ABSTRACT
 

The study was performed in order to test the possibility wheather a single whole body vibration (WBV) session will produce human skeletal muscle response. In 6 female volleyball players movement velocity, muscle force and power were recorded when they performed maximal leg press exercises with extra load of 70,90,110 and 130 kg. The testing took place before and after a 10-min WBV exposure. During WBV subjects were in the standing position with the toes of one leg on the vibration platform (E leg) while the other leg (C leg) was risen from the ground. WBV induced statistically (P
 

INTRODUCTION
 

Skeletal muscle is a specialised tissue which modifies its overall function capacity in response to chronic exercise with high loads (e.g. McDonagh and Davies 1984). Intensive prolonged strength training is known to induce a specific neuromuscular (e.g. Sale, 1988) and hormonal (e.g. Guezennec et al., 1986) adaptive responses in the human body in few months, while the changes in the morphological structure occur later (e.g. Sale,1988). However, the exact mechanism which regulate how the body adapts to the specific demands upon it, is still unknown. Even less knowledge are available in respect to fatigue, relative strength loss and hormonal changes during one acute session of exercises (e.g. Hakkinen & Pakarinen 1995, Bosco et al. 1998, inpress). It should be remind, that specific programs 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 be remind that changes of the gravitational conditions can be produced also by mechanical vibrations applied to the whole body. Whole body vibration applied for ten minutes during 10 days treatment period have induced an enhancement of explosive power performances in physical active subjects (Bosco et al. 1998, submitted for publication). A question arises from these results: how human skeletal muscle response to a single session of 10 minutes application of whole body vibration in well trained athletes? The present study was performed in order to answer of the question.
 

METHODS
 

Six female volleyball players of national level (age : 19.5 ± 2.1 years ; weight : 65.1 ± 3.7 kg ; height : 174.9 ± 3.2) voluntarily participated to the study. They were physically active and were engaged in team sport training program 5 times a week _ Each subject was instructed on the protocol and signed an informed consent to participate in the experiment. Subjects with 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.
 

Procedures:
 

Ten minutes warm up was performed:5 minutes of bicycling at 25 km-h-’ 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, all the subjects, well accustomed with the exercises, performed maximal dynamic leg press exercises on a slide machine (Newform s.p. a., Ascoli Piceno, Italy) with extra loads of 70,90,110 and 130 kg . One leg per time was used for each load. The best trail of three measurements for each load was used for statistical analysis. During the test, the vertical displacements of the loads were monitored with simple mechanics and sensor arrangement (Ergopower ®,Ergotest Technology A. S _, Langensund, Norway). The loads were mechanically linked to an encoder interfaced to an electronic microprocessor (Muscle Lab, Pat. No. 124 1671). When the loads were moved by the subjects a signal was transmitted by the sensor every 3mm 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).
 

Reproducibility of measurements
 

The dynamic exercises reproducibility testing gave a test-retest correlation r = 0.45 for the average power (P) (Bosco et al., 1995).
 

Treatment Procedures
 

Subjects were exposed to vertical sinusoidal whole body vibration (WBV) using the device called GALILEO 2000 ( Novotec, Pforzheim, Germany). The frequency of the vibrations used in this study was set at 26 Hz (displacement = 1Omm ; acceleration = 27 m l s^-2). The subjects were exposed ten times for a duration of 60s with 60s of rest between the treatment each.
 

Type of treatment employed
 

The application was performed in the standing position with the toes of one leg on the vibration platform, the knee angle was pre-set at 100” flexion, while the other was risen from the ground. During all the treatments the subjects were asked to wear gymnastic-type shoes to avoid bruises. The leg which was exposed to vibration was assigned to E group , while the other not exposed was assigned to C group. Thus , in each subject one leg was exposed to vibration (E) and the other was considered as control (C). The leg randomly assigned to each E or c groups demonstrated similar mechanical behaviour exposure (Table 1). Testing procedures were administered at the beginning (Pre) and immediately after (Post) the VT period.
 

Statistical Methods
 

Conventional statistical methods used included mean, standard before the vibration (VT) deviation , paired and unpaired Student’s t-test. The, level of significance was set at P <.05
 

RESULTS
 

Before the VT period, no significant differences was found in the mechanical behaviour between E and C legs in parameters studied (AF,AV, and AP) for all loads used (70, 90,110 and 130 kg) (Table 1) . After the VT period the legs affected by vibration (E) showed statistically significant improvement (Pre vs Post) of the AF, AV and AP developed with all loads used (P < 0.05 - 0.005) (Table 1). In result, the velocity-force (V-F) and the power-force (P-F) curves (Fig. 1), established by the variables shown in Table 1, were shifted to the right after the VT period. Only the AF developed with 70 kg remained unchanged after the VT period . In contrast , the mechanical behaviour of the C legs, demonstrated no changes in mechanical variables studied by the Pre - Post test analysis (Table 1). Only the AV developed with 130 kg showed statistically significant improvement (near 3 % ) in the Post evaluation test (P< 0.05).
 

