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70 PERVASIVEcomputing Published by the IEEE CS and IEEE ComSoc ■ 1536-1268/06/$20.00 © 2006 IEEE

S P O R T S T E C H N O L O G I E S

Rapid FeedbackSystems for Elite Sports Training

Advances in information technologyhave let computer scientists andengineers develop sports-specificfeedback systems in cooperationwith biomechanists, physiologists,

sport psychologists, and strength and condition-ing specialists.1 These feedback systems incorpo-rate embedded sensors and devices into the sportequipment or use sensors attached to the athleteto acquire biomechanical, physiological, and cog-nitive and behavioral parameter values of theexercise performed. Microprocessors and mobile

devices (such as notebooks andPDAs) derive the relevant infor-mation from this data and pre-sent it to coaches and athletes;other systems then use teleme-

try solutions to transmit the collected data to aremote receiver for further processing and appro-priate presentation.

Athletes benefit from such objective, immedi-ate feedback during training2,3 because it lets themknow how well they’re performing an action.However, when designing and constructing feed-back systems, developers must account for sports-specific limitations and peculiarities. Erich Müllerand his colleagues have identified three aspects toconsider when using biomechanical methods intechnique analysis and performance diagnosis:4

� establish precise parameters and accurate mea-surement systems,

� make technique parameters as specific as pos-sible, and

� minimize the degree to which the measurementsystem interferes with the athlete.

We’ve also found that systems should provide fastand comprehensible results with an easily deci-pherable GUI, and they should be mobile andavailable at the actual location of the training.Otherwise, their use is restricted to laboratoryenvironments.

With these criteria in mind, we’ve developedseveral feedback systems—for rowing, table ten-nis, and the biathlon—to demonstrate such sys-tems’ practical application. Our systems, whichdon’t attach any sensors to the athletes, measurecharacteristics the athletes or coaches wish toimprove without restricting the athletes’ motionsin any way.

Feedback in rowingRowing requires a well-coordinated and pow-

erful sequence of actions as well as good tech-nique. Researchers have identified the timing andthe curve shape of the acceleration of the rower’scenter of mass with regard to the boat as well asthe force applied to the oars as important factorsfor qualifying and quantifying a rower’s tech-nique.5,6 Specifically, fluctuations of the boat’svelocity due to the rower’s forward and back-ward motions should be kept low, verticalmotions of the rower’s center of mass are inef-fective, and the rower should apply a force shapedin a bell-shaped curve to the oars.

Analyzing a rower’s technique in the boat isdifficult to perform regularly and is demanding in

Elite sports training programs use rapid feedback systems to acquire andpresent relevant performance data shortly after motion execution usingembedded sensors and devices.

Arnold Baca and Philipp Kornfeind University of Vienna

time and instrumentation. An additionalproblem is the coach sits or stands toofar from the rower to effectively evalu-ate his or her technique. To overcomethese drawbacks, elite rowing teams usefeedback systems incorporated directlyinto the boat. Systems that use standardmobile electronic devices (such as the oneD.J. Collins, Ross Anderson, and DerekO’Keeffe7 proposed, which couples aPDA with Wi-Fi capabilities and a dataacquisition card within an expansionbox) are particularly useful in this case.A PDA can capture the data from sen-sors mounted on the rowing boat andtransmit it to the coach’s laptop, whichprocesses and displays it.

Land-based feedback systems usingrowing simulators (rowing ergometers)provide a good alternative for on-watersystems.8,9 We’ve developed feedbacksystems for use on land that monitor thefactors affecting an athlete’s rowing tech-nique. Figure 1 shows one of our systemsthat simulates single rowing. In this case,we placed a Concept II rowing ergome-ter onto two 9281C force plates. Weconnected a U9B force transducer to thechain attached to the handle. The systemrecords horizontal (in the anterior/pos-terior direction) and vertical groundreaction forces as well as the pullingforce. Disregarding the masses of thehandle and the sliding seat as well as thevibrations of the ergometer, the groundreaction forces measured are directlyproportional to the acceleration of therower’s center of mass with regard to theenvironment (the ergometer). The feed-back system displays the ground reac-tion and pulling forces on a screen infront of the rower during motion execu-tion. We used a Pentium IV notebookcomputer to house a 6062E DAQ Per-sonal Computer Memory Card Interna-tional Association (PCMCIA) card pro-viding the required analog inputchannels, and a LabVIEW application

acquires and processes the data. Rowersget feedback on the quality of their tech-nique, letting them see how changes intheir movement pattern might alter theircurve shape in the desired direction.

