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Abstract

BACKGROUND:

Effects of whole-body-vibration (WBV) training have been claimed to depend on individual characteristics and vibration frequency; however, there have not been any studies focusing on effects of different WBV frequencies on flexibility, jump performance, and dynamic balance ability in the same group.

METHODS:

Twenty healthy men participants were randomly assigned to three conditions [0 Hz, 25 Hz, 40 Hz] prior to the study. They stood barefoot on the non-dominant leg and performed WBV of 5 sets $\times$ 30 seconds. In pre- and post-WBV in each condition, the participants measured ankle dorsiflexion angle; single leg squat jump (SLSJ) height; and dynamic postural stability index (DPSI), anterior-posterior stability index (APSI), medial-lateral stability index (MLSI), and vertical stability index (VSI).

RESULTS:

Ankle dorsiflexion angle at 25 and 40 Hz and SLSJ height at 25 Hz significantly increased after WBV ( $p<$ 0.01). DPSI and APSI at 25 Hz and 40 Hz significantly decreased, except VSI, which was significant only at 25 Hz ( $p<$ 0.05).

CONCLUSIONS:

The acute effects of exposure to WBV on flexibility, jump performance, and dynamic balance ability differ by the selected vibration frequencies.

Keywords

Different whole-body-vibration frequencies flexibility physical performance

1. Introduction

Whole-body-vibration (WBV) training has become a general therapy in athletic training and rehabilitation because it is safer and more practical than other training methods, such as weight training and balance exercise. There are some indications regarding the long-term effects of WBV on improving muscular strength, power, jumping performance, balance ability, and flexibility [1, 2]. In addition, WBV has been shown to produce greater short-term increases in muscle temperature [3], flexibility, muscle strength, dynamic balance ability, and jump ability than other training methods or non-vibration training [4, 5].

The acute effects of WBV have been claimed to depend on the characteristics of vibration, such as vibration frequency and amplitude from transmission of vibration, through their effect on muscle activation [6, 7]. Improvements in neuromuscular performance variables, such as peak moment and muscle activation were confirmed both in studies using lower frequencies, between 25 and 30 Hz (low frequencies) [8, 9], and those using higher frequencies, between and 40 and 50 Hz (high frequencies) [10, 11]. On the other hand, Cardinale and Lim found a 3.9% increase in the vertical squat jump height after a 5-min WBV session at a frequency of 20 Hz, whereas a frequency of 40 Hz produced no improvements [12]. Da Silva et al. have examined the effects of various vibration frequencies on certain measures (squat jump, countermovement jump, maximal strength, and power). At 20 and 30 Hz, squat jump height has shown a significant increase after a 5-min WBV session, whereas WBV with 40 Hz produced a significant decrease [13].

Dallas et al. [4] found a 3.9 and 3.8% increase in sit and reach test score which is index of the flexibility after WBV with 30 and 50 Hz frequency respectively. Cronin et al. [14] reported that the vibration stimulus of 30 s, with four conditions (14, 24, 34, and 44 Hz frequency), significantly increased the flexibility of the hamstring range of motion by 1.6–2.1%, whereas vibration stimulation at 14 Hz did not cause significant increases. However, Cardinale and Lim [12] found an increase in the sit and reach score, by 13.5%, after a 5-min WBV session with 20 Hz frequency, whereas WBV with 40 Hz frequency produced a 3.3% decrease.

It seems from these reports that any frequency makes the peak moment and muscle activation improvements. Additionally, the high improvements for the jump performance might be provided at low frequency, and for the flexibility might be provided at high frequency. However, there is few reports to examine jump performance and flexibility in the same group.

For the balance ability, Borges et al. [15] reported that WBV exercise did not influence the swaying of the center of pressure for healthy women whereas Cloak et al. [5] showed improved dynamic balance ability such as Y-balance test among amateur and elite soccer players. Thus, these studies seem to indicate that WBV exercises do not improve static postural stability but may affect dynamic postural stability. Additionally, the response to acute WBV exercise appears to vary according to participants, exercise program, and vibration characteristics; however, few reports are available on postural stability after landing in healthy subjects of the same group.