DISCUSSION
 

As expected the Pre vs Post test analysis performed for the C legs did not show any modification in the mechanical properties studied. This is not a surprising finding, since, in half -squat exercises performed with extra load (100 % of subject’s body mass) no change has been observed in twelve female and male throwers during same day (Bosco et al., 1995). However , the AV developed with 130 kg showed statistically significant improvement in the Post evaluation test of C leg (P< 0.05). Reasonable explanation for this improvement cannot be easily found, considering that the athletes of the present experiments were well accustomed with this type of exercises and therefore any learning effect of the movement executed could be excluded. The mechanical behaviour of the E legs demonstrated a dramatic alterations in the V-F and P-F relationships after VT lasting only ten minutes. Changes and shifting to the right of force-velocity (F-V) relationship have been observed after several weeks of heavy resistance training (e.g. Coyle et al., 1981: Hakkinen & Komi, 1985). The improvement of the of the F-V relationship has been attributed to the enhancement of the neuromuscular behaviour caused by the increasing activity of the higher motor center (Milner-Brown et al., 1975). Thus , it is likely that also the VT have caused a dramatic enhancement of the neural traffic regulating the neuromuscular behaviour (Bosco et al., 1998, submitted for publication) .
 

During vibration of the body skeletal muscles undergo small changes in muscle length. Facilitation of the excitability of spinal reflex has been elicited through vibration to quadriceps muscle (Burke et al., 1996). Lebedev and Peliakov (1991) pointed on the possibility that vibration may elicit excitatory flow through short spindle - motoneurons connections. Burke et al. (1976), suggested that vibration reflex operates predominantly or exclusively on alpha motoneurons and does not utilise the same cortically originating efferent pathways as are in the performance of voluntary contractions, However, a facilitation of voluntary movement cannot be excluded. In the present study, any neurogenic potentiation has not been demonstrated since no EMG recordings were performed. Nevertheless, enhancement of the mechanical behaviour strongly suggests that a neurogenic adaptation have occurred in response to the vibration treatments. Therefore, even if the intrinsic mechanism contributed, the adaptive response of neuromuscular functions to VT could not be explained by it. The duration of the stimulus seems to have relevant importance_ Adaptive response of human skeletal muscle to simulated hypergravity conditions (1. 1 g), applied for three weeks, caused a drastic enhancement of the neuromuscular functions of the leg extensor muscles shifting the F-V relationship to the right (Bosco, 1985). In the present experiment, even if the total length of the VT application period was only 10 minutes, the perturbation of the gravitational field was rather consistent (2.7 g). An equivalent length and intensity of training stimulus can be reached only by performing 150 times leg press or half squat exercises with extra loads of 3 body mass twice a week for 5 weeks (Bosco,l992).
 

Recent research has shown that vibration training (VT) may improve muscle performance.11 12 By considering this mechanism, we designed this study to find out if VT before eccentric exercise may prevent DOMS by improving muscular strength and power development strategy,13–17 improving kinesthetic awareness,18 and providing insights into the effects of fatigue, 19 20 within the vibrated muscles.

MATERIALS AND METHODS
 

The study was approved by the ethical committee of Semnan University of Medical Sciences. Fifty healthy non-athletic volunteers (25 females, mean (SD) age 21.1 (0.2) years and 25 males mean (SD) age 20.1 (0.5) years) were assigned randomly into two experimental, VT and non-VT groups. Exclusion criteria included a history of cardiac and neuromuscular diseases, undertaking severe sport activity or having received an intramuscular injection during the last week.
 

A computer generated randomisation list was drawn up by the statistician for each group. It was given to the physiotherapy department in sealed numbered envelopes. When the subjects qualified to enter the study and had signed their informed consent forms, the appropriate numbered envelope was opened at the reception; the card inside indicated the subject’s allocation to one of the VT or non-VT groups. This information was then given to the physiotherapist to administer the appropriate intervention.
 

Intervention
 

Both experimental groups walked downhill on a 10° declined treadmill at a speed of 4 km per hour for 30 min. In the VT group, a 50 Hz vibrator apparatus (model VR-7N, ITO, Tokyo, Japan) was used to apply vibration on the middle line of each of the left and right quadriceps, hamstring and calf muscles for 1 min before downhill treadmill walking, while the subjects in the non-VT group did not receive any vibration before downhill treadmill walking.
 

Measurements
 

Measurements were performed before and 24 h after treadmill walking and included isometric maximum voluntary contraction. The IMVC force of the left and right quadriceps muscles in 100°of knee flexion was measured in the sitting position. The subject was asked to sit on a back-supported quadriceps table and a load cell was connected to the distal end of her/his leg by means of a tight sling. The output of the load cell was connected to a digital monitor so it was possible to record and save the maximum tension on the load cell. After bringing to zero the output of the load cell, the subject was encouraged to perform IMVC by pulling the sling tight as hard as she/he could, three times, with 30-second intervals between each pull. The best attempt was recorded and considered as the quadriceps IMVC force in newtons.
 

