BIOMECHANICS AND MEASUREMENT ON ARCHERY ATHLETE: BALANCING BODY POSTURE

Transcription

1 BIOMECHANICS AND MEASUREMENT ON ARCHERY ATHLETE: BALANCING BODY POSTURE by AHMAD TAUFIQ BIN AYUB Thesis is submitted in partial fulfillment of the requirements for the award of degree of Bachelor of Mechanical Engineering Faculty of Mechanical Engineering UNIVERSITI MALAYSIA PAHANG JUNE 2013

2 vi ABSTRACT Body balance is a very important aspect especially in a precision sports like archery. However this balance might be affected by the stance and foot positioning by the archers themselves. The main purpose of this study is to study the correlation between the foot positioning and its effect on postural stability during arrow releasing in archery with the accuracy of arrow being shot. 15 archers ranging from beginners and elite were involved in this study. The time taken for the archers to draw and aim until arrow release was measured using the time frame from the high speed camera. The postural sway and the force changes on front left foot on legs were recorded using force platform for every single arrow shot. The postural sway on the X-axis of force plate is the movement parallel to the target face (side to side) is denoted by X-X and the sway on the Y-axis of force plate is the movement perpendicular to the target face (front-back) is denoted by X-Y. The archers were asked to perform 3 arrow shots from a distance of 7 meters to the target. From the correlation analysis, it was found that the foot angle has a positive correlation with the score shot by the archers. It is also found that archers with lower tendency of X-X and X-Y have a better and consistent result as compared to the archers with higher postural sways changes. An analysis on differences in postural sway changes of recurve and compound bow was also done and it was found that the difference in bow type used caused different effects on the postural sway changes pattern of the archers. Based on the result, it was also found that there was a weight changes acting on the front left foot of the archer after the arrow release.

3 vii ABSTRAK Keseimbangan badan merupakan aspek penting dalam sukan yang memerlukan ketepatan terutamanya seperti sukan memanah. Namun begitu, keseimbangan ini mungkin terjejas disebabkan cara berdiri dan kedudukan kaki daripada pemanah itu sendiri. Tujuan utama kajian ini adalah untuk mengkaji hubungan antara kedudukan kaki dan kesannya terhadap kestabilan postur badan semasa pelepasan anak panah serta kaitannya dengan ketepatan anak panah yang dilepaskan. 15 pemanah yang terdiri daripada pemanah baru hingga elit telah terlibat dalam kajian ini. Masa yang diambil pemanah untuk menarik dan membidik sehingga pelepasan anak panah telah diukur berdasarkan jangka masa daripada kamera berkelajuan tinggi. Perubahan postur dan perubahan daya pada kaki kiri hadapan pemanah telah direkodkan menggunakan platform daya untuk setiap tembakan anak panah. Peralihan postur pada paksi-x plat daya adalah pergerakan selari dengan sasaran (sisi ke sisi) diwakili oleh X-X dan peralihan postur ke arah paksi-y plat daya adalah pergerakan serenjang ke sasaran (depan dan belakang) diwakili oleh X-Y. Para pemanah telah diminta untuk melaksanakan 3 tembakan anak panah daripada jarak 7 meter kepada sasaran. Berdasarkan analisis korelasi, ia didapati bahawa sudut kaki menujukkan hubungan positif dengan skor yang diperolehi oleh pemanah. Ia turut didapati bahawa pemanah dengan kecenderungan X-X dan X-Y yang rendah menunjukkan prestasi yang lebih baik dan konsisten berbanding pemanah yang menunjukkan nilai yang tinggi pada perubahan peralihan postur. Analisis perbezaan pada perubahan postur daripada busur recurve dan busur majmuk juga dilaksanakan dan ia telah didapati bahawa jenis busur yang berbeza menyebabkan kesan yang berlainan ke atas corak perubahan peralihan postur pemanah. Berdasarkan keputusan yang diperolehi, ia turut didapati bahawa terdapat perubahan berat badan yang bertindak pada kaki kiri hadapan pemanah selepas anak panah dilepaskan menuju ke sasaran.

