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Abstract
The aim of this study was to investigate the effects of whole body vibrations on the mechanical behaviour of human skeletal muscles. For this purpose, fourteen physically active subjects were recruited and randomly assigned to an experimental (EG) and a control group (CG). The EG was treated fur ten days with 5 sets of vertical sinusoidal vibrations lasting up to two minutes each, for a total volume of ten minutes per day. The subjects of CG were asked to maintain their normal activity and avoid strength or jumping training. Subjects were tested at the beginning and at the end of the treatment with specific jumping tests performed on
a resistive platform. Results showed remarkable and statistically significant enhancement in the EG of the height of the best jump, the mechanical power of the best jump and the average jumping height during. In contrast, no statistically significant variations were noted in the CG. Consequently, it was suggested that the effect of
WBV treatment elicit fast biological adaptation connected to neural potentiation.
Introduction
The adaptation to the training stimulus is related to the modification induced by the repetition of the daily exercise, which are specific for the movement executed
[12]. Strength training response has been shown to be mediated by both
neurogenic and myogenic factors
[22]. The first phase of adaptation is
characterised by an improvement of neural factors, while the myogenic factors becomes more important as the adaptations continues over several months (e.g.
[20]. Enhancement of explosive power performance (e.g. jumping abilities)
and the
corresponding biological adaptations to a specific training stimulus are still not
understood. Gravity normally provides the major portion of the mechanical stimulus responsible for the development of the muscle structure during everyday life and during training. It should be remind, that strength and explosive power training specific programs are based on exercises performed with rapid and violent variation of the gravitational acceleration In this connection, simulation of hypergravity (wearing vests with extra loads) conditions has been utilised for enhancement of human explosive muscle power [5,6], On the other hand, changes of the gravitational conditions can be produced also by mechanical vibrations applied to the whole body. Thus, in light of the above observations, it was assumed that application of whole body vibration to physical active subjects could influence the mechanical behaviour of the leg extensor muscles
Methods
Fourteen subjects voluntarily participated to the study, they were physically active and were engaged in team sport training program 3 times a week. The subjects were not engaged in strength and explosive power training but participated regularly for tactical and technical training program according to the discipline practised (handball and water polo). They were equally divided into two groups:
an experimental group (EG) and a control group (CG). Each subject was instructed on the protocol and signed an informed consent, approved by the ethical committee of the Italian Society of Sport Science, to participate to the experiment. Subjects with previous history of fractures or bone injuries were excluded from the study together with the ones under the adult age. Table 1 presents physical characteristics of the subjects.
Procedures
Anthropometric measures (height and weight) were recorded together with the age of the subjects. Following this phase a ten minutes warm up was performed consisting of 5 minutes of bicycling at 25 kmh-1 on a cycle ergometer (Newform s.p.a., Ascoli Piceno, Italy) and five minutes of static stretching for the quadriceps and triceps surae muscles. After the warm up, the subjects peformed the followings jumping exercises: counter movement jump (CMJ) and 5s of continuous jumping (5s CJ).The flight time (tf) and contact time of each single jump were recorded on a resistive (capacitative) platform [4] connected to a digital timer (accuracy 0.001s) (Ergojump, Psion XP, MA.GI.CA.Rome, Italy). To avoid unmeasurable work, horizontal and lateral displacements were minimised, and the hands were kept on the hips through the gravity above the ground (h in meters) in were measured from flight time in seconds) applying ballistic laws: where g is the acceleration of gravity (9.81 m
During CJ exercises the subject were required to perform the maximal jumping effort minimising knee angular
displacement during contact. From the recordings of
and the average mechanical power (AP), average rise of center of gravity (AH) were calculated for the total 5s continues jumping. From 5s CJ the best jumping performance was selected and maximal mechanical power (PBJ) as well as the highest rise of center of gravity (HBJ) were obtained using the equation introduced by Bosco et al [4]
The frequency of the vibrations used in this study was set at 26 Hz (displacement = 1 Omm; acceleration = 27 m
The subjects were exposed five times for a duration of 90s with 40s of rest between the treatment each. This procedure was repeated for ten days, each day five seconds were added for each treatment up to a total of 2 minutes per position. Following the ten days the subjects of both groups were again tested and data were statistically analysed.
Type of treatment employed: The first applicaticm was performed in the standing position with the toes on the vibrations platform. The second bout was performed
with the subject in the half squat position. The third application was realised with the feet rotated externally on the vibration platform. The knee angle was pre-set at
flexion. The fourth treatment was performed with the subjects standing on the leg on the right side of the vibration platform with the knee at flexion. Finally the fifth application was given while the subjects standing on another leg on the left side of the vibration platform with the knee at flexion. During the 4th and 5th treatment subjects were allowed to keep themselves in balance with the aid of a bar mounted on the platform. During all the treatments the subjects wear gymnastic-type shoes to avoid bruises. The E group was treated with WBV for ten days, the C group was not treated during the project and was asked to maintain their typical activities. Testing procedures were administered at the beginning and at the end of the experiments for both E and C groups.
RESULTS
After almost two weeks of regular technical and tactical training program, the subjects of the C group, as expected, failed to showed changes in any of the mechanical or anthropometric parameters studied. The jumping height in
CMJ remained the same in E group after 10 days of W B V (Table 2). This treatment, in contrast, produced remarkable and statistical significant enhancement of the HBJ and the PBJ. In addition, the average height during 5s CJ was also improved in E group, demonstrating a statistical
significant difference of. On the other hand, the average power developed during 5s CJ failed to demonstrate statistically significant change after the treatment.
Discussion
Less than two weeks of regular tactical and technical training programme, as expected, did not induce any modification in the mechanical properties and anthropometric profile of the control subjects This is not a surprising findings,
since no changes, in jumping performances, was noted after four weeks either in
physical active subjects [ 14], or in volleyball players [2]. In contrast, a remarkable improvement of the neuromuscular characteristics studied was observed after the WBV period in the E subjects. Significant enhancement was noted for the H
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