Performances of Experienced and Novice Sportball Players

in Heading Virtual Spinning Soccer Balls

Thierry Hoinville∗ Abdeldjallil Naceri† Jesus Ortiz Emmanuel Bernier Ryad ChellaliItalian Institute of Technology, Genova, Italy

ABSTRACT

Using virtual reality for understanding sports performance allowsfor systematic investigation of human sensorimotor capabilities andmeanwhile promotes the design and comparison of realistic immer-sive platforms. In this paper, we propose a virtual reality-based ex-perimental design for studying the human ability to intercept spin-ning balls deflected by the Magnus effect. Compared to the previ-ous approaches, we focused on a tight perception-action coupling.Experienced and novice subjects immersed in a 3D soccer stadiumwere asked to head realistically simulated balls, free kicked withand without sidespin. Consistent with the former studies, qualita-tive results show that the interception performance systematicallyrelates to both the ball sidespin direction and arrival position for allthe subjects, either experienced or not. However, contrary to thoseformer studies where subjects answered only pseudo-verbally, ex-perienced and novice groups differentiate in quantitative perfor-mances, supporting that expertise likely appears when perceptionis coupled to action. Further analyses will be needed to extract thedifferent information-movement relationships governing the behav-iors of experienced subjects and novices.

1 INTRODUCTION

Sports with their strict rules and definite playgrounds provide welldefined case studies for investigations in virtual reality (VR). Bycomparing the performances achieved in virtual environments tothe large amount of statistics available from competitions, one canpossibly assess quantitatively the realism, immersivity, strain and soon of VR platforms. In turn, VR can help to understand the humansensorimotor skills challenged in sports, by allowing for systematicvariation of parameters and repetition of identical trials.

In this paper, we propose a virtual reality-based experimental de-sign for studying the human ability to head spinning balls deflectedby the Magnus effect (Fig. 2). Within some Reynolds numberrange, applying a spin to a flying ball creates a lift force called Mag-nus force able to deflect the ball from the simpler ballistic trajectory.Perceiving or intercepting such deflected trajectories proves to bechallenging in many sports, like in soccer where they are especiallyused for scoring at most direct free kicks.

Beyond potential applications in improving sport training, study-ing human perception of such accelerated trajectories can lead tomore fun gameplays for the growing market of motion-based videogames (eg. Nintendo Wiimote, Microsoft Kinect). In particular,next generation sport and shooting games can benefit from a finecontrol of difficulty level and progression, based on the humanintrinsic skills. Furthermore, understanding human perception ofcomplex realistic motions could improve VR and especially mixedreality designs by suggesting the cues to emphasize in visualizationor the perception-action couplings allowing efficient perception.

2 BACKGROUND

Many studies dealt with perception or interception of moving ob-jects [12, 3], notably using VR immersive sport simulations [14,10, 8]. However, to our knowledge, Craig et al. [5, 6] are the only

∗e-mail: thierry.hoinville@iit.it†e-mail: abdeldjallil.naceri@iit.it

Figure 1: A subject immersed in the virtual stadium

ones who have studied human perception of the complex lifted balltrajectories so common among ball sports.

Specifically, they asked professional soccer players and novicesto predict where free kicks will go, in presence of sidespin or not.Early portions of normal, left- and right-deflected ball trajectorieswere presented to the subjects through a head-mounted display.Subjects viewpoint position was held fixed in the middle of the goalline, only head orientation was tracked. After every stimulus, thesubjects reported by mouse clicking whether they judged the ballwill arrive inside the goal or outside. No performance feedbackwas given to avoid learning bias.

Craig et al. found that, disregarding expertise, all subjects didsystematic judgement errors according to the spin direction. Thisgives support for an inherent human limitation to perceive acceler-ated motion [12, 3]. Moreover, they found no significant differencesbetween experts and novices. However, one can expect that profes-sional players, especially goalkeepers that are trained to interceptcurved kicks, should be more performant than novices.