4 viii TABLE OF CONTENTS Page SUPERVISOR S DECLARATION STUDENT S DECLARATION DEDICATION ACKNOWLEDGEMENT ABSTRACT ABSTRAK TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES LIST OF SYMBOLS LIST OF ABBREVIATIONS ii iii iv v vi vii viii xii xiii xv xvi CHAPTER 1 INTRODUCTION 1.1 PROJECT BACKGROUND PROBLEM STATEMENT PROJECT OBJECTIVES SCOPE OF STUDY 3

5 ix CHAPTER 2 LITERATURE REVIEW 2.1 INTRODUCTION POSTURAL STABILITY LEG STANCE THE IDEAL STANCE VECTOR ANALYSIS TYPES OF BOW Recurve Bow Compound Bow 12 CHAPTER 3 METHODOLOGY 3.1 INTRODUCTION PARTICIPANTS EQUIPMENTS AMTI Force Platforms High Speed Camera (EPIX Silicon Video 643) Digital Cameras EXPERIMENT FLOWCHART EXPERIMENTAL SETUP DATA PROCESSING 19

6 x 3.7 FRONT FOOT ANGLE CALCULATION 19 CHAPTER 4 RESULTS AND DISCUSSION 4.1 INTRODUCTION DATA ANALYSIS Body Weight Changes Correlation of Postural Sway, Front Foot Angle and Scores Below 12 years old Category Below 15 years old Category (Recurve Bow) Below 15 years old Category (Compound Bow) Above 18 years old Category (Recurve Bow) Above 18 years old Category (Compound Bow) Comparison of Mean Postural Sway and Score for Different Classes Archers Recurve Bow Archers Compound Bow Archers Comparison of Mean Postural Sway for Different Bow Type Archers Recurve Bow Archers Compound Bow Archers Recurve and Compound Bow Archer Comparison SUMMARY 36

7 xi CHAPTER 5 CONCLUSION AND RECOMMENDATIONS 5.1 CONCLUSION RECOMMENDATIONS 39 REFERENCES APPENDICES APPENDIX A Average Postural Sway in Different Scores of Each Age Category 42 APPENDIX B Correlation Table for Each Age Category 43 APPENDIX C Graph of Average Postural Sway versus Age Category 45 APPENDIX D Archers Foot Position on Force Platform 47 APPENDIX E Archers Position at Full Draw 48

8 xii LIST OF TABLES Table Num. Title Page 2.1 Suggested drawing weights for recurve bows Suggested drawing weight for compound bows Participants Categories, Number and Experience Range Body Weight Changes Before and After Arrow Release for All Age Categories Data Correlation Summary for Below 12 Age Category Data Correlation Summary for Below 15 Age Category (Recurve Bow Archers) Data Correlation Summary for Below 15 Age Category (Compound Bow Archers) Data Correlation Summary for Above 18 Age Category (Recurve Bow Archers) Data Correlation Summary for Above 18 Age Category (Compound Bow Archers) 28

9 xiii LIST OF FIGURES Figure Num. Title Page 2.1 Pictures of common body posture mistakes done by archers Example of square and open stance Example of an ideal stance Illustration of a recurve bow Illustration of a compound bow Experiment Flowchart Experiment Layout Axis Labels on AMTI Force Platforms Example of Weight Changes in Archers Front Foot During Experiment Mean Postural Sway of Recurve Bow Archers from Different Classes Mean Postural Sway of Compound Bow Archers from Different Classes Mean Postural Sway Vs Num. of Trials for Recurve Bow Archers 32

10 xiv 4.5 Mean Postural Sway Vs Num. of Trials for Compound Bow Archers Postural Sway Comparison Vs Num. of Trials for Different Bow Type Archers 34

11 xv LIST OF SYMBOLS lbs. m cm N Pound Greater than or equal to Less than or equal to Meters Centimeters Delta Newton Degree r Correlation Coefficient

