Biomechanical Analysis of Discus Throwing at the 1996 Atlanta Olympic Games
Gideon Ariel, Ph.D. Ann Penny, Ph.D. and Al Finch, Ph.D.
History was made at the Atlanta games by utilizing the Internet to provide Biomechanical data immediately for use at remote sites. Video cameras were utilized to record the events and the data was transmitted to computers on-site for conversion to digital format. Video clips of individual performances in various events were made available for downloading free of charge from the Ariel Dynamics website usually within hours of the actual performances. The posted events were filmed from various perspectives utilizing numerous cameras providing data capable of yielding three-dimensional biomechanical results. This rapid availability of sporting activities for study by scientist, athletes, coaches, and the general public on the Internet is a history making event. It further illustrates the potential of Internet as a research tool.
The purpose of the research conducted at the XXVI Olympiad in Atlanta was to expand the biomechanical applications and the interactive capabilities of the Internet to make sport performances rapidly available to everyone. The Track and Field events which were performed at the Atlanta Olympics in 1996, were selected for illustrate these procedures because they are unique in captivating an enthusiastic world-wide audience.
In the present study, the Biomechanical analysis of the Discus throwing was performed. Data was collected on Preliminaries and Final performances.
Video cameras were placed in key positions, approximately 45 degrees to the plane of the path of the thrown object or of the athletic performance itself, in order to record the particular event. As few as three and as many as nine video cameras were utilized. One camera was placed to the rear of the event. A second camera was placed at the side, that is perpendicular to the first camera view, and a third camera was placed at approximately 45 degrees to the event. All video cameras recorded at 60 fields per second. Figure 1, illustrate some of the various cameras positions. These are dynamics video clips and one can control and observe the movement. For non compress full video one can log into our web site at: http://www.arielnet.com and download the original video clips through FTP connection.
Dimensions of known factors on the field and various other measured objects in the field of view were used for the calibration points. Because multiple camera were utilized, the best views were selected for further analysis.
The video pictures were grabbed from each view with Intel Smart Video Recorder Plus frame grabber and the files were stored in Audio Video Interlace format (AVI). This data was then uploaded, via satellite, to the Ariel Dynamics website. The stored data was available to all free of charge. The AVI files can be downloaded frame by frame from the Ariel Dynamics FTP Site for digitizing. The files are in compressed video format in order to conserve bandwidth. The resolution of compressed files are lower than the regular files but the data was able to be rapidly available which was the purpose of the study.
To download these files, visit http://www.arielnet.com and click on the FTP Site Button. Then, select the Olympics directory and click on the desired AVI file(s). For a detailed list and explanation of what each file contains, click the 'ATLANTA' link from the middle frame of the main page. The list is also obtainable by clicking on any of the sport icons on various pages of the site.
Video calibration procedures
Since it was impossible to enter a calibration cube into the field, other methods had to be devised. The following is a description of a unique technique that was devised to create a calibration cube from known measurement on the field and the utilization of the athletes body measurement.
The method was checked against known official measurements of the the discus circle area.
From the rear camera view, the circle diameters adjacent to each of the dividing line hash marks were digitized as control points and a scaling factor was determined using the multiplier module. Then the ends of the hash marks, circle diameter, and midline of the athlete were digitized. After conversion to real dimensions, the diameter of the circle was determined and compared to the known displacement. This measurement procedure was repeated 10 times for the top four discus performers in the Atlanta Olympics.
The data coordinate endpoints were then smoothed using a second order low-pass Butterworth digital filter with a 10 hertz (Hz) cutoff frequency. The average error in the 250 cm diameter dimension determined for these 40 measurements was 2.88 cm (1.2%), for a subject to camera distance of over 90 m. The latitudinal position of the athlete's midline was digitized and determined using the multiplier technique. The longitudinal position of the athlete was determined using similar procedures for the side view. The average error in the circle diameter was 3.4 cm (1.4%) for a distance over 90 m.
Next the athlete's standing height which was obtained from the Official Olympic Track and Field guide was entered into the calibration data with the latitudinal and longitudinal coordinates determined from the previously discussed multiplier techniques. Then using the segmental length ratios reported by Dempster (1955), the shoulder, hip, and knee heights were determined for each athlete. These heights were used to create an 3 dimensional cube using 5 data points on the circle (left hash, left and right circle diameter, left and right sector hash) and 4 body control points.
Considering the fact that a calibration objects were not allowed on the field, this method deemed to be superb. Since the variation in throws of the same athlete is more then 10 percent, the error in measurement of less then 1.5 percent was acceptable in the present study.
