Scientific Aspects in the Preparation of Elite Athletes

Gideon B. Ariel, Ph.D.

6 Alicante, Trabuco Canyon, California 92679

INTRODUCTION

 

Biomechanics is the study of the motion of living things and, as an established discipline, it has evolved from a fusion of the classical disciplines of anatomy, physiology, physics, and engineering for measuring and evaluating performances. Biomechanics, then, is built on a foundation of knowledge and the application of basic physical laws. Although anything which moves be it human or machine can be quantified, the application of biomechanics has been successfully applied to great athletes and their world record performances.

Previously, it was common practice to assess athletes in terms of muscular strength, cardiovascular capacity, body composition, or other tests performed on individuals. Individually or collectively, these tests inadequately defined or determined elite qualifications. Biomechanics affords a productive approach to the quantification and assessment of performances of elite athletes.

A biomechanical movement analysis system provides a means to quantity motion utilizing input information from visual inputs (either video or film), as well as additional quantitative measures such as, electromyography (EMG) and force platforms. The theoretical bases of the video technique models the human body as a mechanical system of moving segments upon which muscular, gravitational, inertial, and reaction forces are applied. Although the physical and mathematical model for such a system is complex, it is well defined. (1)

The movement analysis system provides a means for measuring human motion based on a technique of processing multiple high-speed film or video recordings of a subject's performance (2,3,4). This technique demonstrates significant advantages over other common approaches to the measurement of human performance. First, except in those specific applications requiring EMG or kinetic (force platform) data, it is non-invasive. No wires, sensors, or markers need be attached to the subject. In fact, the athlete need not be aware that data is being collected. Second, it is portable and does not require modification of the performing environment. Cameras can be taken to the location of the activity and positioned in any convenient manner so as not to interfere with the athlete. Activities in the gymnasium, field, or any sporting facility can be studied with equal ease. Third, the scale and accuracy of measurement can be set to whatever levels are required for the activity being performed. Camera placement, lens selection, shutter and film speed may be varied within wide limits to collect data on motion of only a few centimeters or of many meters, with a duration from a few milliseconds to a number of seconds. Video equipment technology currently available is sufficiently adequate for most applications requiring accurate motion analysis, although special applications may require very high-speed cameras, powerful lenses, and high levels of illumination. Determination of the problem, error level, degree of quantification, and price will all affect the input device selection. Fourth, film data can be collected during international competition far from the analyzing location and, at a later date, the events can be quantified.

A typical kinematic analysis consists of four distinct phases. The initial step involves "grabbing" the image from the film by means of appropriately programmed software utilizing a specialized board and storing the data in the computer's memory. This eliminates any further need for the video apparatus. The image sequence is then retrieved from computer memory and is displayed, one frame at a time, on the digitizing monitor. The grabbed image can be enhanced or altered in several ways, including zooming the whole frame or a defined, isolated portion of the view. Changing the size may help the digitizer to more accurately determine a particular joint which in the original view could not be identified; (4) the location of at least six fixed noncoplanar points visible from each camera view (calibration points) must be known. These points need not be present during the activity as long as they can be seen before or after the activity. Usually they are provided by some object or apparatus of known dimensions that is placed in the general area of the activity, filmed and then removed; (5) the speed of each of the cameras (frames/second) must be accurately known, although the speeds do not have to be the same; and (6) some event or time signal must be recorded simultaneously by all cameras during the activity in order to provide synchronization.

These rules for data collection allow great flexibility in the recording of an activity. Information about the camera location and orientation, the distance from camera to subject, and the focal length of the lens is not needed. The image space is "self calibrating" through the use of calibration points discussed previously. Different types of cameras and different film speeds can be used and the cameras do not need to be mechanically or electronically synchronized. The best results are obtained when camera viewing axes are orthogonal (90 degrees apart), but variations of 20 to 30 degrees can be accommodated while introducing almost negligible error. A panning camera option is also available.

