Abstract

The aim of this study was to evaluate the acute

responses of blood hormone concentrations and neuro-

muscular performance following whole-body vibration

(WBV) treatment. Fourteen male subjects [mean (SD) age

25 (4.6) years] were exposed to vertical sinusoidal WBV,

10 times for 60 s, with 60 s rest between the vibration sets

(a rest period lasting 6 min was allowed after 5 vibration

sets). Neuromuscular performance tests consisting of

counter-movement jumps and maximal dynamic leg

presses on a slide machine, performed with an extra load

of 160% of the subjects body mass, and with both legs

were administered before and immediately after the WBV

treatment. The average velocity, acceleration, average

force, and power were calculated and the root mean

square electromyogram (EMGrms) were recorded from

the vastus lateralis and rectus femoris muscles simulta-

neously during the leg-press measurement. Blood samples

were also collected, and plasma concentrations of tes-

tosterone (T), growth hormone (GH) and cortisol (C)

were measured. The results showed a signi(r)cant increase

in the plasma concentration of T and GH, whereas C

levels decreased. An increase in the mechanical power

output of the leg extensor muscles was observed together

with a reduction in EMGrms activity. Neuromuscular

e…iency improved, as indicated by the decrease in the

ratio between EMGrms and power. Jumping perfor-

mance, which was measured using the counter-movement

jump test, was also enhanced. Thus, it can be argued that

the biological mechanism produced by vibration is similar

to the e�ect produced by explosive power training

(jumping and bouncing). The enhancement of explosive

power could have been induced by an increase in the

synchronisation activity of the motor units, and/or im-

proved co-ordination of the synergistic muscles and in-

creased inhibition of the antagonists. These results sug-

gest that WBV treatment leads to acute responses of

hormonal pro(r)le and neuromuscular performance. It is

therefore likely that the e�ect of WBV treatment elicited a

biological adaptation that is connected to a neural

potentiation e�ect, similar to those reported to occur

following resistance and explosive power training. In

conclusion, it is suggested that WBV inｯuences prop-

rioceptive feedback mechanisms and speci(r)c neural

components, leading to an improvement of neuromus-

cular performance. Moreover, since the hormonal

responses, characterised by an increase in T and GH

concentration and a decrease in C concentration, and the

increase in neuromuscular e�ectiveness were simulta-

neous but independent, it is speculated that the two

phenomena might have common underlying mechanisms.

Key words EMG ﾃ｡ Growth hormone ﾃ｡ Jumping

performance ﾃ｡ Testosterone ﾃ｡ Whole-body vibrations

Introduction

Recent studies have documented the e�ect of vibration

on the neuromuscular apparatus. Acute treatment with

whole-body vibration (WBV) has been shown to in-

crease leg muscle force (F ) and power ( 標 ), and

movement velocity. After 10 min of vibration treat-

ment the velocity/F and 標 /F curves were shifted to the

right (Bosco et al. 1999a). In 12 well-trained boxers,

treated with 5 repetitions of 1-min vibration that was

applied while their arms were kept in a semi-ｯexed

position, an increase in the mechanical 標 of the arm

was observed. The root mean square of the associated

electromyogram (EMGrms) did not change following

the vibration treatment, but the ratio of EMG/ 標

decreased, showing an enhancement of neural e�-

ciency (Bosco et al. 1999b). Apart from these acute

e�ects, vibration may induce chronic adaptation

changes in the mechanical behaviour of human skeletal

muscles: a daily series of (r)ve vertical sinusoidal vi-

brations lasting 90 s each and imposed for a period of

10 days caused pronounced improvement of jumping

performance (Bosco et al. 1998). These results suggest

that vibration elicits short-term and long-term neuro-

genic adaptation. In accordance with this, previous

studies have demonstrated a facilitation of the excit-

ability of the patellar tendon reｯex by vibration ap-

plied to quadriceps muscle (Burke et al. 1996),

vibration-induced drive of a-motoneurones via the Ia

loop (Rothmuller and Cafarelli 1995), and activation

of the muscle spindle receptors (Kasai et al. 1992).

However, muscle tissue can also be a�ected by vibra-

tion (Necking et al. 1992). In rats, a vibration-induced

enlargement of slow- and fast-twitch (r)bres has been

demonstrated (Necking et al. 1996).

A question arises as to whether vibration e�ects

include adaptive changes and changes in endocrine

functions. It has been shown that short-term intensive

exercises such as 60-s consecutive jumps (Bosco et al.

