Science & Research

Abstract

The aim of this study was to analyze electromyography

(EMG) responses of vastus lateralis muscle to different

whole-body vibration frequencies. For this purpose, 16 pro-

fessional women volleyball players voluntarily participated in the study. Vibration treatment was administered while standing on a vibrating platform with knees bent

at 100 (Nemes Bosco-system, Rome, Italy). EMG root mean

square (rms) and was recorded for 60 seconds while stand-

ing on the vibrating plate in the following conditions: no

vibrations and 30-, 40-, and 50-Hz vibration frequencies in

random order. The position was kept for 60 seconds in each

treatment condition. EMGrms was collected from the vastus

lateralis muscle of the dominant leg. Statistical analysis

showed that, in all vibration conditions, average EMGrms

activity of vastus lateralis was higher than in the no-vibration

condition. The highest EMGrms was found at 30 Hz, sug-

gesting this frequency as the one eliciting the highest reflex

response in vastus lateralis muscle during whole-body vi-

brations in half-squat position. An extension of these studies

to a larger population appears worthwhile to further eluci-

date the responsiveness of the neuromuscular system to

whole-body vibrations administered through vibrating plat-

forms and to be able to develop individual treatment pro-

tocols.

Introduction

Mechanical vibrations applied to the muscle belly

or tendons have been shown able to elicit reflex

muscle contractions (12). This neuromuscular response

has been named ''tonic vibration reflex'' (TVR) and

has been shown to be mediated by mono- and poly-

synaptic pathways (8, 19). Muscle spindle Ia reflexes

have been indicated as the major determinant of this

vibration-induced neuromuscular activation (5) lead-

ing to the TVR. Recent observations have shown the

possibility of utilizing vibrations as a training tool in

athletic settings. In fact, neuromuscular performance

has been enhanced through the administration of vi-

bration treatment (2). These improvements have been

attributed to an enhancement of neural factors deter-

mining neuromuscular performance: recruitment, syn-

chronization, inter- and intramuscular coordination

and also proprioceptors' responses. In this connection,

it should be noticed that vibrations have been shown

to be effective in inducing improvements in vertical

jumping ability (3) and in mechanical power of lower

limbs in elite athletes (4). Moreover, studies conducted

by Issurin et al. (13, 14) have shown increases in ex-

plosive strength and flexibility in athletes. Vibrations

applied to the arm showed enhancement of mechani-

cal power and an increase in neuromuscular efficiency

as indicated by a decrease in EMG/power ratio sup-

porting the evidence that vibrations represent a strong

stimulus for the neuromuscular system (2). Further-

more, EMG activity during vibrations has been shown

to reach values higher than 200% of the baseline in

arm flexor muscles (2). Previous studies have found

vibrations determining an increased EMG activity in

the muscles undergoing the vibration treatment (15, 7).

Vibrations are starting to be used as an alternative

training means for enhancing strength/power char-

acteristics. However, it should be pointed out that

there is a lack of information on the effectiveness of

different vibration frequencies on neuromuscular per-

formance. Moreover, it should also be considered that

currently there is no methodology able to identify the

individual vibration load that an individual can sus-

tain. Muscle activation during vibration can be moni-

tored recording the EMG signal of the target muscles.

With this tool, it is in fact possible to determine muscle

activity in a given task. In light of the above, it is pos-

sible to affirm that EMG can be used to provide an

indication of the muscle activity determined by vibra-

tion. In fact, the EMG signal can be used to measure

the severity of muscle activation following the appli-

cation of vibration. The aim of this study was to ana-

lyze EMG responses in the vastus lateralis muscle

while standing on a vibrating plate producing oscil-

lations of different frequencies to verify the hypothesis

that different vibration frequencies determine different

neuromuscular responses.

