of a single bout of WBV on isometric squat (IS) and counter movement jump (CMJ).	Randomized controlled trial two groups:1. WBV2. Sham	9 Male, Age b/w 19 to 23 yrs, Recreational sporting activities	Knee angle flexed at 100°, 5 min bicycle ergometer warm up, 30 s exposure, 3 min rest time, Maximal isometric contraction for 3 seconds.	A = 2.5 mm F = 30 Hz	isometric squat	Counter-movement jump (CMJ), Jumping Height	WBV resulted in an increase in vertical JH in comparison with the sham group.
Cardinale et al. [16]	To Analyze EMG responses of vastus lateralis muscle to different WBV frequency	Randomized controlled trial four groups:1. WBV at 30 Hz2. WBV at 40 Hz3. WBV at 50 Hz4. No Vibration	16 Female, Mean age 23.5 ± 4.6 yrs, Professional volleyball players	Knees bent at 100°, 60 s exposure, half-squat position, 60 s rest time.	A = 10 mm F = 30–50 Hz	NM	NM	Highest EMGrms activation response was found at 30 Hz vibration frequency, suggesting highest reflex activity in vastus lateralis muscle.

Note: EMG - electromyography, WBV – whole-body vibration, PL- placebo, IS - isometric squat, CMJ - counter movement jump, A - amplitude, F - frequency, a – acceleration, MVC -maximal voluntary contraction, sEMG - surface electromyography, RPE - rate of perceived exertion, PF - peak force, VL - vastus lateralis, RF - rectus femoris, BF - biceps femoris, SOL – soleus muscles, G - gastrocnemius muscle, NM - not measured.

2.1 Search strategy

For the current review, the related studies were searched using the keywords-‘whole body vibration’, ‘strength’, ‘power’, ‘neuromuscular’, and ‘EMG’ in Google scholar, science direct and PubMed. The relevant papers were selected based on whether they fulfilled the eligibility criteria of the review or not.

2.2 Eligibility criteria

Studies were eligible for review inclusion if they fulfilled the following criteria: (i) studies must be randomized controlled trials (RCTs); (ii) studies should be either of WBV alone, or of exercise (placebo), or of WBV combined with the conventional therapy; (iii) at least one outcome of the study should be related to the strength of the muscle.

2.3 Exclusion criteria

For the current review, studies were excluded if any of the following cases were found: (i) studies were non-randomized controlled trials (non-RCTs); (ii) studies were not published; (iii) studies did not utilize WBV.

2.4 Quality of assessment

The assessment methodology for evaluating the quality of the selected studies, based on the guidelines of the International Society of Musculoskeletal and Neuronal Interactions (ISMNI) for WBV intervention studies, consisted of 13 factors [35] that are listed below in Table 3. Based on the response of these factors, which enquired about WBV parameters (e.g., frequency, acceleration, and peak to peak displacement) and the subject’s position (i.e., body position, footwear condition, stance distance, and exercise position), the quality of each study was evaluated. These factors were scored either as ‘yes’ (Y) or ‘unsure’ (U). For example, the type of vibration was scored as ‘unsure’ if it was not clear whether the vibration applied was vertical or oscillatory in nature.

Table 3
Methodological assessment by the recommendations of the ISMNI

Authors (Years) Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 Q10 Q11 Q12 Q13 Quality Score

Samah et al. [33] Y Y Y Y Y U U Y U U Y Y Y 9

Lienhard et al. [32] Y Y Y Y U Y Y U U U Y Y Y 9

Rittweger et al. [34] Y Y Y Y U U U U U Y U Y Y 7

Cormie et al. [31] Y Y Y Y U Y Y U Y U U Y Y 9

Cardinale et al. [16] Y U Y Y U U U Y U U U Y Y 6

Authors (Years)	Q1	Q2	Q3	Q4	Q5	Q6	Q7	Q8	Q9	Q10	Q11	Q12	Q13	Quality Score
Samah et al. [33]	Y	Y	Y	Y	Y	U	U	Y	U	U	Y	Y	Y	9
Lienhard et al. [32]	Y	Y	Y	Y	U	Y	Y	U	U	U	Y	Y	Y	9
Rittweger et al. [34]	Y	Y	Y	Y	U	U	U	U	U	Y	U	Y	Y	7
Cormie et al. [31]	Y	Y	Y	Y	U	Y	Y	U	Y	U	U	Y	Y	9
Cardinale et al. [16]	Y	U	Y	Y	U	U	U	Y	U	U	U	Y	Y	6

The assessment methodology recommended by ISMNI consists of the following questionnaire: Q1) Brand name of vibration platform, Q2) Type of vibration, Q3) Vibration frequency, Q4) Vibration amplitude, Q5) Peak acceleration, Q6) Accuracy of vibration parameter, Q7) Evaluation of skidding off the feet, Q8) Changes of vibration parameters, Q9) Rationale for choosing vibration parameters, Q10) Support devices during vibration exposure, Q11) Type of footwear, Q12) Body position, and Q13) Description of exercise.

