Journal of Sports Sciences, 2003, 21, 87–95
Dynamics of visual feedback in a laboratory simulation of a penalty kick EDGARD MORYA,1* RONALD RANVAUD1 and WALTER MACHADO PINHEIRO2 1
Department of Physiology and Biophysics, Institute of Biomedical Sciences, University of Sa˜o Paulo, Av. Lineu Prestes 1524, Sa˜o Paulo, SP 05508-900 and 2Department of Physiology and Pharmacology, Biomedical Institute, Federal Fluminense University, Rua Hernani Mello 101, Niteroi, RJ 24210-130, Brazil
Accepted 22 October 2002
Sport scientists have devoted relatively little attention to soccer penalty kicks, despite their decisive role in important competitions such as the World Cup. Two possible kicker strategies have been described: ignoring the goalkeeper action (open loop) or trying to react to the goalkeeper action (closed loop). We used a paradigm simulating a penalty kick in the laboratory to investigate the dynamics of the closed-loop strategy in these controlled conditions. The probability of correctly responding to the simulated goalkeeper motion as a function of time available followed a logistic curve. Kickers on average reached perfect performance only if the goalkeeper committed him or herself to one side about 400 ms before ball contact and showed chance performance if the goalkeeper motion occurred less than 150 ms before ball contact. Interestingly, coincidence judgement – another aspect of the laboratory responses – appeared to be affected for a much longer time ( 4500 ms) than was needed to correctly determine laterality. The present study is meant as groundwork for experiments in more ecological conditions applicable to kickers and goalkeepers. Keywords: decision-making, motor program, penalty kick, point of no return, soccer, visuo-motor integration.
Introduction One of the most characteristic features of association football is the low scores with which matches are frequently decided. Penalty kicks can thus play a decisive role, even more so since the introduction of the penalty shoot-out (FIFA, 2002) to decide draws in competitions such as the World Cup. Several recent major football events were won or lost on penalties (Williams and Burwitz, 1993; McMorris and Hauxwell, 1997; Lover and Blatter, 1998; Franks et al., 1999; McGarry and Franks, 2000). Given the determining role of this fundamental of the game, relatively little attention appears to have been devoted to it in the scientific and the technical literature. Bonizzoni (1988) indicated that only half of first league professional coaches in Italy included specific penalty drills in their routines. Considering the accuracy with which professional soccer players can place the ball (Lees and Nolan, 2001) and the time required for a goalkeeper to * Author to whom all correspondence should be addressed. e-mail:
[email protected]
reach the edges of the goal (Morris and Burwitz, 1989), it is surprising that between 25 and 33% of penalty kicks in official games are missed (Kuhn, 1988; Franks and Hanvey, 1997). The statistics would indicate that scoring from the penalty spot is not as easy as it might seem and that defending penalties is more within the capabilities of goalkeepers than might be thought (Whitfield, 2002). Scientific laboratory investigations can thus be used to obtain specific technical information on human performance as it relates to penalty kicks. Controlled experiments in the laboratory contribute to, support and complement field studies (Tenenbaum and Summers, 1997; Hastie, 2001). Generalization of these results must be done with caution, since many factors in the field – psychological, physiological and biomechanical – are important in match-play (Davids et al., 2000). According to the laws of the International Football Association Board (FIFA, 2002), during a penalty kick the goalkeeper must remain on his or her goal line, facing the kicker, between the goalposts until the ball has been kicked from a distance of 11 m (12 yards). Typical ball speed is about 75 km×h71, a speed
Journal of Sports Sciences ISSN 0264-0414 print/ISSN 1466-447X online # 2003 Taylor & Francis Ltd DOI: 10.1080/0264041031000070840
88 generally taken as the break point between what is considered slow or fast (Kuhn, 1988; Morris and Burwitz, 1989). This means that the ball takes a little under 600 ms to reach the goal line from the instant it is kicked. Kuhn (1988) identified two strategies for penalty kickers in the German Bundesliga and the European Cup. The first, which was followed by 22% of kickers, he called ‘open loop’; here, the kicker acts according to a plan and ignores any action the goalkeeper might take. The second strategy, followed by 78% of kickers, he called ‘closed loop’; with this strategy, the kicker tries to take into account the goalkeeper’s actions, so as to place the ball to the opposite side of the goal. Kuhn (1988) also identified two strategies for goalkeepers. In the first, the goalkeeper decides which side to dive to at