Regarding dynamic postural stability, subjects jump and land on single-leg on a force plate, and measurements are made using the dynamic postural stability index (DPSI) [16]. This is a comparative measure for dynamic postural stability that reflects the maintenance of balance as the participant transitions from the dynamic to the static state. It serves as a functional measurement of neuromuscular control, as it is calculated during a single-leg jump stabilization maneuver [17]. Therefore, DPSI might show difference of dynamic postural stability caused by different vibration frequencies, because DPSI evaluates postural stability of minute change using force plate.

To our knowledge, most of the studies related to this topic have examined parallel studies. On the other hand, Gerodimos et al. showed the effects of WBV intervention with different vibration frequencies on flexibility, jump height in the same group [18], but this study only low frequency (15, 20, 30 Hz). In addition, no study has observed effects on posture stability after WBV with different vibration frequency in the crossover study.

The purpose of this study was thus to examine the acute effects of WBV with low and high vibration frequencies on jump height, flexibility, and dynamic postural stability. We hypothesized that WBV with low frequency would improve jump height and dynamic postural stability more effectively than that with high frequency and non-vibration and that WBV with low and high frequencies would improve flexibility more than that with non-vibration.

2. Methods

2.1 Participants

Twenty healthy men (age: 22.7 $\pm$ 1.6 years; height: 172.4 $\pm$ 5.7 cm; body weight: 65.2 $\pm$ 7.2 kg; BMI: 21.9 $\pm$ 1.8) were recruited from the local university. Participants with injuries to the lower extremities and/or neuromuscular disease, were excluded. All participants have been exercising 1–3 times per week but were not exposed to WBV before. This study protocol met the requirements of the Declaration of Helsinki and was approved by the Hiroshima University Committee on Ethics in Clinical Research (approval number C-195). All participants signed an informed consent form.

Figure 1.

Flowchart of this study.

Figure 2.

Positioning for the intervention of whole-body-vibration on the vibrating platform.

2.2 Intervention of WBV

Three conditions including two different vibration frequencies (0 Hz [control], 25 Hz, 40 Hz) were used in this study. To reduce bias the order of the trial was randomized using the method of randomly permuted blocks (www.randomization.com) while participants were blinded to the order (Fig. 1). In addition, we conducted two different vibration frequencies conditions and control condition on separate days, with at least one week between them. In each condition, after pre-tests of ankle dorsiflexion angle, vertical jump height, and DPSI, participants performed 5 sets $\times$ 30 s with a 30 s rest period between sets on a vibrating platform (Sonic Wave Vibration Exercise System; Sonix, Inc., Irvine, CA, USA) which uses electromagnetic technologies and a speaker mechanism to generate precise vertical vibrations. The platform vibrated continuously at a frequency of 25 Hz or 40 Hz, with intensity set at 30 for each frequency. Figure 2 shows positioning for the WBV intervention on the platform. Participants stood barefoot on the non-dominant leg in the center of the vibration platform [15] with the knee positioned at 50 ${}^{\circ}$ flexion as flexion angles smaller than 50 ${}^{\circ}$ lead to more vibration being transmitted to the head [19]. Participants completed measurement of the ankle dorsiflexion angle, vertical jump height, and DPSI 2 minutes after the final set of intervention; we measured the values three times and averaged them. All measurements were completed within 15 minutes after the end of the intervention [18].

2.3 Measurements items

We measured three items of passive dorsiflexion angle, vertical jump height and DPSI.