A minor problem in the instrumenta-tion is the wire that transmits the forcedata from the sensor connected to thechain attached to the handle, which candisturb the rower. We intend to imple-ment a wireless telemetry transmitter onthe handle similar to the one Jeremy Lohand his colleagues described.9

To develop a cheaper and more mobilefeedback system that doesn’t requireforce plates, we also constructed twounits that measure horizontal and verti-cal reaction forces directly at the footstretcher for both feet (see figure 1).Employing these units, we may alsoattach slides to the ergometer’s legs, let-ting it roll back and forth during therowing stroke.

Feedback in table tennisIn table tennis, the factors that affect

the quality of the ball played are the spin,the position, where the ball hits the table,and the intervals between the ball im-pacts on the table. Systems that giveimmediate optical or acoustic feedbackon the quality of a player’s hit can helpdirect and condition the player’s tech-

nique and provide some positive moti-vation. A similar situation occurs whenplayers interact with a computer game.

One exercise, for example, is to playthe ball as near to the edge of the table aspossible. In this case, the frequency of anacoustic feedback signal might dependon the distance of the ball to the edge ofthe table. Another exercise is to serve theball in a way that the interval betweenthe ball’s first and second impact on thetable is as short as possible.

We’ve developed two types of feed-back systems. The first detects the ball’simpact position on the table in realtime,10 and the second acquires the ball’simpact intervals.

Detecting impact positionsThe first of our systems, which detects

impact positions on one half of a table,can tell us how accurately a playerplaced the ball when performing certaintasks. It also gives us feedback on impactpositions and impact times during ser-vice. In addition, the system will let usevaluate a series of trials and give sum-mary feedback.

Figure 2 shows an example applica-tion of the training system. A table ten-nis robot with fixed adjustments servesthe ball in short intervals. The player cansee a monitor, which displavs an image

OCTOBER–DECEMBER 2006 PERVASIVEcomputing 71

Force sensor

Foot stretcher

Force plate

Figure 1. Feedback system for rowing.The rower observes how his center ofmass accelerates during each stroke.

of the table and the impact positionsimmediately after each shot (see figure3). During the training, the player mustreturn each ball into the marked areaand alternatively return three balls long-line (along or adjacent to the edge of thetable) and each fourth ball into themarked area.

After each series of trials, the monitorgives the player feedback on the ballimpact positions (see figure 3). Our sys-tem automatically detects impact pointcoordinates in almost real time (less than

0.1 second).10 The player can selectbetween two different modes of presen-tation: single-shot mode presents theposition of the last ball played only andcontinuous mode presents the positionsof all balls played in a series.

We use a triangulation method basedon acquiring and processing the vibra-tion signal caused by the ball spreadingthrough the table (figure 4). The vibra-tion signal originates in the impact pointand spreads out. It arrives at four mea-suring sensors (at least three are required)

at different instants. From these instants,we calculate the impact position.

We use 8632C10 accelerometers(Kistler, 5134A1 amplifier), which areabout 14 � 14 � 14 mm in size, as vibra-tion sensors. We fixed four accelerome-ters onto the underside of half the tableand connected them to an amplifier,which itself is connected to a data acqui-sition system. This system consists of anotebook computer and a DAQ record-ing with a high sampling frequency (aNational Instruments’ DAQ 6062 witha maximum sampling frequency of 500kHz). We require such performancebecause of the high velocity of signalpropagation through the wooden table:534 m/s for the table we used in oursetup (a JOOLA Rollomat).

The system doesn’t disturb the play-ers, nor does environmental noise influ-ence the system. We obtain an average

72 PERVASIVEcomputing www.computer.org/pervasive


Figure 2. Table tennis feedback system. (a) A table tennis robot with fixed adjustments serves the player balls in short intervals. (b) The enlarged picture shows the robot and the player’s target (the white rectangle on the table) in greater detail.

(a) (b)

Figure 3. Presentation of target area andimpact positions to a table tennis player. (a) Results of the player trying to returneach ball into the marked area and (b) those of alternatively returning threeballs longline and a fourth ball into themarked area.

