|
|||||||||||||
Abstract
Information technology provides manifold means to develop sports specific
systems, which present feedback information to athletes immediately
or shortly after motion execution. In many cases, the system developer
may even select between very different approaches to acquire the relevant
parameter values to be presented. When designing and constructing systems
of that kind, special emphasis should be given to the performance goal.
Athletes should be able to recognize deficits. Information should be
given, which allows to notice improvements or deteriorations.
Feedback systems have been developed for assisting table tennis players and rowers, a system to be used in biathlon is under development. In the case of table tennis, tools have been set up to assist players in training. The first system allows to detect the impact position of the ball on the table. Vibration sensors are fixed onto the underside of the table. Positional and timing information is acquired in real time and may be presented shortly after the shot. The second, low cost system, allows to detect and display the times between first and second as well as between second and third impact of the ball on the table. Different service techniques may be trained using this system.
In the case of rowing, a measuring station has been constructed in order
to supply coaches and athletes with KP (knowledge of performance)-information
based on kinetic parameters. A rowing ergometer is placed onto two force
plates. A force transducer is connected to the chain attached at the
handle and generates a signal proportional to the athlete’s pulling
force. Horizontal (in motion direction) and vertical ground reaction
forces and the pulling force are recorded simultaneously and may selectively
be displayed in real time, after a short delay or as summary feedback
about a series of strokes on a monitor in view of the rower. The application
of the system in the training process of elite rowers has shown that
subjective observations of the coach could objectively be verified and
vividly be presented. Certain deficits in the movement technique could
be visualised allowing the athlete to study the effect of changes in
the movement pattern.
In the case of biathlon a video based system is under development, which
allows to visualize the motion of the barrel of the rifle just before
and immediately after shooting.
The success of systems of that kind depends on how exact characteristics
to be improved are measured and on how fast and how comprehensible the
results can be made available to coaches and athletes Introduction
Fractions of a percentage point often decide upon success in modern
elite sport. Effective ways to improve sport performance are required.
Computer scientists and engineers in cooperation with biomechanists,
physiologists, sport psychologists and strength and conditioning specialists
may interactively develop systems, which provide coaches and athletes
with innovative and effective support as they strive for success at
the highest level [1].
Time for motor skill acquisition can be reduced by suitable feedback
systems. If augmented feedback (Knowledge of Results – KR, Knowledge
of Performance – KP) is provided appropriately, sport performance
may improve significantly [2]. Consequently, feedback systems have a
high value in training. Advances in information technology have made
it possible to develop tools and methods, which are specially oriented
on the motion task to be performed [3]. Powerful technology and methods
are available for developing systems, which allow to provide athletes
objectively with supplementary information rapidly and immediately according
to Farfel’s principle [4]. Biomechanical and physiological parameter
values may be acquired and adequately presented shortly or immediately
after motion execution within an effective pre-KR interval [5].
In the design and construction process of a feedback system, special
emphasis should be given to the performance goal of the athletes using
it. Sports specific limitations and peculiarities should be taken into
consideration. The athletes should be able to recognize improvements.
It must be evident, what to aim at.
Information technology often provides different methods to assess the
problem and to realize a specific feedback system. Pros and cons should
thoroughly be analysed before starting the development and implementation.
When high performance hardware is used, immediate availability of the
results may often be achieved.
In particular powerful data acquisition and analysis as well as video
processing tools allow to construct sophisticated sports specific feedback
systems. This will be underlined by the following examples from table
tennis, rowing and biathlon. Example 1 – Table Tennis
Factors affecting the quality of the ball played are the spin, the position,
where the ball hits the table, and the intervals between the ball impacts
on the table. Systems that give immediate optical or acoustic feedback
on the quality of the ball just played are applicable in training. Besides
of directing and conditioning the technique some motivational effects
can be expected.
One exercise, for example, is to return a specific service into a certain
area on the opposite side of the table (Figure 1). Another is to play
the ball as long as possible. In this case the frequency of an acoustic
feedback signal may depend on the distance to the edge of the table.
A third exercise is to serve the ball in a way that the interval between
the first and second impact of the ball on the table is as short as
possible.