First, maximum passive dorsiflexion angle was measured using an electronic device with a resolution of 1 ${}^{\circ}$ (Rakkun walk-R-1, Maruzen Industrial Co., Ltd. and Hiroshima University, Japan), and we measured the ankle dorsiflexion angle in a standing position, because it is more reliable than measurements in a supine or sitting position [20]. We defined the maximum angle as the angle that precedes the point where the knee and hip joints begin to bend. This measurement has good test-retest reliability (ICC $=$ 0.97) [21].

Vertical jump height was measured next. This study defined vertical jump height as single leg squat jump (SLSJ) height on the non-dominant leg, since the intervention posture was one of single-leg standing on the non-dominant leg. Participant performed maximal voluntary SLSJ on the floor, using a jump instrument (Myotest SA, Sion, Switzerland). They were instructed to jump straight up and to make an effort to land with the same body form and in the same place as at takeoff, without moving laterally or vertically. Participants failed the SLSJ they performed it from a semi-squat position and with a preparatory countermovement. They kept their hands on their hips during the jump task and maintained their trunk in an upright position to emphasize the use of the leg-extensor muscles [22]. Thirty second of rest were allowed between jumps. This measurement of squat jump height has an ICC range (3,1) of 0.82 to 0.84 while the coefficient of variation is 4.25% [23].

Figure 3.

Equations for calculations of APSI, MLSI, VSI and DPSI. $\sum$ , sum; GRFx, medial-lateral ground reaction force; GRFy, anterior-posterior ground reaction force; GRFz, vertical ground reaction forces; DPSI, dynamic postural stability index; APSI, anterior-posterior stability index; MLSI, medial-lateral stability index; VSI, vertical stability index.

Finally, we assessed the DPSI using a single-leg jump landing in the anterior direction. This measure has good test-retest reliability (ICC3, 1 $=$ 0.96), and the standard error of measurement for interrater reliability of the composite test is 0.03 [16]. The starting position of the participants was 40% for participant’s body height distance at the end of the force plate (AccuGait, AMTI, Hiratsuka, Japan) from which they jumped a 30 cm hurdle placed at the midpoint between the starting place and the force plate [17]. For the anterior direction jump task, participants were instructed to use both legs when they jumped over the hurdle before landing in the force plate on the non- dominant leg only, stabilizing as rapidly as possible, placing their hands on their hips after posture stabilized, and maintaining this posture for 10 s while looking forward. Upper extremity movement was unlimited during the jump. Participants were allowed three practice trials beforehand for each condition to become familiar with the task only before measurement began. They took rest time of 1 min between trials to prevent fatigue. Participants repeated the task if they lost balance or made limb contact with the hurdle between jump and landing. Similarly, if the non-dominant limb touched the dominant leg or the ground around the force plate, the trial was repeated. Measurements from three successful trials were collected from before and after the intervention for each condition and used for analysis.

The ground reaction force values were recorded with sampling frequency of 200 Hz. A Microsoft Excel macro was used to process the ground reaction force values in order to calculate the DPSI; ground reaction values were passed through a zero-lag fourth-order low-pass Butterworth filter with a frequency cut-off of 20 Hz. As shown in Fig. 3, the DPSI is a composite of the ground reaction forces in all planes, and thus, is sensitive to force changes in the anterior-posterior ( $y$ ), medial-lateral ( $x$ ), and vertical ground reaction forces ( $z$ ) axes. Accordingly, the ground reaction force in each direction, respectively, provides the anterior-posterior stability index (APSI), medial-lateral stability index (MLSI), and vertical stability index (VSI). The DPSI was calculated using the first 3 seconds of the ground reaction forces instantly following initial contact, identified immediately when the vertical ground reaction force exceeded 5% of the body weight. The force plate values were normalized to body weight. Maximum vertical ground reaction force (vGRFmax) was calculated using raw value signals; the study then used normalized vGRFmax by each body weight of the subject. The average of three successful data for each condition was used for additional analysis.