Pressure pain threshold (PPT) on the 5, 10 and 15 cm above the left and right patellae and also on the middle line of calf muscles was measured by a 20 ml syringe with a spring inside which was scaled from 0 to 10. The rounded tip of syringe was placed at the above points in a vertical position and the piston was pressed down. The subject was asked to announce any unpleasant sensation (pain), and then the indicating number on the syringe was recorded as the PPT.
 

The level of muscle soreness was evaluated by mean of a Visual Analogue Scale (VAS). The day after treadmill walking, the subject was asked to indicate her/his feel of the level of muscle soreness in each lower limb along a 10 cm line ranging from 0 (‘‘no muscle soreness at all’’) to 10 (‘‘the most severe muscle soreness that I can imagine).
 

The serum level of creatine kinase (CK) was measured 24 hours after treadmill walking by taking 3 cc blood samples from the brachial artery in the front of the elbow and then the level of CK enzyme was measured in the laboratory.
 

Statistics
 

To compare the possible effect of VT on DOMS, an intention to treat analysis was used which involved all subjects who were randomly assigned to their group. Student’s t tests were used to compare the mean changes in the IMVC force, PPT values and the mean of CK level and muscle soreness between the experimental groups.
 

RESULTS
 

Fifty healthy subjects were randomly assigned into two experimental VT (n=25, 12 male and 13 female) and non-VT (n=25, 13 male and 12 female,) groups and the study was then completed. The mean age was 20.6 (1.9) years (mean (SD)) for the VT group, and 20.6 (2.1) years (mean (SD)) for non-VT group, without any significant differences between the two groups.
 

Isometric maximum voluntary contraction force
 

A comparison of the mean change in the IMVC force in the right quadriceps showed a significantly higher decrease (P=0.001) in the non-VT group (-39.6 (46.6) years, mean (SD)) compared with the VT group (37.2 (100.1) years, mean (SD)). This reduction was also seen in the left quadriceps (non- VT group -16.5 (77.6) years vs VT group 56.8 (100.9) years, P=0.006), fig 1.

Pain pressure threshold
 

Table 1 shows the mean changes in PPT at 5, 10 and 15 cm above the right and left patella. Comparison of these values from non-VT and VT groups showed significant reduction of PPT in the non-VT group (P=0.0001). The same significant reduction (P=0.0001) of PPT was seen in the calf muscles of the non-VT group (right -1.1 (1.3) and left -1.3 (1.4)) compared with the VT group (right 0.4 (0.8) and left 0.4 (1.2)).
 

Level of muscle soreness
 

The comparison of mean level of muscle soreness recorded the day after treadmill walking showed higher soreness in the non- VT group (right 2.3 (1.9) and left 2.3 (2.1)) vs VT group (right 0.4 (1.1) and left 0.5 (1.1)). These differences were significant in both lower limbs (P=0.0001), fig 2.
 

Level of CK enzyme
 

The higher mean of the CK enzyme was found in the non-VT group (195.2 (109.2)) compared with the VT group (116.1 (27.8)) which was statistically significant (P=0.001), fig 3.

 

 

DISCUSSION
 

Delayed onset muscle soreness and reduced muscle strength after eccentric exercise5 21 may decrease functional activities in athletics.7 22 Because of this, different methods have been investigated and recommended to prevent these symptoms.2 This study was designed to find the possible effects of VT to control and prevent DOMS after eccentric exercise.
 

A previous study showed muscle strength reduction after eccentric activities,9 while our findings showed no muscle strength reduction in the IMVC force of quadriceps in the VT group, which may be due to the establishment of optimum neuromuscular function in the quadriceps muscles by applying VT. This has been reported by other researchers,12 15 who showed better muscle performance after vibratory stimulation. Thompson and Belanger (2002) also showed that VT may increase muscle spindle activities and establish motor unit activity synchronisation that may optimise neuromuscular function.23 By contrast, it has been shown that muscle spindle stimulation by vibration may increase the afferent activities of muscle spindles which may increase background tension in the vibrated muscles.24 25 This increased background tension and motor unit activity synchronisation in the vibrated muscle may prevent sarcoma disruption or damage to excitation–contraction coupling, which may happen due to tension development during eccentric exercise.4 Therefore, this optimised muscle performance may control and prevent muscle damage and so reduce DOMS. This reduction in DOMS was seen in our study, as we found increased PPT in the right and left quadriceps and calf muscles, lower muscle soreness, and lower levels of CK enzyme in the VT group compared with the non-VT group.
 

The CK enzyme has been defined as an index for muscle damage and its level will be increased within 24 to 48 hours after eccentric activities,5 22 which is a sign of eccentric muscle damage. However this increase was seen only in the non-VT group, and not in the VT group. In fact, the lower CK level in the VT group may indicate lower muscle damage in this group, while the non-VT group showed a higher CK level and so higher muscle damage, which was accompanied by higher muscle soreness.
 

