- Email: [email protected]
Comparison of offensive agility techniques in Australian Rules football
Available online at www.sciencedirect.com
Journal of Science and Medicine in Sport 14 (2011) 65–69
Original paper
Comparison of offensive agility techniques in Australian Rules football Ryan J. Bradshaw a,∗ , Warren B. Young a , Andrew Russell b , Peter Burge b a
School of Human Movement and Sport Science, University of Ballarat, Ballarat, Victoria, Australia b Hawthorn Football Club, Victoria, Australia Received 20 October 2009; received in revised form 3 June 2010; accepted 11 June 2010
Abstract Agility skill is important in Australian Rules football (ARF) as it enables an attacking player to successfully evade an opponent. To date, no research has examined offensive agility techniques in ARF. There were two purposes of this study: first, to compare the change of direction (COD) speed of three offensive agility techniques, and second, compare the reaction speed and accuracy when observing the same techniques from a defensive perspective. The techniques included the side-step, shuffle, and split-step. Seventeen players from an Australian Football League club were required to perform four trials of each technique. COD speed was expressed as a total time, and divided into entry, foot plant preparation, and exit time. In addition, nineteen players from the same club were assessed on a video-based reaction test. Players were required to respond by depressing a thumb switch to indicate whether the player on screen changed direction to the left or right. From an offensive perspective, the split-step foot plant preparation time (0.66 s) was significantly slower than both the side-step (0.7 s) and shuffle (0.75 s) (p < 0.05), but there were no significant differences in entry and exit times between techniques. From a defensive perspective, the players were significantly slower and less accurate when reacting to the player in the video performing the split-step (0.19 s) and shuffle (0.15 s) compared to the side-step (0.12 s) (p < 0.05). In a one-on-one situation in ARF, the split-step may be the most effective offensive technique. Not only was it slower and less accurate to react to, the exit speed following the change in direction was not significantly slower than the side-step. However, a performer would need to consider the cost of a slower foot plant preparation time versus the potential to deceive an opponent when under time stress. © 2010 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved. Keywords: Perception; Change of direction speed; AFL
1. Introduction In Australian Rules football (ARF), an attacker in possession of the ball may either kick or handball to a team mate in an attempt to move the ball forward and create a scoring opportunity. Alternatively, the offensive player may ‘take-on’ the opposition through the evasive action of rapidly changing direction or velocity. Examination of the physical factors which influence agility performance demonstrates that running technique can play a key role in maximising change of direction (COD) speed. Young et al.1 suggested that foot placement, trunk lean, stride adjustment and running posture are important technical components of agility technique. For example, it is ∗
Corresponding author. E-mail address: [email protected] (R.J. Bradshaw).
likely that a lateral foot placement would be ideal to generate a large medio-lateral ground reaction force. In addition, a forward lean may shift the centre of mass outside the base of support, allowing gravity to assist motion during the change of direction. This can only be speculated however, as specific performance based research on agility technique remains non-existent.2 To date, the major focus of research has been on injury prevention. In a one-on-one situation, an offensive player’s goal is to evade and outrun his opponent. As the opponent’s attempt to defend is reactive in nature, certain features of the offensive player’s movement will influence the defender’s action. Filmbased visual search and anticipation studies have shown that, coupled with superior pattern recognition and task specific knowledge, highly skilled athletes are able to successfully predict the action of an opponent before it is carried out.3 However, a major criticism of traditional perceptual test-
1440-2440/$ – see front matter © 2010 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jsams.2010.06.002
66
R.J. Bradshaw et al. / Journal of Science and Medicine in Sport 14 (2011) 65–69
ing has been the focus on observing consistent performance rather than enhancing uncertainty.4 Many of the display samples used may not accurately represent competitive situations where performers may attempt to disguise their intentions. Deception is a fundamental part of an offensive player’s movement in a one-on-one situation.5 An offensive agility technique that is fast to perform, and disguises a performer’s intention, may give the offensive player an advantage when performing under pressure from the opposition. Jackson et al.5 made the first attempt to compare non-deceptive and deceptive agility techniques from a defensive perspective. They found expert rugby player’s anticipation accuracy was equal when observing deceptive and non-deceptive sidesteps. Findings suggest that a deceptive side-step offers no advantage over a non-deceptive side-step. However, the results may have been a reflection of the closed-nature response required by players and familiarity with the agility technique. Further research examining unfamiliar techniques in a time-stressed situation may result in different findings. To date, no research has examined the COD speed and predictability of offensive agility techniques in ARF. As a result, there were two purposes of this research: first, to compare the COD speed of three offensive agility techniques, and second, to compare the reaction speed and accuracy when observing the techniques from a defensive perspective. The ultimate aim was to find the most effective offensive agility technique for a one-on-one situation in ARF (i.e. fastest speed exiting the change in direction, hardest to predict accurately and slowest to react to).
