2017 | VOLUME 13 | 23© ARCHIVES OF BUDO | SCIENCE OF MARTIAL ARTS This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (http://creativecommons.org/licenses/by-nc/4.0), which permits use, distribution, and reproduction in any medium, provided the original work is properly cited, the use is non-commercial and is otherwise in compliance with the license. Effects of local vibrations on muscle strength and roundhouse kick performance of taekwondo athletes Mariana P OliveiraABCDE, Sara A RodriguesABCDE, Leszek A Szmuchrowski ADE, Maicon Rodrigues AlbuquerqueCDE, Reginaldo GonçalvesDE, Cristiano AG FlorBD, Luiza F VieiraBE, Marcos DrummondBE, Márcio PrudêncioDE e Bruno P CoutoABCDE Laboratório de Avaliação da Carga, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil Received: 15 June 2016; Accepted: 21 October 2016; Published online: 30 January 2017 AoBID: 11202 Abstract Background and Study Aim: Exercise with vibration exposure is a type of neuromuscular training that has been used with athletes and non- athletes. The vibration is used in the development of sports performance by applying only oscillatory move- ments in the human body or combining those oscillations with conventional strength training exercises. No studies were found that verified the effects of applying vibration on the performance of roundhouse kick. Thus, the aim of this study was the possibility of optimising performance of kick speed and maximal isomet- ric strength through the application of mechanical vibrations. Material and Methods: Ten taekwondo athletes (6 males and 4 females) with mean age 18.3 ±2.5 years, mean body mass 62.1 ±6.6 kg, mean height 173.3 ±7.0 cm and mean practice time 5.5 ±3.2 years volunteered to participate in this study. All the volunteers participated in two different sessions. In the sessions, they performed a warm-up, speed kick pre-test, maximal voluntary contraction (MVC) intervention (with or without vibration) and a speed kick post-test. Results: No significant differences were found between pre-test and post-test (immediately, 3, 5 and 8 minutes after intervention) for speed kick. For the force recorded during MVC with local vibration (LV), a significant differ- ence was found between moments, during and after vibration. The force produced during LV exposure was higher compared with after vibration. However no significant difference were found between the moments after and during LV compared to MVC without LV (in corresponded moments in the given range curve), in addition, the moment after LV was higher in MVC without LV. The impulse without vibration was significant higher than impulse with vibration. Conclusions: These data suggest that training with the vibration parameters used in this present study was not able to in- crease the performance of speed kick (subacute) and maximal force (acute). In addition, reduced the maximal force subacutely and impulse acutely. Key words: kick speed • mechanical vibration • sports training • strength training • maximal voluntary contraction Copyright: © 2017 the Authors. Published by Archives of Budo Conflict of interest: Authors have declared that no competing interest exists Ethical approval: The study was approved by the Ethical Committee for Research at the Federal University of Minas Gerais, Brazil (49553815.4.0000.5149) Provenance & peer review: Not commissioned; externally peer reviewed Source of support: Fundação de Amparo à Pesquisa do est estado de Minas Gerais (FAPEMIG – Minas Gerais / Brazil) and the Pró-Reitoria de Pesquisa (PRPQ) [Research Pro- Rectory] from the Universidade Federal de Minas Gerais Author’s address: Bruno P Couto, Universidade Federal de Minas Gerais, Av. Antônio Carlos, 6627 Pampulha, Belo Horizonte, Minas Gerais, Brazil; e-mail: brunopena@yahoo.com.br Authors’ Contribution: A Study Design B Data Collection C Statistical Analysis D Manuscript Preparation E Funds Collection  ORIGINAL ARTICLE 24 | VOLUME 13 | 2017 www.archbudo.com Original Article INTRODUCTION Exercise with vibration exposure is a method of neuromuscular training that has been used both with athletes and non-athletes [1]. The vibra- tion is used in the development of sports perfor- mance by applying only oscillatory movements in the human body or combining those oscillations with conventional strength training exercises. Vibration exposure may be applied in two differ- ent ways: using local vibration (LV) or by whole body vibration (WBV). LV is normally applied per- pendicularly to a muscle or tendon of the target muscle and indirectly through devices (dumbbells, bars and cables). In sports training, the most com- monly used method is WBV, its application is per- formed indirectly, through vibration platforms. Tankisheva et al. [2] reported that when WBV is used, the vibration energy can be attenuated when transmitted through body