These findings may indicate that vibration training before eccentric exercise may help the muscles to build up a background tension and optimal neuromuscular activity to overcome the increased passive tension within the exercised muscles during eccentric activities. Thus, vibration training could be used before eccentric activities to control and prevent delayed onset muscle soreness and it might be a useful method for athletes to prevent any DOMS in their sports activities.
 

CONCLUSION
 

DOMS is a major complication faced by athletes after eccentric activities, which may compel them to postpone their sports activities, thus prevention and treatment of DOMS is of great concern to coaches, trainers, and therapists. In this study, we investigated the effect of vibration on muscle before downhill treadmill walking and our results showed that applying vibration before eccentric activities may prevent DOMS and so it may help non-athletic people to follow and complete their sport activities without any delay. Further studies are needed to investigate these results to find the possible application in athletics.
 

        Whole body vibration (WBV) is a novel training intervention performed on a commercially manufactured machine known as the Galileo Sport. This instrument was originally developed to increase power and strength in athletes1–3 and its vibration production has been described elsewhere.4 However, in summary the Galileo Sport machine has a tilting platform that delivers oscillatory movements to the body of varying frequencies (0–30 Hz) around a horizontal axle. The subject stands on the machine with the feet placed on either side of the axle and maintains a steady position while the oscillations of the platform produce a vertical ground reaction force to each foot alternately. The rapidly repeating eccentric-concentric contractions presumably evoked, result in muscular work evidenced by a significant elevation in metabolic rate.4

Proven to be effective in improving strength,5 6 bone density,7 and body composition,5 this novel training intervention is becoming popular for conditioning, rehabilitation, and general health. However, the acute effects of WBV on power, strength, and flexibility are still largely untested.

Superimposed cable vibration increases maximal muscular force8 and an improved residual post-vibration effect has been noted.9–11 The latter is suggested to be the joint result of increased sensitivity of muscle stretch receptors to excitation, elevated muscle temperature, and improved blood flow.8 The improvement in muscular power following acute exposure results in performance gain that would otherwise require weeks of training.10 11

From these observations, it would be expected that a functional measure of muscular power, such as maximal vertical jump height, would be significantly enhanced by acute exposure to WBV. However, the data are equivocal, showing either no change,12 a decrease,13 or an increase14 15 following acute exposure to WBV. Nevertheless, improper control, differences in vibration duration, frequency, and amplitudes, lack of or use of a warm up, and differing exercise positions, may explain the discrepancies in the results. In addition, previous studies have not permitted arm movement in the vertical jump test in an attempt to isolate lower limb muscular performance. In sport, jumping movements generally involve arm drive, so information regarding vertical jump performance with arm countermovement (arm countermovement vertical jump, ACMVJ) after acute WBV is required.

The paucity of well controlled studies investigating the acute effect of WBV on vertical jump height is surprising considering the number of sporting activities which would be enhanced by an improvement in vertical jump height. Furthermore, the lay literature suggests that other measures, such as flexibility, are influenced by acute WBV, but there is little information in the scientific literature in this regard

Therefore, the primary aim of this study was to quantify the acute effect of WBV on ACMVJ performance in elite female field hockey players. To better understand the mechanism by which WBV may exert an ergogenic effect, a secondary aim of this study was to observe whether muscle groups exposed less proportionally to vibration are capable of enhancing muscular performance. Finally, a third aim was to test the effect of WBV on flexibility using a sit and reach test.
 

METHODS
 

Participants
 

Eighteen healthy female elite field hockey players volunteered to participate in the study. They were (mean¡ standard deviation) 21.8¡5.9 years old and 1.66¡0.06 m tall, and weighed 63.7¡7.6 kg. The protocol was approved by the Massey University Human Ethics Committee.
 

ACMVJ, isometric grip strength, and sit and reach flexibility were measured before and after the following three interventions, which each lasted 5 min:
 

Within 15 s of completing the intervention, participants were retested on ACMVJ, grip strength, and flexibility measures in the same order as before the intervention.
 

The intervention order was allocated in a randomised, balanced design, with 24 h recovery between each testing session. During the course of the study, participants were not permitted to undertake any power or strength training and timing of the menstrual phase was accounted for. The participants were strictly instructed to refrain from undertaking any vigorous activity 24 h prior to the interventions, and, to prevent interference from variations in daily biorhythms, participants performed the tests at the same time each day. Each participant was familiarised with the equipment and tests prior to the study and performed pre and post measures of countermovement vertical jump, grip strength, and sit and reach. Every participant wore exactly the same pair of shoes to standardise vibration dampening.
 

Performance tests
 

Warm up was prohibited prior to the performance tests to reduce the possibility of influencing the outcome of the study.