The 21 data points digitized were left foot (5th metatarsal), ankle, knee, hip, right, hip, knee, ankle, left wrist, elbow, shoulder, right shoulder, elbow, wrist, hand, discus, base of the neck, mastoid process, top of the head, left and right circle diameters at the hash marks.
This composite control cube consisting of 9 points and 21 data points were digitized and entered into the 3 dimensional linear transformation (DLT) module and converted to real displacements. The real coordinate endpoints were smoothed using a 10 Hz cutoff frequency in a low-pass digital filter. The 3 dimensional displacements of the circle diameter were compared to the actual 250 cm displacement. The top 4 performers' trials yielded an average error of 2.9 cm (1.2%) using the DLT transformation algorithm.
This multi staged approach created an 3 dimensional cube of control points from field dimensions and human anthropometric measures. This made it possible to overcome the limitation of not having a pre-determined calibration cube set in the field of view and yet, obtain accurate 3-dimensional track and field data from the Olympics.
From the present Kinematic data, enormous amount of results could be analyzed. However, only few parameters were selected. The parameters to be analyzed were segment velocities and Moment Arm calculations.
Figures x to y illustrates the stick figures resulted from the DLT measurements.
The resultant release velocities calculated the best 4 throws were 3080.1 cm/sec for Riedel (GER), 2718.5 cm/sec for Dubrovschchik (BLR), 2599.0 cm/sec for Kaptyukh (BLR) and 2498.0 cm/sec for Washington (USA).
The projection angles in the YZ plane representing the angle in respect to the horizontal were 21.9, 29.1, 37.3, and 29.9 degrees for Riedel, Dubrovschchik, Kaptyukh, and Washington, respectively.
The heights of release of the discus were 1.5 m, 1.75 m, 1.6 m, and 1.21 m for Riedel, Dubrovschchik, Kaptyukh, and Washington, respectively.
The elapsed times to complete the turns of the throw were 3.0 seconds for Riedel, 2.3 sec for Dubrovschchik, 1.9 sec for Kaptyukh, and 1.6 seconds for Washington.
The combined effect of the projection velocity, projection angle, and height of release resulted in medalist throws of 69.4 m (Olympic record) by Riedel (GER), 66.6 m by Dubrovschchik (BLR), 65.8 m for Kaptyukh (BLR), followed by 65.4 m for Washington (USA). The aerodynamic variable of angle of attack was not determined for these throwing trials.
The throwing velocities determined were similar to the velocities calculated for in analyses performed by Ariel in 1976 ? on Silvester and Oerter. There were negligible differences in the projection angles used by the 4 best discus throwers in the Atlanta Olympics but there were significant differences in the resultant projection velocities between the top 4 contestants analyzed. Riedel, the gold medallist generated the greatest projection velocity of 3170.1 cm/sec and Washington had a projection velocity of 2484.9 cm/sec, which represented an 28% increase in solely the speed of the discus over the fourth place finisher.
Interestingly enough, Washington performed the throwing movement in 46% less time, while Riedel took the longest amount of time to release the discus. This may indicate that Washington moved across the circle too quickly, thus not allowing enough time for the storage of elastic energy in the arm during the turns and then consequently a lower energy return was observed at the release of the discus.
The study successfully demonstrated that digitization is a biomechanical task which can be performed between different geographical locations using the Internet as the interfacing medium. The applications of this technique and intellectual resource appear unlimited.
For example, a golf teacher in New York can video his students' swings. These video clips can be transmitted digitally in AVI format to a server in one part of the world and then interfaced to the biomechanical program for further analysis. Many Olympic events make fixed laboratory studies difficult, including equine events, sailing, and cross-country skiing. Coaches can film actual performances on site using cameras with direct AVI format input attached to Laptop computers. These files can then be digitized or transmitted through Internet protocols.
Biomechanical quantification has developed far beyond the pioneers who relied upon visual observations of animation to describe movement. The revolution continued with improvements in cameras, the introduction of computers, development of various algorithms to better fit the data, and expansion beyond sports studies. Additional innovations in the process are expected as the Internet further evolves into newer presentation technologies involving animation and virtual reality (e.g. Java and VRML).
The ability to quantify motion has appeal to many groups and at many different levels. Access to global resources via the Internet expands biomechanics beyond a fixed geographical location. This has direct applications in medical research and industrial engineering where, frequently, transmission and processing of research data between remote sites has to occur in a real-time mode.
Thus, the subject presented and studied in this document represents a significant threshold in furthering the accessibility and applicability of Biomechanics to several scientific, medical, industrial and aeronautical endeavors far beyond its present reach.