2.4 Statistical analysis

Data was analyzed using the SPSS Statistics for Windows, version 20.0 (IBM Japan Co Ltd, Tokyo, Japan). A two-way ANOVA 3 [condition] $\times$ 2 [time] with repeated measures on both factors was applied. When the main effect was statistically significant, follow-up analyses were carried out with paired $t$ -test between pre- and post- mean value and Bonferroni-adjusted (0.05/3) paired $t$ -test for pre- and post- mean values within each condition. For all statistical analyses, $\alpha$ of 0.05 was the criterion for rejection of the null hypothesis. Effect size was calculated using the $F$ value ( $F$ ) and the calculation result was reported as the $r$ value ( $r$ ).

3. Results

The mean values and standard deviations of ankle dorsiflexion angle, single leg squat jump height, and DPSI are shown in Tables 2 and 2. The pre-WBV value within each condition was not significant for any measured items. In addition, no improvements by standing only (control) were found.

3.1 Ankle dorsiflexion angle

The ICC value for test-retest was 0.97 respectively, with no significant ( $p>$ 0.05) differences between the mean values of each condition. There was a significant condition $\times$ time interaction with respect to ankle dorsiflexion angle ( $F=$ 16.053, $p=$ 0.001, $r=$ 0.545). Follow-up analyses with paired $t$ -test revealed significant pre-post differences for 25 Hz ( $p=$ 0.001) and 40 Hz ( $p=$ 0.001) but not for control ( $p=$ 0.266). Significant differences of the post-mean value within each condition were revealed between control and 40 Hz ( $p=$ 0.016).

3.2 Single leg squat jump (SLSJ) height

The ICC value for test-retest was 0.82, with no significant ( $p>$ 0.05) differences between mean values of each condition. The SLSJ height had a significant condition $\times$ time interaction ( $F=$ 10.832, $p=$ 0.001, $r=$ 0.471). Follow-up analyses with paired $t$ -test revealed significant pre-post differences for 25 Hz ( $p=$ 0.001) but not for control ( $p=$ 0.095) or 40 Hz ( $p=$ 0.892). The post-mean values within each condition showed no significant differences.

3.3 DPSI, APSI, MLSI, VSI, and VGRFmax (%BW)

The ICC values for test-retest were 0.83 at DPSI, 0.82 at APSI, 0.54 at MLSI, 0.84 at VSI, and 0.91 at VGRFmax (%BW) with no significant ( $p>$ 0.05) differences between mean values of each condition.

DPSI showed a significant condition $\times$ time interaction ( $F=$ 3.933, $p=$ 0.028, $r=$ 0.306). Follow-up analyses with paired $t$ -test revealed significant pre-post differences for 25 Hz ( $p=$ 0.001) and 40 Hz ( $p=$ 0.008) but not for control ( $p=$ 0.335). The post-mean values within each condition showed no significant differences.

APSI showed a significant condition $\times$ time interaction ( $F=$ 6.227, $p=$ 0.001, $r=$ 0.375). Follow-up analyses with paired $t$ -test revealed significant pre-post differences for 25 Hz ( $p=$ 0.004) and 40 Hz ( $p=$ 0.001) but not for control ( $p=$ 0.279). The post-mean values within each condition showed no significant differences.

VSI showed a significant condition $\times$ time interaction ( $F=$ 3.300, $p=$ 0.048, $r=$ 0.283). Follow-up analyses with paired $t$ -test revealed a significant pre-post difference for 25 Hz ( $p=$ 0.001) but not for control ( $p=$ 0.520) or 40 Hz ( $p=$ 0.183). The post-mean values within each condition showed no significant differences.

MLSI and VGRFmax showed no significant condition $\times$ time interaction ( $F=$ 1.954, $p=$ 0.167, $r=$ 0.221 and $F=$ 0.425, $p=$ 0.657, $r=$ 0.105).