Vertical jump test
 

Six countermovement jumps with arm swing (ACMVJ) were performed according to the protocol of Harman et al.¹⁶Each jump was recorded to 0.1 cm and was separated by a rest period of 10 s. The ACMVJ was measured by a system consisting of a portable hand held computer unit connected to a contact timing mat (Swift Performance, NSW, Australia). It has been previously reported that this system has a validity and reliability similar to a force plate.17
 

Hand grip strength test
 

Hand grip strength was determined by a hand grip dynamometer (Smedlay, Tokyo). Participants stood upright with arms down the side of the body. The dominant hand performed three repetitions of a 2 s grip contraction which was recorded to the nearest 0.1 kg.
 

Sit and reach test
 

The sit and reach test was conducted on a sit and reach apparatus (Figure Finder Flex Test, Novel, Rockton, IL) and followed the procedure as outlined by Church et al18 with the maximum end position being held for 2 s. Two repetitions were recorded to the nearest centimetre and separated by a rest period of 10 s.
 

Interventions
 

WBV
 

This was performed on a commercial Galileo Sport machine (Novotec, Pforzheim, Germany). The participants stood on the machine and positioned their feet around the centre of the oscillating platform, that equated to a peak to peak amplitude of 6 mm of vertical vibration. The frequency was set at 26 Hz. The positions taken by the subject were: (1) standing upright with knees semi-locked; (2) isometric squat at a knee angle of approximately 120˚; (3) kneeling on the ground with arms straight and hands placed on the platform equating to a peak to peak amplitude of 6 mm of vertical vibration; (4) squatting at a tempo of 2 s up and 2 s down at a knee angle of approximately 120˚; (5) lunge position with left leg on platform and right leg on ground; and (6) lunge position with right leg on platform and left leg on ground (fig 1). Positions 1–4 were held for 1 min and positions 5 and 6 for 30 s.

 

Control
 

The control intervention was performed on the Galileo Sport machine (0 Hz, amplitude 0 mm) with the exact same six body positions and time constructs as described for the WBV intervention.
 

Cycling
 

In the seated position, the subject pedalled at a cadence of 50 rpm for 5 min at 50 W on a friction braked cycle ergometer (Monark 818 E, Varberg, Sweden).
 

Statistical analysis
 

All statistical analyses were performed using a specialised statistical software package (SPSS for Windows version 10; SPSS, Chicago, IL). Dependant variables were compared using a three way, repeated measures analysis of variance (ANOVA). Each test data measure (six for ACMVJ, three for hand grip, and two for sit and reach) was included as part of the ANOVA analysis. Pairwise comparison between means was performed using post hoc contrasts to identify intervention difference. Intraclass correlation coefficients (ICC) assessed the test-retest reliability of comparing the mean of the dependent variables between testing sessions. Significance was considered to be at or greater than the 95% level of confidence (p<=0.05).
 

RESULTS
 

ACMVJ
 

There was a significant (p<0.001) (intervention6pre-post) interaction (fig 2) such that WBV produced an ergogenic effect resulting in an 8.1±5.8% increase in ACMVJ height (fig 3) compared to both control (–2.0±3.7%) and cycling (–0.3±3.7%).

There was no significant interaction effect for WBV, control, and cycling interventions (fig 4). There was a significant effect in that each repetition of grip strength decreased (p<0.05).

Sit and reach
 

The interaction between treatment and pre-post sit and reach (p,0.05) was significantly greater after WBV (8.2¡5.4%) compared to the control and cycling interventions (5.3¡5.1% and 5.3¡4.9%, respectively; fig 5). Post hoc analysis revealed that for all interaction interventions (WBV, control, and cycling) sit and reach increased for each repetition (p,0.001). The ICC test-retest reliability analyses revealed that jump height (0.916), grip strength (0.804), and sit and reach (0.934) were consistent between sessions.
 

DISCUSSION
 

ACMVJ
 

The primary purpose of this study was to examine the acute effects of WBV (eccentric-concentric loading), control, and cycling (concentric only loading) interventions on ACMVJ performance. Our results show that ACMVJ performance is enhanced by 8.1% immediately following 5 min of WBV exposure when compared to control (no vibration) conditions. These observations are in accordance with the findings of Bosco et al¹⁴ and Torvinen et al¹⁵ who reported increases of 2.5% and 3.8% in vertical jump height, respectively, but without arm action.
 

The ACMVJ is a common test used by conditioners and coaches in a variety of game sports to monitor the effects of training and/or rehabilitation. Additionally, it is an established measure of lower body explosive power and, as such, it should be used as part of a battery of tests for assessing the field hockey player.19 Although the ˜8% improvement in ACMVJ height seen in this study cannot be directly extrapolated to predict field hockey performance, it would have relevance in game situations such as changing direction, lunging, and acceleration, where maximal explosive power is important. In this context, only small enhancements in muscular performance are needed to provide the necessary edge in elite competition.
 