12 xvi LIST OF ABBREVIATIONS COP Center of Pressure

13 1 CHAPTER 1 INTRODUCTION 1.1 PROJECT BACKGROUND Precision sports such as archery is a fine, steady and coordinated action of many physiological organs, like the visual organs, the proprioceptors, the motor effectors and systems like the neurogenic, respiratory, cardiovascular, endocrinous and locomotor system. Unlike to most sport activities, target sports like archery requires the elimination of any movement that could perturb the stability of the system bow, to achieve the best performance on the target (Gianikellis K., 2000). According to many trainers, athletes and training books, postural stability is one of the most important factors that influence on the performance in precision sports. As stated by KiSik L. and Robert D.B (2005), the foundation of the shot is the static alignment of the body and the equipment that leads to a stable shot platform thus helps the archer s shots to be more consistent and less frustrating. The elements of the foundation of the shot includes the stance, including the foot position and leg alignment; the body position, including the orientation of the hips and rib cage; the archer s center of gravity and maintaining stability; the position of the bow shoulder; the position of the draw shoulder; the position of the bow arm; and the archer s

14 2 hand position and grip on the bow handle are also all part of the foundation. The foundation of a shot sets the stage for good shot performance (KiSik L. and Robert D.B., 2005) Keeping a stable body posture along the overall shooting process is very important as it might significantly affect the results and accuracy of arrow being shot. A good posture is extremely important for balance, string clearance, effective posterior chain use, and shooting mechanics. An ideal posture would see the centre of gravity of the archer centered between the feet (Logan O., 2011). A proper stance is very important to ensure a stable centre of gravity for the archers. The stance in an ideal position at full draw should be upright, balanced and comfortable position (Archery Australia, 2007). The objective of this research is to study the correlation between the front foot angle and its effect on postural stability with the archer s performance. An experimental work was setup with the cooperation by the archers from Majlis Sukan Negeri Terengganu (MSNT) with an age range of 10 to 20 years old. AMTI Force Plate set is used to obtain the data on the force exerted and postural sway of the archers throughout the arrow shooting process. AMTI Net Force and Bioanalysis software is then used to analyze the data from the force platforms which in turn helps to identify the position or angle of the foot of the archer in order to achieve the research objectives. 1.2 PROBLEM STATEMENT The main problem of sport technique in precision sports consists in maintaining the body segments as stable as possible in position and orientation. Experimental data in shooting sports proved that the posture that shooters adopt is mechanically unstable as consequence of the interactions among the body segments. In this way shooters and archers try to make their posture more consistent and reproducible reducing the variability of their actions during the aiming and shooting (Gianikellis et al., 1999). In the effort of achieving the best stability, archers also sometimes tend to adjust their body posture according to their instinct which sometimes leads them to adapt this mechanically unstable posture.

15 3 The archers normally will take position and place themselves in a comfortable and stable position prior to the drawing phase. The body placement normally starts with the foot placement on the ground to help them gather a stable base as they draw, aim and shoot at the target face. In order to achieve the best performance in archery, having a stable and comfortable stance plays a very important role because the stance will act as a base which supports the whole body structure of the archer throughout the whole arrow shooting process. Having a weak stance can cause the instability of the body thus causing the whole body posture to sway. This mistake can heavily affect the archers stability especially during aiming phase which requires a very stable and minor movement from the body system. In this research, an experimental work is executed to study on the relation of front foot angle and the postural sway with the accuracy of the arrow shot by archers. The front foot orientation is analyzed in terms of the angle while the postural sway is analyzed based on the data obtained from the force plates. The performance of the archers is observed based on the score the shot on the target face. 1.3 PROJECT OBJECTIVES 1. To study the correlation between the front foot angle and its effect on postural stability with the archer s performance. 2. To study the differences in the postural sway changes between recurve bow archers and compound bow archers. 1.4 SCOPE OF STUDY The scope of this research is focused on studying the front foot orientation and the postural sway and its effect on the accuracy of the archers as they draw the bow until the arrow is released. Front foot orientation is analyzed in terms of its angle from the X-axis of