or immediately before the point at which the ball is struck by the penalty taker. In the second, the goalkeeper tries to anticipate the direction of the kick from earlier cues (Abernethy, 1987; Kuhn, 1988; Morris and Burwitz, 1989; McMorris et al., 1993; Williams and Burwitz, 1993; Franks and Hanvey, 1997; McMorris and Hauxwell, 1997; Franks et al., 1999; Savelsbergh et al., 2002) and dives to that side before the kicker strikes the ball. When adopting the second strategy, goalkeepers must also choose when to dive. Diving too early might allow kickers who adopt a closed loop strategy time to react and place the ball to the opposite side (Franks et al., 1999). Diving too late might not allow the goalkeeper enough time to get to the ball (Morris and Burwitz, 1989), especially if it is placed near the goalpost or if it is struck very hard. Using an innovative apparatus to examine differences in anticipation between expert and novice goalkeepers during penalty kicks, Savelsbergh et al. (2002) showed that expert goalkeepers initiated a joystick correction around 300 ms before kicker–ball contact (versus 500 ms for novices). They suggested that either expert goalkeepers try to pick up more information before initiating a movement or the kicker’s crucial postural cues emerge around 300 ms before striking the ball. The questions addressed in this paper are relevant for goalkeepers that adopt the second of Kuhn’s strategies, since knowing the time required by the kicker to be able to respond to the goalkeeper action is very useful in deciding when to dive (Williams and Burwitz, 1993). The closer the kicker gets to the ball, the harder it is to change the planned kick or to react appropriately to the goalkeeper action. Thus, if possible, the goalkeeper should not dive until, but no later than, the moment the kicker can no longer modify the direction in which the ball will be hit. In combination with the ability to use advance postural cues from the kicker (Abernethy, 1987; Kuhn, 1988; Morris and Burwitz, 1989; McMorris et al., 1993; Williams and
Morya et al. Burwitz, 1993; Franks and Hanvey, 1997; McMorris and Hauxwell, 1997; Franks et al., 1999; Savelsbergh et al., 2002), this might increase the probability of the goalkeeper defending a penalty successfully. Morya et al. (2001) presented preliminary results on the point of no return during a penalty kick. The term ‘point of no return’ has been used by several workers in the study of when the response to a stimulus can no longer be stopped (Osman et al., 1986, 1990; Logan, 1994; Band and Van Boxtel, 1999). The moment beyond which the response cannot be inhibited has been investigated mainly using countermanding paradigms. Countermanding paradigms probe a participant’s ability to inhibit the initiation of an intended movement by infrequently presenting an imperative stop signal in a reaction time task (Lappin and Eriksen, 1966; Logan and Cowan, 1984; Osman et al., 1986; DeJong et al., 1990; Logan, 1994). Clearly, the probability of not being able to stop the response – that is, responding even when the stop signal appears – increases with stop signal delay (Logan and Cowan, 1984; Logan, 1994). The stop process has been analysed using a ‘horse-race model’, in which the response process and the inhibitory process run in parallel (Logan and Cowan, 1984). If the response process finishes before the stop process, then the response is executed; however, if the stop process finishes first, the response is inhibited (Logan and Cowan, 1984; Osman et al., 1986; Logan, 1994; Kok, 1999). Some authors have indicated that hundreds of milliseconds are necessary to inhibit a simple key press response (Osman et al., 1986, 1990; Logan, 1994; Band and Van Boxtel, 1999). Others have reported that a graded response could be inhibited at any stage (DeJong et al., 1990; McGarry and Franks, 1996). Clearly, the ability to react quickly to changing stimuli and inhibit or modulate one’s action accordingly is an important aspect of skilled behaviour in sport, but little research has focused on the ‘point of no return’ in sport (Tenenbaum and Summers, 1997). For penalty kicks, rather than establishing the point beyond which action can no longer be inhibited, it is important to determine the instant at which it is still possible to alter the side of the goal to which the ball is struck based on cues gleaned from the goalkeeper. This is closely related to the time involved in closing the visual feedback loop (Keele and Posner, 1968; Carlton, 1981; Kuhn, 1988), which should be of considerable interest to coaches in preparing training sessions for penalty kicks. The lack of research on how long kickers require in a penalty kick to process goalkeeper cues and thus determine to which side the ball should be struck (or, alternatively, to change their initial choice because of goalkeeper cues) motivated this psychophysical laboratory study in preparation for future field experiments.