The fact that the protocol of the present study enhanced jump performance to a greater degree than in previous studies using an acute WBV intervention is difficult to explain. However, the pre-intervention measures of ACMVJ in the present study were performed without a warm up, whereas the Bosco et al¹⁴ and Torvinen et al15 studies which revealed a smaller ergogenic effect of WBV, employed a cycling warm up prior to the first measure. Although in the present study the effect of seated cycling alone on ACMVJ did not reach significance (p=0.07), it is likely that, with greater participant numbers, we would have observed improved jump performance compared to control conditions. Thus, it is possible that cycling alone may provide sufficient warm up to enhance vertical jump performance to a small degree, and this may explain the smaller ergogenic effects of the aforementioned studies in which cycling exercise was used as a warm up. It is clear, however, from the results of the present study and those of Bosco et al14 and Torvinen et al,15 that the effects of vibration are additive to any cycling based warm up.
 

In the present study different muscle contractions were utilised in the different interventions: WBV exposure elicits both concentric and eccentric contractions, whilst seated cycling requires only concentric muscle action. ACMVJ involves activation of the stretch-shortening cycle, where the stretch receptors are activated under the eccentric loading phase. Given the significant enhancement of ACMVJ by WBV compared to cycling, it could be speculated that the additional effect of WBV over cycling was due to the eccentric stimuli it provides.
 

The enhanced muscle power observed following acute vibration is suggested to occur via potentiation of the neuromuscular system whereby stimulation of muscles spindles (Ia afferents) results in reflex activation of motor neurones with increased spatial recruitment.20 21 The continued enhancement of the stretch-reflex pathway can also be attributed to the ÿ motor neurone input causing an increase in sensitivity of the primary endings. Furthermore, the tonic vibration reflex can recruit additional motor units via muscle spindle and polysynaptic activation.15
 

This current study did not include EMG recordings and therefore it is not possible to directly assess any neurogenic enhancement. However, it has been reported that the stretchshortening cycle of the ACMVJ activates the spinal reflexes to enhance jump height compared to that of a concentric squat jump.22 Therefore, it is reasonable to propose that any neurogenic changes are ably detected by ACMVJ. The accentuated jump height following acute WBV suggests that neural enhancement occurred through an increased sensitivity of the stretch reflex mechanism and is in agreement with other investigators.3 10 14 23 24 However, in the absence of measures of neural function, such an explanation remains speculative and a better understanding awaits further investigation.
 

The use of arm countermovement in the vertical jump measure in the present study is unlikely to have contributed to the greater enhancement of jump performance, unless WBV somehow improves the coordination of arm and leg movements. Additionally, arm swing in the countermovement jump has been proven to require less practice than a countermovement jump with no arm swing and has been described as a natural practiced movement.19
 

Hand grip strength
 

Maximal grip strength was not improved with either intervention compared to control conditions. The level of vibration exposure sustained by the forearm muscles during the protocol in the present study is difficult to quantify, but would have been small. The results confirm the findings of Torvinen et al,12 15 illustrating the fact that muscles not directly exposed to vibration do not show a concomitant performance enhancement as does the vibrated muscle. Therefore, the improved ACMVJ performance seen in the present study is not related to a vibration induced centrally mediated phenomenon but confirms that the effect is localised to the spinal level and/or the muscle itself.

 

Flexibility
 

The significant improvement of 8.2% in the sit and reach flexibility following WBV is comparable to the 8% increase in leg split flexibility reported by Issurin et al.2 However, their protocol of flexibility training was performed with a vibrating cable simultaneously attached to the lower limb of the participant.2 In the present study, no flexibility exercises were conducted concurrently with the three interventions.
 

As stated previously, vibration enhances the stretch reflex loop through the activation of the primary endings of the muscle spindle, which influences agonist muscle contraction while antagonists are simultaneously inhibited.25 The enhanced flexibility measure following WBV was greater than that after the control and cycling interventions, which suggests that the vibration exposure may have activated the Ia inhibitory interneurones of the antagonist muscle. This in turn may have caused changes to intramuscular coordination to decrease the braking force around the hip and lower back joints and potentiate the sit and reach score.3
 

Increases in static and dynamic muscular contractions have been attributed to muscle stiffness, which is a function of muscle and tendon components.26 The magnitude of the stretch load and the condition of the musculotendinous complex ultimately determine which reflexes dominate.27 For pre-stretching to enhance concentric muscular contraction, excitatory responses of the muscle spindle must exceed the inhibitory effects of the Golgi tendon organ (GTO). This is normally achieved through potentiated neural input of muscle spindle sensitivity or suppression of GTO neural activity.
 

In strength and power training, performing heavy sets of squats has been shown to augment jump squat height.28 Equally in WBV, fast joint rotation and muscle stretching occur, which is likely to increase muscle stiffness following the purported neural potentiation of the stretch reflex pathway and motor neurone input.29 Moreover, vibration causes more excitatory responses to the primary endings of muscle spindles compared to secondary endings and GTOs. The joint, skin, and secondary endings also detect the vibratory stimulus whereby the neural activity of the primary endings is potentiated through the activation of the ÿ motor neurone.3 This post activation potentiation, known in the strength conditioning field as ‘‘tuning up’’, may explain the concomitant enhancement of ACMVJ and flexibility performance.
 