tissues, espe- cially for more distant muscles (e.g. upper body), because of the distance between the vibration source and the target muscle. On the other hand, during LV this attenuation is reduced by decreas- ing this distance between the vibration source and the target muscle and, therefore, optimizes the vibratory stimulus that reaches the muscles. According to Cardinale and Bosco [5], an increase in muscle strength from mechanical vibration is pur- ported to occur because of the TVR. The contractile response, which is investigated by means of elec- tromyographic (EMG) activity, produces an invol- untary component of force production. Bosco et al. [5] evaluated the influence of vibration on mechan- ical properties of elbow flexors in twelve elite box- ing athletes. The results showed a significant acute increase in average power in the arm exposed to vibration. The EMG activity analysis showed a sig- nificant increase in neural activity during vibration exposure, compared with the recorded EMG activ- ity before treatment. Similar Griffin et al. [6] also found a significant increase in strength and EMG activity of triceps for elbow extension when the vibration was applied. Bongiovanni and Hagbarth [7], Hazell et al. [8] found that muscle vibration provided acute increases in strength, EMG activ- ity signals and the stimulation rate of motor units during maximal isometric contractions. However, Humphries et al. [9] applied vibration (50 Hz and 5 mm) in the belly of the rectus femoris during mus- cle contraction, did not find any change in muscle activation (EMG) and the force produced by maxi- mal voluntary contraction (MVC). There are three different responses to a train- ing program: acute and subacute physiological responses and chronic adaptations. There are in the literature different explanations for the positive subacute effects induced by vibration exposure: increased proprioceptive feedback – increasing the sensitivity of muscles spindles [10]; increase in muscle temperature – increas- ing nerve conduction velocity [11] and the post- activation potentiation (PPA) – increasing the intracellular sensitivity Ca²+ due to prior mus- cle activation [12]. However, its effects on per- formance of some specific movements are still unknown [13]. According to Ogiso et al. [14], the mechanical response of stretch reflex depends on functional aspects relating to the motor task. In this way, probably the exercise performed during vibration exposed influences the response to this vibra- tion stimulus. In addition, Rittweger [15] suppose that the complexity of the action may be influ- ence the results of vibration exposure. The more complex action, smaller the effect of vibration to optimize performance. Therefore, it is still neces- sary to verify the effect of vibratory stimulation on the performance of specific techniques, such as taekwondo kick. Taekwondo (TKD) is a Korean martial art sport, became a full-medal sport at the 2000 Summer Olympics in Sydney and has been an Olympic sport since then. Taekwondo can be character- ized by short durations, high intensity and require specific fast, high and spinning kicks [16]. Bandal chagui (roundhouse kick) is the most used kick technique [17] and can be defined as a semi-circu- lar kick performed with foot dorsum on the abdo- men height of the opponent. This kick requires high precision and power of lower limbs muscles [18]. Because of its high velocity and impact force, during a competition, the opponents have less time to react and more likely to concede points. Therefore, the explosive turning kick is a principle focus of TKD training [19]. No studies were found that verified the effects of applying vibration on the performance of roundhouse kick. Thus, the aim of this study was the acute and subacute effects of mechanical vibration on the roundhouse kick speed, muscle strength and impulse of lower limbs in taekwondo athletes. We set the following hypothesis: the athletes after the maximal isometric contraction performed Roundhouse kick (bandal tchagui) – a type of kick executed to the chest that generally starts in the sagittal plane and finishes in the lateral plane. Vibration – is a mechanical oscillation which usually is sinusoidal though others form of oscillations may be applied. In case of sinusoidal oscillations it is defined by frequency measured in the unit of hertz (Hz) and by amplitude. The number of Hz denotes number of cycles per seconds. Amplitude is a half difference between the maximum and the minimum of the oscillation [24]. Strength training – noun training that aims to build muscle strength, usually resistance training [25]. Stretch reflex – noun a reflex reaction of a muscle that contracts after being stretched [25]. Tonic – adjective relating to or affecting muscular tone or contraction [25]. Isometric – adjective 