(a)

(b)

accuracy of 0.020 ± 0.011 m.10 In addi-tion, we can easily adapt the system forindividualized tasks.

Detecting ball impact intervalsTo give feedback on the quality of

serve techniques, we developed a low-cost, microcontroller-based system (Mi-crochip’s PIC16F628 microcontroller)that lets us determine and display thetime interval between the ball’s first andsecond impact on the table immediatelyafter a serve. In the case of short serves,it also determines and displays the timeinterval between the second and thirdimpact.

We use two microphones to record thesignals caused by the ball impact. Wefixed both in metallic boxes and placedthem onto both halves of the table.Because we’re interested in the intervalsbetween impacts, it isn’t essential to deter-mine the times of impact with the sameaccuracy as it would be if we were calcu-lating impact positions. We preprocessed

the signals from the microphones elec-tronically and then fed them to the micro-controller, which is also connected to theserial port of a PC, notebook, or PDA. ALabVIEW application program acquiresthe data from the serial port and displaysthe results (see figure 5a).

The system, which doesn’t require userintervention between successive serves,is fully automatic. If we connect it to twomonitors, two players standing on oppo-

site sides of the table can use it simulta-neously. (In this case, there are two mon-itors facing opposite directions.) Figure5b shows a typical training session.

Players typically use the system to helpthem minimize impact intervals, whichreduces the opponent’s time to reactproperly. The time interval is stronglyaffected by the degree of spin on theball—that is, higher forward spin resultsin shorter impact intervals.


Figure 4. The triangulation method’s fluctuation principle. Given thedifferences between the different instants of time (t1, t2, t3, and t4), we can estimate the impact position.

t3

t1

t4 t2

Figure 5. Table tennis feedback session. (a) The system detects the ball impact interval during a short table tennis serve. Time 1gives the time between first and second impact on table, and Time 2 gives the time between the second and third impact. (b) Thelaptop computer gives the player immediate feedback on the quality of specific serve techniques.

(a) (b)

Feedback in the biathlonBiathlon coaches and athletes are

interested in the motion of the rifle’s bar-rel just before and after a shoot. This isa crucial factor because of the precedinghigh exertions of the athletes due tocross-country skiing. Rapid feedbacksystems can present this informationduring or shortly after a shot.

Researchers have attached a laserdevice attached to the rifle in combina-tion with a laser-sensitive grid to obtainvisual information on the deviation fromthe center of a target in real time.11 Adrawback of this method lies in thenecessity of attaching the laser device tothe rifle and the expense of calibratingthe system.

Alternatively, we could use auto-matic tracking systems to track andrecord the rifle and the athlete’s 3Dmovements in real time by attachingmarkers to both rifle and athlete. How-ever, such systems are expensive anddifficult to use outdoor.

We’re currently investigating a low-cost video-based system. In a first step,we developed a method for tracking therifle’s muzzle in a user-selectable timeinterval before and after the shot usingimage-processing algorithms. A video

camera set up approximately 7 m infront of the athlete in a laterally dis-placed position records the barrel. Weconnected the camera to a Pentium IVnotebook computer. A LabVIEW appli-cation lets the user start and stop acquir-ing the video data and specify the shapeof the muzzle to be tracked so the sys-tem can track this shape automatically.

Applying the results of a calibrationprocedure, the system converts the imagecoordinates obtained by the trackingalgorithm to object space coordinates.From the soundtrack the video camerasimultaneously records, we estimate theinstant of shooting. Figure 6 shows animage of the barrel and the muzzle’s re-constructed trajectory.

The results of our work are promis-ing. A standard video cameras’ resolu-tion (720 � 576 pixels) can sufficientlyacquire the muzzle’s small motion. With-in approximately a minute of the shot,the trajectory is available.

ExperiencesWe tried to follow Müller’s design

principles4 when developing and imple-menting our sports feedback systems. Allthe systems we’ve described worked wellduring deployment. Our users could

understand the information we presen-ted, and the measuring devices didn’trestrict the athletes’ motions in any way.This is particularly the case in the bi-athlon feedback system. Coaches andathletes have told us that they expect thefinal systems will be easier to use thanour current systems. Using the system,they can study the effects of differentaiming techniques.