That result could arise from the experimental design based onpseudo-verbal responses. This hypothesis is supported by a formerstudy [7] which suggested that loose coupling between perceptionand action, such in the case of verbal or pseudo-verbal answers,impedes expertise from affecting anticipatory performance.

3 METHOD

The present study complements the work done by Craig et al. [5, 6],in that it addresses the perception of spinning ball trajectories in thecontext of motor-based responses. To this end, we replaced the‘goal-or-not’ judgement task by a ball heading task, while retainingmost of the previous procedure unchanged. Hereby, we imposed atight coupling between perception and action, the head being boththe viewpoint and the effector. Interception using hands would havebeen an alternative approach, but such task allows less perception-action coupling in that some subjects could move their hands only,while keeping their viewpoints static. Ball heading in VR has al-ready been studied [10], but for trajectories without spin effect.

Our experimental design contrasts with Craig et al.’s one ac-cording to two main differences. Firstly, rather than using a head-mounted display device, we chose to use a widescreen 3D projec-tion which is more suitable for moving freely and provides a largefield of view. Secondly, as detailed below, we adapted the ball ar-rival positions to be always comfortably reachable by each subject’shead (ie. it did not require a high level of physical skill).

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IEEE Virtual Reality 2011

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Figure 2: A spinning ball can be deflected (depending on theReynolds number) by the Magnus lift force (Fl), perpendicular to boththe velocity (u) and spin (w) vectors.

3.1 Apparatus

Subjects were immersed in a virtual soccer stadium (Fig. 1) com-pliant with the FIFA Laws of the Game, through a widescreen 3Dprojection (130” diagonal, giving a field of view of 79◦ at 2 m dis-tance). 3D images of resolution of 1024×768 pixels, generatedfrom a NVIDIA Quadro FX 3800 graphics card, were displayed at120 Hz (60 Hz each eye) using a Viewsonic PJD6221 projector andfiltered back to the subjects by NVIDIA 3D Vision shutter glasses.

Left and right eye projection viewpoints were updated in real-time (60 Hz, latency < 50 ms) according to the subject’s head lo-cation and orientation (only five degrees of freedom were used forthe tracking because no pitch angle is necessary to determine botheyes positions). This was achieved using a WorldViz PPT opticalmotion tracking system of six cameras (H series) sensing two LEDmarkers mounted on the shutter glasses sides (Fig. 3).

3.2 Stimulus

Trajectories of the free kicks presented to the subjects were com-puted using the Cyberbotics Webots simulation software (ver-sion 6.2.4 PRO). The ball was modelized as a solid sphere of radiusR = 11 cm and mass m = 440 g (FIFA compliant values), submittedto gravity, drag and lift (Magnus) forces (Fig. 2) defined resp. as:

Fg = mg (1)

Fd = −1/2 ρACd|u|2u (2)

Fl = 1/2 ρACl|u|2 (w×u) (3)

where g is the gravity acceleration vector, ρ = 1.2 kg/m3 is the air

density, A = πR2 is the ball cross sectional area, Cd and Cl are resp.the drag and lift coefficients, u is the ball velocity vector, w is theball spin vector and e = e

|e| is the vector normalization operator.

Like in [5, 6], all ball trajectories (Fig. 4) started from the sameinitial position (laterally centered and at 30 m far from the goal linewhere stood the subjects) with a constant initial speed of 36 m/s.We also investigated the same three conditions of pure sidespin:no spin (NS); clockwise spin of +600 rpm (CS); counter clock-wise spin of −600 rpm (CCS). For each subject and spin con-dition, we computed the right initial velocity vector direction tomake the ball arrive at an easy altitude for interception (defined as:subject′s height−7.6 cm) and at one of eight different lateral posi-tions along the goal line: −35, −25, −15, −5, 5, 15, 25 and 35 cmaround the goal center (ie. subject initial position). In [5, 6], displayof the ball was disabled at variable cutoff distances from the goalline (ie. 10 or 12.5 m), in order to enforce anticipatory behavior.Here, the proposed motor task intrinsically requires anticipation.Yet, we varied display cutoffs at 6 or 2 m for three reasons: (1)To maintain homogeneity with the former studies; (2) To avoid theimmersion conflict when the ball becomes bigger than the screen;(3) To narrow what subjects can guess on their performances andthus limit their learning. To summarize, we presented to each sub-ject 48 different conditions in total (three sidespins × eight arrivalpositions × two cutoff distances).