Table 1
The value of ankle dorsiflexion angle and single leg squat jump height at pre- and post-whole-body-vibration between each condition

$n=$ 20	0 Hz		25 Hz		40 Hz		(condition $\times$ time)			(condition)			(time)
							Interaction effect			Main effect			Main effect
Variable	Pre	Post	Pre	Post	Pre	Post	$F$	$p$	$r$	$F$	$p$	$r$	$F$	$p$	$r$
Ankle dorsiflexion angle (degrees)	19.2 $\pm$ 5.0	19.4 $\pm$ 5.3	18.6 $\pm$ 5.1	20.4 $\pm$ 5.4*	18.8 $\pm$ 5.2	20.9 $\pm$ 5.9*	16.053	0.001	0.545	0.786	0.463	0.142	53.785	0.001	0.860
Single leg squat jump height (cm)	20.7 $\pm$ 1.6	20.2 $\pm$ 1.9	20.3 $\pm$ 1.5	21.1 $\pm$ 1.5*	20.7 $\pm$ 2.0	20.8 $\pm$ 1.6	10.832	0.001	0.471	0.729	0.489	0.137	0.346	0.563	0.134

Table 2

Mean and standard deviation of dynamic postural stability index at pre- and post-whole-body-vibration between each condition

							Interaction effect			Main effect			Main effect
$n=$ 20	0 Hz		25 Hz		40 Hz		(condition $\times$ time)			(condition)			(time)
Variable	Pre	Post	Pre	Post	Pre	Post	$F$	$p$	$r$	$F$	$p$	$r$	$F$	$p$	$r$
DPSI	0.265 $\pm$ 0.028	0.263 $\pm$ 0.027	0.265 $\pm$ 0.033	0.250 $\pm$ 0.031*	0.265 $\pm$ 0.042	0.258 $\pm$ 0.042*	3.933	0.028	0.306	0.549	0.582	0.119	27.916	0.001	0.771
APSI	0.128 $\pm$ 0.011	0.127 $\pm$ 0.010	0.129 $\pm$ 0.010	0.124 $\pm$ 0.009*	0.130 $\pm$ 0.011	0.124 $\pm$ 0.012*	6.227	0.005	0.375	0.221	0.755	0.074	23.651	0.001	0.745
MLSI	0.027 $\pm$ 0.006	0.027 $\pm$ 0.007	0.027 $\pm$ 0.006	0.025 $\pm$ 0.005	0.026 $\pm$ 0.005	0.024 $\pm$ 0.006	1.954	0.167	0.221	0.396	0.676	0.102	15.200	0.001	0.667
VSI	0.230 $\pm$ 0.029	0.228 $\pm$ 0.027	0.230 $\pm$ 0.035	0.215 $\pm$ 0.034*	0.229 $\pm$ 0.045	0.224 $\pm$ 0.044	3300	0.048	0.283	0.480	0.622	0.112	17.816	0.001	0.696
vGRF (%BW)	2.668 $\pm$ 0.596	2.717 $\pm$ 0.510	2.717 $\pm$ 0.510	2.711 $\pm$ 0.517	2.739 $\pm$ 0.621	2.749 $\pm$ 0.529	0.425	0.657	0.105	0.250	0.780	0.081	0.008	0.929	0.021

4. Discussion

The results of the present study partly support our hypothesis, specifically for the ankle dorsiflexion angle and SLSJ height. This indicates that the effect of WBV on flexibility and physical performance differs by vibration frequency. More specifically, two crucial discoveries were reported in this study. First, vibration frequencies of 25 and 40 Hz increased ankle dorsiflexion angle in the same way, compared with control condition. Second, WBV at 25 Hz improved single leg squat jump height, and dynamic postural stability improved at both vibration frequency.