1. involving equal measurement 2. used for describing muscle contraction in which tension occurs with very little shortening of muscle fibres 3. used for describing exercises in which the muscles are put under tension but not contracted [25]. Isometrics – noun a form of exercise in which the muscles are pushed against something fixed or against other muscles to strengthen them [25]. Oliveira MP et al. – Effects of local vibrations on muscle strength and roundhouse... © ARCHIVES OF BUDO | SCIENCE OF MARTIAL ARTS 2017 | VOLUME 13 | 25 with vibration, would produce a greater subacute performance in the kick speed and there would be an increase in acute and subacute maximal isometric strength. MATERIALS AND METHODS Participants Ten taekwondo athletes (6 males and 4 females) with mean age 18.3 ±2.5 years, mean body mass 62.1 ±6.6 kg, mean height 173.3 ±7.0 cm and mean practice time 5.5 ±3.2 years volunteered to participate in this study. Subjects were informed about the nature of this study and signed an informed consent form according to the International Review Board for the use of human subjects at research. All proce- dures were approved by the Ethical Committee for Research at the Federal University of Minas Gerais, Brazil (49553815.4.0000.5149). Procedures and measures Each subject visited the laboratory on two sepa- rate sessions: isometric intervention and isomet- ric with vibration intervention - in a randomized order. In both sessions all the volunteers per- formed a warm-up, speed kick pre-test, isometric intervention or isometric with vibration interven- tion and a speed kick post-test (Figure 1). Pre-test After the warm-up (3 sets of 3 submaximal roundhouse kicks and an interval of 30 seconds between each set), all the athletes performed the speed kick pre-test composed of 3 maximal roundhouse kicks with an interval of 15 seconds between each repetition. There was a 1-minute recovery interval between the warm-up and the speed kick pre-test. To perform the roundhouse kick, the volunteer placed the foot that would kick on a contact mat. A taekwondo target pad was positioned at the optimum individual height and distance (Figure 2). Figure 1. Roundhouse kick test. Figure 2. Roundhouse kick test. Figure 1. Study Design 26 | VOLUME 13 | 2017 www.archbudo.com Original Article The athletes were also instructed to perform the movement as quick as possible. An inertial sen- sor was coupled to the target pad for measuring the movement time, which corresponded to the loss of contact of the kicking foot to the mat until contact with the target pad. At the end of each kick, the volunteer received as feedback the per- formance achieved during the attempt. For the roundhouse kick test, it was used a contact mat fixed to the ground. The contact mat was con- nected to a computer containing the Multisprint Full program version 3.5.7 (Hidrofit Ltd., Brazil). The mat fixed to the floor was marked with the length of centimetres, so the initial position for all kicks were the same, and to keep the dis- tance between the mat and the target pad also the same during the test. An armoured inertial sensor was inside to a specific taekwondo tar- get pad (Figure 3). The sensor, in its interior, has a mass in the form of coil spring. When the foot loses contact with the mat on the floor, it opens an electric circuit, and when the foot makes contact with the tar- get pad, it closes the electric circuit. The vari- able obtained in the roundhouse kick test was kick time. The volunteers were instructed to kick as fast as possible, with the preferred member, the taekwondo target pad was positioned at the height of the iliac crest of the individual, and the kick should be performed as soon as allowed. The horizontal distance, defined by the length of lower limbs plus the volunteer basis size, and the height of the iliac crest of the individual were used to calculate the hypotenuse, through the Pythagorean Theorem (Figure 4) [20]. The hypot- enuse was considered as the distance between the initial position of the kicking foot and the tar- get pad. As this distance represents the displace- ment, it was used to calculate the kick speed: KS = √(HD² + HC²) / KT Since KS is the kick speed, in m/s, HD the hori- zontal distance, in meters, HC the height of the iliac crest of the individual, in meters, and KT the kick time in seconds. Isometric Intervention Each volunteer executed four MVCs with 3 min rest between each MVC. The MVCs were per- formed in a specific kick position (Figure 5). To that end, each volunteer tried to perform a round- house kick and then maintained the contraction against an insurmountable resistance with a dura- tion of 8 seconds, starting from the moment when the volunteer reached the peak strength. At the isometric with vibration intervention, the first MVC was performed without vibration; this