Figure 3: Tracked shutter glasses are used to update the positionand orientation of a simplified human head model.

Drag and lift coefficients are known to vary at least with the ballspeed and spin rate [11, 13, 1]. However, Craig et al. [5, 6] showedquite realistic trajectories using constant coefficients during wholeball flights. Since they did not report the values they used, weset Cd = 0.25 and Cl = 0.25 which appear to be realistic values(Fig. 4) in our case of fast moving and spinning balls [9, 2, 4].

3.3 Procedure

Subjects stood at two meters from the screen, corresponding onthe virtual soccer pitch as standing over the goal line. They wereasked to intercept the ball with their heads as if they were ‘handlessgoalkeepers’. We informed them that this task required only lateralmoves, up to one sidestep (no need to jump, crouch, move forwardor backward). Indeed, the ball always arrived at the height of theirheads on the goal line where they stood.

After each free kick, subjects were constrained to return backto the initial neutral position facing the middle of the screen (ie.corresponding to the goal line center). This was achieved by re-leasing the next free kick only when subjects head position had re-mained for one second within an imaginary 10 cm wide corridorpointing perpendicularly to the screen center. Most of the time,recentering was natural for the subjects. But, when needed, we en-couraged them to use several cues (most preferable first): (1) Visualcue: checking for the vertical alignement of the penalty spot and theball; (2) Verbal cue: operator signaling too much left/right shifting;(3) Physical cue: putting their feet on the ground mark (Fig. 1).

During each entire ball flight, subject’s head trajectory wasrecorded. For each trial, success was assessed according to whetheror not the simplified head model (Fig. 3), whose pose was updatedaccording to the head tracking information, had touched the ball.If not touched, ball trajectories were stopped after entering 50 cminto the goal. In addition, as we said before, the ball was no longerdisplayed beyond either 6 or 2 m from the goal line.

For each subject, the whole experiment including pauses lastedabout one hour. It was divided in three phases: (1) Adaptingphase. For few minutes, the subject was encouraged to move inthe VR room to adapt himself to the tracking-based 3D projec-tion. (2) Training phase. In order to become familiar with theheading task, the subject was asked to intercept 32 non spinningballs (eight arrival positions, two cutoff distances, presented twiceand randomly). A spoken performance feedback was played on aloudspeaker, saying if the subject whether or not managed to touchthe ball. This phase did not entail free kicks with spin to limit thelearning effects. All the subjects performed well during this phase.(3) Experiment phase. All the 48 trajectory conditions were ran-domly presented ten times without any performance feedback, giv-ing a total of 480 trials per subject (three spin conditions, eightarrival positions, two cutoff distances, ten repetitions).

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Figure 4: The 24 ball trajectories from 3 spin conditions× 8 arrival positions. Two balls are shown before disappearing at the two cutoff distances.

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Figure 5: Interception performances of the novices (NOV) in blue and the sportball players (SBP) in red according to the eight ball arrivalpositions, the three spin conditions (rows) and the two cutoff distances (columns). Significances of the differences were obtained by independent1-way ANOVAs (thin lines: p < .05; thick lines: p < .01). Without spin, there is a slight left-right asymmetry, more significative with the NOV. Withspin, there is no significant left-right differences, except one with the NOV, for 6m cutoff, between arrival positions = +25cm [CS], −25cm [CCS].

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Figure 6: Crossing and non crossing kicks

3.4 Subjects

We performed our experiment with 17 subjects. Their heights weremeasured in order to adjust all ball arrival positions at head levels.Subjects were also asked to report their experience in ball sports.We defined a criterion to split them into two groups: experiencedand novices. They were considered experienced if they did morethan one hour a week of ball sports (soccer, volleyball, basketball,tennis, squash...), for more than one year and less than one year ago.It gave two groups: the sportball players (SBP group, N = 9, ages ∈[26,33], heights ∈ [168,179] cm, 2 females) and the novices (NOVgroup, N = 8, ages ∈ [26,33], heights ∈ [157,188] cm, 6 females).