2. Methods This study used a within subjects design to investigate two separate qualities (COD speed and perceptual skill). The three techniques were: 1. Side-step: Player runs straight ahead and in one step, places a leg to the side of the body to push off and change direction to the opposite side. 2. Shuffle: Player moves in a forward direction whilst performing a series of small ‘side-steps’ without any significant lateral displacement of the trunk. Final movement to change direction is similar to a side-step. 3. Split-step: Whilst moving forward, the player performs a small jump, landing with both feet approximately shoulder width apart and body weight evenly distributed. Upon landing, the foot opposite the intended direction is used to initiate a COD. Change of direction speed test: Seventeen players (mean (M) ± standard deviation (SD) age: 21.2 ± 2.7 years; height: 184.9 ± 6.7 cm; weight: 84.8 ± 6.5 kg) from an Australian Football League (AFL) club volunteered to participate in the COD speed test. Informed signed consent was obtained from each player before testing to comply with the University’s ethics committee.
Before testing, players were allocated five familiarisation sessions within their normal training schedule. Players were required to run around a stationary defender performing eight trials of each technique. Practice was equally distributed during the session to prevent a skill bias of the two less familiar techniques (shuffle and split-step). Testing was conducted on a carpeted indoor surface at the training venue of the club midway through the 2008 preseason (January). Players were tested individually, and were first required to warm-up under the supervision of the fitness staff. After the warm-up, players were required to perform four trials of each technique (two left and right) at maximal, game-like intensity. The order of technique and direction to change (left or right) was given to the players at the beginning of each trial. The test order was counterbalanced, and designed to prevent the order favouring any one technique. Each trial was separated by a 30 s interval which was considered adequate for full recovery following the physical exertion during the test (approximately 2 s). During the test, players were required to hold a football in both hands. Beginning with toes on the start line, players were instructed to accelerate forward towards a static defensive subject positioned 3.6 m from the start. The subject represented a physical obstacle similar to that encountered in a competitive situation. Once forward movement commenced, players triggered a pair of dual beam infrared timing lights (Swift, Lismore, Australia) and initiated the recording of COD speed (to the nearest 0.01 s). A second pair of timing lights positioned 1 m from the start measured entry speed. Players were instructed to use the defender to determine where to change direction so that they were not too close to be ‘tackled’. Once the change in direction was made, players accelerated for 3 m through a final pair of timing lights, positioned approximately 40–50◦ to the original running direction (left and right), completing a test trial. Previous research on player movement patterns in the AFL has shown that typical changes of direction during games are less than 90◦ .6 Each trial was recorded with a high speed digital camera (Redlake PCI 2000S) operating at 125 Hz. The camera was linked with the timing system so recording began once the first light gate had been triggered. COD speed was expressed as a total time (TTOT ). The video footage was used to divide TTOT into the following dependant variables for further analysis: Entry time (TENT ): Time between first and second pair of timing lights (1 m). Foot plant preparation time (TFP ): Time between the second pair of timing lights and initial ground contact of the foot used to change direction. This included the preparation of each technique prior to the COD, for example, the small jump of the split-step. Time to ground contact was determined by counting frames (to the nearest 0.008 s) from the beginning of the test. TENT was subtracted from this time to calculate TFP .