Figure 3. Coupled inertial sensor inside the target pad: (A) – before the kick contact; (B) – after the kick contact (closed electronic circuit between the coil spring and the armoured inertial sensor). Oliveira MP et al. – Effects of local vibrations on muscle strength and roundhouse... © ARCHIVES OF BUDO | SCIENCE OF MARTIAL ARTS 2017 | VOLUME 13 | 27 was to verify if the volunteers were in the same recovery condition. Subsequently, the mechani- cal vibration was superimposed during the last 3 MVCs. The sinusoidal LV was applied through a steel cable in the direction of the resultant mus- cle forces’ vector addition [21]. The steel cable was attached to the athlete’s back foot. The ath- lete was placed with their back to the engine and forced to flex the hip and extend the knee while grasping the railing in front of them. The vibra- tion stimulus were applied during the 6 seconds of the MVC. To the application of LV, it was used a motor, three-phase induction, brand WEG (IP55 model, 2 CV power, frequency 60Hz and speed 3400 rpm) coupled to an eccentric shaft. A cable passes through this eccentric shaft, which transmitted vibration to the athlete’s ankle. Values of maxi- mal force and impulse (force versus time curve area) were obtained using a force cell by JBA(Zb Staniak, Poland) connected to an amplifier of sig- nals (WTM 005–2T/2P, Jaroslaw Doliriski Systemy Mikroprocesorowe, Poland). The amplifier itself was connected to a computer with a MAX (ver- sion 5.1, JBA) interface that enables analysis of the strength curve as a function of time (frequency of data input: 1000 Hz). The measurements obtained in this study were impulse (MVC with and without vibration), maximal force: MVCs with vibration (1st moment – before vibration - 2nd moment – during vibration – 3rd moment – after vibration) (Figure 6). And MVCs without vibration - three corre- sponding moments of the MVCs with vibration (1st moment - peak strength - 2nd - highest force value registered in the range of 6 seconds, after reached the peak strength - 3rd moment - highest force value after the 6 seconds). The strength vari- ables were calculated using the software Dasylab (11.0). It was considered the highest value of force registered in the given range curve analysis. Post-test The athletes performed speed kick test com- posed of 3 maximal roundhouse kicks with an interval of 15 seconds between each repetition in each moment - immediately, 3, 5 and 8 min- utes- after the last MVC of each session with vibration (repeated measurements). The com- parison between the pre-test and post-test val- ues of kick speed was performed to verify the subacute effects of the isometric intervention with and without vibration in the roundhouse kick speed. Statistical analysis The normality of all data was verified using the Shapiro-Wilk test. A Paired Student’s t-test was KS = √(HD² + HC²) / KT Figure 4. Roundhouse kick (model of the test and measurement criteria). 28 | VOLUME 13 | 2017 www.archbudo.com Original Article done to compare the impulse during MVC with and without vibration. The Shapiro-wilk test and Paired Student’s t-test was performed using SPSS (version 20.0). To compare the results of kick speed and force along the MVC, Two-way ANOVA (interventions vs. time) with repeated measures with post hoc LSD (vibration) and with post hoc Scott-Knott (time) was implemented using Sisvar (version 5.6) software. For the data analysis, the mean of the three MVCs (maximal force and impulse) and the mean of the three kicks were used. The significance level was set at p<0.05. RESULTS Concerning the results from kick speed, no sig- nificant differences were found between the repeated measurements, pre-test, immediately after, three, five and eight minutes MVC with LV (F(1,5)= 0.366, p = 0.832) and MVC without LV (F(1,5)= 0.0364, p = 0.991) and between interven- tions (MVC’s without or with vibration exposure) for kick speed (F(1,5)= 0.117, p = 0.9764). Figure 7 shows the repeated measurements values of speed kick with and without vibration. No significant differences were found between the maximal force values obtained during the first MVC in the first (713.83 ± 164.8) and second session (607.11 ± 122.8) (p = 0.293). All the vol- unteers were under the same recovery conditions between sessions. The impulse without vibration was signifi- cant higher than the impulse with vibration (p = 0.001). For the force recorded during MVC with LV, a significant difference was found between moments, 1st, 2nd and 3rd (F(1,9)= 15.143, p = 0.0000). The force produced in the MVC during LV exposure was higher com- pared with 1st and 3rd moment (before and after vibration). There were no significant dif- ference between moments (1st,2nd and 3rd) in MVCs without LV (F(1,9)= 2.363, p = 0.101) and between MVC with and without vibration in the 1st moment (F(1,9)= 