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					        Application of tactile displays in sports: where to, how and
                                when to move
                    J a n B . F. v a n E r p 1              Ian Saturday 2                      Chris Jansen 3
                    TNO Human Factors                      Utrecht University                TNO Human Factors



ABSTRACT                                                                rowing motion feedback system as compared to traditional
                                                                        feedback systems.
In this paper we explore the possibilities of tactile displays in
sports applications, and report an experiment that shows that a
tactile feedback systems improves rowing efficiency compared
to traditional feedback systems. Earlier papers have shown that
localized vibrations provide intuitive cues for orientation and
navigation, i.e. where to move to, and motion initiation, i.e. how
to move. In the first part of the paper we will give examples for
the spin-off of these applications of tactile displays to the sports
domain, including tactical guidance for soccer players (where)
and body posture feedback for speed skaters and cyclists (how).
In part two we report a study that extends the ‘where’ and ‘how’
examples with coordinated movement patterns. These systems
also provide cues on when to move. In a laboratory experiment
we showed that motion feedback with a localized and timed
tactile cue resulted in better performance than the current
methods of motion feedback.

Keywords: tactile, motor behavior, sport, motion feedback,
training.

1   INTRODUCTION
Challenging environments such as those encountered by                   Figure 1: Helicopter pilot showing the TNO Tactile Torso Display designed
                                                                           for orientation and navigation in challenging environments but with a
athletes, astronauts, pilots or individuals with a disability are a                              possible spin-off to sports.
major thrust for ergonomic innovation. An example of this
ergonomic thrust is the introduction of tactile torso displays to
counteract the danger of visual and auditory overload for
military pilots. Tactile displays use the skin as an information
channel. A widespread example is the vibration function on
mobile phones. Tactile torso displays consist not of one, but of a
matrix of vibrating elements and use a localized vibration on the
torso to present spatial information. Research has shown that
these displays can intuitively present spatial information, like
the proverbial tap on the shoulder: left is left, front is front, etc
[13]. These displays have shown their value in, for example,
counteracting spatial disorientation [14], waypoint navigation
[15, 16] and low-level flight [17], see Figure 1 for an example.
    Besides orientation and navigation information (i.e.,
information on where to move to), a localized vibration can also
be used to initiate limb movement (i.e., information on how to
move). This means that tactile displays could be used to learn
new or to fine tune existing co-ordination patterns [6]. Figure 2
gives an example in which three people perform a coordinated
dance based on tactile signals given on their individual tactile
displays.
    Both application areas are relevant for sport as well. Over
the past few years a spin-off of (military) tactile technology to
the sports domain is emerging. In the first part of this paper we
will describe several chances for tactile technology and describe       Figure 2: Three ‘dancers’ showing the concept of multi-player coordinated
examples in which top coaches and sportsmen and women                    movement based on signals displayed on each participant’s tactile body
explored the technology. In part two we will report a controlled         display. Picture taken during the Worldhaptics conference 2005 in Pisa.
laboratory study in which we investigated the effects of a tactile



    1
               e-mail: jan.vanerp@tno.nl
    2
               e-mail: iansaturday@casema.nl
    3
               e-mail: chris.jansen@tno.nl
                                                                        Zijlaard-van Moorsel (see Figure 5) has been involved in
2   TACTILE DISPLAYS IN SPORT                                           testing innovative posture feedback systems for cyclist in a
                                                                        wind tunnel. For cyclists, the idea is that a tactile system could
Recently several tactile display applications have been                 improve seating and more importantly back position which is
investigated in the field of sport. We distinguish three classes: a.    critical in maintaining a good aerodynamic posture.
tactical information in field sports, b. posture information, and c.
motion co-ordination pattern, that is where to, how and when,
respectively.
     An application that can be considered a spin-off of the
orientation and navigation displays is a system developed for
soccer players (see Figure 3). This system provides the soccer
player with tactical information on where to look and where to
move to through vibrators at specific locations on the body. One
of the problems that coaches experience during training is the
difficulty of communicating that kind of information verbally in
a timely and efficient manner, a. because of the distance
between coach and player, and b. because of possible language
barriers. Currently, top coaches may easily have five or more
languages spoken in a single team. The system uses the same
principles that are so successfully tested in mobility systems. A
vibration in a belt around the torso indicates the direction to
move to. This set-up is extended with four other signals: a                   Figure 4: Olympic champion speed skater training with position
vibration high up the back means “keep your head up”, one on                               measurement and feedback system.
the left shoulder means “look left” the one on the right shoulder
means “look right”, and one at the middle of the chest means
“stop”. By a wireless link, the coach can give an individual
player direct and language-barrier-free directions. The system
was tested by soccer coach Guus Hiddink while coaching
Champions League team PSV.