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Visual feedback of the penalty kick
4.2°
Methods Participants Ten participants were chosen from among male university soccer players with experience in taking penalty kicks and with a minimum of 10 years practising soccer. They were all right-handed (Oldfield, 1971), were aged 23.7+4.2 years (mean+s), had normal or corrected-to-normal vision, and had no experience as a goalkeeper facing penalty kicks. They provided informed written consent before beginning the experiment.
1.4°
Goalkeeper 0.1°
4.1° Ball
t = 1344 ms 6.3°
Kicker
t = 0 ms Apparatus The participants were tested in an acoustically isolated booth with dimmed lights. The commercial software MEL Professional v. 2.01 (Psychology Software Tools, PST Inc., Pittsburgh, PA), installed on a Pentium 100 MHz computer, generated visual stimuli on a 14 inch monitor (60 Hz) and recorded the volunteers’ responses. MEL Professional re-programs the standard IBM hardware clock to maintain millisecond acquisition accuracy (Schneider, 1988). Data were generated by inclining a vertical manual lever to the right or to the left, tripping off optical sensors connected to the game port. The participants had their eyes centred 57 cm from the computer screen by a chin and forehead support. They were seated in a comfortable chair with adjustable height. To one side of the computer screen, a Sony video camera (model CCD-TR900) permitted us to check the volunteers’ gaze during the trials on a monitor outside the booth. If a participant had not followed the instructions, the trial was cancelled and automatically re-done. Experimental design A goalmouth with a goalkeeper, a ball and a kicker (Fig. 1) were displayed on a computer screen. On a black background, the goalkeeper, ball and kicker were represented by white disks, each 0.48 in diameter, and the goalposts were represented by straight vertical lines 1.48 high and the crossbar by a straight horizontal line 4.28 wide. The goalkeeper was in the middle of the goalmouth, 4.18 vertically above the ball, which in turn was 6.38 vertically above the kicker. The kicker moved vertically upwards at a speed of 81.02 mrad×s71 (4.7 cm×s71) towards the ball. Thus the kicker coincided with the ball 1344 ms after the beginning of the trial (Fig. 1). This coincidence lasted for the next 17 ms, the duration of a frame on the computer screen. Occasionally, the goalkeeper moved laterally, also at a speed of 81.02 mrad×s71, whereas the ball was always immobile at the centre of the display.
0.4° Fig. 1. Frontal view of the experimental design (not to scale). Dashed lines represent distances in degrees (18 = 1 cm, at 57 cm from the monitor). The solid line represents the motion of the kicker, from the beginning of the trial (t = 0 ms) to 1344 ms later, when perfect superposition with the ball begins, lasting 17 ms. The speed of the kicker was 4.7 cm×s71.
