introducing wearable force sensors in martial...

1536-1268/05/$20.00 2005 IEEE Published by the IEEE CS and IEEE ComSoc PERVASIVEcomputing 47

Introducing WearableForce Sensors in Martial Arts

Technology has long been alteringsports, from the introduction ofcorked baseball bats to novel ten-nis rackets to long-flight golf ballsand swimsuits made with low-drag

material. However, the acceptance of technologyin sports has typically been littered with contro-versy. This is in line with prior experiences inintroducing new technologies: its a complicated

process that raises many issues.So, although introducing sen-sor technology in sports canhelp promote fairness and spec-tator enjoyment, a resistance to

change exists, partly because such technologyalso tends to alter player behavior. To succeed,the introduction of ubiquitous technology insports equipment must therefore account forsocial acceptance issues, including the playersperception of fairness and rule changes.

In a collaborative effort, the Stanford Tae-kwondo Program, the Palo Alto Research Center,and Impact Measurement recently developed andintroduced sensor technology for the unforgiv-ing environment of the martial arts sparring ring.Our system, called SensorHogu, uses piezoelectricforce sensors on body protectors to help taek-wondo judges and referees score tournamentmatches.1 We have two objectives for the tech-nology: it should support the judges in accuratelyscoring the sparring matches, and it should merge

with and minimize changes to existing equipmentand thus blend into the activitys background.

Introducing SensorHogu to tournament playersand judges raised many challenges. First, the sys-tem must work technologically without signifi-cantly affecting the play of the game. Second, toaccept it, the players must perceive the technologyas fair and unobtrusive. Third, judges must feelcomfortable converting to a new scoring system.Finally, to accommodate the technologys use, rulechanges are required. We are currently workingwith the World Taekwondo Federation (WTF) andUnited States Taekwondo Union (USTU) to vali-date and certify the equipment for tournamentadoption.

Here, I chronicle our experiences and comparethem with technology adoption issues in othersports. To evaluate technology acceptance, weuse a framework developed to evaluate ubiqui-tous computing applications.2

SensorHogu: Technological challenges Taekwondo has enjoyed enormous popularity

for decades and was inducted as an officialOlympic sport at the 2000 Sydney Games. Thesports popularity has created increasing pressureto ensure fairness in judging and to make it morespectator friendly. This pressure resulted in sev-eral rule changes3 and an openness to using tech-nology to ameliorate problems inherent in judg-ing a match. The utmost challenge to accurate

Sensors can offer accurate and transparent judging in sports, but as thisdiscussion of the SensorHogu system for taekwondo shows, introducingthem raises complicated issues.

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

Ed H. ChiPalo Alto Research Center

scoring is the subjective judgment ofwhat constitutes a valid scoring bodykick. According to current rules, a scor-ing kick must be delivered accuratelyand powerfully to the legal scoring areaof the body.3 The subjective nature ofthis judging criteria has been a majorimpediment to the sports development,sometimes resulting in accusations ofbiased judges favoring players from cer-tain countries (for examples, see the newsstories at www.indiavarta.com/olympics/newHeadlines.asp?cat=Taekwondo).

As figure 1a shows, taekwondo is afull-contact sport that uses both headand body protectors. Figure 1b showsthe sports sparring set up: two com-petitors (one in a blue body protector,the other in red) move around asquare, 12-meter-wide padded mat.Three judges score the hits and a ref-eree conducts the match. As the figureshows, the three judges are placedaround the mat in a triangular shapeand currently use wired handsets toscore the points.

System context and overviewFigure 2 shows the relationships

among the SensorHogu devices. A sin-gle base station connects to a laptop,three judges scoring handsets, and twoTrueScore SensorHogu wireless bodyprotectors (hogu is the Korean wordfor body protector). SensorHogu usespiezoelectric sensors to detect the amountof force delivered to a competitorsbody protector and wirelessly transmitsthis signal to a computer that scores anddisplays the point. Our hope is that theforce sensors will help judges achievegreater accuracy and help eliminatescoring controversy.