                                                                              Figure 5: Olympic champion cyclist during a wind tunnel session to
                                                                                                 optimize her back posture.


                                                                             One step beyond posture is movement coordination [10,
                                                                        22] or simply put a combination of posture and timing. The
                                                                        sequence of limb motions and the forces exerted is critical in
                                                                        many sports, including golfing, throwing the javelin, rowing,
    Figure 3: The PSV soccer team evaluating position measurement and   tennis etc. For these applications, not only motion initiation is
                            feedback systems.                           critical (how to move), but also the exact timing (when to
                                                                        move)[5]. An example that will be used in the second part of
     The second application is related to movement initiation           this paper is rowing. Although rowing seems a relatively
applications. Body posture is a very critical aspect in virtually       simple, cyclic motion, the timing of the different phases of the
all forms of sports, for instance in darts, golf, gymnastics,           rowing motion has large effects on the force exerted on the
skiing, shooting, skating, dancing, etc. Important in the training      oars [1, 2].
of posture is watching an expert and yourself taking a specific
posture. However, most coaches will also physically push and            3     TACTILE MOTION FEEDBACK IN ROWING
pull their student’s limbs and other body parts until the posture
is to his or her satisfaction. However, in a dynamic situation
(e.g. in speed skating or cycling) this is impossible. It is not so     3.1       Introduction
difficult to imagine a situation in which the hand of the coach is
                                                                        The success factors of sports training have been extensively
replaced by a vibrating element. This concept was introduced in
                                                                        studied. The four factors mentioned by Thorndike almost 80
training the posture of speed skaters (see Figure 4). Especially
                                                                        years ago [12] are generally still accepted [4, 8, 11, 19],
for long distances, skaters tend to neglect their posture because
                                                                        although debated by some researchers [9, 20, 21]. These four
of tiredness. An important “error” is that the skater raises his or
                                                                        factors are: a. frequency of feedback (more is better), b. the
her shoulder, which has large negative effects on performance.
                                                                        accuracy (higher is better), the timeliness (sooner is better),
As a spin-off of the movement initiations studies, a system was
                                                                        and d. the information richness. Looking at the characteristics
developed that provides corrective instructions to the skater
                                                                        of a tactile feedback system, it provides cues on when and
based on an advanced posture measuring system and a wireless
                                                                        how to make a specific motion. Although Thorndike did not
link to the coach. Several Olympic skaters recently tested this
                                                                        explicitly include cues on how to move as a success factor, we
system. Olympic and world champion cycling Leontien
argue that this is also a potential advantage of a tactile feedback              right one for the back (see Figure 7, left panel). The only
system. The design of the tactile feedback system must be such                   difference with Group I was the location and modality of the
that it explores these potential advantages.                                     information. Group III received delayed, non-positional
    We studied these potential advantages in a rowing                            feedback. This group also received feedback on each stroke,
experiment. We choose rowing because it is cyclic and not too                    but only once every 25 strokes (see Figure 7, right panel). This
fast of a movement pattern while requiring accurate timing of                    latter condition mimics a human coach, although a human
the different phases. Also, the current indoor rowing machines                   coach would probably summarize the 25 strokes in one
allow to run the experiment in a controlled lab setting and to                   instruction.
automatically register several performance measures. Without                          The apparatus (see also Figure 8) consisted of a Concept 2
going into much detail, we will give a short description of                      model D indoor rowing machine. The built-in performance
rowing motion (see [3, 7] for more information). The critical                    monitor only depicted the remaining time of a session and the
part of the coordinated motion is the relation between the legs                  tempo (number of strokes per minute). Three Xsense MT9-B
and the back (or the knee angle and back angle), both in the pull                motion trackers were used to measure the knee and back
phase (going backwards) as in the recovery phase (going                          angles. The system calculated both angles at 100 Hz. Figure 8
forward). An optimal rower will start with stretching the knees                  depicts the most relevant parts of the set up, including the
with a steady back until a specific knee angle is reached and the                rowing machine, the motion sensors and the control panel of
knees and back work together (see Figure 6).                                     the experiment leader. The tactile feedback was provided by
                                                                                 pager motor based tactile actuators (TNO model JHJ-3). These
                                                                                 actuators have a contact area of 1 by 2 cm and vibrate at
                                                                                 approximately 160 Hz. Heart rate of the participants was
                                                                                 measured with a Polar device.