Two feedback systems have been developed. The first is based on the
detection of the impact position of the ball on the table in real time
[6], the second on the acquisition of the ball impact intervals.
![]() Figure 1. Feedback system in table tennis. An acoustic signal is given,
if the ball hits the marked area. Impact position detection
The base system – allowing to detect impact positions on one table
half – was developed to give feedback on the accuracy of the placement
when performing certain tasks and to give feedback on impact positions
and impact times during service. Moreover, it should be possible to
evaluate a series of trials and to give summary feedback. For this purposes
it was essential to detect impact point coordinates automatically in
almost real time.
In service strategy training, however, not only the placement on the
opposite half of the table is of interest. The impact positions on both
halves of the table and eventually (in the case of short services) that
of a third impact point allow additional conclusions to be drawn. The
system was therefore designed to be easily extendible to both sides
of the table. This (potential) extension is also usable in applications,
where the times, when the ball hits the table and the impact positions
shall be recorded during a whole match. These data can then be used
as input for a match analysis software used in table tennis [7].
Information technology provides different methods to assess the problem.
A decision had to been made between methods based on image processing
and two different triangulation methods.
Image processing methods would either require the use of a very expensive
high speed video camera or that of a (likewise expensive) asynchronous
camera taking an image at the very instant when the ball hits the table,
thus requiring an additional vibration sensor for triggering. Pre-experiments
with video based methods showed that problems might arise from reflections
on the table and from the boundary (white) lines on the edges of the
table top.
Triangulation methods could either be based on the acquisition and the
processing of acoustic signals (sound) or on the acquisition and the
processing of mechanical signals (vibrations). The principle is shown
in Figure 2. The (acoustic or mechanical) signal originates in the impact
point and spreads out. It arrives at the measuring sensors (at least
three are required) at different instants of time. From these instants
the impact position can be calculated.
To acquire acoustic signals microphones would
be applicable. However, due to the high propagation velocity of sound
the microphones should allow a very accurate estimation of the instant
the signal arrives. Surrounding noise would influence the system in
a very negative way.
![]() Figure 2. Principle of the triangulation method.
Given the different instants of time (t1, t2, t3) the radii
(r1, r2, r3) are determined.
A triangulation method based on the acquisition
of vibration signals was realized in the end. Four vibration sensors
(one redundant) are fixed on the underside of one half of the table
and connected to an amplifier, which itself is connected to a computer
based data acquisition system. This system does neither influence the
players nor does environmental noise influence the system. It is, however,
rather expensive, since very precise vibration sensors (accelerometers;
Type: Kistler 8632C10; amplifier 5134A1) and a data acquisiton board
recording with a high sampling frequency (National Instruments DAQ 6062;
maximum sampling frequency: 500kHz) have to be used. The high sampling
frequency required (125 kHz per channel) results from the high velocity
of signal propagation through the wooden table (534 m/s; Table: JOOLA
Rollomat). Because of different propagation velocities of the signal
depending on the material properties of the table top, the system has
to be calibrated. An additional problem of the method lies in its dependency
on the construction of the table. In the case of the JOOLA Rollomat,
a vibration damping material of 6-9 mm thickness is therefore used as
an elastic interlayer between the table top and the metallic parts,
which are used to stiffen it [6]. This decoupling is necessary due to
the fact that vibration signals propagate much faster through the metal
than through the wood. Because of that, the signals may arrive earlier
at the sensors when taking a way through the metal, even if this is
longer. A schematic presentation of the method including the four signals
recorded is shown in Figure 3. ![]()
Figure 3. Detection of impact points in table tennis using vibration
sensors (accelerometers).
The accuracy of the method was tested by drawing a matrix consisting
of 165 points (coordinates (in mm) (62+100i, 62+100j); i=0,..14 and
j=0,..10; origin of coordinate system: corner of table, coordinate axis
in parallel to table edges) and dropping a ball, which was previously
pressed into an ink pad, from a height of about 0.3 m onto each of these
matrix points (details in [6]). The mean deviation of the centres of
the marks and the reconstructed coordinates was 0.020 ± 0.011
m. Ball impact interval detection
Another system was developed for analysing the quality of service techniques
[8]. This (low cost) microcontroller based system (microcontroller PIC16F628)
allows to determine and display the time interval between first and
second impact of the ball on the table immediately after service execution.