Method

Subjects: Sixteen professional women volleyball players volunteered as subjects for the present study. All of the subjects had competed for several years at a high level and were regularly training at the time of the experiment. Full advice was given to the volunteers regarding the possible risk and discomfort that might be involved, and all the subjects gave theirwritten informed consent, approved by the ethical committee of the Italian Society of Sport Science, to

participate in the experiment. Subjects with a previous

history of fractures or bone injuries were excluded

from the study. EMG Analysis The signals from the vastus lateralis of the dominant leg were recorded with bipolar surface electrodes (interelectrode distance, 1.2 cm) including an amplifier

(gain, 600; input impedance, 2 G ; CMMR, 100 dB;

band-pass filter, 6-1500 Hz; Biochip, Grenoble, France)

fixed longitudinally over the muscle belly. The

MuscleLab converted the amplified EMG raw signal

to an average root-mean-square (rms) signal via its

built-in hardware circuit network (frequency response,

450 kHz; averaging constant, 100 milliseconds; total

error, 0.5%). The EMGrms was expressed as a func-

tion of the time (millivolts or microvolts). EMG cables

were secured with an appropriate setup to prevent the

cables from swinging and from causing movement ar-

tifact. A personal computer (PC Celeron 400) and the

MuscleLab software were used to collect and store the

data. The EMGrms was collected during each repeti-

tion, lasting 1 minute each, of isometric half squat. In

a previous study, the reliability of the EMG measure-

ments was shown to be 0.91 (2).

Treatment Procedures

Subjects were exposed to a vibration treatment (VT)

using a vibration platform called Nemes Bosco-system

(OMP, Rieti, Italy). The amplitude allowed by the vi-

bration platform was (peak-to-peak) 10 mm. The sub-

jects were exposed randomly to three different VTs.

The frequencies used in the experiment were 30, 40,

and 50 Hz. Each vibration treatment lasted 60 seconds,

with 60 seconds of rest allowed between each VT fre-

quency. The subjects were asked to stand in half-squat

position on the vibration platform (knee angle 100 ) as

indicated in previous studies (3). The EMG signal was

collected from the vastus lateralis muscle during the

60-second duration of the testing position. A total of

4 sets lasting 60 seconds each were performed with 60

seconds of rest between sets allowed. EMGs were re-

corded for 5 seconds before starting the vibration

treatment to verify the absence of residual muscle ac-

tivity. If the EMG was different from the baseline mea-

surement, a further 30 seconds of rest were allowed.

The subjects then performed the isometric half squat

in the following conditions: no vibrations, and ran-

domly 30-, 40-, and 50-Hz vibration frequency.

Statistical Analyses

Ordinary statistical methods were employed, includ-

ing means (X) and standard deviation ( SD). Average

EMGrms values for vastus lateralis were considered

for analysis. To analyze differences in EMG between

vibration frequencies, a repeated measures ANOVA

was computed to identify significant differences for

the dependent variables. Significant F values were fol-

lowed by paired t-tests for within- and between-group

comparisons. Significance was set at p

0.05.

Results

Whole-body vibration treatment lead to an increase of

EMGrms activity of vastus lateralis muscle as com-

pared with baseline values (p0.001) collected in the

no-vibration condition (see Figure 1). The highest

EMGrms activity was found at 30-Hz vibration fre-

quency ( 34%, p0.001). Between-treatment com-

parisons showed statistically significant differences be-

tween 30- and 50-Hz frequencies (20%, p0.05) and

40- and 50-Hz frequencies (10%, p0.05). EMGrms

activity between 30- and 40-Hz vibration frequencies

did not show any statistically significant difference

(9%, n.s.).

EMG Activity of Vastus Lateralis Muscle 623

Discussion

The results of the present study demonstrated an in-

creased EMG activity in vastus lateralis muscle at var-

ious vibration frequencies when subjects were stand-

ing on a vibration platform. An increase in EMG has

been observed in quadriceps muscle undergoing vi-

brations and it has been attributed to a facilitation of

the excitability of spinal reflex (7). Based on these find-

ings, it was possible to verify that whole-body vibra-

tions transmitted through a vibrating platform in half-

squat position were able to determine a higher EMG

activity compared with the nonvibrating condition.

This effect has been related to excitation of primary

endings of muscle spindles and activation of -moto-

neurons as indicated elsewhere (i.e., 12, 16).

