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New Technologies in Sport and Coaching |
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Dr
David Lloyd and Dr Thor Besier Introduction In the current age of digital technology, there is increasing demand for coaches to become proficient at integrating the latest technological advancements in video, computers and ‘gadgets’ into their daily coaching practise. But is it all worth it_ And what is actually gained (or indeed lost) from becoming a ‘techno-coach’. As this area of sport science has traditionally been the realm of the ‘biomechanist’, we are in a unique position to provide some insights into the current technologies that are being applied around the world by coaches and sport scientists. But first, we must explain the biomechanist and where they are coming from, as they are typically the people that drive the new technologies. What is Biomechanics_ Biomechanics is the study of the mechanics of biological systems, which obviously includes humans, at least most of the time. As musculoskeletal biomechanists, we have a particular interest in the mechanics of human muscle and the skeletal system during movement. This can range from macro or segmental mechanics (how the limbs move, what causes them to move, and the external forces applied to the system), to cellular or tissue biomechanics (understanding the movement and loading of human tissue such as muscles, ligaments, and cartilage). Typically, the biomechanist examines forces and motion to understand mechanisms of injury or to enhancement performance. The Analysis Process Before describing all of the weird and wonderful tools available to the coach, let us first consider the context in which these tools should be used. As a coach or biomechanist, it is your role to provide meaningful feedback to your athlete regarding their movement and performance. The analysis should form a continual process between the coach and athlete and can be simplified into a 4-stage model, as shown below in Figure 1. The analysis process involves understanding your athlete, the movements that you wish to understand and having a goal that you wish to achieve. Then you must observe and evaluate the performance before providing feedback to the athlete. The tools that we describe in this article relate to steps 2,3, and 4 of this model, as they can help you observe, evaluate, and provide feedback to your athlete.
The Tools of the Trade This section covers some of the tools that can be used to describe (qualitative) and measure (quantitative) movement. A qualitative analysis can prove invaluable for understanding or recognising a problem, whereas a quantitative analysis might be essential for tracking changes in your intervention, or comparing between athletes. The Eyeball The human eye is still perhaps the most powerful motion analysis tool that we all have at our disposal. A trained coach can often pick up subtle changes and differences in technique and explain these to the athlete, providing instantaneous feedback. And because we have two eyes, we also have the ability to visualise movements in three dimensions (3D)! However, this skill takes time and practise to develop and is still a subjective measure by the coach. Also, if you are wishing to analyse faster movements such as the position of a tennis racquet prior to contact with a ball, then you are likely to miss important information with the naked eye. Unfortunately, we cannot capture any information we see and play it back in slow-motion so we must look towards using some sort of image capturing system. Video Camera Before explaining how video can be used effectively for coaching, we shall briefly indulge in some technological issues. Standard video cameras we use in Australia work in PAL format, which means they operate at 25 frames/sec. The time between each frame is therefore 0.04 sec and this frame rate may limit the motions analysed using standard video. However, each frame of a video image is actually made up of two separate images or fields, which are 0.02 sec apart. Therefore, standard PAL video footage operates at 25 frames/sec or 50 fields/sec. When you play a video in slow-motion through some VCRs you see each field, so one second of video gives you 25 frames or 50 fields. For example, if you are interested in the motion of a foot of a runner as it strikes the ground, you might get only 5 frames of video footage over this time period. This may be better analysed using a VCR that has slow motion field play back mode that would provide 10 fields of the foot strike event. However, if you wanted to analyse the motion of a tennis racquet at ball impact, you would have to be extremely lucky to catch this motion on standard video even at 50 fields/sec, so you would need to use ‘high-speed’ cameras Video is a very powerful form of feedback for the athlete, and perhaps is not used enough in everyday coaching practise to qualitatively track performance and technique. A limitation of video is that it can only give you information in two-dimensions (2D). Therefore, when you are interested in movements and rotations of the body, the camera must be perfectly perpendicular to the plane of motion or you will get perspective errors. A prime example of this was the apparent arm extension (or throw) of the Indian cricket spin bowler, Muttiah Muralitharan during a recent tour of Australia. From a 2D-camera view, it appeared as though Muralitharan extended his elbow during the bowling delivery, however, following a 3D analysis (see below), it was found that the elbow angle did not change during the bowling action, ie. he did not throw. Rather, there was considerable upper arm and shoulder rotation, which, when viewed in 2D, appeared as though his arm was straightening. Computer Analysis using Video There are many different software packages available that allow you to capture video onto a computer for further analysis. These packages allow you to manipulate images, draw on the screen and take some simple two-dimensional measures such as angles, distance and speeds. Apart from being able to take measures from video, these packages are useful in providing split-screen views so that you can display multiple camera views simultaneously to provide a better representation of the three-dimensional motion that is occurring. You can also use the split screens to display technique changes before and after an intervention, or compare novice and elite performers side by side. The current cost of digital cameras and laptop computers is beginning to make these type of portable motion analysis systems more attractive to the average coach, who would previously not have access to such equipment. High-speed Video If you are videoing fast sporting movements, then you need to use specialised cameras that are capable of high frame rates (ranging from 100 to 5000 frames/sec) to ensure that you capture the frames of interest. With some of these cameras the videos can be replayed through a normal VCR, but playback is in slow-motion and can be used to perform some qualitative analysis and provide feedback for the athlete. However, high-speed cameras are mostly incorporated with a computer system in what is called a motion analysis system, which enables digitising of the