16 4 the force platform. This angle can be calculated using vector analysis. Postural sway is observed based on the Centre of Pressure (COP) changes obtained from the force platform where the postural sway on X-Axis of the force platform (side to side) is denoted as X-X, meanwhile the postural sway on Y-Axis of the force platform (front-back) is denoted as X- Y. The performance of the archers is observed based on the score shot by them. In this study, apart from the age categories differences, there are also the differences in the bow types used by the archers which are the recurve and compound bow. Hence the data from recurve bow and compound bow archers are recorded and compared in order to study if there were any differences in their postural sway changes. The study regarding body weight changes in archers front leg is also incorporated in this research. The experiment is executed in an indoor environment with using a standard size target face for an indoor archery. The results obtained from this research met the main objective which is to study the correlation between the foot positioning and its effect on postural stability with the archer s performance.

17 5 CHAPTER 2 LITERATURE REVIEW 2.1 INTRODUCTION Archery can be described as static sport, which involves stable sequence of performed movement patterns (Hayri E. and Rafet I., 2010) and there exist a strong relationship between the stability of the archer and the accuracy of the arrow being shot. In order to get a good record in an archery competition, one requires a well-balanced and highly reproducible release during the shooting. In the literature, some of the researchers describe the shot as a three-phase movement: the stance, the aiming, and the sighting (Leroyer et al., 1993; Martin et al., 1990; Pekalski, 1990). The foundation of the shot is the static alignment of the body and the equipment that leads to a stable shot platform. When the body is stable, the archer s shots will be more consistent and less frustrating (KiSik L. and Robert D.B., 2005) The elements of the foundation of the shot include: the stance, including the foot position and leg alignment; the body position, including the orientation of the hips and rib cage; the archer s center of gravity and maintaining stability; the position of the bow shoulder; the position of the draw shoulder; the position of the bow arm; and the archer s

18 6 hand position and grip on the bow handle are also all part of the foundation. The foundation of a shot sets the stage for good shot performance (KiSik L. and Robert D.B., 2005). In target archery, there are normally two types of commonly used bows by archers which are the recurve bows and compound bows. Apart from the shape and design differences, these two types of bow have also had different mechanics applied to them. The recurve bow requires a smooth increase of force from beginning to peak weight at the end; meanwhile compound bow design causes the increment of peak weight in the first half of the draw stroke (Ruis S. and Stevenson C., 2004) 2.2 POSTURAL STABILITY Postural stability can be defined as the ability of an individual to maintain the position of the body, or more specifically, its centre of mass, within specific boundaries of space, referred to as stability limits (McCollum and Leen, 1989). Postural stability is a very important aspect in archery. A strong and stable posture will help the archer to achieve better performance throughout the repeated arrow release process. KiSik L. and Robert D.B (2005) stated that by establishing stability will reduce movement during the shot and increase the archer s sense of confidence. The foundation of the shot must, in every instance, be established precisely the same way every time to create consistency. This in turn helps to sets the tone for the shot and supports the shot, making it easier to execute and thus easier to duplicate. Logan O. (2011) also stated that a good posture is very important for balance, string clearance, effective posterior chain use and shooting mechanics. An ideal posture would show the centre of gravity of the archer centered between the feet. However archers tend to make body posture mistakes especially in the drawing and aiming phase. Common mistake that occurs is that the chest of the archer is leaning forward or the spine is curved backwards.

19 7 (a) (b) (c) Figure 2.1: Pictures of body posture common mistakes done by archers Figure 2.1 (a) shows an example of a good body posture of the archer where the centre of gravity is centered between the feet of the archer. Figure 2.1 (b) and Figure 2.1 (c) shows a bad example of body posture where in Figure 2.1 (b), the archer s body is leaning forward meanwhile in Figure 2.1 (c) the archer s body is curved backwards. Note the differences of the centre of gravity position between these three postures. This mistake can lead to an unstable position when the archers shoot the arrow thus affecting the accuracy. 2.3 LEG STANCE The stance is one of the most important basics in archery. It begins with the placement of the feet where the feet are placed in a comfortable and stable position. The legs of the archer should be relaxed as putting too much force in the muscles might cause the instability and affect the performance of the archer. A study done by Tinazci C. (2011) states that lower activity of muscles results in a better performance.