The vibratory stimulus of the Ia neural drive and proprioceptive loop may also replicate a warm up effect by increasing pain threshold, blood flow, and muscle elasticity.2 Kerschan-Schindl et al30 have reported an increase in mean blood flow of the popliteal artery after acute WBV. They cite a combination effect of possible vasodilation and thixotropism for reducing the viscosity of the blood and improving the mean speed of blood flow. Acute vibration exposure has also been shown to reduce pain affected areas of muscle or tendon,31 which may allow a greater tolerance of the stretch threshold.
 

As yet, there are no set guidelines for WBV exercises, hence different investigators have used very different protocols 14 15 32 with few comparisons with other controls. In this study, WBV elicited a greater increase in ACMVJ and flexibility compared to cycling, which suggests that WBV may be an effective intervention for warming up. Numerous warm up mechanisms have been described to increase performance,33 therefore it is difficult to explain how WBV may accelerate the warm up effect. However, given that WBV results in concentric-eccentric contraction but cycling in solely concentric, WBV may provide an additional eccentric stimulus that is currently overlooked in conventional warm up procedures. Incorporating a greater eccentric component in warm ups may be beneficial for enhancing performance in sporting activities that rely on the eccentric-concentric interaction, which with WBV requires little effort and time. Further investigation is required before any conclusion can be drawn that acute WBV may be used as a potential warm up intervention.
 

Conclusions
 

In conclusion, this study further substantiates the claims of other investigators14 15 34 that acute WBV causes neural potentiation of the stretch reflex loop as observed by the improved ACMVJ and flexibility performance. Additionally, muscle groups less proportionally exposed to vibration do not exhibit physiological changes that potentiate muscular performance.
 

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 (1.6 %, P
 

INTRODUCTION
 

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 [8 ] 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 (tJ 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 (tf in seconds) applying ballistic laws:
 

h=t_f^2.g.8^-1(m)
 

where g is the acceleration of gravity (9.81 m . s^-2) During CJ exercises the subject were required to perform the maximal jumping effort minimising knee angular displacement during contact. From the recordings of t_f and t_c 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] :
 

AP = T_f . T . 24.06 e ( T_c )^-1 (W *.kg brn-‘) 
 

where P is the mechanical power per kilogram of body mass, T_f the sum of the total flight time, T_t the total working time (5s), and T_c the sum of the total contact time. The average height during 5s CJ and the HBJ were computed using formula 1. 
 

Reproducibility of measurements  
 

The reproducibility of the mechanical power test (5s CJ) and CMJ performances were high with respectively r =.95 and r =,90 [4,27] 
 

Statistical Methods: 
 

Conventional statistical methods used included mean, standard deviation and paired Student’s t-test. The level of significance was set at p<.O5. 
 

Treatment Procedures: 
 

Subjects were exposed to vertical sinusoidal whole body vibration (WBV) using the device called GALiLEO 2000 (Novotec, Pforzheim, tiermany) . The frequency of the vibrations used in this study was set at 26 Hz (displacement = 1 Omm; acceleration = 27 m.s^-2). 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 900 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 90” 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 90” 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 (P>0.05). The jumping height in 
 

CMJ remained the same in E group after 10 days of WBV (Table 2). This treatment, in contrast, produced remarkable and statistical significant (P< 0.05) enhancement of the HBJ ( Fig. 1) and the PBJ ( Fig. 2). In addition, the average height during 5s CJ was also improved in E group, demonstrating a statistical significant difference of P< 0.01 (Table 2). On the other hand, the average power developed during 5s CJ failed to demonstrate statistically significant change after the treatment (Table 2). 
 

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 HBJ (Fig. 1), PBJ (Fig. 2) and the average jumping height during 5s CJ (Table 2). On the other hand, no changes were noted for the AP during 5s CJ. It should be remind that, during the continues jumping test [4], the average jumping height possessed higher significance and sensitivity than AP in differentiating athletes [28] or in revealing the effect of creatine supplementation [9]. In addition no changes in CMJ were noted after the vibration treatment in E group. Apparently these are contradictory results. However, a reasonable explanation can be found analysing the mechanical behaviour of the leg muscles during CMJ and 5s CJ. In fact, both exercises are characterised by the so called stretch- shortening cycle (SSC). This means that, before the concentric work (pushing phase), leg extensor muscles are actively stretched (eccentric phase) in both exercises. Nevertheless, the neuromuscular activation in CMJ is different than that found in 5s CJ. The CMJ is characterised by large angular displacement and slow stretching speed (3- 6 rad . s-l) [3], while 5s CJ are performed with fast stretching speed (1 O-12 rad . s-l) and small angular variation [7]. This means that, only in 5s CJ the leg extensor muscles experience fast stretching which may elicit a concurrent gamma dynamic fusimotor input that would enhance primary afferent discharge. This notion is supported by the studies of Bosco, et al. [3], who showed that during eccentric phase of drop jumping exercises (similar to 5s CJ), EMG activity was high and comparable to maximal concentric ballistic movements. Thus there is a possibility of enhanced neural potentiation either via spinal or cortical reflex. On the other hand, it is likely that CMJ is not a suitable activity to elicit stretch reflex, since high EMG activity has not been recorded during the stretching phase (e.g. [3]) 
 