video. These will now be discussed. Two Dimensional Motion Analysis of Video Quantitative analysis of a images from one video camera is different to qualitative analyses simply because a calibration rod (eg. a one metre ruler) is videoed so that image pixel positions can be converted to actual measurements, ie. metres, millimetres etc. Once this is done the 2D positions of joints or markers on the subjects can be digitised and measured using the motion analysis software that comes with these systems. The big step from here is to create a link-segment model (the old "stick figure") of the person, which is also performed using the motion analysis software. You can then measure the segment angles and joint angles between segments to help analyse what might be going wrong. An example of an output is shown in figure 2. By comparing the person's curves with the average curves from a "normal" population of people some quantitative difference could be identified and targeted for change by training. By doing the same analysis after a period of training, data would be available to quantify outcomes and ensure changes have been achieved. One of the main problems with 2D quantitative analyses is that the data is only as good as the video images recorded. More importantly, the movement being videoed must occur mainly in one plane, like walking or running. The camera must be perfectly perpendicular to this plane of motion or you will get perspective errors. If there is any large rotations out of the plane of movement then you will generate large errors in the data that you collect. In these cases you need to resort to 3D motion analysis methods. Three Dimensional Motion Analysis from Videos These systems use multiple cameras connected to a computer to enable 3D quantitative analysis of movement. The cameras used can be normal PAL 25 frames/sec units, but the computer software can be split the video images into the individual fields so that the analyses can be performed at 50 fields/sec. This may be adequate for analysing many sporting movements. If this is not the case high speed cameras can also be used. Any of these systems tend to be expensive as these synchronise all the cameras so that all the frames from each video camera are taken at exactly the same point in time. The computers have motion analysis software that allow you to digitise points such as joint centres or markers. To enable 3D measurements to be taken from the multiple cameras a calibration grid is videoed that permits calibration of the 3D space that person moves in. This enables us to reconstruct 3D "rigid body" models of the athlete’s motion that was measured (see Figure 3). The Biomechanics team at our department recently used 200 frames/sec cameras and Peak motion analysis system to reconstruct and analyse the fast bowling action of Shoaib Akhtar. Advanced Three Dimensional Motion Analysis In advanced 3D motion analyses, multiple specialised video cameras are used to measure motion, which have sets of lights (usually infra-red) located around the lens of the camera. These illuminate markers covered with a retro-reflective film, similar to the material on road signs so these can be seen at night. The 3D position of markers attached to the person are used to create a "rigid body" model. These systems are all computer controlled and collect data from other types instrumentation additional to motion data. Typical instrumentation would be force plates and electromyography (EMG) systems and data produced is synchronised to the motion data collected. Force plates measure the forces that people exert, for example, on the ground or against a wall. When analysing a person walking, running or side stepping force plates are set into the ground, flush with the surrounding surface. The subject then walks or runs across the surface and one foot strike is measured with the force plate. More foot strikes could be measured if more force plates are available. EMG systems measure muscle activity by using electrodes placed over the muscles (the same used to take a electrocardiogram - ECG). This enables one to assess how active a person's muscles are when they perform some movement. These motion analysis systems are very expensive , but currently available systems essentially operate in real time in that the 3D movement data and other data collected are produced before the system is ready to capture the next field of video data. We used a similar system (a 6 camera Vicon system) with the marker set and model shown in figure 3 to analyse the bowling action of Muttiah Muralitharan (Lloyd et al, 2000). We also use this system, with force plates and an electromyography system, to determine the loading of knee when athletes do side steps as seen in AFL, rugby, soccer or netball (Besier et al., 2001a,b). This is helping us to identify the reason for knee ligament ruptures and ways to reduce the incidence of these injuries in sport. The data collected with these motion analysis systems can be input into musculoskeletal computer models of the human body. We use such models to estimate the loads experienced by the knee ligaments and muscles during movements such as walking, running and side stepping. These models are becoming increasingly important, as there is no other way to assess the forces that have to be supported by these internal tissues, which is necessary to determine the cause of injury. Conclusion We have presented a range of new digital technologies that can be used to assess sporting movements. Many of these are relatively low cost and readily available. Some of the more advanced equipment is very costly, but there are a few institutions around Australia that have such facilities (for example, Department of Human Movement and Exercise Science, University of Western Australia and Australian Institute of Sport) with people that can help collect and process data, and offer interpretation of the results. However, this would come at a cost. There is no doubt that some learning is involved in use of each systems presented above, and the more "higher tech" systems require greater amounts of learning, but with that may come greater understanding and potential for performance improvement and/or injury prevention. All this technology may help you the coach provide enhanced and meaningful feedback to your athlete(s) about their movement and performance, but in the end it depends on the coaches ability to integrate the "technology" into the whole analysis process. References Knudson, D.V. and Morrison C.S. (1997). Qualitative Analysis of Human Movement. Human Kinetics, Champaign, IL, USA. Lloyd D.G., Alderson J. and Elliott B.C. (2000) An Upper Limb Kinematic Model for the Examination of Cricket Bowling: A Case study of Mutiah Muralitharan. Journal of Sports Sciences, 18(12), 975-982. Besier T.F., Lloyd D.G., Cochrane J.L., and Ackland T.R., (2001a) External loading of the knee joint during running and cutting maneuvers. Medicine and Science in Sports and Exercise, Vol 33(7), 1168-1175. Besier T.F., Lloyd D.G., Cochrane J.L., and Ackland T.R., (2001b) Anticipatory effects on knee joint loading during running and cutting maneuvers. Medicine and Science in Sports and Exercise, Vol 33(7), 1176-1181. |
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