1996a), anaerobic cycle exercises (Adlercreutz et al.

1976; Naﾃ�veri et al. 1985; Buono et al. 1986; Farrell

et al. 1987; Brooks et al. 1988; Kraemer et al. 1989;

Schwarz and Kindermann 1990) and weight lifting

(Kraemer et al. 1990; Schwab et al. 1993) evoke rapid

hormonal responses. At the same time, certain rela-

tionships seem to exist between plasma concentrations

of hormones and short-term performance: athletes

with better explosive strength and sprint-running per-

formances have a higher basal concentration of tes-

tosterone (T, Kraemer et al. 1995; Bosco et al. 1996b).

It has been demonstrated that exercise-induced hor-

monal responses are signi(r)cant not only for acute

adaptation, but also for triggering long-term training

e�ects (Inoue et al. 1994; Viru 1994; Kraemer et al.

1996). Similarly, the vibration-induced hormonal

changes may be signi(r)cant for chronic improvement

of neuromuscular function in repeated exposure to

vibration.

The aim of the present study was to test the possi-

bility that WBV induces changes in the plasma concen-

tration of hormones that are known to be associated

with the adaptation of muscular activity.

Methods

Subjects. A group of 14 male subjects voluntarily participated in the study. They were physically active and were engaged in a team sport training program three times a week. Each

subject was instructed on the protocol and gave their written

informed consent to participate in the experiment, which was

approved by the ethical committee of the Italian Society of Sport Science.

Subjects with a previous history of fractures or bone injuries were excluded from the study, as were those under the age

of 18 years. The protocol consisted of performing jumping and

mechanical 標 testing together with electromyographic (EMG)

analysis of leg extensor muscles, as well as blood collection for analysis, before and immediately after the 10-min WBV treatment.

Testing procedures

The first blood sample was taken after the measurement of height and the subjects then performed a 10-min warm-up, consisting of 5 min of bicycling at 25 km/h on a cycle ergometer (Newform s.p.a., Ascoli Piceno, Italy), followed by 5 min of static stretching for the quadriceps and triceps surae muscles. Jumping measurements After the warm-up, as well as after the vibration exposure, the subjects performed three trials of a counter-movement jump was recorded on a resistive (capacitative) platform (Bosco et al.

1983) that was connected to a digital time.

To avoid unmeasurable work, horizontal and lateral displacements were mini-mised, and the hands were kept on the hips throughout the test. During CMJ the angular displacement of the knee was standardised so that the subjects were required to bend their knee to approximately 90ｰ.

measurements recorded were averaged and used for statistical analysis, as recommended by Tornvall (1963) and Bosco et al. (1995). During the test, the vertical displacement of the load was monitored with a sensor (encoder) machine

System, Ergotest Technology, Langensund, Norway) that was interfaced to a PC. When the loads were moved by the subjects, a signal was transmitted by the sensor at every 3 mm of displacement. Thus, it was possible to calculate several parameters, such as av-

erage velocity, acceleration, average F, and average power ( 標 ),

corresponding to the load displacements (for details see Bosco et al.

1995). However, since it has been shown that 標 is the most sensitive parameter among all of the mechanical variables studied, it was the

only one considered for statistical analysis (Bosco et al. 1995).

EMG analysis

The signals from the vastus lateralis and rectus femoris muscles of one leg were recorded during the leg-press measurements, with bipolar surface electrodes (inter-electrode distance 1.2 cm) that

were (r)xed longitudinally over the muscle belly. An ampli(r)er (gainｴ600, input impedance 2 GW, common-mode rejection ratio

100 dB, band-pass (r)lter 6ｱ1500 Hz; Biochip Grenoble, France) was used. The MuscleLab encoder converted the ampli(r)ed 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 ms, total error �0.5%). The EMGrms is

expressed as function of the time (mV or lV). Since the EMGrms signals were used in association with the biomechanical parameters

measured with MuscleLab, they were sampled simultaneously at 100 Hz. The subjects wore a skin suit to prevent the cables from swinging and from causing movement artifacts. A personal com-

puter (PC 486 DX-33 MHz ) was used to collect and store the data. The values obtained for both the vastus lateralis and rectus femoris muscles were averaged for statistical analysis, as suggested by Bosco and Viitasalo (1982) and Viitasalo and Bosco (1982).

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