3.254, p = 0.0769) and 2nd moment (F(1,9)= 1.708, p = 0.197). However, the force produced in the MVC without the application of LV, 3rd moment, were higher than MVC with LV (F(1,9)= 4.850, p = 0.0320). Intervention: MVC (with and without vibration) position. ł ń Figure 5. Intervention: MVC (with and without vibration) position. Oliveira MP et al. – Effects of local vibrations on muscle strength and roundhouse... © ARCHIVES OF BUDO | SCIENCE OF MARTIAL ARTS 2017 | VOLUME 13 | 29 DISCUSSION The principal issue of the present study is related to the possibility of optimizing the performance of kick speed through the application of mechan- ical vibrations. Based on the results, the hypoth- eses have not been confirmed. No statistical differences were found between the values of kick speed at the pre-test and post-test (imme- diately, 3, 5 and 8 minutes after intervention) in the isometric and isometric combined with vibra- tion intervention. Considering both intervention characteristics used in this present study, it is possible that the magnitude of the stimulus was not sufficient to produce effects on speed of specific techniques. According to Rittweger et al. [1] depending on the frequency of vibration, the mechanical vibration can be specific training of type of type II muscle fibres. Additionally, athletes can be more sensi- tives to acute vibration effects compare to non- athletic people. Nevertheless, the complexity of the action maybe influence the effects of vibra- tion exposure. The more complex action, smaller the effect of vibration to optimize performance [15]. Rittweger [15] suggests that power gains obtained with vibrations intervention were not observed in sprint running. Rittweger [15] sug- gests that power gains obtained with vibrations intervention were not observed in sprint run, because, sprint run is a more complex task than vertical jump and involves other factors, such as Figure 6. Maximal force along the MVC with vibration (1st, 2nd and 3rd moment). Figure 7. Kick speed (m/s) before and after MVC’s without and with vibration exposure. 30 | VOLUME 13 | 2017 www.archbudo.com Original Article Table 1. shows mean ± Standard Deviation values for impulse during MVC and maximal force in moment 1st and 2nd moment. Maximal force Maximal force (MF) 1st moment 2nd moment 3rd moment MF without LV (N) 541.6±99.4 646.8±130.6 628.4±151.2** MF with LV (N) 452.0±65.7 711.7±128.9≠ 512.5±119.9 Impulse Impulse without LV (N.s) 250.7* ± 53.7 Impulse with LV (N.s) 210.1±41.0 Legend: Maximal force (MF) in moment 1st, 2nd and 3rd moments without and with local vibration (LV). *Significate difference between impulse without vibration and with vibration (p=0.001). ** Significate difference between MF without LV and MF with LV. ≠ Significate difference between 1st, 2nd and 3rd moment in MF with LV (p=0.0000). reaction time, motor patterns and tendon force trans- mission that are not improved by vibration training. This explanation can justify the results found in the present study; kick speed is a complexity motor task. Supporting our results, Couto et al. [13] and Cochrane et al. [11], did not find positive effects of vibration exposed on the speed on a run protocol. Moreover, Bogaerts et al. [4] found no significant increases in speed of knee exten- sion after a WBV protocol. Most of the previous studies, which have investigated the effect of vibration in the lower limbs, evaluate using performance on the vertical jumps. The vertical jump constitute the most common method of assessing the power of lower limbs in athletes [22]. Despite being a universal method of assessing the power of lower limbs, it is not a specific test to evaluate the physical condition of taekwondo athletes [23]. Furthermore, is a simpler test compared to the kick technique. For this reason, it is expected different results to speed kick, compared with the results reported in the liter- ature for vertical jumps. Moreover, contrasting to the type of vibration used in these previous studies (WBV), this study applied LV simultaneously with the isomet- ric action, producing consecutive movements oppo- site to the muscular action. Still discussing the speed kick results, it is also important to know that the pre- test was conducted after a warm-up and the post-test performed immediately after the isometric vibration intervention, consequently, the athletes were proba- bly fatigued. During the post-test, conducted 3, 5 and 8 minutes perhaps they were potentiated, nevertheless already without the warming effect. So, this condition is maybe equating the results from pre and during the repeated measure in the post-test. Our results revealed no difference between maximal forces registered in the 1st and 2nd moments of MVCs with and without vibration and values of 3rd moment were higher in MVC without LV. We investigated