Experiment I consisted of two blocks of 40 trials without any goalkeeper motion. Experiments II, III and IV consisted of two blocks of 110 trials in which the goalkeeper moved sideways on 81.8% of trials and remained stationary on 18.2% of trials. The goalkeeper could start to move randomly at one of nine possible instants (51, 102, 153, 204, 255, 306, 357, 408, 459 ms) before ball contact. These nine intervals between goalkeeper motion and ball contact were called ‘available times’, since they corresponded to the times between stimulus onset and time at which the response should occur. Each participant responded to 20 trials for each ‘available time’, with the goalkeeper moving ten times to the right and ten times to the left. On 40 trials, the goalkeeper did not move. Procedure The participants were positioned in a chair in front of the monitor with their eyes held 57 cm away by a chin and forehead rest and with the right hand gripping the lever. They undertook two sessions of about 40 min on separate days. In the first session, they responded to four blocks of tests: two in Experiment I and two in Experiment II. In the second session, they again responded to four blocks of tests: two in Experiment III and two in Experiment IV. Intervals of 5 min were allowed between blocks and experiments. Before taking part in each experiment, the participants received verbal and written instructions and 25 training trials (data discarded). Throughout all trials, the participants were
90 instructed to fixate their eyes on the ball; their gaze was monitored to ensure that they were all tested in the same condition and that they followed the instructions. While fixating the ball, they were clearly aware of goalkeeper and kicker motion through peripheral vision. The participants were told the experiment was supposed to represent a penalty kick from the kicker’s point of view, never looking at the spot to which the ball should be struck. Experiment I The instructions emphasized that the participants should tilt the lever to the right (one block of 40 trials) or to the left (another block of 40 trials) at the exact moment the kicker struck the ball. There was no goalkeeper motion and incorrect responses could only be due to grossly anticipated or delayed responses. Experiment II The participants were instructed to tilt the lever to the right (one block of 110 trials) or to the left (another block of 110 trials) at the exact moment the kicker struck the ball but ignoring any goalkeeper movement, which occurred in 81.8% of the trials. Again, incorrect responses could only be due to grossly anticipated or delayed responses. Experiment III The participants were again instructed to tilt the lever at the exact moment of kicker–ball contact but, whenever possible, to the opposite side of the goalkeeper motion. If the goalkeeper did not move, they were free to tilt the lever to either side. Incorrect responses could now be due not only to overly anticipated or delayed responses, but also to moving the lever to the same side as the motion of the goalkeeper, or a combination of these.
Morya et al. goalkeeper; (c) tilting the lever to the side opposite to the default (predetermined side) when the goalkeeper did not move; or (d) a combination of these. Data analysis Precision of response in the four experiments was analysed using a repeated measures analysis of variance (ANOVA) or paired t-test (a = 0.05). When necessary, the data were submitted to a post-hoc Student-NewmanKeuls test (a = 0.05). In Experiments III and IV, the average percentage of correct responses as a function of ‘available time’ was fitted to a logistic curve model (Fig. 2) as a description of the dynamics of the closed-loop strategy. This model considered average performance for each available time and adjusted the best curve starting with chance performance for a short available time and reaching perfect performance for a long available time. The mid-point between chance performance and perfect performance (75% chance of tilting the lever to the correct side) can be taken as a psychophysical measurement of the mean time required to respond correctly to goalkeeper motion when adopting the closed-loop strategy.
Results In all experiments, superposition of kicker and ball occurred during the frame that lasted from 1343 to 1360 ms after the kicker started to move, the mid-point of this frame being at 1351.5 ms after the kicker started to move. The participants’ mean response times in all experiments (Table 1) were not significantly different (ANOVA: F3,39 = 1.62, P = 0.203). In Experiment III, legitimate errors, inevitable when the available time was short, corresponded to 19.9% of responses, and they occurred on average at 1359+3 ms (mean+sx ), no different from the average of correct responses (t9 = 70.583, P = 0.574). When the goalkeeper
Experiment IV The main difference in relation to Experiment III was that before starting each trial the participant had selected a predetermined side to which the lever should be tilted at the time of kicker–ball contact. In one block, the predetermined side was to the right (110 trials) and in the other block it was to the left (110 trials). Whenever the goalkeeper moved to one side, correct responses consisted in tilting the lever to the opposite side. If the goalkeeper did not move, then correct responses consisted in tilting the lever towards the predetermined side. Incorrect responses could now be due to: (a) overly anticipated or delayed responses; (b) tilting the lever to the same side as the motion of the
Fig. 2. Logistic curve model fitted to the mean percentage of correct responses as a function of available time. The variable AT = available time; the parameter AT0 = the available time for the mid-point between chance and perfect performance; b = unknown parameter to be estimated.