The methodology of quality assessment rating was originally formulated by Van Tulder et al. [36], which was later on modified by Niklasson et al. [37]. This modified quality rating score has been employed in the present review, as depicted in Table 3. The mean quality score of all the included studies was found to be 8 ± 1.41 (ranging from 6 to 9) out of 13 points. Among the studies included, three of them [31 –33] exhibited a high-quality score of 9 points which indicated good quality of research; while the remaining two [16, 34] indicated moderate quality. This quality index rating was also used by Rehn et al. [38].

3 Effect of WBV and PL on knee extensor isometric strength

Three of the five studies [31 , 34] (Table 2) have investigated the short-term effect of vibration on knee extensor isometric strength, with the duration of vibration exposure varying from 20 to 30 seconds. Whereas in the remaining two studies, the duration of vibration exposure was 60 seconds [16, 33]. Moreover, the vibration frequency reported in the study of Rittweger et al. [34] was 26 Hz, which was lower as compared to the remaining studies which used frequencies ranging between 30 to 50 Hz, resulting in an increase in neuromuscular excitability of the recruited muscles. This study, further, reported that there was no significant improvement in perceived rate of exertion (PRE) between the two treatments, i.e., with and without WBV. During the investigation, the subjects were asked for knee extension at an angle of 100° and maximum voluntary contraction (MVC) of 70% , during a 30 seconds exposure of vibration therapy. It was reported that there was a significant decline in torque after hard squatting exercise as compared to that without the exercise. Also, both the training protocols produced comparable levels of exhaustion and neuromuscular fatigue; and there were also differences observed in the neuromuscular function.

Samah et al. [33] reported that there was a significant improvement in knee extension isometric strength in WBV as compared to PL (half squat exercise was performed in both the cases). In addition, there was significant improvement reported in WBV group for counter movement jump (CMJ), but no significant improvement was found in PL group for CMJ. During CMJ, the jumper starts from an upright standing posture, makes an initial downward movement by flexing the knees and hips, then instantaneously extends the knees and hips again to jump vertically up off the ground. It was also reported that there was significant improvement between post-test performance of both WBV and PL groups for CMJ. Therefore, this affirms the effectiveness of the WBV intervention on muscular performance and strength for CMJ.

4 Effect of WBV on knee flexion isometric strength

During WBV intervention, with static squat exercise for 20 seconds, there were considerable improvements in the knee flexion isometric strength, when a frequency of 30 Hz with amplitude of 2 mm was applied on the knee flexed at an angle of 70° [32]. Samah et al. [33] tested the subjects for anthropometry and isokinetic dynamometry in a position with the knee flexed at an angle of 100°; and for CMJ with the knee flexed at an angle of 90° during half squat exercise. The investigations were carried out using frequencies ranging from 35 to 40 Hz, and the results reported a significant enhancement in muscles performance of both WBV and PL group, with better improvement for WBV [33]. Therefore, it can be concluded that short-term exposure of vibration on the knee flexion, in combination with some additional squat exercises, helps in significant enhancement in muscles strength and performance.

5 Effects of WBV on EMG response at different frequencies

Cardinale and Lim [16] reported an increase in EMG response of vastus lateralis muscles during WBV exposure with different frequency levels, i.e., no vibration, 30 Hz, 40 Hz, and 50 Hz in random order. The study was performed on 16 female professional volleyball players, who were asked to bend their knees at 100° during the trial in half-squat position, with a vibration exposure for a duration of 60 seconds, followed by 60 seconds rest in between each trial. They investigated that WBV treatment led to an increase of EMG RMS activity of vastus lateralis muscles, as compared with the baseline values (p < 0.001) at no vibration condition. The highest EMG RMS response was reported at a vibration frequency of 30 Hz (p < 0.001), suggesting this frequency to be the one eliciting the highest reflex response in vastus lateralis muscle during WBV in half-squat position. Also, they concluded that there was significant improvement in EMG RMS at 30 Hz and 40 Hz, in comparison to no vibration and 50 Hz vibration.