20 8 The elements of the foundation of the shot include: the stance, including the foot position and leg alignment; the body position, including the orientation of the hips and rib cage; the archer s center of gravity & maintaining stability; the position of the bow shoulder; the position of the draw shoulder; the position of the bow arm; and the archer s hand position and grip on the bow handle (KiSik L. and Robert D.B., 2005). There are several stances that are commonly used by the archers. A square stance would be the most common and basic stance used by archers. Other stances such as open stance show a different orientation of feet in with a wider angle of front foot from the center line of the body. (a) (b) Figure 2.2: Example of a square and open stance Figure 2.2 shows the example of the leg stance commonly used by archers. Figure 2.2 (a) is an example of square stance meanwhile the Figure 2.2 (b) shows an example of open stance. KiSik L. and Robert D.B (2005) states that a more open stance is acceptable for higher level archers who have found that the resulting twist of the torso increases archer stability in windy conditions.

21 9 2.4 THE IDEAL STANCE The idea of an ideal stance is that it is upright, balance and comfortable for the archer (Archery Australia, 2007). The leg should be in a position which allows the knee to be relaxed but it is not bent. An archer s weight distribution on the feet should be about 70% on the balls of the feet because keeping the weight forward improves the balance and can reduce the tendency of the body to rock while standing at the line (KiSik L. and Robert D.B., 2005). This might be due to the fact that the frontal part of the feet is broader, giving better stability with its wider surface area. This might also because of the fact that the body balance receptors are also located here. The more stable the archer is in the stance and the stronger they are at full draw, the shorter the time they took to shoot the arrow with more confidence (FITA Coaching Manual, 2003). Figure 2.3: Example of an ideal stance

22 10 From Figure 2.3, an example of ideal stance and body posture can be seen. Notice the vertical and upright line alongside the upper body and lower body of the archer. The stance of the archer is also a bit opened and angled to the side, which gives a bit twist to the torso of the archer. This position helps to bring out the best stability and what s most important is that it provides comfort to the archer during arrow shooting. 2.5 VECTOR ANALYSIS The angle for the front foot can be calculated by applying the vector analysis. When two forces F 1 and F 2 act at right angle angles to each other, the direction of a their resultant R that is θ can be found by trigonometry (Avison J., 1989) tan θ = F 2 F 1 (2.1) Where θ = Angle of resultant force F 1 = Force 1 F 2 = Force 2

23 TYPES OF BOW Recurve Bow A recurve bow is characterized by the slender elastic arms or limbs and it is braced by putting a string shorter than the bow between the tips of the limbs. Additional deformation energy is stored in the elastic limbs by drawing the bow into the fully drawn position. Part of this amount of energy is transformed into kinetic energy of a light arrow (Kooi B. W., 1991). Figure 2.4: Illustration of a recurve bow Recurve bow also has its own draw weight. The list on the suggested draw weight range is described as in Table 2.1.

24 12 Table 2.1: Suggested drawing weight for recurve bows Archer's Weight Small Children ( lbs.) Larger Children ( lbs.) Small-Frame Female ( lbs.) Medium-Frame Female ( lbs) Small-Frame Male ( lbs) Medium-Frame Male ( lbs) Large-Frame Females (160+ lbs) Large Frame Men (180+ lbs) Suggested Drawing Weight lbs lbs lbs lbs lbs lbs lbs lbs Referred from: Compound Bow Compound bow is a modern bow that uses a system of wheels, cables and the leverage of cam design, compound bows allow the shooter to draw the bow back so that the peak draw weight of the bow is reached and held for part of the draw stroke, and then drops off into a valley at the back of the draw stroke so that the shooter is holding only part of the peak draw weight when at full draw (Alex G., 2011).