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Visual feedback of the penalty kick remained stationary or moved less than 204 ms before kicker–ball contact, there was a tendency for the participants to respond late, as if waiting until the final moment to commit themselves (Table 2). Interestingly, with the two longest available times (408 and 459ms), responses were significantly anticipated (F9,81 = 11.2, P 50.001). In Experiment IV, legitimate errors (no change from the predetermined side when there should have been, inevitable when the available time was short) represented 15.9% of all trials, with response times on average at 1356+2 ms (mean+sx ), no different from the average
Table 1. Mean responses of participants (mean+sx ) Experiment
Mean responses (ms)
I II III IV
1350+2 1354+1 1357+3 1357+2
Table 2. Mean response times (mean+sx ) and performance (percentage correct response) as a function of available time in Experiment III Available time
Response time (ms)
% correct response
Goalkeeper still 51 ms 102 ms 153 ms 204 ms 255 ms 306 ms 357 ms 408 ms 459 ms
1369+3 1362+4 1363+3 1368+5 1363+5 1359+4 1355+3 1353+4 1343+5 1339+4
100.0 52.5 50.5 49.5 62.5 81.0 90.5 96.0 99.0 100.0
Discussion and conclusions The results of Experiments I and II revealed a baseline for when participants tried to tilt the lever to coincide with kicker–ball contact. In both experiments, the (b)
(a) 100
Correct responses (%)
Correct responses (%)
for correct responses (t18 = 0.09, P = 0.927). The mean percentage of correct responses as a function of time available was fitted to a logistic curve in Experiments III (Fig. 3a) and IV (Fig. 3b). From the fits, the times for 75% correct responses were determined in Experiments III (241+16 ms; mean+sx ) and IV (246+11 ms). These times represent the half-way mark between 50% correct responses (random performance) and 100% correct responses (perfect performance). The difference in the point of no return in Experiments III and IV was smaller (5 ms) than the 11–16 ms standard error in estimating these points from the fits. Again, there was a tendency for the participants to respond later when the goalkeeper did not move or when they had too little time to decide which side to strike the ball (Table 3). Little, if any, overall difference was noted between the conditions in which the participants started the trial with or without a preferred side in mind (Experiments IV and III, respectively; t9 = 0.40, P = 0.695). Responses for the two longest available times (408 and 459 ms) were again significantly anticipated, as in Experiment III. These responses were different from those for the other available times (F9,81 = 11.2, P 50.001). Correct responses revealed a statistically significant interaction between predetermined side and goalkeeper laterality (F2,18 = 13.7, P = 0.001). The implication of this interaction is that responses were rather late (1360 ms on average) when goalkeeper motion forced the participants to incline the lever opposite to the predetermined side (changing response), but were perfect (1352 ms on average) when goalkeeper motion allowed them to carry on and incline the lever to the predetermined side (Fig. 4).
90 80 70 60 50 0
51 102 153 204 255 306 357 408 459
100 90 80 70 60 50 0
51 102 153 204 255 306 357 408 459
Available time (ms)
Fig. 3. Logistic fits of success rate (tilting the lever towards the correct side) in (a) Experiment III (75% probability of success at 241 ms, sx = 16 ms) and (b) Experiment IV (75% probability of success at 246 ms, sx = 11 ms) as a function of available time.
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M Mean response time (ms)
Morya et al.
Fig. 4. Interaction between predetermined side (PrS) and goalkeeper laterality showing later responses when the goalkeeper moved to the predetermined side, requiring the participants to change their response (~), and accurate responses when the goalkeeper moved to the non-predetermined side, allowing the paticipants to continue (&).