Figure 3 shows the body protectorwith its wireless transmitter on the upperleft shoulder. For the force sensor, weneeded something low cost, low pow-ered, and rugged. We didnt considerusing accelerometers because we wantedto directly measure the impacting force.The most common dynamic force detec-tor is the piezoelectric sensor. Piezo sen-sors stiffness and strength makes them

particularly suitable in harsh environ-ments such as a protective suit. In ourapplication, we mounted a single longpiece of piezoelectric sensor onto a plas-tic backing and inserted it into the centerof a WTF-approved body protector. Wedidnt instrument the head and facial areabecause a mask would obscure theplayers vision and facial attacks are easyfor judges to score.

The SensorHogu handsets connectwirelessly to a single base station. Eachjudge has two handsets: the left-handhandset scores the red player (see figure4) and the right handset scores the blueplayer. As the figure shows, each hand-set has a trigger button to score one pointfor a body hit, and a side button to scoretwo points for a head blow. Force sen-sors on the body protector must coordi-nate with the scoring handset, whichcomplicated the system design. Accord-ing to existing rules for electronic scor-ing, at least two judges must press thesame handset buttons within a one-sec-ond window for the point to score.

48 PERVASIVEcomputing www.computer.org/pervasive


Judge 1

Judge 3Judge 2

0.5 m1 m

Bluecoach

Redcoach

Redcompetitor

Bluecompetitor

1 m1 m

0.5 m

12 m

12 m

(b)(a)

0.5 m

10 m

1.5 m

Referee

Figure 1. Taekwondo basics. (a) Two players match up with each other, one wearing a blue body protector and the other wearingred. (b) A standard competition ring, which includes three judges and a referee. The one-meter attention boundary helps the players orient themselves.

Design goalsWe set several design goals when devel-

oping the SensorHogu system. First, tofacilitate ease of use, we developed a com-pletely wireless system. The standard foil-fencing scoring system tethers competi-tors.4 That system, which has been usedfor decades, is based on electrical switchesthat complete a circuit when the switchestouch a metal vest. Because the tetheredsetup requires linear fencing forms, it hasrestricted the sports natural development.Essentially, human activity has been mod-ified to suit the available technology. WithSensorHogu, our goal was not only totransmit contact wirelessly to facilitateease of use but also to accurately measurethe amount of force applied.

Second, our system functions well inreal time. Multiple signals from eachbody protector are transmitted at aboutthe same time, and the system interpretsthese signals according to the rules.Third, the overall system meets severalrobustness criteria to withstand anextremely hostile environment. Not onlyis it small and secure, but the system canresist physical abuse and potential radiointerference.

Finally, our system is modular, lowcost, and low powered. Both the hoguand transmitter units can be individuallyreplaced, and both can run for up to fourdays of competition on two AAA-sizedbatteries.

Gaining trust Our next step was to see how well our

SensorHogu system worked in practice.

We used two strategies for gaining thetrust of players and judges. First, wetested the system with real users andpublished the results for examination.

Second, we performed live demonstra-tions at major tournaments.

Judging whether a hit scores a pointdepends on a variety of factors. Judges

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Bluehogu

Redhogu

Wirelessaynchronous

signals

Basestation

collector

Scoredisplayoperator

Judge #1 Judge #2 Judge #3Figure 2. SensorHogu system devices.The judges scores and the SensorHogudata feed into the base station, whichconnects to a computer that keeps trackof scoring according to the rules and displays the matchs progress.

Figure 3. The SensorHogu bodyprotector. The wireless transmitter isstored in a protective pouch on the leftshoulder.

modify their criterion for a point accord-ing to the weight division, gender, andtype of kicking technique used. There isno easy way to standardize judges opin-ions on which kicks are delivered accu-rately and powerfully. The resultinginconsistency in judging has erodedsome peoples confidence and trust, andit was one of the major reasons we devel-oped SensorHogu.

To test the system with users, we out-fitted an anatomically correct anthropo-morphic dummy of a male upper torsowith the SensorHogu. We then asked awide variety of taekwondo players toattack the dummy. The system recordedand analyzed the resulting data using rea-sonable settings for the scoring thresh-old. We published our results in a con-ference paper.1 At the same conference,

we also published live footage of the sys-tem in action, and independent judgesanalyzed the videotapes in slow motion.For example, an analysis of the action infigure 5 shows that a very fast kick mightappear to be blocked by the arm, but itactually slipped underneath the elbow;our system scored these cases accurately.