In another study, Cormie et al. [31] found that during half squat position (knee angle at 100°) there was no change in average IEMG (Integrated EMG) activity of the muscles, viz., vastus medialis, vastus lateralis, and biceps femoris, during vibration intervention (f = 30 Hz, A = 2.5 mm,VV, Power Plate) than without vibration. Futhermore, Rittweger et al. [34] showed an increase in EMG median frequency after exhaustive squatting training with vibration (f = 26 Hz, A = 12 mm, SV, Galileo) and extra load (40% of body mass), suggesting that type I motor units were recruited during fatigue. Moreover, they also reported an improvement of the patellar tendon stretch reflex after continuous vibration training (f = 26 Hz, A = 6 mm, SV, Galileo) with dynamic squatting to exhaustion (349 seconds), and concluded that α-motoneurons were augmented by vibration, which recruited high thereshold units and muscles fibres. However, it was found that there was no effect of vibration on EMG power.

Lienhard et al. [32] investigated that due to WBV, the raw EMG signal was contaminated with motion artifacts and reflex activities during a randomized controlled trial of two groups: (i) WBV with no load condition, and (ii) WBV with a load of 33 kg. The participants were asked to flex their knees at an angle of 70°, and vibration was induced for 20 seconds at a frequency of 30 Hz, with 60 seconds rest between each session; and the subjects performed static squats exercise during all the trials. The test was performed for sEMG of the five lower limb muscles: VL, RF, BF, SOL and G; and it was found that during WBV, the EMG RMS of the fundamental as well as that of the harmonics were significantly higher when the additional load was used as compared to the no load condition in all the measured muscles (p < 0.05). Further, no significant difference was observed between the two loads for the no-vibration condition (p > 0.05). In addition, the EMG RMS of the fundamental as well as that of the harmonics showed significant correlations with the interpolated EMG RMS of the corresponding muscle (p < 0.05). However, the EMG RMS of the patella was found to be uncorrelated with the EMG RMS of the interpolated sEMG of any muscle that was reported (p > 0.05).

Therefore, in view of the aforementioned studies, it can be concluded that at lower frequencies, there is more improvement in the muscles strength and EMG rms during whole body vibration, in combination with some additional exercises. Furthermore, the sEMG signals during WBV contaminate with vibration induced motion artifacts; therefore, it is suggested that proper filtering techniques need to be implemented in order to eliminate the motion artifacts so that there are no deviations in the EMG parameters and consequently in muscles activities.

6 Discussion

An increase in muscle strength as a result of whole-body vibration intervention has been widely documented. However, some additional exercise protocols help in further improvement of the muscles performance. Due to vibration exposure, there was an enhancement in the muscles activities as reported in numerous studies [4 , 53]. Samah et al. [33], during an investigation, observed that the WBV group was superior with respect to the isometric strength of the knee (extension-flexion) as well as the ankle (planter-dorsiflexion). In a study, Eftekhari et al. [41] reported that with a low frequency, ranging from 2 to 20 Hz, due to WBV there was improvement in knee extensor muscle strength. Also, Ahlborg et al. [42] reported the same improvement in leg muscle strength and walking ability, while performing a broad survey of spastic, strength, and walking variables after 8-week WBV or 8-week resistance training in adults with cerebral palsy (CP). They found that WBV was more effective in reducing spasticity of the knee extensors as compared to the resistance training. Further, Cheng et al. [43] reported that there was improvement in walking ability and decrease in spasticity after the WBV intervention due to which there was increase in muscles strength. However, De Ruiter et al. [18] measured muscles strength immediately after the vibration exposure and found no effect of vibration on isometric knee extensor maximal force or rate of force development. Their findings seem to be in contradiction with that of the other studies [26 , 43]. But, in general, it can be suggested that vibration exposure causes increase in muscles strength and hence the walking ability.

Kang et al. [44] conducted a review and meta-analysis on the effect of whole body vibration on muscle strength and functional mobility in persons with multiple sclerosis and found that three studies [41 , 46] reported the positive effect of WBV on the knee extension strength (p = 0.003) in comparison to without vibration. However, two studies [45, 46] reported that WBV, with a frequency ranging between 25 to 45 Hz, did not improve knee-flexor strength (p = 0.65). Therefore, Kang et al. [44] suggested, in line with most of the discrete studies, that WBV along with some additional exercise considerably enhances the knee-extensor strength, but not the knee-flexor strength. Similar findings have been reported by some other studies [14 , 31–34] as well.