Table 3. Mean response times (mean+sx ) and performance (percentage correct response) as a function of available time in Experiment IV Available time
Response time (ms)
% correct response
Goalkeeper still 51 ms 102 ms 153 ms 204 ms 255 ms 306 ms 357 ms 408 ms 459 ms
1360+2 1366+4 1367+3 1355+3 1365+3 1362+4 1358+3 1355+3 1348+2 1342+4
78.7 49.5 49.5 47.5 65.0 74.0 93.5 97.5 98.5 100.0
participants performed remarkably well, which suggests that there was no appreciable overall involuntary influence of the goalkeeper’s motion on the responses; the participants were, by and large, successful in ignoring the goalkeeper’s motion, as instructed. The instructions to fixate the ball caused participants to use peripheral vision to pick up goalkeeper information. This was not considered a problem, since in the field peripheral view is often used, given that fixating the point aimed at would be a very useful cue to the goalkeeper. Moreover, there is evidence to suggest that peripheral vision is more sensitive to movement than the fovea (Williams and Davids, 1997, 1998; Williams et al., 1999; Savelsbergh et al., 2002). Finally, in a real penalty kick, kickers also often attend to the ball during most of the run up.
Fitting the mean percentage of correct responses as a function of available time with a logistic curve provided a laboratory measurement of the dynamics of the visual feedback loop for laterality determination. From a psychophysical point of view, the halfway point from random performance to 100% success in determining laterality can be considered as the mean point of no return to alter the laterality (Morya et al., 2001). However, from a practical point of view, kickers would wish to be more than halfway from random to perfect performance, and the logistic curves (Figs 3a,b) show the dynamics of how the time cost increases with improved performance. The point of no return in the laboratory was 240–245 ms before the ball was struck. Performance improved from random to near 100% reliability as goalkeeper movement changed from 150 to 400 ms before the ball was struck by the penalty taker. When there was no goalkeeper motion, or when goalkeeper motion occurred less than 153 ms before kicker–ball contact (Experiments III and IV), the participants inclined the lever 11 ms later, on average, than they did in Experiments I and II. These results suggest that kickers waited 11 ms longer to trigger their motor response when the situation was ambiguous compared with when they knew which side the lever should go. Beyond this point, there was not enough time to use any visual information from the goalkeeper. In Experiment IV, comparing correct responses when volunteers had to change sides and when they had not to change sides showed that changing laterality delayed responses by about 8 ms. Responses in which laterality was not changed were identical to those in Experiments I and II. These results raise the question of whether performance might be affected when, having chosen which side to place the ball, the kicker then adopts a closed-loop strategy and is forced to change. This might be related to Kuhn’s (1988) finding that waiting for the goalkeeper to reveal his or her intention might result in a weak shot. An unexpected result was found when the goalkeeper remained stationary. Kickers did not always incline the lever to the predetermined side as instructed. They sometimes suddenly changed side, as if they were trying to anticipate or predict the goalkeeper’s movement without any real cue. This might be explained by the model of Kim and Shadlen (1999), who examined why monkeys sometimes make wrong decisions in psychophysical tasks. They recorded cells from the middle temporal area and found that these cells are inherently ‘noisy’, as other neurons are too. This means that the number of times they fired varied from trial to trial, even when the same stimulus was presented to the monkey. Thus, from time to time a burst of firing by chance occasionally overwhelmed a signal from stimuli that indicated another response.