In our opinion, two factors could account for the

observed increase in EMG activity: i) the initial length

of analyzed muscles and ii) the frequency of vibratory

stimulation. In fact, it is already known that vibratory

stimulation is more effective in stretched muscles (21,

9). Vibration sensitivity of human muscle spindles has

also been demonstrated in single human spindle af-

ferent by Burke and Gandevia (5). These observations

support the utilization of the half-squat position on the

vibrating platform as an effective position for trigger-

ing vastus lateralis stimulation. Escamilla et al. (10)

reported that the two vasti muscles produce 40-50%

more activity than the rectus femuris during half

squat. Moreover, compared with each other, the vastus

medialis and vastus lateralis produce approximately

the same amount of activity (10, 11, 24). The position

chosen in the experiment could then be considered op-

timal for stimulating the vastus lateralis muscle be-

cause of the lengthened position and the activation rel-

ative to the quadricep muscles, as suggested elsewhere

(1).

Vibrations-induced increases in EMG activity and

the consequent degree of motor unit synchronization

have been shown to be dependent on the vibration fre-

quency (6, 17, 19, 23). This was not observed in our

experiment because the highest EMGrms activity was

found when the frequency was 30 Hz. However, pre-

vious observations at constant displacement amplitude

have shown that monosynaptic inhibition does not

vary with vibration frequency (18). The missing EMG

augmentation with vibration frequency may be due to

inhibitory mechanisms mediated by mechanoceptors

and skin receptors, which have been shown to be ac-

tivated during whole-body vibrations and contribute

to the EMG activity (22). The results of our study sug-

gest that, in the specific position used, the frequency

able to cause the highest EMG response in vastus la-

teralis muscle was 30 Hz. In our opinion, vibrations

are strong perturbations that are perceived by the cen-

tral nervous system, which modulates the stiffness of

the stimulated muscle groups. The reflex muscle activ-

ity could then be considered a neuromuscular tuning

response to minimize soft-tissue vibrations. These re-

sponses are individual and probably could be popu-

lation-specific and could be based on mechanical and

reflex factors. Natural frequencies of muscle groups in

athletes' legs have been reported to be between 5 and

65 Hz (20); the input frequencies used in our study are

in this range and it suggests that individual responses

could be related to individual capabilities in damping

external perturbations to avoid resonance effects.

These adaptations have been hypothesized during

running in humans (20), and our opinion is that the

same principles apply to vibrations superimposed

with vibrating plates.

In conclusion, this study indicates, first, that stand-

ing on a vibrating platform in half-squat position elic-

its higher EMG responses in vastus lateralis muscle as

compared with the same position without vibrations

being transmitted. Second, EMG recordings could rep-

resent a means for individualizing training protocols

for whole-body vibrations. Further studies are needed

for elucidating the mechanisms determining individ-

ual responses and the effectiveness of individualized

treatments on neuromuscular performance.

Practical Applications

Vibrations exercise has been shown to be effective in

enhancing strength/power capabilities (i.e., 2-4, 13,

14). However, it should be pointed out that there is no

current knowledge about effective exercise protocols

or measurements to which to refer when prescribing

a vibration exercise program. One way for individu-

alizing vibration treatments could be the use of surface

EMG to assess muscle responsiveness to different vi-

bration frequencies. In fact, the results of this study

support the idea that vibrations elicit higher EMG ac-

tivity compared with isometric conditions. Also, dif-

ferent vibration frequencies elicit different EMG re-

sponses in the stimulated muscles.

This new technique could be used separately or in

combination with conventional strength-training rou-

tines. In particular, due to the effects of vibration on

stretch reflexes, it can be suggested for use in a com-

plex training-type routine in place of plyometric drills.

Vibration in fact stimulates reflex muscle responses

due to the small and quick changes in length of the

muscle-tendon complex and the small and fast joint

rotations associated with the oscillatory motion. Those

stimuli are similar to the ones experienced during ply-

ometric exercise and place less stress on joints due to

the reduced impact load. Future studies are needed in

order to develop safe exercise procedures on vibrating

platforms and in order to understand the effectiveness

of different vibration exercise protocols.

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