Dakota [47], while executing a 10 day exercise program at a frequency of 26 Hz, reported that WBV training resulted in an increase in neuromuscular adaptation, similar to the result that was produced by explosive strength exercises. Bosco et al. [48] also reported a significant improvement in the height and mechanical power, during a 5 seconds continuous-jumping test, while investigating the effect of a 10 days WBV training program (5×90 seconds) at a frequency of 26 Hz. The same tendency of improvement in neuromuscular performance was reported by Rittweger et al. [34] when WBV was exposed at a frequency of 26 Hz. Furthermore, Delecluse et al. [14] reported that the strength, and more specifically isometric and isokinetic strength, significantly improved after WBV training. Moreover, they found an enhancement in CMJ (7.6% ) comparable to the 8.5% increase in jump height in the study of Torvinen et al. [17]. In addition, Torvinen et al. [17] recorded an increase of 3.7% in isometric knee-extensor strength after 2 months of WBV training, but a 16.6% increase in isometric knee-extensor strength was reported by Delecluse et al. [14]; however, this improvement in muscles strength vanished partially in the next 2 months of WBV training. Bosco et al. [40] also observed an increase in JH (3.9% ) after exposure of 10 bouts of 60 seconds using WBV protocol. In line with these studies, Cormie et al. [31] also reported that WBV resulted in a significantly higher JH (p < 0.05) during the CMJ immediately following vibration, as compared to no vibration condition.

But these results were in contradiction with that of the findings of Rittweger et al. [39], which reported a significant decrease in vertical JH (9.1% ) after the vibration exposure. In the same way, Ronnestad et al. [49] also found no substantial improvement between groups with respect to relative jump height increase. Furthermore, Cochrane et al. [50] also found no improvement in muscles strength when vibration was applied during vertical jump, sprint, or agility performance. However, they mentioned that the performance variables were not measured until 2 days after the last exposure to vibration. Therefore, it may be suggested that the possible reason for no improvement in muscles strength was that the vibration stimulus was not utilized in the same context as that was used in the previously mentioned studies [51, 52].

Torvinen et al. [53] reported an increase in isometric muscle strength and vertical jump height by the end of 4 months of WBV training. However, after continuing the training for 8 months, they found that only the vertical jump height increased, but not the muscle strength; indicating that the training of the subjects was carried out unsupervised and/or the participants in the control group did not perform any exercise. De Ruiter et al. [54] also reported that there were no significant improvement in muscles strength in healthy physically active students, who had undergone 11 weeks of WBV training. A possible reason for this observation could be the small sample size of the control as well as the exercise group (n = 10), and also a light exercise protocol. Thus, there may be an ideal dose–response paradigm by which a certain amount of vibration can result in increased muscle strength and performance.

Sale [55] reported that full activation of the muscle may lead to motor unit fatigue, and consequently an increase in the strength of the muscles. Also, the EMG recordings showed the impact of WBV on muscle activity, which recommended that a prolonged span of standing on the WBV platform results in full motor unit activation. Cardinale et al. [16], while investigating EMG responses of vastus lateralis muscle to different WBV frequencies, found that the maximum EMG RMS was observed at 30 Hz; suggesting this frequency as the one eliciting the maximum reflex response during WBV in half-squat position. However, there was an increase in EMG activity in quadriceps muscle during vibrations and it has been attributed to a facilitation of the excitability of spinal reflex [56]. Hazell et al. [57] found an increase in EMG activity in muscles exposed to WBV with a frequency of 45 Hz, while performing loaded or unloaded dynamic squats. Marin et al. [58] reported a significant increases in upper body EMG activity when WBV was applied via a ground-based platform, with frequency ranging between 0 to 46 Hz. These results are in agreement with those of the cited studies [16 , 34] that WBV causes increase in muscle activity. Therefore, based on these findings, it can be concluded that WBV can be used to enhance EMG activity of the muscles and improve their performance.