Digitizing is the second phase of analysis. Initially, the video image is captured by the computer and stored in memory. This eliminates any further need for the video apparatus. The image sequence is then retrieved from computer memory and is displayed, one frame at a time, on the digitizing monitor. using a video cursor, the location of each of the subject's body joints, such as ankle, knee, hip, shoulder, elbow, is selected. In addition, a fixed point, which is a point in the field of view that does not move, is digitized for each frame as an absolute reference. This allows for the simple correction of any registration or vibration errors introduced during recording or playback. At some point during the digitizing of each view, a synchronizing event must be identified and, additionally, the location of the calibration points as seen from that camera must be digitized. This sequence of events is repeated for each camera view.

Digitizing is primarily a manual process. It is performed, however, under computer control and the digitizing of video images is computer assisted. User participation in the digitizing process, however, provides an opportunity for error checking and visual feedback which rarely slows the digitizing process adversely. A trained operator with a reasonable knowledge of anatomy and a consistent pattern of digitizing can rapidly produce high-quality digitized images. Because all subsequent information is based on the data provided in this phase, it is essential that the points are selected precisely. However, an automated digitizing option is available.

The computation phase of analysis is performed after all camera views have been digitized. The purpose of this phase is to compute the true three-dimensional image space coordinates of the subject's body joints from the two-dimensional digitized coordinates of each camera's view. Computation is performed using a direct linear transformation. This transformation is determined by first relating the known image space locations of the calibration points to the digitized coordinate locations of those points. The transformation is then applied to the digitized body joint locations to yield true image space locations. This process is performed under computer control with a small amount of timing information provided by the user. This information includes starting and ending points if all the data are not to be used, as well as a frame rate for the image sequence that may differ from the frame rates of the cameras used to record the sequence.

When transformation is complete, a smoothing or filtering operation is performed on the image coordinates to remove small random digitizing errors and to compute body joint velocities and accelerations. Smoothing options include cubic and quintic splines as well as a Butterworth second-order digital filter (5,6,7). Smoothing may be performed automatically by the computer or interactively with the user controlling the amount of smoothing applied to each joint. In addition, error measurements from the digitizing phase may be used to optimize the amount of smoothing selected. At the completion of smoothing, the true threedimensional body joint displacements, velocities and accelerations have been computed on a continuous basis throughout the duration of the sequence.

At this point, optional kinetic calculations may be performed to complete the computation phase. Body joint displacements, velocities and accelerations are combined with body segment mass distribution to compute dynamic forces and moment at each of the body joints. muscular contribution to these forces and moments can then be computed by selectively removing the inertial and gravitational kinetic components.

The presentation phase of analysis allows computed results to be viewed and recorded in a number of different formats. Body position and motion can be presented in both still frame and animated "stick figure" format in three dimensions. Multiple stick figures may be displayed simultaneously for comparison purposes. Joint velocity and acceleration vectors may be added to the stick figures to show the magnitude and direction of body motion parameters. Hard copies of these displays can also be produced for reporting and publication.

Results can also be reported graphically. Plots of body joint and segment linear and angular displacements, velocities, accelerations, forces and moments can be produced in a number of format options. An interactive graphically oriented user interface makes the selection and plotting of such results simple and straightforward. In addition, results may also be reported in numerical form. All quantities that can be selected for graphing may also be printed in tables of body motion parameters. Results can also be exported for processing with other software applications, such as spreadsheets, graphic packages, and statistical programs.

The preceding discussion has described a computerized biomechanical system which can be utilized for the quantification of activities and performance levels. The need to identify and measure a task with subsequent application and evaluation for elite athletes and their training program was raised in the introduction as an important need.

One example for the application of biomechanical analyses to elite sports occurred during the Third IAAF World Championships in Tokyo in 1991 in the long jump competition between Mike Powell and Carl Lewis. Both of these outstanding athletes broke the longest existing world record in athletics, Bob Beamon's 8.9m jump. The purpose of this study was to analyze the components of the jumping techniques during different phases of one of each athlete's performances: Mike Powell (8.95m) and Carl Lewis (8.91m).

Three consecutive parts of the jump were considered: (1) approach, (2) take-off, (3) flight and landing.