We demonstrated the judges scoringhandsets starting at local tournaments,then gradually introduced these hand-sets locally, nationally, and internation-ally. Among the recent tournamentsusing wireless scoring handsets are theUS Taekwondo Union National Cham-pionships, US Junior Olympics, andWorld University Championships. Wealso recently demonstrated the wholeSensorHogu scoring system at the Stan-ford Open 2004 tournament.

Systems that play such potentially con-troversial roles in determining match out-comes must be proven valid to playersand judges in a variety of ways. Even inthe large taekwondo community, word-of-mouth reputation is extremely impor-tant to gaining the trust of players andjudges.

Certain details of our design played acrucial role in forming community opin-ion. For example, our handset grip isbased on industrially designed joystickgrips that reduce fatigue during pro-longed use. Judges really appreciatedthese, as some of the earlier electronicscoring systems used cylindrical plastichandles. Also, players appreciated thatour new chest protectors have the samelook-and-feel as their normal gear, withfew modifications. We also ensured that



Figure 4. A judges handset. Because competitions often run for an entire day,the devices are ergonomically designedto eliminate hand fatigue.

Figure 5. Screenshots from the sparringtest video. Because of the kicks speed, itwould have been difficult for judges toclearly see that apparently blocked kicksactually hit the body.

the systems scoring beep isnt distract-ing to the players.

To use the SensorHogu in tournamentmatches requires rule changes. In May2005, WTF changed the rules to specifytwo rather than four judges when elec-tronic hogu technology is in use. In fact,WTF had changed the rules in anticipa-tion of the new technology and had calledfor possible technology partners. We willbe demonstrating our technology to WTFtaekwondo officials in Seoul in July 2005,in hopes of becoming the first electronichogu system on the market. Generally,organizational changes are extremely dif-ficult to push forward, however, particu-larly when they involve the rule changesrequired to allow electronic scoring equip-ment. Fortunately, there is pressure fromthe Olympic committee to introduce newscoring technology in taekwondo, andwe hope to introduce the system in the2008 Olympics.

Evaluation framework forsports technology

To evaluate SensorHogu and compareit to other marketed sports technologies,we used an evaluation framework forubiquitous computing applicationsdeveloped by Jean Scholtz and SunnyConsolvo.2 They suggested nine critical

evaluation areas for understanding aubiquitous computing technology: atten-tion, adoption, trust, conceptual mod-els, interaction, invisibility, impact,appeal, and application robustness. Theyalso suggested a set of metrics for evalu-ating each area (34 metrics in all).

After reviewing the SensorHogu, DarioSalvucci of Drexel University applied theScholtz and Consolvo metrics to varioussports technologies and roughly estimatedhow different technologies performed. Idiscussed these ideas with him in depth atthe 2004 HumanComputer InteractionConsortium Symposium in Frasier, Col-orado. Based on these discussions and hisunpublished slides, I offer here a subsetof the applicable metrics as applied to

three existing sport technologies. I thenevaluate the SensorHogu system using thesame metrics. Table 1 summarizes howSensorHogu and systems in bowling, ten-nis, and baseball faired in relation to thesimplified evaluation framework.

Bowling: Foul-line detectorIn bowling, when a player steps over

the foul line, a simple system detects theinfraction and issues a beep (see figure 6).The system also automatically counts thepins and scores the game. (A bit of trivia:as a new startup in Silicon Valley, HewlettPackard tried to sell a bowling foul-lineindicator as far back as 1938.)

This foul-line technology is a posterchild for a sensor systems successful


Figure 6. The bowling foul-line detector iswidely used in bowling alleys around theworld.

TABLE 1Evaluating various sports technologies using a ubiquitous computing evaluation framework.

The final column is our initial evaluation of SensorHogu.