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Visual feedback of the penalty kick A second unexpected finding was that, in Experiments III and IV, responses with the two longest available times (408 and 459 ms) were both significantly anticipated and different from all other available times. The two longest available times were long enough for 100% accuracy in perceiving and responding to goalkeeper motion and reliably moving the lever to the opposite side. One might reasonably expect that when 100% of the responses were in the correct direction, the coincidence times would not be very different from those in Experiments I and II. It would appear, however, that the effort in determining which side the lever should be moved interferes with determining exactly when it should be moved. Such an effect is present for a surprisingly long time, more than 50 ms (and probably more than 100 ms) beyond the moment responses approach 100% reliability. The results presented here suggest that goalkeeper cues might be ‘early’ when about 500 ms (or perhaps more) before the kicker strikes the ball, since on average 400 ms were required to respond to goalkeeper motion with near 100% reliability. Accuracy in placing the shot will not be important if the kicker is successful in reading the goalkeeper’s intention and is able to place the ball into the empty half of the goalmouth. From the kicker’s point of view, however, there is always the risk that the goalkeeper will not reveal early enough the side to which he or she intends to dive or provide false cues. In such cases, the shot must be accurate because the goalkeeper might have gleaned the kicker’s intention from advance cues as described by several authors (Abernethy, 1987; Kuhn, 1988; Morris and Burwitz, 1989; McMorris et al., 1993; Williams and Burwitz, 1993; Franks and Hanvey, 1997; McMorris and Hauxwell, 1997; Franks et al., 1999; Savelsbergh et al., 2002). Goalkeepers can take advantage of the knowledge that there is a moment beyond which kickers cannot change their actions, or at least cannot do so accurately. Diving at that moment, but not earlier, would give the goalkeeper maximum time to reach the ball (if they dive to the correct side) while simultaneously not giving enough time for the kicker to change predetermined plans. Goalkeepers might combine this information with that found by Franks et al. (1999), who looked for cues that might enable goalkeepers to predict the placement of the shot (Abernethy, 1987; McMorris et al., 1993; Williams and Burwitz, 1993; Lees and Nolan, 1998). Franks et al. (1999) examined whether goalkeepers could be trained to use cues from the position of the non-kicking foot, and found significant improvement in the correct prediction of the direction of the penalty kick. Williams et al. (1993) examined the proposition that experienced soccer players exhibit greater task-specific cognitive knowledge than novice players. They demonstrated that the amount of
information extracted from a briefly presented soccer action sequence was a function of the soccer-specific knowledge of the participants, and the same might apply to goalkeepers for penalty kicks (Morris and Burwitz, 1989; Williams and Burwitz, 1993; Williams, 2000; Parravicini, 2001; Savelsbergh et al., 2002). Recently, Savelsbergh et al. (2002) found that expert goalkeepers initiated joystick movement about 300 ms before foot–ball contact, which was later than novices, who started the movement almost 500 ms before foot– ball contact. The present results, combined with those of Savelsbergh et al. (2002), suggest that kickers might have a better chance of changing a motor program when the goalkeeper provides cues about 500 ms before the kicker strikes the ball. On the other hand, the kicker might be at a disadvantage when the goalkeeper provides cues only about 300 ms before kicker–ball contact, because he or she will not be able to react appropriately. It is also important that postural cues of the kicker should be available by about that time (Williams and Burwitz, 1993; Franks and Hanvey, 1997; Savelsbergh et al., 2002). It would appear that the strategy adopted by many players in taking a penalty kick is not based on detailed technical or scientific understanding, but on empirical and subjective choice. We thus believe that, with specific training, there is room for considerable improvement. Williams et al. (1992) advised that laboratory tests should be as similar to field tasks as possible, or that research should be performed in the natural environment, before generalizing outcomes and providing advice to athletes and coaches. The work reported here might be a valid preparatory study for such fieldwork from the kicker’s point of view. We believe that there is good reason for such scientific investigation, since it offers the hope of enhancing the performance of top athletes in what appears to be an area neglected not only by researchers but also by many athletes and coaches.
Acknowledgements The authors are grateful to the participants and to Gilberto Paula, Joa˜o Sato and Fa´bio Nogueira, from the Institute of Mathematics and Statistics of the University of Sa˜o Paulo, who helped with data analysis. We would like to extend our thanks to the anonymous reviewers for their helpful suggestions. Financial support was provided by CAPES.
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