Optimization of the Raiz Skill Training Methodology Based on 3D Kinematic Analysis

Transcription

1 Optimization of the Raiz Skill Training Methodology Based on 3D Kinematic Analysis PETR HEDBÁVNÝ, MIRIAM KALICHOVÁ Faculty of Sport Studies Masaryk University Kamenice 5, , Brno CZECH REPUBLIC Abstract: - Capoeira is a martial art based on kicks as well as acrobatics. Using a 3D kinematic analysis this work aims to determine the technical basis of the selected acrobatic skill, the raiz, and subsequently to draw methodological recommendations for the practice of this motion structure. The basis of the observed skill is formed by an aerial cartwheel with full turn around the longitudinal axis of the body. This is a case study in which Contramestre Sazuki, a capoeira Master from Brazil and a Grupo Candeira member who is currently devoted to training activity in the Czech Republic, acted as a tested person. The kinematic analysis was performed using SIMI Motion system. The record was used to evaluate temporal characteristics of individual phases of the movement, spatial and velocity characteristics of lower extremities, the centre of mass as well as angular characteristics of the body rotation around its longitudinal axis. The evaluation showed the degree to which individual segments are involved in the movement and their temporal sequence. Based on the detection of mechanical principles affecting the execution of the raiz skill we were able to draw recommendations for the training practice which, if used, may facilitate the practice of this complex acrobatic skill and enhance safety in motor learning. Key-Words: - Biomechanics, kinematic analysis, technique, performance, capoeira, acrobatics 1 Introduction The art called capoeira most probably came into existence as a result of African slaves travelling to Brazil in the colonial era. [1] Despite the difficulties it was faced with over centuries, capoeira was established as a national martial art in Brazil, received uniform rules and became one of the most popular sports there second only to football. [2] The basics of capoeira consist in the mastering of a ginga which is a repeated rocking movement from side to side. Besides being a basis for every kick as well as acrobatic skills, a ginga is also a basic fighter s movement which he resumes following the execution of any other skill. The continuous movement is designed to confuse the opponent, disguise the actual intention and make an attack more difficult for an opponent to handle [3]. Kicks are usually circular and are performed in all possible positions when lying, on one hand, on the head, etc. Most recently capoeira has also included acrobatic skills, e.g. cartwheels, front handsprings, back handsprings, saltos, pirouettes and other. These acrobatic skills often are of complex spatio-temporal structure. Requiring an accurate execution, they put high demands on coordination abilities. Understanding of biomechanical principles which directly affect the performance of this skill may facilitate the training process of a given motion structure. Methodology optimization is thus expected to lead to improved safety and higher training efficiency. In the area of martial arts and fighting sports an increasing number of authors deal with the biomechanic essence of punches [4-7], kicks [8-12], falls [13,14] and other techniques. To understand the above-mentioned spatiotemporal changes it is essential to utilize the most modern methods, a 3D kinematic analysis being one of them. Deficiencies in the technique of complex motion structures may not be observed by the coach s naked eye. Therefore, it is necessary to utilize high frequency cameras as well as special software for the subsequent digitization of the performed movement. To understand the technique of the monitored motion structure, it is important to evaluate the velocity and angular characteristics of individual parts of the body, especially their maximum values. The treatment of capoeira from the perspective of technique utilization and biomechanics is not, however, sufficiently represented in the available ISBN:

2 sources. For this reason and in cooperation with a capoeira Master from Brazil we have decided to broaden our findings in this area. Previously we carried out a biomechanic analysis of the bencao skill [15]. In this paper we have focused on a raiz as one of the acrobatic capoiera skills. This is a skill composed of an aerial cartwheel with full turn around the longitudinal axis described in gymnastics as Butterfly with 1/1 twist or Tong Fej [16] (Fig. 1). Fig.1 Kinogram of the aerial cartwheel with full turn (from FIG [17]) To facilitate the biomechanic analysis we will break the skill into three phases: take-off phase, flight phase and landing phase where the take-off phase may be further subdivided into swing and the actual take-off. For the proper performance of the swing, active movement of the trunk and arms against the mat is necessary thus obtaining the part of energy needed to perform the entire skill. This is followed by the stop of the trunk movement, the backswing of the swing leg and dynamic take-off where the latter has to be sufficiently large to allow the performance of the skill in its entirety. On the basis of the momentum change equation we may state that the body receives momentum through the impulse of the ground reaction force. Based on the principle of momentum conservation we know that the momentum gained during take-off is the same throughout flight and cannot be changed. Through the movements of individual body segments during the take-off phase the body receives the rotation around the central longitudinal axis, as the muscle forces are applied to lever arms and produce a torque. For faster rotation during the flight phase, it is necessary to reduce the moment of inertia. This is done through the approach of the body weight to the rotation axis, which is achieved through sufficient straddle and extending arms out. When landing the swing foot is the first one to come into contact with the ground with the take-off foot to follow. and subsequently to make methodology recommendations for the practice of this motion structure. We have formulated these questions to which answers will be provided later: Question n. 1: What are the temporal characteristics of individual microphases? Question n. 2: In which macrophases do upper and lower extremities tend to achieve their peak velocity values? Question n. 3: What trajectories are followed by the centre of mass and the ankles? Question n. 4: What are the angular parameters characterizing rotations during the take-off phase? Contramestre Sazuki whose real name is Vanilson Alessandro de Areu served as a tested person selected by purposive sampling. Having practised capoeira for 29 years this Master originally from Brazil currently dedicates his time to coaching activity in several cities in the Czech Republic. Being a Grupo Candeira member he represents this group at workshops in Europe, e.g. in France, Spain, Poland, Ireland and other countries. The kinematic analysis was performed using the SIMI Motion system made by a German company SIMI Reality Motion Systems based in Unterschleisseheim. Thirteen retro reflex markers that are used to identify individual anatomic points were placed on the body of the athlete. The raiz skill was performed five times by a tested person from which the best three attempts were chosen by the Master himself for subsequent analysis. 3 Results If the below results are not specifically attributed to any of the attempts, the given values are to be treated as equal to the arithmetic average of all three analyzed attempts. Figures and graphs have been created using the first attempt record. 2 Methods Goals of this work. On the basis of the 3D kinematic analysis this work aims to determine the key technical characteristics of the raiz skill movement Fig. 2 Raiz skill kinogram: a) take-off iniation, b) take-off completion, c) flight phase, d) landing iniation, e) landing completion ISBN:

3 3.1 Temporal characteristics of movement The kinogram of the raiz skill (Fig. 2) shows the position of the body at the beginning and end of each phase. At the beginning of the take-off phase there occurs an active movement of the trunk and arms against the mat while the swing movement of the right lower extremity is initiated. During the second take-off microphase, the right ankle finishes the swing while the lower extremity becomes involved in the take-off through concentric muscle activity. The entire take-off phase lasted s on average. The flight phase lasting on average s is initiated upon the left foot leaving the ground. The body rotates around its longitudinal axis where its centre of mass is moving along the projectile motion trajectory. The landing phase is initiated upon the touchdown of the right foot on the mat and finished with the left doing the same. This is followed by putting both legs together and the execution of the basic capoiera step, the ginga. The whole of the landing phase, i.e. up until the moment of contact of both of the lower extremities with the mat, lasted s on average. Individual time values are given in Tab. 1 Table 1 The length of each phase Movement Phase Time Take-off phase 0.208s ± Flight phase 0.425s ± Landing phase 0.191s ± Total 0.824s ± Speed characteristics of movement Table 2 shows the maximum velocity values obtained from selected anatomical points of the body, which were the distal joints of extremities, i.e. the right and left wrists, and the right and left ankles. Table 2 Maximum velocity values of the selected anatomical points of the body First Second Third Average attempt attempt attempt ± SD Left ± wrist Right ± wrist Left ± ankle Right ± ankle In addition to the maximum magnitude of the monitored speeds it is also important to note times when they were achieved. The maximum speeds of the right ( s) and the left wrists ( s) were recorded in the first part of the take-off phase (Fig. 3), i.e. during the active movement of the arm against the mat. The right wrist, which is the first one to initiate movement, reached its peak a little earlier than the left wrist. The graph shows that as soon as the wrists achieve their maximum speed, the dynamic back swing of the right lower extremity is initiated. Fig. 3 A) maximum right wrist speed, b) maximum left wrist speed The maximum speed of the right ankle ( s) was achieved in the second part of the take-off phase, just before the completion of the actual takeoff (Fig. 4). The maximum speed of the left ankle ( s) was recorded in the landing phase, just after the right foot comes into contact with the ground (Fig. 4). Fig. 4 A) maximum right ankle speed, b) maximum left ankle speed 3.3 Distance characteristics of movement Due to the nature of the observed motion structure it was of particular interest to us to find out trajectories the ankles and the centre of the body mass are moving along. As is shown in the graph (Fig. 5) the centre of mass follows the projectile motion trajectory, i.e. a parabolic curve. It reached its maximum height of m with a time of s which is indicated in the graph by a vertical line. Furthermore we can see that the ankles were able to considerably exceed the maximum height of the centre of the body mass with almost the same difference. The right ankle was able to ascend to the height of m with a time of s, the left ankle was able to ascend to the height of m with a time of s. The delay of the left ankle therefore amounted to s, which corresponds to the non-simultaneous temporal involvement of lower extremities in the take-off phase. A parabolic shape of the ankle movement curves are affected by ISBN:

4 the fact that these two joints follow trajectories which are almost circular in shape (Fig. 6). Fig. 5 Changes in the position of selected anatomical points on the vertical axis Fig. 7 Changes in the angles formed by the shoulder axis and the horizontal plane and by the hip axis and the horizontal plane Fig. 6 Ankle trajectory 3.4 Angular movement characteristics The body rotation around its longitudinal axis is an important feature of the analyzed raiz skill. For this reason we have also focused on the angular characteristics of this rotation at its start. Other noteworthy features include changes in the angle made by the shoulder axis and the horizontal plane and changes in the angle made the hip axis and the horizontal plane. As shown by the green curve in the graph (Fig. 7), the shoulder axis turns around first, followed by the hip axis with a small delay. The vertical line marks the moment when the shoulder axis is making almost a right angle (82.7 ) with the horizontal plane, while the angle formed by the hip axis and the XY plane, is less than 45 (Fig. 7). 4 Discussion Question n. 1: What are the temporal characteristics of individual microphases? The measured data, especially their standard deviations indicate the stable movement stereotypes in tested individuals when performing the raiz skill. To achieve its intended goal - to surprise an opponent, the whole skill should be performed as quickly as possible. At the same time, however, the flight phase should be high enough for the jump to overcome an obstacle or an opponent. A dynamic take-off lasting approximately 0.2 s is therefore particularly important. The flight phase is the most significant not only in terms of the spatial but also temporal characteristics. When performing this acrobatic skill, for more than half of the time a capoeirista goes through the unsupported phase, during which his body must make a 360 turn around the axis which passes through the centre of mass. This phase cannot be accelerated which is not desirable anyway. What is desirable, however, is to achieve maximum height of the centre of mass ISBN:

5 which is related to the growing flight duration. Landing is the last phase, which again should be carried out as fast as possible in order to launch the next skill and to reduce the opponent s reaction time. The results showed that, similarly to the takeoff phase also the landing phase takes approximately 0.2 s to perform. Question n. 2: In which macrophases do upper and lower extremities tend to achieve their peak velocity values? The measured velocity values for distal anatomical body points are essential for understanding of the causes of the observed movement. Both the wrists reached their maximum velocity in the swing phase against the mat. On the basis of mechanical laws, we believe that through this movement the pressure force against the ground increases. Based on the Newton s Third Law of Motion the growth of this force is accompanied by the growth of the ground reaction force which is directed towards the centre of the body mass and which sets it in motion. The maximum speed of the right ankle was also observed in the take-off phase. With the increasing swing speed of this lower extremity there is a growing momentum of its segments. If the swing is stopped just prior to the other lower extremity leaving the ground, the effects of the law of conservation of momentum within the body may be observed. That is to say the momentum of the swing lower extremity segments is transferred to the heavier body segments, resulting in the movement acceleration of the centre of the body mass upwards. The left ankle speed reached its peak following the right foot landing on the mat. It is evident that no movement performed during the flight phase can change the centre of mass trajectory. It may, however, change the position of indivual segments to each other. The accelerated movement of the left lower extremity downwards just before landing, therefore, in our opinion, helps, on the basis of the law of momentum conservation inside the body, to erect the trunk while maintaining the centre of mass parabolic curve. Following the right foot landing on the ground, the left leg swing is used to rapidly put the legs together and complete the skill. Question n. 3: What trajectories are followed by the centre of mass and the ankles? Of all the observed trajectory curves the ones representing the centre of mass and the ankles are the most important. Simultaneously the body performs a sliding movement along a parabolic curve and a rotational movement around an axis passing through the centre of mass. The results primarily refer to the temporal coordination of these movements while the parabolic trajectory of the centre of mass clearly shows the change of its position on the vertical axis as well as the moment when the highest point is reached. Through simultaneous observation of the ankles s position in space, we found that at the maximum height of the centre of mass level one of the ankles is already below its maximum height while the other is still rising and is approximately at the centre of mass level. These data indicate that when the centre of mass rises to its maximum height, the rotation of the body is not even half way through yet. We can also observe that at this moment the centre of mass starts to decline, with the take-off leg further rising by about 0.5 m and thus reaching a height corresponding to the previous maximum height of the swing leg ankle. Thanks to this capoeiria performer looks as if not affected by gravity temporarily. This is because the observer tends to perceive especially those body parts that are currently situated most highly. Thus he does not realize the fall of other segments of the body, especially of the swing leg, which is made possible only through an athlete s considerable flexibility. Question n. 4: What are the angular parameters characterizing rotations during the take-off phase? The existence of biomechanical principles means that the initial rotation of the body must be initiated even during the supported phase. Under certain circumstances it is possible to partially transfer the initial rotation of the body to the secondary rotation as late as the flight phase, this being a procedure often used in gymnastics for acrobatic skills with rotation around the longitudinal axis of the body. Through a kinematic analysis we found that in the case of the raiz skill the rotation impulse to move the body around the longitudinal axis is given in the take-off phase. The start of rotation is initiated by the movements of the shoulders and is followed by the rotation of the hip axis. 5 Conclusion Raiz, an acrobatic capoeira skill was observed during its three stages: take-off, flight and landing. To make the processing of the given skill easier to follow we have further subdivided the take-off and landing phases into two microphases. The former was divided into the swing initiation and the actual take-off subphases while the latter was broken down into the first foot landing on the mat and the other foot landing on the mat (putting both legs together). The nature of this motion structure indicates that the take-off phase serves as a preparation for the next unsupported phase, which is crucial to the entire skill. Quality performance of the flight phase ISBN:

6 directly affects the quality of landing, the performance of which is key to the fluent establishment of the following skills. The skill was analyzed from four different perspectives. First, we focused on the temporal characteristics, then on the speed, spatial and trajectory-based as well as spatial and angular characteristics. We would have not been able to conduct such a detailed spatio-temporal analysis without the use of our technical equipment. The necessary digitization of the analyzed motion structure provided us with answers concerning the technical basis of the movement which is transferrable to the training methodology of this skill. Having resolved the research questions we were then able to get deeper into the mechanical nature of this coordination-intensive skill. Temporal characteristics showed that a well acquired skill may be repeated with minimal differences in performance, which is essential for its use in unexpected and constantly changing combat conditions. Furthermore and from the abovementioned reasons, the temporal characteristics suggest an effort to minimize the duration of the take-off phase and landing phase. The speed of the execution of these phases is not only affected by temporal characteristics, but it is also supported by the measured speed values for the observed anatomical points. The swing of both arms downwards against the ground and the swing of one leg upwards during the take-off phase contributes to the dynamic take-off and the maximum height of the flight phase. The fast swing of the other leg at the moment of landing helps to complete the skill and to rapidly acquire a stable position using the support of both feet. By following the trajectories of the centre of the body mass and the ankles in space, we found that during the flight phase lower extremities are kept apart almost in the same way as upon the takeoff completion. The whole of the acrobatic skill is largely spatially symmetrical, as shown by almost identical ankle trajectories which differ only in the time delay with which the other ankle movement was performed. In addition to the rotation around the transverse axis of the body, the raiz skill is also typified by the rotation of the body around the longitudinal axis passing through the centre of mass. According to our measurements this rotation is initiated by the movement of the shoulder axis and passes through the body to the hips only with a time delay. It is necessary to draw conclusions from these findings for the training practice, and thus to optimize the motor training phase of this coordination-intensive skill. For the take-off stage it is essential to focus not only on the upward motion of the centre of mass, but also on the dynamic swing of the arms towards the ground while bending forward. This is followed by the dynamic swing of one leg where obtaining the maximum velocity is subject to a high degree of flexibility of the lower extremities. This flexibility is desirable also in the following flight phase as the increasing straddle is accompanied by the decreasing moment of inertia of the body and the increase in its rotation speed. The dynamic swing is accompanied by the other leg take-off. This activity is dependent on the explosive strength of the extensor joints of the take-off lower extremity. These movements set the centre of the body mass in projectile motion and simultaneously launch a rotation of the body around its transverse axis, i.e. into aerial cartwheel it may be said. The biomechanical analysis suggests that the capoeirista should first master this simplified version of the skill both from the coordination and strength perspective. Only after mastering this body rotation around its transverse axis which leads to the achievement of the required height of the centre of mass during the flight phase can we proceed to the training of the complex motion structure of the raiz skill and add rotation around the longitudinal axis of the body, i.e. to perform a twist. An impulse to this rotation is provided through the movement of the shoulder axis aided by the asymmetrical swing of the arms against the ground. Mastering of this skill also requires a high level of kinaesthetic - differentiation ability and spatial orientation. Prior to the training of the skill itself, we recommend to complete gymnastics training, especially jump and rotation training [18, 19]. In the actual capoiera training, it is also suitable to use a trampoline which will facilitate force exertion during take-off and thus enable the athlete to concentrate on the correct performance of individual movements from the coordination point of view. 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