Members of the Austrian NationalTeam successfully used our rowing feed-back system to prepare for the 2004Athens Olympic Games. The team’scoach made some subjective observa-tions, which we’ve been able to objec-tively verify. For example, during thefeedback sessions, the system helped himpoint out certain peculiarities of the row-ers’ techniques. Coaches could also usethe measuring device to distinguish be-tween different rowing styles, so we be-lieve it might be particularly useful in se-lecting crews for rowing teams.

We observed some minor problemswith the table tennis system that detectsball impact intervals. Specifically, whileserving, some players stamp a foot ontothe floor at the same time as the ball hitsthe table. The system sometimes mis-took the acoustic signal caused by thisstamping as the ball’s impact. By pre-processing the signals from the micro-phones, we were able to reduce thenumber of occurrences of this erroneousdetection drastically.

Also during our table tennis system tri-als, we observed a competition atmos-phere where young players attempted toachieve shorter impact intervals thantheir mates. The players assessed theirunused potential by comparing times intraining (service errors allowed) to timesin simulated competition conditions.This positive motivation during trainingshould help them achieve shorter impacttimes in competition.



Figure 6. Tracking the barrel in thebiathlon. The plotted line in the diagramrepresents the muzzle’s reconstructedtrajectory.

Time and cost considerationsOur experiences throughout the years

have shown us that

• coaches will only apply easy-to-usefeedback systems and

• sports federations and associationsoften have small budgets and there-fore require low-cost solutions.

The first observation means thatcoaches are unwilling to spend muchtime training with the system (no morethan two hours). System developers alsoshould keep in mind that many coacheshave little computer knowledge. There-fore, easily understood GUIs are essen-tial. All our systems present a graphicalvisualization instead of numbers (seefigures 1, 3, and 6; most of the impacttime intervals in figure 5 appear in bothnumerical and graphical form.) Theprogramming language LabVIEWoffers numerous tools and options thathelp us meet these goals.

In addition, most coaches are unwill-ing to spend more than a few minuteson time-consuming installation or as-sembly procedures or to interact withthe system during the exercise. For thisreason, we chose the video-based solu-tion for the biathlon. For that feedbacksystem, we only need to set up a tripodand a camera, the athletes and coachdon’t have to attach any devices to thebarrel, and no complicated calibrationprocedure is required.

In the case of table tennis, we onlyneed to put the boxes containing themicrophones onto the game table. A pre-liminary version of this system requiredthe user to click a start button beforeeach serve trial. The coaches and athletesrequested that we remove this require-ment, so the present system needs nouser intervention between trials.

The following lists the costs of deploy-ing and maintaining all our systems:

� The rowing system based on forceplates costs US$73,000, including two$35,000 Kistler force plates.

� The rowing system based on the mobilemeasuring system costs $5,700, includ-ing two $1,800 units for measuring thereaction forces at the foot stretcher (seefigure 1).

� The table tennis system that measuresimpact position costs $6,100, includ-ing $2,500 for the DAQ card.

� The table tennis system that measuresimpact interval costs $300. (Unlike theearlier version, this system doesn’trequire a DAQ card.)

� The biathlon system costs $400,excluding cost of video camera.

The figures for the rowing systems ex-clude the cost of the ergometer (approx-imately $1,900), and all the figuresexclude the costs of any necessary PCs,notebooks, and PDAs. Considerablecosts of repair could arise in the case of rowing. A change of one of the foot-stretcher force measuring units, for ex-ample, would cost $1,800.

To demonstrate such systems’ cost limitations, we can look at the publicfunds available for each sport we’ve discussed here. In 2005, for example, the Austrian Rowing Association re-ceived approximately $11,000 for sport-scientific (including biomechanical) andsport-medical treatments. Therefore, theassociation can only finance low-costfeedback sessions. Because of this andthe mobility arguments we mentionedearlier, we developed the cheaper mobilesystem.

The Table Tennis Association has asimilar budget for training systems. Thelow price of the table tennis system forimpact interval detection enables us toconstruct several systems and simulta-neously use them at different tables.Conversely, a broader application of thesystem for detecting impact positionswill depend on our ability to drastically

reduce the costs of its components,installation, and calibration.10

The biathlon program we workedwith had no such funds available. Thelaser-device based feedback system,which costs approximately $3,600, iscurrently used by Austrian coaches andathletes in elite training. Unfortunately,young athletes quite often shoot anddestroy one component of this system, areflector that costs $850. This cost issueprompted us to develop our cheapervideo-based system.