RESULTS

The basic or generalized biomechanical characteristics for both athletes are presented in

Table 1: Parameters of long jump

• Official Distance (m) 8.95 8.91
• Effective Distance (m) 8.98 8.91
• Wind Velocity (m/s) * +0.30 +2.90
• Average Running Speed between:
• 11-6m. to the board (m/s) * 10.79 11.23
• 6-1m. to the board (m/s) * 10.94 11.26
• The Length of the: Third-last Stride (m) 2.40 2.23
• Second-last Stride (m) 2.47 2.70
• Last Stride (m) 2.28 1.88
• CM Horizontal Velocity (m/s) 9.27 9.11
• CM Vertical Velocity (m/s) 4.26 3.37
• Angle of Projection (degrees) 24.60 20.30
• Angle of Body Lean (degrees) at:
• Touch-down 71.80 77.00
• Take-off 73.90 67.50
• Maximum CM Height (m) 2.05 1.84
• Height of CM at Touch-down (m) 0.54 0.49
• Horizontal Distance between the CM and Foot Mark (m) 0.41 0.42

It should be noted that the actual results used in this analysis were achieved under different ambient conditions, the most important being the difference in the wind speed. According to official measurements, Lewis' jump was assisted by a favorable wind of considerable velocity. It is very difficult to give a reliable assessment of the relative contribution of the wind speed to the jump distance, although the most prudent estimates suggest that the favorable wind was responsible for at least 10 to 15 cm in Lewis, final result. Although this factor should be considered in evaluating the actual value of the results obtained, for this paper the main task will be to examine the biomechanical aspects of the performance.

The most pronounced differences in the jumping techniques for the two athletes are clustered within the last two strides preceding the take-off. Attention will, therefore, be restricted to that portion of the jump.

The first athlete to be evaluated was Mike Powell. The length of Powell's second-to-the-last stride was 2.74 m, which placed it near the lower boundary of the statistical interval for high performance jumpers. Shortening of the stride was clearly seen on the video recording and showed a breaking of the running tempo at this stride. The flight phase of this stride marked the beginning of the lowering of the center of mass in preparation for the final push. This motion was accompanied by a decrease in both horizontal and vertical velocities of the center of mass. Nevertheless, this "shortened" stride allowed Powell to attain a very straight body position, nearly vertical, with a slight offset of the support leg and a stiff landing. There was virtually no flexing in the knee joint and with the prevailing "straight-downwards" motion, the toe was forward. This allowed Powell to enter the last stride with no reduction in velocity by powerfully swinging the left, or leading leg, and with an active thrust of the trunk forward.

The flight phase of the last stride started, consequently, with a very high horizontal velocity of the center of mass (11.8 m/s), but with zero vertical velocity due to the knee flexors absorbing the impact. During the flight, the position of Powell's center of mass dropped by 8 cm, reaching it's absolute minimum of 97 cm and the angle of the trunk changed rapidly. The landing of the take-off leg was marked by it's very long forward offset from the projection of the center of mass. The landing of the take-off leg was very stiff, with active body motion forward-upward. Instead of an eccentric motion in both the hip and the knee joints, both joints worked in a concentric mode with high angular velocities at the hip (1100 deg/s at the interval from 160 to 188 degrees) and lower and decreasing angular velocities at the knee which, in this case, performed as the only impact absorbing element. The knee performed actively as well as with the angle changing from 160 to 178 degrees. At this phase, the velocity of the lead leg began to increase in an undulating motion. After passing the vertical, the motion of the swing leg prevailed. The angles in the support leg changed considerably, although with lower angular velocities. Thus, the final push was performed mostly by the motion of the swing leg. The final surface interaction was characterized by dramatic changes in the center of mass velocity. The horizontal velocity of the support leg decreased by more than 2 m/s, but at the expense of the vertical velocity increase of 4.26 m/s, which was largely assisted by the very energetic motion of the pelvis at this stage. The resulting take-off angle was 24.6 degrees.

Examination of Carl Lewis' jump yielded different movement patterns. Lewis commenced the preparation for the final push during the support phase of the second to last stride. This is expressed in the lowering of the center of mass by 3 cm. Lewis left the surface interaction with noticeable forward trunk lean and a high horizontal velocity of the center of mass (11.8 m/s). At the same time, the vertical component of the center of mass speed was negative. The next stride was considerably longer than the statistical average by more than So cm. During this elongated stride, the position of the center of mass was lowered by 10 cm. and the trunk was nearly vertical. During this stride, the left knee absorbed the impact with the change in the knee angle approximately 10 degrees. The center of mass reached it's absolute minimum in height at 94 cm. With the knee flexed, active extension of the left hip was attained with the angle changing by 19 degrees and the angular velocity reaching 600 deg/s. The right hip swung at an angular velocity of 850 deg/s in order to assist the raising of the body from the deeply bent stride.

