“Kinematics is the branch of mechanics which describes the motion of objects using words, diagrams, numbers graphs and equations” (Sand 1984). Using this branch of mechanics it becomes possible to accurately study and record flight paths and release positions of objects used in sporting situations. For example studying a javelin throwers throw can determine if the athlete is releasing the javelin at the optimum angle, these results can be taken to the coach who can then work on the areas needed to improve the performance.
“The biomechanical analysis of different events can help to understand the critical points of the technical performance, thus helping coaches and athletes in their preparation” (Grimshaw 1998). This report is based around the values received from the analysis of a netball shot. Netball shooting uses only the upper limbs simplifying the shot thus reducing the margin for error. Wilma Shakespeare (1998) said that ‘Netball has a unique shooting style: the ball is released high and is usually released when the shooter is stationary’.
Aim To retrieve kinematic values for the joint centres and ball characteristics involved in a netball shot using digitilisation. Method The participant selected went through the preperation procedure which included filling out the par-Q and consent form, being weighed, height, age, sex and sport. Sport – Football Height – 186cm Weight – 83kg Age – 21 Sex – Male The participant’s joints required were clearly marked with a black cross using tape.
The video camera was positioned at the correct height and angle to clearly record the entire body position of the participant. The scale stick was then recorded in order to put the participant and ball in scale, with the rest of the frame. The participant was then recorded taking three shots, the clearest of these was used for the digitalisation process. The digitalisation process was then recorded using seven frames, three prior and three post to the release frame. Using these frames the joint positions were plotted and recorded on a piece of OHP plastic.
The positions were then calculated and put into a microsoft excel programme to calculate the joints and objects velocity and acceleration. These results were then put into tables and graphs for interpretation and analysis. Results Table This table shows the centre points of the joints recoreded throughout the seven frames, in respect to the kinograph used. Graph 1 This graph has plotted the increases and decreases in velocity of the different joint centres between the frames captured. Table 2 Release velocity 8. 84m sec Height of release 2. 3m Angle of release 44. 96 deg This table includes three vital pieces of information. As with these three figures it is possible to predict the flight path of the ball.
The release velocity was calculated by the preset excel programme. The height of release was established by measuring the scale, then measuring from the bottom of the scale up to the centre of the point of release then dividing them. The angle of release was established through the use of pythagoras theorem, then trigonometry. The length of the velocity vector was calculated, then through this an inversion cosine sum was used to find the release angle. Graph 2 This graph clearly indicates the changes in acceleration of the joints between frames.
Discussion Refering to Graph1 the velocity of all the joints except the wrist is very varied, there is no constant speed to maintain a good posture. If the velocity was constant then the posture and angle of release would have been better. As the velocitys change a lot this means there was no fluency in the shot and that the participant may have been jurking causing an unevency in the shot. At the point of release however the instant velocity increases in only one out of the four joints, it decreases in the other three.
This could mean that the participant thought the shot would be too powerful, or that they were readjusting themselves at the last second. Starting from the point of release the shoulders velocity increases, then in the next frame the elbow and the fingers increase. This is more than likely due to the necessaty for a follow through on the shot. Graph 2 shows that once again at the point of release three of the four joints produce a reduction in their acceleration. However they also show that post to the release phase all the joints increase their acceleration causing the follow through.
The velocitys and accelerations of the joint centres almost mirror each other, this is due to the phasing they are going through. The shot starts with the shoulder building the power then pushing through the elbow, wrist then hand. The release variables in Table 2 show the characteristics of the ball at the time of release. These results are the ones used to determine the flight path of the ball and thus determining the success of the shot. The aim of scoring in netball is to get the ball in the hoop which is ten feet high.
The variables in this instance is that the players are only allowed to shoot within a zone known as the “goal circle” which is 16ft is circumfrance. Therefore the players can shoot from different distances thus affecting their release variables. The optimum release angle for any thrown projectile landing at the same level as the release height will be 45 deg, this is confirmed by V. Zatsiorsky, 2000, “The optimum angle for maximum range for zero relative release height is 45 deg”.
As already established the netball hoop is at a height of ten feet, therefore the optimum release angle for a netball shot will be greater as its release height is 7 foot. Therefore the projectile must be released at a greater angle in order to compencate for the difference in height. The release angle of the projectile in this report was 45 deg therefore the angle would need to be increased in order to achieve the goal. A netball shot is mainly taken without the use of the legs nutralising the legs in order to allow the higher limbs to contol the shot.
The use of legs in this situation would complicate the matter as the player is so close to the goal they margin for error is minimal, and keeping the legs out of the action reduces the margin of error. If the legs were involved within the action this may cause too much power resulting in a greater velocity from the arm joints thus causing the participant to overshoot the goal. However this is argued by Dr B. Mason, 2001 “the forces applied through the legs while shooting provide most of the acceleration for the shot. ”
Due to the subject’s height and weight the release height and velocity were greatly reduced compared to that of a participant of smaller scale. A participant that was for example, 5’6″ and weighed 65kgs they would have to increase their release velocity in order to compensate for the lack of release height. If this report were to be carried out on an elite netball player then the results are greatly beneficial to them and their coaching. As these results can determine whether the participant is releasing the ball at the right angle or if their release velocity is not as fast as it should be.
This can be taken to their coach who can then define the areas that need refining and work on them, thus improving overall performance. Conclusion The results received from this test are accurate and give true values for the velocity and accelerations of the joint centres and projectile. The participants movements and the release values have ben obtained allowing an accurate analysis of the shot to be carried out.
Reference List Dr B, Mason (2001), Services provided to netball, this can be found at: http://www. ais. org. au/biomechanics/sports. asp#netball, accessed on 9/1/2005.P, Grimshaw, (1998), The Use of Motion Analysis as a Coaching Aid to Iprove the Individual Technique in Sprint Hurdles, this can be found at: http://www. coachesinfo. com/article/215/, accessed on 8/1/2005. P, Sand (1984), Kinematics in one dimension, this can be found at: http://optimus. msc. net. ph/mike/acd/kinematics. htm, accessed on 9/1/2005. V, Zatsiorsky (2000).
Biomechanics in sport performance, published by Oxford Blackwell Science 2001. W, Shakespeare (1998), Netball, steps to success, published by Drayton publishers Ltd 1999. ASCDO5 9/1/2005.