Bowling: Tennis: Baseball: Taekwondo:Metric Foul-line detector Cyclops QuesTec SensorHogu

Adoption rate/value Good Fair Fair Too early to tell

Accuracy Good Good Good Good

Predictability Good Good Good Good

Awareness of capability Good Good Good Good (for players and judges;

too early to tell for spectators)

Minimal distraction Good Fair Fair Good

Minimal behavior changes Good Good Poor Good

Social acceptance Good Fair Poor Good

adoption within a sport. The systems areeverywhere, and theyre extremely accu-rate. Players understand how they workand can easily predict system behavior.Because the beep occurs after playershave released their balls, the system isnot distracting and requires no behav-ioral changes. Socially, the systems areas much part of the sport as any otherpart of the bowling alley.

Tennis: Auto serve-line detector Major tennis tournaments have used a

system with functionality similar to thebowling technology since the 1990s.Cyclops detects whether serves are inor long (see figure 7). It doesnt detectwide serves, and referees can overrule itscalls. Cyclops created quite a stir whenit was first introduced but eventuallywon reluctant acceptance.

Cyclops systems are highly accurateand consistent in their detections, andhow they work is easily understood.Despite sharing these characteristics withthe bowling foul-line detector, Cyclopshas achieved only limited adoption.While it doesnt directly change the serveor the receiving players strategy, the sys-tem is distracting. The beep occurs justbefore the receiving player is about tostrike the ball. This breaks both playersconcentration and focus and, therefore,its social acceptance has been limited. Toremedy this, researchers must furtherstudy the beeps ergonomics and its effecton players to perhaps better optimize itstiming. The beep might be less distract-ing, for example, if it occurred after thereceiving player returned the ball.

Baseball: Video analysis of ballsand strikes

The QuesTec Umpire Information Sys-tem offers video analysis of balls andstrikes in baseball (see figure 8). In theUS, a huge dispute over its use put thesystem at the center of labor negotiationsbetween Major League Baseball and itsumpires. This dispute wasnt settled untilrecently,5 and the resolution is still sub-ject to ratification. Thirteen of the 30major league stadiums have installedQuesTec, and baseball officials haveused its data to grade umpire perfor-mance. As a reporter noted in a NewYork Times article, If an umpires callsdisagree with the computers more than10 percent of the time, his performancewill be considered substandard and pos-

sibly held against him in future promo-tion considerations and when lucrativepost-season assignments are made. Theumpires are, naturally, freaked out byQuesTec.6

Players are just as unhappy about thesystem. Arizona Diamondbacks pitcherCurt Schilling was fined for smashing aQuesTec camera after being told umpiresare changing their strike zones to matchthe machine.7 Atlanta Braves pitcherDarren Holmes was quoted as saying thatthis system is one of the worst things thathas happened in baseball.7

Players and umpires understand howthe system works. It has nonethelessproven a major distraction for hitters andpitchers alike. The reason is that QuesTecforced definitive behavioral changes.Umpires had to change their strike zonesto conform to QuesTecs zones, whichforced pitchers to deliver pitches to thenew zones and modify their mix ofpitches. In turn, hitters had to adjust tonew strike zones and pitching styles.These effects resulted in extremely poorsocial acceptance of the technology.Thus, despite its fairly accurate and con-sistent performance, most parks haventinstalled the system, creating inconsis-tencies across the league.

Taekwondo: SensorHogu After discovering what happened to

the QuesTec system, our developmentteam was naturally concerned because,as with QuesTec, SensorHogu sets athreshold standard that players mustmeet to earn a score. Although inconsis-tent and poorly understood, the originalstandard of a hit being delivered accu-rately and powerfully had been usedfor a long time and a culture of under-standing had been built around it. Play-ers and judges made adjustments basedon their personal interpretations of therules and match situations; inconsisten-cies became somewhat a part of thegame. As with QuesTec, winning Sen-



Cyclops system

Figure 7. The Cyclops system. When players serve, the technology calls longfor a single line on the tennis court.

Figure 8. The QuesTec video analysis system for baseball. (figure courtesy of InTech magazine/ISA)

sorHogu acceptance required that sportsofficials not only buy in to it but alsobelieve in our systems accuracy and fair-ness. Moreover, it was crucial that Sen-sorHogu not force any unwanted behav-ioral changes on players or judges. Ourdesign mantra was every design is forthe benefit of the players and the fairnessof the game.

