Scientific Aspects in the Preparation of Elite Athletes
Gideon B. Ariel, Ph.D.
6 Alicante, Trabuco Canyon, California 92679
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
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
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
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.
The basic or generalized biomechanical characteristics for both athletes are presented
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
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.
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:
- Shortened second-to-last stride
- Vertical lead leg landing
- Stiff landing of the take-off leg with a large offset
- Incorporating the pelvis in the locomotion due to powerful trunk muscles
- Energetic swing. The lowest center of mass position was reached in the beginning of the last surface interaction
- 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:
- Significant elongation of the second-to-last stride
- Early lowering the center of mass
- Entering a very short last stride with zero vertical velocity
- A very short last stride
- 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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