Rittweger et al. [34] reported an improvement of the patellar tendon stretch reflex after continuous vibration training (f = 26 Hz, A = 6 mm, SV, Galileo) with dynamic squatting to exhaustion (349 seconds), and concluded that α-motoneurons were augmented by the vibrations, which recruited high thereshold units and muscles fibres. However, Hopkins et al. [59] observed some contradictory results and reported no effect of patellar tendon stretch reflex after intermittent vibration exposure (f = 26 Hz, A = 4 mm, SV, Galileo). This contradition was also supported by Lienhard et al. [32], whose results suggest that the EMG RMS of the patella was found to be uncorrelated with the EMG RMS of the interpolated sEMG of any muscle that was reported (p > 0.05). In addition, Rittweger et al. [34] results showed an increase in EMG median frequency after exhaustive squat training, with vibration (f = 26 Hz, A = 12 mm, SV, Galileo) and extra load (40% of body mass), suggesting that type I motor units were recruited during fatigue. Furthermore, in a previous study by Rittweger et al. [39], it was again reported that during the isometric contraction, the increase of EMG median frequency over vastus lateralis muscles was more after exercise with vibration, than without vibration. However, it was reported that there was no effect of vibration on EMG power.

In another study, Cormie et al. [31] found no change in average IEMG (Integrated EMG) activity of the muscles during the vibration intervention (f = 30 Hz, A = 2.5 mm,VV, Power Plate), in half squat position (knee angle at 100°). On the contrary, Abercromby et al. [60] examined muscles activity of different lower limb postures of static squat (18.5°) and dynamic squat (10–35° knee angle) and found that EMG RMS increased in all the muscles with vibration (f = 30 Hz, A = 4 mm,VV, Power Plate). Also, Delecluse et al. [14] reported that there was an increase in gastrocnemius and rectus femoris muscles EMG RMS while standing in static half-squat position with vibration (f = 35 Hz, A = 5 mm,VV, Power Plate) as compared to no vibration. Therefore, it can be suggested that there is an increase in EMG activity after exercise with vibration than without vibration, during half squat position at frequency ranging between 20 to 50 Hz.

Various EMG characteristics such as RMS, work done, power, and slope of median frequency can be used as common parameters to control the appropriate stimulation frequency as well as the vibration effectiveness during the treatment. However, motion artifacts constantly obstruct the recording of raw EMG signals. It may, primarily, be due to the relative motion among electrodes and skin or between skin layers, skin stretching, power line, and cable interference due to which there may be changes in the distribution of the internal charge, causing variation in electrode potential [61 –64]. Therefore, in order to reduce the effect of motion artifacts during WBV trainings, the electrodes and their cables must be taped properly to the shaved and cleaned skin, the amplifier/preamplifier should be located near to the recording electrodes, and small length cables must be used to connect the electrodes with the amplifier [61].

Lienhard et al. [32] reported that the sEMG signal during WBV may include muscle activity phase-locked to the vibration frequency and its multiple harmonics because the RMS of the spikes increased with increasing background activation of the muscle, and was highly correlated to the RMS of the sEMG signals without the spikes. Therefore, they concluded that sEMG signals measured during WBV were contaminated with motion induced artifacts and that the artifacts were more distinct in the fundamental than in the harmonics. Fratini et al. [65] and Sebik et al. [66] also confirmed the existence of motion artifacts by finding excessive peaks at the vibration frequency and its multiple harmonics. The presence of motion artifacts caused overestimation of EMG parameters and consequently muscle activity during vibration. It was reported that deleting the unnecessary spikes in the sEMG spectrum not only eliminates the motion artifacts, but also some portions of the reflex activity induced by the vibration [67]. Hence, the raw surface EMG signals need to be filtered using high pass (cut off frequency of 10 to 20 Hz), low pass (cut off frequency of 500 Hz), full wave rectification, harsher filtering or as Fratini et al. [65] recommended, a series of sharp notch filters at the vibration frequency and its superior harmonics. But, by using sharp notch filters, some accurate EMG signal also vanishes, however, by using this filter there was a chance to hide likely deviation of the EMG parameters in those bands due to mechanical activated synchronization of muscles during the vibration [65]. Therefore, it can be suggested that proper filtering techniques are to be implemented in order to eliminate the motion artifacts, without causing any deviation of EMG parameters and consequently muscle activity during vibration.

Protocols for the vibration stimulus in numerous previous studies have utilized different frequencies and amplitudes, ranging from 20–50 Hz and 1–12 cm respectively, making interpretation of the effectiveness of vibration difficult. Whatsoever may be the mechanism behind it, it is clear that WBV elicits muscle contraction involuntarily and induces an increase in muscles strength and power of the participants within a short period of time.