Figure 6: Sketch used by trainers to explain the (sequential) relation between
           the acceleration of the legs, trunk, and arms in rowing.


    The two most common faults are moving the back either too
early or too late. In a standard feedback situation a rowing coach                Figure 7: Feedback display for group II with flashing circles for the knees
will give verbal feedback (e.g., “you are too late with your                      and back (left) and group III with an overview of the back timing after 25
                                                                                              strokes (right, figure truncated after 15 strokes).
back”) every ten or so rowing motions. A tactile feedback
system could provide this feedback during (and not after) every
pull and recovery. Moreover, making the feedback positional, it
could also indicate how to move. We call this direct and
positional, respectively. To assess the effect of these two
potential success factors we compared the training results of
three groups: group I with direct and positional feedback via
vibrators on knees and back, group II with direct but non-
positional feedback via a visual display, and group III with
delayed (after 25 strokes), non-positional feedback. The
feedback was held constant with respect to the other success
factors accuracy, frequency, and information richness.

3.2      Method
A total of 22 female rowers participated in the experiment with a
mean age of 20.1 year (SD 1.06). All participants trained at                      Figure 8: Participant in the rowing machine and with the motion sensors
                                                                                     attached to the knees and back. The monitor in front allowed the
student league level and had at least two supervised training                         experiment leader to check the motions and other parameters.
sessions each week. The participants were randomly assigned to
one of the three experimental groups.                                                 Each participant rowed eight sessions on one day: a pre-
    The three groups differed in the way feedback was given.                     test, six training sessions and a post-test. Feedback was only
However, all three groups received feedback on whether and                       given during the six training sessions. Each session was ten
how much their back was too early or too late in the pull phase                  minutes in length with a 20-minute break in between sessions,
of each stroke. Thus all three forms of feedback had the same                    and a 1-hour lunch break after the fourth session. The
accuracy, frequency and information richness. Group I received                   participants were instructed to maintain tempo of 21 strokes
direct positional feedback. This was provided by two sets of                     per minute which they could check on the display of the
vibrators, one on the knees and one on the back. These vibrators                 rowing machine. They were also instructed to maintain a heart
gave a burst of vibration when a set knee and when a set back                    rate of around 160 beats per second. The experiment was run
angle was reached, respectively. A correct coordination would                    in the presence of their own experienced and certified rowing
result in simultaneous vibrations while the situation of back too                coach. The starting point for the feedback was an 80° knee
early / too late would result in the back vibrators signaling                    angle and a 110° back angle, but individual adjustments of
before / after the knee vibrators. Group II received direct, non-                these angles could be made by the coach. However, once the
positional feedback. For this group, the vibrators were replaced                 values were chosen, they were used throughout the
with two circles on a monitor, the left one for the knees and the                experiment.
3.3                                Results                                                                            pattern, potentially even during each stroke (for the direct,
                                                                                                                      non-positional group). Although high performance levels can
We calculated the following dependent variables: timing (the
                                                                                                                      be maintained through these (continuous) corrections, it comes
absolute difference between reaching the set knee angle and the
                                                                                                                      at a price, that is it requires more effort which is reflected in
set back angle in ms), heart rate (beats/minute) and speed
                                                                                                                      an increased heart rate over the day. The second is that the
(km/hour). All data were analyzed with an analysis of variance
                                                                                                                      increased heart rate reflects higher mental effort in the groups
(ANOVA): test (pre-test, post-test) × feedback group (direct                                                          with non-positional feedback [18]. This explanation would be
positional, direct non-positional, delayed, non-positional). The                                                      in line with the findings with other tactile displays that apply
timing data showed no significant results. Table 1 gives the                                                          an intuitive, localized cue and that reduced the mental
means of all groups and sessions.                                                                                     workload of the user. Concluding, we can say that the very
                                                                                                                      high performance level can be maintained with significantly
Table 1: Timing accuracy (ms) as function of feedback condition and session.
                         tr means training session.
                                                                                                                      lower effort when trained with tactile feedback.