R.J. Bradshaw et al. / Journal of Science and Medicine in Sport 14 (2011) 65–69
Approach time (TENT + TFP ): Combination of TENT and TFP (represents first forward portion of COD speed). Exit time (TEX ): Time from initial ground contact of foot used to change direction to the instant the player passed through the final timing gate. This was calculated by subtracting TENT + TFP from TTOT . Video-based agility reaction test: Nineteen players (M ± SD age: 20.9 ± 2.7 years; height: 185 ± 6.6 cm; weight: 84.7 ± 7.2 kg) from the same club volunteered to participate in the reaction test (conducted 1–2 weeks following COD speed test). Players were tested individually in a closed off area to prevent others memorising the video sequence. A total of 21 clips were presented to the players. The first three served as practice trials to familiarise the players with the testing procedure. Each technique was presented equally during the sequence in a randomised order. The test video (edited using Windows Movie Maker version 5.1) included a sub-elite ARF player accelerating for 5 m then performing one of the three techniques to change running direction (approximately 40–50◦ ) either left or right. Footage was taken from a defensive perspective so when viewing the video the player was running towards the observer. The camera (JVC GR-DV 5000, recording at 50 Hz) was positioned approximately 170 cm from the ground to approximate eye level height of a defending player. The video was displayed on a large projection screen (4 m × 4 m) so the subject appeared approximately life-sized. A custom made film sequencer (Windows XP) was used to play the clips on a laptop computer linked with a digital projector. Players stood 5 m away facing the projection screen, holding custom made buttons in their left and right hands. The buttons were interfaced to the computer and linked with a digital timer which measured the player’s response to the nearest 0.01 s. At the beginning of the test, participants were instructed as follows: “You will see a player run towards you who will change direction to the left or right using either the side-step, shuffle or split-step techniques. If he runs to your right, press the right button and if he goes to your left, press the left button as fast and accurately as possible like in a game”. Players began each clip by activating one of the two hand held buttons with their thumb. The digital timer began when the first button had been depressed, and stopped when a decision and response had been made. Once the response time and accuracy was recorded for each trial, the next clip was manually loaded and the digital timer reset. Two response measures were used: Decision time (TDEC ): Interval between initial ground contact of foot used to change direction (stimulus onset) and the beginning of the player’s response. TDEC was calculated by subtracting the time to stimulus onset from the time recorded by the digital timer for each individual trial. Time to stimulus onset was found by counting frames from the beginning of the video (to the nearest 0.02 s). Decision accuracy: Comparison between the player’s response and the actual change of running direction in the
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clip. Incorrect responses were expressed as a total number, and a percentage of the total number of trials per technique. 2.1. Statistical analysis 2.1.1. Change of direction speed test First, descriptive statistics (M ± SD) for each dependant variable were calculated. Then, for the purpose of comparing COD speed, performance data from the group was entered into a technique (side, shuffle, split) × movement time (TTOT , TENT , TFP , TENT + TFP , TEX ) analysis of variance (ANOVA) with repeated measures, with the mean of four trials used to represent each technique. In the event of finding a significant difference, simple contrasts were performed to find which conditions were different. To determine the magnitude of difference between conditions, effect size (ES) calculations were performed using Hopkins (2002)7 thresholds: 0–0.2 trivial, 0.2–0.6 small, 0.6–1.2 moderate, 1.2–2.0 large, and 2.0–4.0 very large. To determine whether exit speed was related to approach time, the Pearson’s correlation coefficient was calculated. 2.1.2. Video-based agility reaction test First, descriptive statistics (M ± SD) for each dependant variable were calculated. Then, to compare the players TDEC , a repeated measures ANOVA was performed in which technique was the within-subjects factor. In the event of finding a significant difference, simple contrasts were performed to find which conditions were different. To determine the magnitude of difference between conditions, effect size (ES) calculations were performed using Hopkins7 thresholds. It should be noted that incorrect trials were found to be outlying values (i.e. either much faster or slower than the mean) and were removed during TDEC analysis. The mean calculated with the exclusion of these trials gave a better representation of the player’s decision time. For the purpose of comparing decision accuracy, a mean number of errors per player × technique ANOVA with repeated measures was performed. Significance was set at p < 0.05. All statistical analysis was performed on SPSS version 15 software.
3. Results Table 1 presents the descriptive statistics for the COD speed test and reaction test. ANOVA revealed a significant difference between the mean TTOT of each technique (p < 0.05). The side-step was performed fastest followed by the shuffle and split-step. Based on the effect size values, the greatest difference was between the side and split-step (1.34), with only a moderate and small difference between the side-step and shuffle (0.97), and shuffle and split-step (0.46) respectively. In its component parts, the side-step had the fastest TFP followed by the shuffle and split-step (p < 0.05). According to
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R.J. Bradshaw et al. / Journal of Science and Medicine in Sport 14 (2011) 65–69
Table 1 Descriptive statistics for COD speed test and video-based agility reaction test. Side-step
Shuffle
Split-step
M
SD
M
SD
M
SD
COD speed test TTOT a TENT TFP a TENT + TFP a TEX
1.57 0.41 0.66 1.07 0.51
0.10 0.06 0.07 0.08 0.08
1.67 0.42 0.70 1.12 0.55
0.10 0.06 0.06 0.08 0.04
1.71 0.42 0.75 1.17 0.55
0.11 0.05 0.07 0.10 0.04
Reaction test TDEC a Decision accuracya
0.12 0.84b
0.03 0.77
0.15 0.32
0.05 0.67
0.19 0.05b
0.03 0.23
a b
Significant difference between means (p < 0.05). Significant difference between side-step and split-step only (p < 0.05).