In the case of short services, it also determines and displays the time
interval between the second and the third impact.
Two microphones are used for recording the signals
caused by the ball impact. Both are fixed in metallic boxes. The boxes
are put onto both halves of the table. Since intervals between impacts
are of interest, it is not essential to determine the times of impact
with the same accuracy as it would be necessary when using the data
for calculating impact positions. The signals from the microphones are
electronically preprocessed [8] and then fed to the microcontroller,
which is also connected to the serial port of a PC or laptop. A LabVIEW®
6.1 program acquires the data from the serial port and displays the
results (see Figure 4).
![]() Figure 4. Ball impact interval detection during a short table tennis
serve. Left: Time between first and second impact on table. Right: Time
between second and third impact.
The system works fully automatic. No user intervention is required
between successive serves.
Feedback on the quality of different service techniques may be obtained.
Typical exercises to be performed by the players include the task to
achieve intervals, which are as short as possible, thus reducing the
time for the opponent to react properly. It is obvious that this time
interval is strongly affected by the degree of spin of the serviced
ball. Example 2 – Rowing
Biomechanical analysis in rowing involves the consideration of the
kinematics and kinetics of the boat-rower system. Different authors
[9, 10] identify the curve shapes of the horizontal and vertical acceleration
of the rower’s centre of mass with regard to the boat and the
curve shape of the force applied to the oars as important factors for
qualifying and quantifying a rower’s technique. Fluctuations of
the boat velocity due to the forward/backward motions of the rower should
be kept low, vertical motions of the centre of mass of the rower are
uneffective from an energetical point of view, the shape of the force
applied to the oars should be bell-shaped.
Analyses of the rowing technique in the boat are difficult to realize
and are very demanding in time and instrumentation. In many cases analyses
are therefore based on rowing simulators (rowing ergometers) on land.
A feedback system considering the factors mentioned
above has been developed for use on land (Figure 5). A rowing ergometer
(Concept II®) is placed onto two (Kistler) force plates. A force
transducer is connected to the chain attached at the handle. Horizontal
(in anterior/posterior direction) and vertical ground reaction force
as well as the pulling force are recorded. If the masses of the handle
and the sliding seat as well as the vibrations of the ergometer are
neglected, the ground reaction forces are directly proportional to the
acceleration of the body’s centre of mass of the rower with regard
to the environment (the ergometer). Kinematic motion data may be measured
simultaneously using a highspeed video system. From a detailed biomechanical
analysis of the kinematic data technical deficits may be detected and
related to the force data. During successive motion executions on the
ergometer (without simultaneous measurement of the kinematic parameters)
feedback can be given to the athletes on the quality of their technique.
For this purpose, the time histories of the relevant kinetic parameters
are displayed on a monitor in view of the rower during motion execution.
The rower is thereby able to discover how changes in the movement pattern
may alter the shape of the curves in the desired direction. In addition,
a series of successive strokes may be evaluated. In this case the results
are presented in the form of a summative feedback. A graphical programming
language (LabVIEW 6.1) has been used to realize and implement the system.
![]()
Figure 5. Feedback system for rowing.
From case studies it has been concluded that the horizontal forces
are particularily informative when investigating individual techniques
in elite rowing. They show much more inter- and intraindividual differences
than those representing the pulling force. As an example, two typical
shapes are shown in Figure 6, both from elite rowers. Large differences
in the horizontal forces during the pulling phase can be observed (marked
area). ![]() ![]() Figure 6. Example: Different shapes of horizontal
ground reaction forces from two elite rowers.
The system was successfully applied in elite rowing.
Subjective observations of the coach could objectively be verified and
vividly be presented. To obtain a more mobile system, which is not restricted
to the use in combination with force plates, a system, which measures
horizontal and vertical reaction forces directly at the foot stretcher
(see Figure 5) is under construction.
Example 3 – Biathlon
Coaches and athletes in biathlon are interested in the motion of the
barrel of the rifle just before and immediately after shooting. This
is a crucial factor because of the preceding high exertions of the athletes.
