Summary

The aim of this study was to investigate the effects of whole-body vibrations (WBV) on the mechanical behaviour of human skeletal muscle. For this purpose, six female volleyball players at national level were recruited voluntarily. They were tested with maximal dynamic leg press exercise on a slide machine with extra loads of 70, 90, 110 and 130 kg. After the

testing, one leg was randomly assigned to the control treatment (C) and the other to the experimental treatment (E) consisting of vibrations. The subjects were then retested at the end of the treatment using the leg press. Results showed remarkable and statis-tically signi(r)cant enhancement of the experimental

treatment in average velocity (AV), average force (AF) and average power (AP) (P<0á05±0á005). Consequently, the velocity-force and power-force relationship shifted to the right after the treatment. In conclusion, it was af(r)rmed that the enhancement could be caused by neural factors, as athletes were well accustomed to the leg press exercise and the learning effect was minimized. Keywords: force/velocity, muscle mechanics, vibration.

Introduction

Skeletal muscle is a specialized tissue that modi(r)es its overall function capacity in response to chronic exercise with high loads (e.g. McDonagh & Davies, 1984). Intensive prolonged strength training is known to induce a speci(r)c neuromuscular (e.g. Sale, 1988) and hormonal (e.g. Guezennec et al., 1986)

adaptive response in the human body in a few months, whereas changes in morphological structure occur later (e.g. Sale, 1988). However, the exact mechanism that regulates how the body adapts to the speci(r)c demands upon it is still unknown. Even less knowledge is available in respect of fatigue,

relative strength loss and hormonal changes during one acute session of exercises (e.g. Hakkinen & Pakarinen, 1995; Bosco et al., 1998). It should be remembered that speci(r)c programmes for strength and explosive power training are based on exercises performed with rapid and violent variation of the gravitational acceleration (Bosco, 1992). In this connection, it should also be borne in mind that changes in gravitational conditions can be produced by mechanical vibrations applied to the whole body. Whole-body vibration applied for 10 min during a 10-day treatment period induced an enhancement in explosive power performances in physically active subjects (Bosco et al., 1998). After the latter experiment, the present study was conducted to observe how human skeletal muscle responded to a single session of 10-min application of whole-body vibration in well-trained athletes.

Methods

Six female volleyball players of national level participated in the study voluntarily. They were physically active and were engaged in a team sport training programme (r)ve times a week. Each subject was instructed on the protocol and gave informed consent to participate in the experiment. Subjects

with a previous history of fractures or bone injuries were excluded from the study. The study design was approved by the ethical committee of the Italian Society of Sport Science.

Ten minutes warm up was performed consisting of 5 min of bicycling at 25 kmh on a cycle ergometer (Newform, Ascoli Piceno, Italy) and 5 min of static stretching for the quadriceps and triceps surae muscles. After the warm up, all the subjects, well accustomed to the exercises, performed maximal dynamic leg press exercises on a slide machine (Newform) with extra loads of 70, 90, 110 and 130 kg. One leg per time was used for each load. The best trial of three measurements for each load was used for statistical analysis. During the test, the vertical displacements of the loads were monitored with a simple mechanics and sensor arrangement (Ergopower; Ergotest Technology, Langensund, Norway). The loads were linked mechanically to an encoder interfaced to an electronic microprocessor (Muscle Lab, patent no. 1241671). When the loads were moved by the subjects, a signal was transmitted

by the sensor for every 3 mm of displacement. Thus, it was possible to calculate average velocity (AV), acceleration, average force (AF) and average power (AP) corresponding to the load displacements (for details, see Bosco et al., 1995).

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