The increase in ankle dorsiflexion angle by 11.0% at 25 Hz and by 11.4% at 40 Hz were congruent with previous studies that revealed increases of 3.8 and 13.5% [4, 24], respectively. On the other hand, the improvement at 40 Hz contradicted the findings of Cardinale and Lim [12] who have reported a decrease of 3.3% after WBV at 40 Hz. However, many previous studies showed improvement in flexibility following WBV [14, 25]. This improvement might be attributed to two known mechanisms: influence of pre-synaptic inhibition through the Ia afferent fiber [26] and influence of the excitatory restraint of the spinal cord exercise cell via the activity of the Ib centripetal tendency fiber from a Golgi tendon organ [27, 28]. These mechanisms may have plausibly caused the increase in flexibility after vibration stimulation in the present study.

The increases in SLSJ by 4.2% at 25 Hz was similar to than in previous studies, which revealed improvements of 5.0% at 26 Hz [29]. Jump height improvement may be related to an increase in muscle temperature [30] and blood flow [31]. During vibratory stimuli skeletal muscles undergo small changes in muscle length called tonic vibration reflex (TVR) [32]. TVR is reflex muscle contraction, through the excitation of muscle spindles which can lead to an enhancement of Ia loop activity [33]. Additionally, transmission of vibration of the ankle, knee, and hip joint differs by vibration frequencies, as does muscle activity during WBV exercise [6, 34]. Such a factor might influence improvement of jump height, but the details are unclear in the present study.

DPSI decreased 5.4% at 25 Hz and 2.7% at 40 Hz after vibration stimulation. A previous study indicated that improvement in the range of motion for ankle, knee, and hip joints is crucial for improvement in DPSI [21]. McKinley and Pedotti indicated that participants with greater and earlier preparatory muscle activity demonstrated better-quality dynamic postural stability scores [35]. Additionally, Borges et al. found improvement of muscle response time regardless of the influence of the vibration frequency after vibration stimulation [15]. Therefore, we speculated that these factors improved DPSI at 25 and 40 Hz. On the other hand, no significant difference was shown in DPSI between 25 and 40 Hz condition, but major differences in rates of decline were shown in DPSI. Interestingly, VSI was significant decreased only at 25 Hz although VGRFmax demonstrated no significant difference. These results show that VGRF during landing decreases after having carried out WBV at 25 Hz.

We hypothesized that improvement in jump performance and postural stability after vibration stimulation varied according to the vibration frequency of 25 Hz or 40 Hz and that improvement of ankle dorsiflexion angle and earlier preparatory muscle activity also varied as a result. Furthermore, our results showed that the effect on exercise performance differed with vibration frequency. In other words, it might be necessary to change vibration frequency depending on the situation, e.g. a warm-up and cool-down.

A limitation of this study was that measurements were not available for all effects across 25 Hz and 40 Hz vibration range. In addition, the results of our study are valid only at the acute phase; long-term training effects of different vibration frequencies are yet to be explores. Further, it is necessary to recall that all the subjects were men, and that jump performance following vibration training differs between women and men [36].

Nevertheless, the findings indicate the importance of the appropriate choice of WBV frequency muscle training.

5. Conclusions

This is the first study to examine the effect of acute WBV exposure at different frequencies on ankle dorsiflexion angle, single leg squat jump height, and dynamic postural stability in the same group. Our suggest that the effect of WBV on flexibility and physical performance differed by vibration frequency. This finding may be of practical importance for the development of training programs for athletes, older people, and rehabilitation patients in general.

Footnotes

Conflict of interest

None to report.

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The effects of two different whole-body-vibration frequencies on ankle dorsiflexion angle,vertical jump height,and postural stability after landing

Abstract

BACKGROUND:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Methods

2.1 Participants

2.3 Measurements items

3. Results

3.1 Ankle dorsiflexion angle

3.2 Single leg squat jump (SLSJ) height

3.3 DPSI, APSI, MLSI, VSI, and VGRFmax (%BW)

Table 1 The value of ankle dorsiflexion angle and single leg squat jump height at pre- and post-whole-body-vibration between each condition

5. Conclusions

Footnotes

Conflict of interest

References

Table 1
The value of ankle dorsiflexion angle and single leg squat jump height at pre- and post-whole-body-vibration between each condition