4 RESULTS

We conducted a 4-way ANOVA between subjects to compare theeffects of sidespin, arrival position, cutoff distance and experiencein ball sports on heading performance in all the possible 48 con-ditions. All factors except the cutoff distance were found to havesignificant effects on the performance:

• Spin: F2,8148 = 219.79 p < .0001

• Arrival position: F7,8148 = 83.87 p < .0001

• Experience: F1,8148 = 91.22 p < .0001

• Cutoff: F1,8148 = 0.57 p = .4488

Results (Fig. 5) show that, without spin the performance of allthe subjects is maximal for frontal shots and decreases as the ball ar-rival position gets away laterally. Also, we notice a slight left-rightasymmetry. In case of spinning balls, we notice systematic shiftsin how performance distributes among the arrival positions. Theseshifts clearly relate to the spin direction: CS and CCS giving riseto quite symmetric performance distributions. Low performancesare likely in case of ‘crossing’ free kicks (ie. trajectories whichlaterally cross the initial line of sight toward the ball, see Fig. 6),whereas non crossing trajectories result in high performances.

Finally, we performed a 1-way ANOVA to compare sportballplayers and novices performances on each condition independently(Fig. 5). In the case of non spinning balls, performance of both sub-ject groups are similar for frontal kicks, but sportball players signif-icantly outperform novices for lateral kicks. In the case of spinningballs, both subject groups manage to intercept non crossing trajec-tories, but sportball players fail significantly less than novices inheading crossing trajectories.

5 DISCUSSION

Our results are consistent with the previous studies [5, 6]. Theyconfirm that curving of ball trajectories by the Magnus effect sys-tematically induces significant errors in perception. Thus, it sup-ports the hypothesis that human visual system would be inherentlylimited in perceiving accelerated motion [12, 3].

The slight left-right asymmetry in performance for non spinningkicks may come from several biases: eye dominance, handedness,errors in the tracking system calibration. Also, the subject’s andball’s initial positions on the soccer pitch dictate the visual back-ground and may bias the performance. Why novices seem moreaffected and why spinning balls filter out that asymmetry are stillunknown.

Interestingly, our results show a significant influence of expe-rience on the performance that has not been found in the formerstudies [5, 6]. It is still unknown what allows in our case the ex-pression of subjects expertise. Do sportball players rely on better

perception skills only? This seems unlikely because otherwise theformer studies based on advanced stereoscopic display too shouldhave also permit expertise to show up. Do sportball players rely ontheir faster and more accurate motor response? Our approach basedon motor only response could make such skills crucial. However,the arrival positions of the ball were chosen close enough to thesubjects initial location to be easily reached even by novices. Dosportball players exploit the tight perception-action coupling char-acterizing our heading task? They may for instance rely on activeperception strategies. This would then support that expertise be-comes apparent only when perception is coupled to action [7, 6].

As we said before, the subjects self-reported their experience invarious ball sports. This and the expertise criterion we defined maybe subject to bias and make our results preliminary. A complemen-tary experiment involving professional soccer players is necessary.

6 CONCLUSION

We extended the work of Craig et al. [5, 6] by studying perceptionof spinning ball trajectories in the context of a strong sensorimotorcoupling enforced by the heading behavior. Our results validatethe former observations that the Magnus effect induces systematicperception errors. In addition, we found that our VR setup allowsfor experience to become apparent.

Further analyses on the recorded head trajectories will be neededin order to extract the different information-movement laws [14,10, 8] that explain the differences in performance of experiencedsubjects and novices. Also, we plan to repeat our experiment withprofessional soccer players.

ACKNOWLEDGEMENTS

Thanks to all the experiment participants and the reviewers thatcontributed to this article.

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