At that moment, the center of mass began it's upward motion. After passing the vertical, the motion of the segments became less active. The raising of the center of mass was achieved mostly by the extension in the hip, with a nearly constant knee angle. Some gain in the vertical speed of the center of mass was achieved due to the foot push. With minimal vertical speed, the last stride was very short (1.88 m) and with a negligible flight phase. The height of the center of mass was maintained constant. This body position was made possible by placing the support leg on the surface with a downward vertical motion without a pronounced "thrust" and with a very fast inward motion of the hips with the angular velocity of this motion reaching 1250 deg/s. The next phase of the final push was characterized by a whip-like motion of the swing leg and the extension of the support leg in the knee. The change in the height of the center of mass during the last interaction was 26 cm. The horizontal speed dropped by 1.4 m/s (from 10.5 to 9.1), and the gain in the vertical speed was 3.37 m/s. The take-off angle was 20.3 degrees. It should be noted here that the angle in the knee of the support leg at the moment of take-off was only 158 degrees, which demonstrated "under extension". This observation suggests the presence of a concealed reserve in the performance recorded.

The flight techniques used by both athletes were practically identical. They both employed a 3 1/2 steps of "run-in-the-air" and the maximum elevation of the center of mass was 2.05 m for Powell and 1.84 m for Lewis.

CONCLUSIONS

Comparison of the individual techniques employed by the two athletes leads to the conclusion that the fundamental distinction lies in the different approach used in the run and take-off phases. The athlete who maximizes the velocity and the direction of the take-off while, at the same time, maintaining the optimum combination of body spatial positions, angular movements, and the relative movements of the segments will produce the longest jump. Powell and Lewis used different techniques during the take-off phase of the jump: (1) between the velocity and the height of the center of mass, and (2) between the accumulated muscle force and kinematics of the speed transformation in order to achieve the maximum efficiency with precise timing. The "jump formula" of Powell might be summarized as follows:

1. Shortened second-to-last stride
2. Vertical lead leg landing
3. Stiff landing of the take-off leg with a large offset
4. Incorporating the pelvis in the locomotion due to powerful trunk muscles
5. Energetic swing. The lowest center of mass position was reached in the beginning of the last surface interaction
6. Considerable loss in the horizontal velocity with compensation in a large gain in the vertical component.
   

The "jump formula" of Lewis can be summarized as follows: .

1. Significant elongation of the second-to-last stride
2. Early lowering the center of mass
3. Entering a very short last stride with zero vertical velocity
4. A very short last stride
5. Take-off with fast inward hip motion

Comparison of the original video recordings of the two attempts described above and confirmed by the biomechanical quantification indicate that there are sufficient differences to consider these jumps as belonging to different jumping styles. One of the most fundamental differences lies in the timing of the beginning of the center of mass upward motion. It is this point which defines how long or how short the process of the transformation of speed will take and, consequently, how the type of motion will be executed. Lewis started the motion much earlier and divided the process of the speed transformation between two strides. As a result, this process was not accompanied by application of accumulated momentum in a single high force surface interaction. Conversely, Powell preserved all the momentum gained until the last surface interaction and converted it into the launch velocity of a single motion.

These conclusions allow us, to some extent, to speculate about the type of physical training of the two athletes. Lewis, in effect, adapted the high speed sprinter's techniques to the long jump. The Powell formula focused on a more pronounced force orientation. Powell's technique required greater efforts for the force exertion capabilities in his training. These two different approaches might be possibly used as guidelines in selecting and structuring the training basis for an athlete, with due consideration of the individualities, training and competitive background.

This presentation has focused on a method for quantification of elite athletes and a specific example of its application. The world of athletics and the personnel who participate can easily be selected, trained, and coached through the techniques of biomechanical analysis. Ultimately, however, the athlete is the only individual who can actually perform; we as scientists, coaches, and spectators can only try our best and then sit back and watch.

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