the effect size values, there was a large difference between the side and split-step (1.33), and a moderate difference between the side-step and shuffle (0.6), and shuffle and split-step (0.84). As a result, the side-step had the fastest approach time (TENT + TFP ), followed by the shuffle and split-step (p < 0.05). No statistical difference was found between the TENT and TEX of the three techniques (p > 0.05). The relationships between TEX and TENT + TFP were low and not statistically significant (p > 0.05): side-step (r = −0.23), shuffle (r = 0.07), split-step (r = 0.06). Results from the reaction test revealed players made significantly slower decisions when observing the split-step followed by the shuffle and side-step (p < 0.05). Based on the effect size values, there was very large difference in TDEC between the side and split-step (2.35), and a moderate difference between the side-step and shuffle (0.78), and shuffle and split-step (0.91). Analysis of decision accuracy revealed more decision errors were made per player when reacting to the split-step compared to shuffle and side-step. However, the only statistically significant difference was between the side and split-step (p < 0.05). According to effect size values this was a large difference (1.4). When analysed collectively, a total number of 16 incorrect decisions out of 114 trials were made for the split-step (14.0%), 6 for the shuffle (5.3%), and 1 for the side-step (0.9%).
4. Discussion Change of direction speed test: A breakdown of total time found that differences in COD speed were a reflection of the foot plant preparation times, as no significant differences were found in entry and exit times. The differences in foot plant preparation times were likely due to the nature of footwork involved in the shuffle and split-step, which both require a greater change of running pattern compared to the side-step. If performing under no time pressure, differences in preparation or total time would be irrelevant. However, in a situation where an opponent was giving chase,
it may disadvantage a player if the movement speed during and following a COD was compromised for the purpose of technique preparation. Despite taking longer to perform, results showed that approach times were not significantly related to the player’s movement speed following the change in direction. Thus, there appeared to be no exit speed disadvantage associated with the slower foot plant preparation times. The largest difference in foot plant preparation times was between the side and split-step, followed by the side-step and shuffle. Compared to the side-step, the actual COD technique for the shuffle and split-step was similar. However, small variations in foot placement, trunk lean, and posture observed, likely reflect the specific foot plant preparation techniques. For example, the series of small side-steps performed during the shuffle results in only a small displacement of the centre of gravity. It is speculated that a low centre of gravity and trunk lean would be beneficial to maximise COD speed. Although foot plant preparation times were slower, results showed the approach time was not significantly related to the player’s movement speed following the shuffle. It is possible the small stretch-shortening cycle generated from right and left legs conserves running velocity, contributing to an effective impulse during the final foot plant. In contrast, the whole-body jump performed during the split-step generates a bilateral stretch-shortening cycle, which is likely to produce a greater braking force against the ground. Despite taking longer to perform, results show the approach time was not significantly related to the players exit speed. It should be noted that players began the test from a stationary start. The results from this study may not represent all situations during a game when players are running at various speeds. In addition, due to individual differences in strength, there may be different optimal approach speeds8 and jump heights9 for maximising force development during the stretch-shortening cycle. The true benefit of all the offensive agility techniques may be dependant on the approach speed and jump height used during a change in direction. A limitation of the COD speed test is that it measured movement speed only, and did not include an element of perceptual skill. In a game situation, an offensive player may change his strategy during a change in direction according to the movement of the opposition. Biomechanical analysis suggests that the temporal pressure experienced during reactive conditions may influence technique.10 Testing the shuffle and split-step under reactive conditions may provide further insight into the practicality of their use as offensive agility techniques. Video-based agility reaction test: The slower TFP times of the shuffle and split-step may not disadvantage the performer if the deceptive foot-pant preparation techniques mislead the observer and cause a slower reaction time. Results showed TDEC was significantly slower when observing the offensive player performing a split-step, followed by the shuffle and side-step (p < 0.05). It is likely the difference in decision time
R.J. Bradshaw et al. / Journal of Science and Medicine in Sport 14 (2011) 65–69