Feedback systems are required, which are able to present this information
shortly after having shot.
Again, different strategies may be followed when developing systems
capable of these requirements.
On the one hand, a small laser device can be attached to the rifle.
By tracking the point caused by the laser beam using position sensitive
laser measurement devices, information on the motion of the barrel can
be reconstructed.
On the other hand, opto-electronic systems may be
used to track the motion of the barrel itself. A method of this kind,
based on image processing, is currently under development. The barrel
is recorded using a video camera. Tracking algorithms are applied to track
the shape of the rifle. Applying the results of a calibration procedure
the image coordinates obtained by the tracking method can be converted
to object space coordinates. In Figure 7 an image of the barrel is shown.
The reconstructed trajectory is shown. ![]() Figure 7. Tracking the barrel in biathlon. The plotted line in the
diagram represents the trajectory.
From the sound track recorded by the video camera simultaneously, the
instant of shooting may be estimated. First results are promising. It
appears that the resolution of video cameras usual in trade (720 ×
576 pixel) is sufficient for acquiring the rather small motion of the
barrel. Summary
Real time and rapid feedback systems as well as sophisticated systems
for collecting and analysing sports-specific data provide innovative
and effective support to coaches and athletes. Powerful IT-tools and
wireless technology facilitate the development of user-friendly systems,
which are specifically oriented towards their needs. It is of central
importance for the success of systems of that kind that the characteristics
to be improved are measured exactly and that the results can be made
available to coaches and athletes fast and comprehensible. The latter
aspect implies that special care has to be taken in the design of the
presentation component of the system. A graphical visualisation form
should, for example, be preferred to a presentation of pure numbers.
If these aspects are considered, novel and rapid performance measurement
and feedback tools based on modern information technology will become
more and more pervasive in the training environment. References
[1]. Broker J. P. & Crawley J. D. (2001). Advanced sport technologies:
Enhancing Olympic Performance. In Proc. XIX Int. Symposium on Biomechanics
in Sports, J.R. Blackwell (Ed.), University of San Francisco, 323-327.
[2]. Schmidt R. A. & Lee T. (1999). Motor Control and Learning,
Champaign, IL: Human Kinetics.
[3]. Liebermann D.G., Katz L., Hughes M. D., Bartlett R. M., McClements
J. & Franks I.M. (2002). Advances in the application of information
technology to sport performance. Journal of Sports Science, 20, 755-769.
[4]. Farfel W.S. (1977). Bewegungssteuerung im Sport, Berlin: Sportverlag.
[5]. Müller E. (2002). Challenges in optimising human sport performance.
In Kinesiology – New Perspectives, Proc. 3rd Int. Scientific Conference,
D. Milanovic, F. Prot (Ed.) University of Zagreb, 2-8.
[6]. Baca A. & Kornfeind P. (2004). Real time detection of impact
positions in table tennis. In The Enginering of Sport 5, Vol. 1, M.
Hubbard, R. D. Mehta, J. M. Pallis (Ed.) Sheffield: ISEA, 508-514.
[7]. Baca A., Baron R., Leser R. & Kain H. (2004). A process oriented
approach for match analysis in table tennis. In Science and Racket Sports
III , A. Lees, J.-F. Kahn, I. Maynard (Ed.) London: Routledge, 214-219.
[8]. Kornfeind P. (2004). A microcontroller based system for detecting
ball impact intervals during a table tennis serve. Internal Technical
Report TR-BMCS-01-2004, University of Vienna.
[9]. Nolte V. (1985) Die Effektivität des Ruderschlages. Berlin:
Bartels& Wernitz.
[10]. Hill H., Damm K. & Buchholtz C. (1995). Die Optimierung des
Kraft-Zeit-Verlaufs des Ruderschlages durch objektives visuelles Feedback.
In Rudern lehren, lernen und trainieren, W. Fritsch (Ed.) Wiesbaden:
Limpert, 95-104.
Arnold Baca
Section of Biomechanics Kinesiology and Applied Computer Science Institute of Sport Science University of Vienna, Austria Email: arnold.baca@univie.ac.at ![]() http://www.icsspe.org/portal/bulletin-january2005.htm |