On the background of these considerations it is likely that the effect of WBV treatment elicits a biological adaptation connected with neural potentiation. Thus, it can be argued that, the biological mechanism produced by vibration treatment is similar to the effect produced by explosive power training (jumping and bouncing exercises). In fact, this suggestions is consistent with knowledge that mainly the specific neuronal components and its proprioceptive feedback mechanism are the first structure to be influenced by specific training [2,14]. 
 

Training with high stretching loads may improve stretch-reflex potentiation and increase the threshold of firing for the Golgi tendon organs (GTO). The latter one, would then improve the possibility to recruit greater amount of motor units during eccentric phase [2]. Furthermore, there are several ways in which the explosive power training can infIuence neural activation, for example by increasing the synchronisation activity of the motor units [21]. I t cannot be excluded also an improvement of co-contraction of synergist and increased inhibition of antagonist muscles. In any case, what ever it is the intrinsic mechanism which enhance neuromuscular activation after specific explosive power training, it is likely that, the vibration treatment have to improve the proprioceptors’ feedback mechanism, since it is filly operating and elicited during 5s GJ performance, which was enhanced after WBW. On the other hand, the lack of modifications observed in GMJ test after the VBV treatment suggests that the proprioceptors’ feedback mechanism is not strongly operating in CMJ . In fact, this exercise is strongly influenced by the voluntary recruitment capacity and by the fiber type composition of leg extensor muscles [ 1]. However, there is no doubt that stretch reflex play an important role in stiffness regulation [IS], and that muscle spindles and GTO operate actively in the control of muscle length and tension [ 16]. Consequently, it can be suggested that WBV treatment may affect dramatically the neuromuscular functions and properties which are regulating muscle stiffiess through the control of length and tension. 
 

During vibration the body and the skeletal muscle undergo to small changes in muscle length. Facilitation of the excitability of spinal reflex has been elicited through vibration to quadriceps muscle [ 11). The idea that vibration may elicit excitatory flow through short spindle - motoneurons connections in the overall motoneuron inflow has been suggested also by Lebedev and Peliaksv [IX] pointed on the possibility. It has been shown also that vibration drives alphamotoneurons via la loop, producing force without descending motor drive [25]. Burke et al. [ 101, suggested that vibration reflex operates predominantly or exclusively on alpha motoneurons and that it does not utilise the same cortically originating efferent pathways as are in the performance of voluntary contractions. In addition, the results of Kasai et al. [ 17] are consistent with vibration induced activation of muscle spindle receptors not only in the muscle where vibration is applied, but also to the nearest muscles. Mechanical vibration (10 - 200 Hz) applied to the muscle belly or the tendon can elicit a reflex muscle contraction (e.g. [ 13]). This response has been named tonic vibration reflex (TVR). It is not known wheather it can be elicited by low WBV frequency (l-30 Hz), even if it has been suggested to occur [26]. 
 

Finally, it should be remind that not only nervous tissue, but also muscle tissue can be affected by vibration [ 23]. In fact, 5 hours daily for 2 days of vibration exposure at two different frequencies were sufficient to induce enlargement of slow and fast fibers in rats [24]. 
 

In the present study, no neurogenic potentiatian or modification in the morphological structure of the muscles was demonstrated since neither EMG recordings nor muscle biopsy sampling were performed. However enhanced mechanical behaviour during 5 s CJ, strongly suggests that a neurogenic adaptation have occurred in response to the vibration treatments. Even if the intrinsic mechanism of the adaptive response of neuromuscular functions to WBV could not be explained, importance. Adaptive hypergravity conditions the effectiveness of the stimulus seems to have relevant response of human skeletal muscle, to simulated (1 . 1g), applied for only three weeks, caused a drastic enhancement of the neuromuscular functions of the leg extensor muscles [6]. Chronic centrifugal force (2 g) for 3 months [ 19] has initiated conversion of fiber type. In the present experiment, the total length of the WBV application period was not very long (only 100 minutes), the perturbation of the gravitational filed was rather consistent (2.7 g )_ An equivalent length and intensity of training stimulus can be reached only by performing 200 drop jumps from 60 cm, twice a week for 12 months. In fact, the time spent for each drop jump is less than 200 ms, and the acceleration developed can hardly reach 2.7 g [8]. This means to stimulate the muscles for 2 min / week for the total amount in one year of 108 minutes, which is almost the total time of vibration applied to the E subjects.