                                                                                                                      4     CONCLUSIONS




                                                                                                              Post-
                                             Pre-




                                                                                                        Tr6
                                             test




                                                                                                               test
                                                      tr1

                                                            tr2

                                                                   tr3

                                                                              tr4

                                                                                            tr5
     Feedback                                                                                                         In this paper we gave examples of the possible use of tactile
       Group                                                                                                          displays in the sports domain. Proof-of-concepts were
      I: direct,
                                             129      89    90     88        83            82           87    85      described of a tactical display (or where to move to) for soccer
     positional
  II: direct, non-                                                                                                    players, a posture display (or how to move) for speed skaters
                                                83    93    71     53        60            64           52    79      and cyclists, and a movement coordination display (or when to
     positional
   III: delayed,                                                                                                      move) for rowers. In the lab experiment on a rowing machine,
                                                78    75    72     73        60            56           79    84
  non-positional                                                                                                      we implemented a tactile feedback system to train the timing
                                                                                                                      of the limbs involved in rowing. In rowing, the efficiency of
   The ANOVA of the speed resulted in no significant effects.                                                         the action is determined by the ability to time leg, trunk, and
But the ANOVA of the heart rate showed a significant test ×                                                           arms movements. An important aspect in this sense is the
group interaction; F(2, 16) = 5.14; p < .02. The means are                                                            relation between the knee angle and back angle as function of
depicted in Figure 9.                                                                                                 stroke phase.
                                                                                                                          Although both coach and trainees liked the system, future
                                                                                                                      studies have to confirm the potential of localized tactile
                                   170
                                                                                                                      feedback in learning and improving motion patterns. When
      heart rate (beats /minute)




                                   168                                                                                people can execute a movement pattern guided by tactile cues
                                   166                                                                                alone, the possibilities in the sports and in other domains are
                                   164
                                                                                                                      sheer endless. For example, motion sensors could register the
                                                                                                                      movement pattern of an expert which can be replayed on the
                                   162
                                                                                                                      tactile guidance system of the student.
                                   160                                                                                    In general, the applications are based on relatively simple
                                                                         I: direct, positional
                                   158                                   II: direct, non-positional                   technolgy and concepts but have proven to be quite effective.
                                                                         III: delayed, non-positional
                                                                                                                      They have shown to be able to solve pratical problems such as
                                                pre                                       post                        language barriers and longer distances and to be able to
                                                            test                                                      provide direct and intuitive feedback. The displays are not
                                                                                                                      only hands, eye, and ears free, but also mind free.
Figure 9: Heart rate differences between the pre- and post-test for the three                                             We conclude that the recent developments in the field of
                             experimental groups.                                                                     tactile displays driven by challenging situations (e.g., pilots
                                                                                                                      and astronauts) have a potentially beneficial spin-off to sports
3.4                                Discussion                                                                         applications.
None of the three groups showed either a positive or negative
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