7 Conclusions

Whole-body vibration has got the potential to bring about considerable improvements in muscles strength and performance. The main factors that are associated with improvement in muscles performance are range of amplitude and frequency, the types of vibration and its method of application, training intensity, exercise protocol, and the characteristics of the participants. However, futher investigation is required to identify the optimal parameters and protocol of WBV training, in combination with additional exercises, for improving the muscles strength and power. Moreover, WBV may also be a plausible warm-up training protocol for increase in vertical Jumping Height. However, the optimal dose of vibration frequency, amplitude, and exposure duration is still unclear. Furthermore, vibration can have a potentiating effect on JH and isometric muscle strength and power during CMJ, but it may also induce fatigue. Therefore, the exact mechanism for the effect of vibration on increasing vertical JH needs more investigations.

Further, it can be concluded that WBV, with frequency ranging between 20–50 Hz, can elicit higher EMG activity as compared with the non-vibrating condition and hence enhance muscles strength. Moreover, sEMG signals measured during WBV were contaminated with motion induced artifacts and need to be eliminated using proper filtering techniques, since the presence of motion artifact caused overestimation of EMG parameters and consequently muscle activity. Also, the present review suggests that addition of WBV intervention, along with some additional exercise program, can cause greater improvements in knee extensor muscle strength and countermovement jump performance than without WBV intervention. Finally, it can be concluded that WBV intervention has great prospects in therapeutic situations, where it may enhance neuromuscular performance in patients who are either not attracted to, or are unable to perform the conventional exercise programs.

8 Limitation

There are several limitations in the present review. Of them, one of the most important one is that the sample size of selected studies used for the review was small, whereas a higher sample size could have had a better and strong influence on the objective of the review. In addition, studies on long-term intervention of WVB have not been included in the review so we did not investigate the longer duration effects of WBV on muscles strength and power. Moreover, several other limitations of the present study need to be mentioned. Firstly, the present review included participants with varying characteristics (e.g. gender, age, weight, height, trained/untrained, and fitness level) and involved evaluation of combined results in order to examine muscle strength and power. Secondly, the present review did not recommend the optimal vibration parameters or exercise protocol due to the lack of uniformity in methodologies of the studies included in it. Thirdly, in the various cited studies, while examining the effect of WBV on muscle strength, power, and, EMG activity, various parameters of the experiment, such as, frequency, amplitude, displacement, posture, and exercise protocol, were not the same; resulting in non-execution of the strict sub-group analyses because of these differences in the parameters. Lastly, the method used for the assessment of the muscle strength (e.g. based on type of muscle contraction, range of movement of the knee, duration of exposure, and warm-up duration) varied among the studies included for knee muscle strength; and also, only CMJ and serial isometric jumping were used for the measurement of muscle power in the studies selected for this review.

9 Future scope

The present review focused on the neuromuscular performance in patients when WBV was applied with or without additional exercise protocol on the lower limb, i.e., on the knee flexor/extensor muscles; therefore, in future it would be required to investigate the muscles performance on the upper limb, viz., on the forearm muscles, shoulder, finger, and wrist, regarding WBV intervention in combination with some additional exercise protocol. Moreover, it was seen that certain circumstances related to WBV may result in the adoption of different postural control approaches, which could in turn, confound with the effects that have been attributed to WBV. Therefore, further research is required to inspect other methodologies that could cause the physiological response and adaptations to WBV, and how these response and adaptations may change between individuals suffering from different neuromuscular disorders. Furthermore, WBV intervention could also involve controls for motion artifacts as well as changes in postural control strategies. In addition, long term effect of WBV, with controlled factors and well-designed exercise protocols, in people with various disorders related to lower limb or upper limb, with higher sample size are required to be investigated in future.

Conflict of interest

None to report.

Footnotes

Acknowledgments

The author would like to thank the Council of Scientific & Industrial Research (CSIR), Human Resource Development Group, New Delhi, India, for awarding Senior Research Fellowship (SRF), having Ack. No. 141530/2K15/1, File No. 09/112(0553)2K17. Also, the authors are grateful to Ms. Saima Zarrin, UGC-Junior Research Fellowship, Department of Statistics and Operations Research, Aligarh Muslim University, Aligarh (UP), India and Mr. Basharat Jamil, Doctoral research student, Department of Mechanical Engineering, Aligarh Muslim University, Aligarh (UP), India for their kind suggestions during the editing and improvement of the manuscript. The authors are also indebted to the editors and reviewers of WORK: A Journal of Prevention, Assessment, and Rehabilitation, for their valuable comments which led to an overall improvement in the manuscript.

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