We’re concentrating ourfuture efforts on develop-ing high-quality, low-cost,easy-to-use systems. In

addition to training support, we intendto use variants of these or similar systemsto gather comparable data in competi-tions. We believe that such an adaptationcould be productive because athletes


the AUTHORS

Arnold Baca is an associateprofessor; head of the Sec-tion Biomechanics, Kinesiol-ogy, and Applied ComputerScience; and on the facultyof Sport Science at the Uni-versity of Vienna. His researchinterests include computer

science in sports, human motion analysis, mo-tion tracking, modeling the behavior in gamesports, and multimedia. He has a PhD in com-puter science from the Technical University ofVienna. Contact him at the Faculty of Sport Sci-ence, Univ. of Vienna, Auf der Schmelz 6, A-1150 Wien, Austria; [email protected].

Philipp Kornfeind is a lec-turer in the Section Biome-chanics, Kinesiology, andApplied Computer Scienceand on the faculty of SportScience at the University ofVienna. His research inter-ests include sports technol-

ogy, hardware and software development (Lab-VIEW), CAD for prototype development,finite-element-method modeling, and simula-tion. He has an engineering degree in sportsequipment technology from the University ofApplied Science “Technikum Wien.” Contacthim at the Faculty of Sport Science, Univ. ofVienna, Auf der Schmelz 6, A-1150 Wien, Aus-tria; [email protected].

could better compare performance intraining and competition. Sports broad-casts could even use the data we acquireto help illustrate informative segments.

ACKNOWLEDGMENTSWe thank Armin Blaha and Emanuel Preuschl fortheir valuable contribution in constructing and pro-ducing the device for measuring reaction forces inthe foot stretcher.

REFERENCES1. J.P. Broker and J.D. Crawley, “Advanced

Sport Technologies: Enhancing OlympicPerformance,” Proc. 19th Int’l Symp. Bio-mechanics in Sports, J.R. Blackwell, ed.,Univ. of San Francisco, 2001, pp. 323–327.

2. W.S. Farfel, Bewegungssteuerung im Sport[Control Over Movement in Sport],Sportverlag, 1977 (in German).

3. R.A. Schmidt and T. Lee, Motor Controland Learning, Human Kinetics, 1999.

4. E. Müller et al., “Specific Fitness Trainingand Testing in Competitive Sports,” Medi-cine and Science in Sports and Exercise, vol.32, no. 1, 2000, pp. 216–220.

5. V. Nolte, “Introduction to the Biomechan-ics of Rowing,” FISA [International Feder-ation of Rowing Associations] Coach, vol.2, no. 1, 1991, pp. 1–6.

6. A. Baudouin and D. Hawkins, “Investiga-tion of Biomechanical Factors AffectingRowing Performance,” J. Biomechanics,vol. 37, no. 7, 2004, pp. 969–976.

7. D.J. Collins, R. Anderson, and D. O’Keeffe,“The Use of a Wireless Network to ProvideReal-Time Augmented Feedback for On-

Water Rowing,” Proc. Am. Soc. Biomechan-ics 28th Ann. Conf., 2004, pp. 590–591.

8. P.N. Page and D.A. Hawkins, “A Real-TimeBiomechanical Feedback System for Train-ing Rowers,” Sports Eng., vol. 6 no. 2,2003, pp. 67–79.

9. J.M.H. Loh et al., “Instrumentation of aConcept II Rowing Ergometer for Kineticand Kinematic Data Acquisition,” The Eng.of Sport 5, M. Hubbard, R.D. Mehta, andJ.M. Pallis, eds., Int’l Sports Eng. Assoc.(ISEA) , vol. 2, 2004, pp. 173–179.

10. A. Baca and P. Kornfeind, “Real TimeDetection of Impact Positions in Table Ten-nis,” The Eng. of Sport 5, M. Hubbard,R.D. Mehta, and J.M. Pallis, eds., ISEA, vol.1, 2004, pp. 508–514.

11. D.G. Liebermann, “Advances in the Appli-cation of Information Technology to SportPerformance,” J. Sports Science, vol. 20,2002, pp. 755–769.



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