the effects of isometric with and without vibration on the impulse, as well. The impulse without vibration was significant higher than the impulse with vibration (p = 0.001). An explanation for these results maybe be related to the intervention used - vibrations characteristics (frequency, amplitude and exposure time) and the technique per- formed to superimpose LV, as already presented. Additionally, although the volunteers were athletes and had a high level of familiarity with the kick technique, they had no experience with isometric training com- bined with vibration application. Therefore, perhaps, the volunteers had to create strategies to maintain balance/ stability during MVC, which may have induced a reduc- tion in mean values of force (impulse). As well, although the MVC with and without vibration have a similar con- dition, what actually happens is that during the concen- tric phase of return to the isometric position, the force production tends to reduce with vibration exposure. In other words during the application of vibration what happens are successive eccentric-concentric actions, these sequences of muscular actions cause cycles of increased (eccentric) force production and subsequent abrupt (concentric) force drop, which probably gener- ate, on average, lower values of force. As observed, the exposure to vibration had a signifi- cant reduction of the impulse compared to MVC with- out vibration and did not produce acute increases in maximal force. In addition, the subacute effect usually founded after the application of mechanical vibration Oliveira MP et al. – Effects of local vibrations on muscle strength and roundhouse... © ARCHIVES OF BUDO | SCIENCE OF MARTIAL ARTS 2017 | VOLUME 13 | 31 [11,21], also, was not observed in this present study. Therefore, maybe for strength capabil- ity, training with this procedure (e.g. LV, 26Hz and MVC position) adopted is not useful for Taekwondo athletes. Another point to discuss is the vibration trans- mission characteristics. In general, the transmis- sion of vibration is attenuated when propagated through the body [2], and this attenuation depends on the joints angles. Although vibration has been applied direct to the ankle, we specu- lated that the joints angles of MVC position - spe- cifically ankle and knee - may have attenuated the vibration transmission and for this reason, the acceleration that reached the target muscle group (e.g. knee extensors and hip flexors) was probably less than the real acceleration that was generated by the vibration source. The influence of vibration stimulus on sports performance depend on many factors such as, the characteristics of the training protocol ( exercise, characteristic of vibratory stimu- lus, muscle action, exposure time and inter- val between the application the application of vibration and data collection), as also the samples of characteristics (trained, not trained, elderly, etc.). According to Luo et al. [24] trained individuals have higher acute effects to vibration training compared with untrained subjects. The volunteers recruited for this pres- ent study were national taekwondo athletes, so it was expected that they respond acutely and subacutely to the vibration stimulus. However, this hypothesis was not confirmed. The prin- cipal issue discussed in the present study has related the importance to understanding the stimulus performed. If the objective of the training is, generate PPA or to train on the PPA effects, the stimulus proposed here was not suitable. Nevertheless, if the intention is to use the mechanical vibration as an additional resource to conventional strength training, the coach need to know clearly about what kind of stimulus he intends to apply. If the impulse is important, use this type of vibration is not ade- quate. If the objective is to diversify the stimu- lus for maximal force, perhaps, the application of this type of vibration becomes an interest- ing training resource. CONCLUSION These data suggest that training with vibrations at a frequency of 26 Hz, amplitude at 6mm, directed to the resulting muscle forces, was not able to increase the performance of speed kick (subacute), maximal force and impulse (acute). In addition, reduced the maximal force subacutely. ACKNOWLEDGEMENTS Fundação de Amparo à Pesquisa do est estado de Minas Gerais (FAPEMIG – Minas Gerais / Brazil) and the Pró-Reitoria de Pesquisa (PRPQ) [Research Pro- Rectory] from the Universidade Federal de Minas Gerais. REFERENCES 1. Rittweger J, Beller G, Felsenberg D. Acute physi- ological effects of exhaustive whole-body vibra- tion exercise in man. Clinical Physiology 2000; 20(2): 134-142 2. Tankisheva E, Jonkers I, Boonen S et al. Transmission of Whole-Body Vibration and Its Effect on Muscle Activation. J. Strength Cond. 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