SensorHogu has yet to be officiallyintroduced and sanctioned. Its thereforetoo early to discern or predict Sen-sorHogus adoption rate. As our pub-lished results suggest,1 the system is con-sistent and fairly predictable to judgesand players, and the sensors are reliableenough to be used for fair play. Also,players and judges easily understandhow the system works. In our testmatches, players performed as if the sys-tem wasnt there and have not yet mod-ified behaviors or strategies while wear-ing it. So far, players and judges seem toenjoy using the technology and appreci-ate its added value to the game. Many ofthese early opinions are based on the sys-tems reputation and inherent reliability.Opinions might change, however, if Sen-sorHogu experiences a high-profile fail-ure, which we are working to prevent.Table 1 summarizes our initial evalua-tion of the SensorHogu system.

In taekwondo, the audience is a thirdparty and has an indirect stake in thisissue. Given this, it might be valuable todisplay how hard someones hogu is hitduring each encounter. This could addentertainment value and make theopaque culture of scoring thresholdsmore transparent and easily verifiable.Another uncertainty is that we currentlydo not know if judges are more likely toscore a kick that has a higher degree ofdifficulty but generates less power thanother types of kicks. If this is currentlytrue, SensorHogus use might alter kickselection and thus change player behav-ior by making the player favor morepowerful kicks.

Reviewing how technologyalters sports raises severalbroad questions: How do wewant technology to augment

sports and help referee them? Can we sys-tematically understand when a technol-

ogy works well or not? When does a tech-nology start to ruin the spirit of a sport?

Researchers have envisioned a varietyof application areas for sensors, includingeveryday environments such as the homeor office. Evaluators have studied tech-nology adoption in such areas2 but notyet in sports. Sensors offer many possi-bilities in this area, including their use in

hockey pucks, to detect when theycross goal lines;

golf balls, to better track their trajec-tory; and

footballs, to better sense the last pointof forward momentum.

Adopting sensor technology in sportsappears to operate on a similar set of eval-uation factors as other areas of ubiquitouscomputing. Blindly developing sports tech-nology without thinking how it affectsplayers behavior, judges scoring choices,and spectator perceptions is a clear recipefor disaster. We hope that our SensorHoguwork points to future work necessary inunderstanding when a ubiquitous com-puting application in sports has the poten-tial for being a killer app.

ACKNOWLEDGMENTSI thank Dario Salvucci for his figure ideas and gen-erous discussions on sports technology evaluations,which contributed significantly to this article.

REFERENCES1. E.H. Chi, J. Song, and G. Corbin, Killer

App of Wearable Computing: WirelessForce Sensing Body Protectors for MartialArts, Proc. 17th Annual ACM Symp. UserInterface Software and Technology, ACMPress, 2004, pp. 277285.

2. J. Scholtz and S. Consolvo, Toward a Frame-work for Evaluating Ubiquitous ComputingApplications, IEEE Pervasive Computing,vol. 3, no. 2, 2004, pp. 8288.

3. World Taekwondo Federation, Competi-tion Rules & Interpretation, 2003.

4. C. Freudenrich, How Fencing EquipmentWorks, 2003, www.howstuffworks.com/fencing-equipment.htm.

5. M. Chass, Baseball and Umpires SettleGrading Dispute, New York Times, 24Dec. 2004; www.questec.com/q2001/spfe_nytimes_122404.htm.

6. H. Lindgren, The Foolproof Umpire,New York Times, 14 Dec 2003; www.questec.com/q2001/spfe_nytimes_1203.htm.

7. B. Wilson, Schilling Fined about $15,000for Destroying Questec Camera, Sports Illus-trated, 2 June 2003; http://sportsillustrated.cnn.com/baseball/news/2003/06/02/schilling_fined_ap.


the AUTHOR

Ed H. Chi is a senior re-search scientist at Palo AltoResearch Centers UserInterface Research Group.His research interests are inuser interface software sys-tems. He received his PhD incomputer science from the

University of Minnesota. He is a senior memberof the IEEE. Contact him at PARC, 3333 CoyoteHill Rd., Palo Alto, CA 94304; [email protected].

Blindly developing sports technology without

thinking how it affects players behavior, judges

scoring choices, and spectator perceptions

is a clear recipe for disaster.

introducing wearable force sensors in martial...

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