is related to the nature of the unfolding cues. For example, before a change in running direction is made, the performer must identify and interpret the relevant information, and select a response that fits the requirements demanded by the situation.3 An increase in decision time may result as the complexity of the stimulus increases (e.g. deceptive actions).11 During the side-step, the player’s intention may be more obvious as the goal is to move with speed rather than unpredictability. The small ‘side-steps’ of the shuffle exaggerate the kinematics of the COD making it difficult to identify the ‘genuine’ action. In contrast, the whole-body jump performed during the split-step delays any cues relating to the performers intention until ground contact has been made. Consequently, the observer cannot make an accurate judgement until after the performer has landed. Another explanation for the slower reactions to the shuffle and split-step may be the relatively little experience the players have had in observing these techniques, as they are not typically used in ARF. Players in this present study had to find a balance between decision speed and accuracy. Results showed that more decision errors were made when reacting to the split-step, followed by the shuffle, and side-step. This is interesting considering players took longer to respond to the shuffle and split-step (p < 0.05). The uncertainty created by the exaggerated kinematics of the shuffle, may have misled some players to perceive one of the fake movements as genuine. In addition, it appeared the combination of temporal pressure and a delay in cues resulted in some players being genuinely fooled by the split-step. Jackson et al.5 found expert rugby player’s anticipation accuracy was equal on deceptive and non-deceptive side-step trials. However, clips were separated by a 4 s interval during which participants were required to indicate their response. Different responses may have been triggered if the participants performed under temporal pressure. Previous research has shown that, as a by-product of superior task specific knowledge and experience, highly skilled performers are able to successfully anticipate the action of an opponent, and organise movements in advance.3 Our findings show that despite this, players were still susceptible to deceptive movements under time-stressed situations. A logical follow on from this study would be to examine visual search and the trainability of perceptual skill to deceptive changes of direction. It should be noted that responses were based on the movement pattern of one performer. It is not known to what extent individual differences in movement pattern of other performers may influence perceptual skill.
5. Conclusion Under the conditions of the present study, the split-step appeared to be the most effective offensive agility technique.
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Despite having a longer total time, the split-step was slower and less accurate to react to and the exit speed following the change in direction was not significantly slower than the side-step.
Practical implications • In a one-on-one situation in ARF, when the player has no better option, the split-step would be a good choice for ‘taking-on’ the opponent. • However, when surrounded by opposition players, the shuffle or split-step may not be appropriate because of the time and space needed to execute the preparatory movements. In addition, opposition players may come from all angles, making deceptive cues ineffective. Thus, the choice of techniques may be situational. • If running at, or close to full pace, a player would need to consider the cost of a loss in running velocity for purpose of technique preparation versus the benefit of deceiving an opponent.
References 1. Young W, James R, Montgomery I. Is muscle power related to running speed with changes of direction? J Sports Med Phys Fitness 2002;42(3):282–8. 2. Sheppard J, Young W. Agility literature review: classifications, training, and testing. J Sport Sci 2006;24(9):919–32. 3. Williams A, Davids K, Williams J. Visual perception & action in sport. London: E & FN Spon; 1999. 4. Passos P, Araujo D, Davids K, et al. Manipulating constraints to train decision making in rugby union. Int J Sport Sci Coach 2008;3(1):125–40. 5. Jackson R, Warren S, Abernethy B. Anticipation skill and susceptibility to deceptive movement. Acta Psychol 2006;123(3):355– 71. 6. Dawson B, Hopkinson R, Appleby B, et al. Player movement patterns and game activities in the Australian Football League. J Sci Med Sport 2004;7(3):278–91. 7. A new view of statistics. A scale of magnitude for effect statistics. http://www.sportsci.org/resource/stats/effectmag.html; 2009 [accessed 7.04.09]. 8. Young W, MacDonald T, Fitzpatrick J. An evaluation of the specificity, validity, and reliability of jumping tests. J Sports Med Phys Fitness 1997;37(4):240–5. 9. Mradakovic V, Ilic D, Jankovic N, et al. Pre-activity modulation of lower extremity muscles within different types and heights of deep jump. J Sport Sci Med 2008;7(2):269–78. 10. Besier T, Lloyd D, Ackland T. Muscle activation strategies at the knee during running and cutting manoeuvres. Med Sci Sports Exerc 2003;35(1):119–27. 11. Vickers JN. Perception, cognition, and decision training: the quiet eye in action. Champaign, IL: Human Kinetics; 2007.
Journal of Science and Medicine in Sport 14 (2011) 65–69
Original paper
Comparison of offensive agility techniques in Australian Rules football Ryan J. Bradshaw a,∗ , Warren B. Young a , Andrew Russell b , Peter Burge b a
School of Human Movement and Sport Science, University of Ballarat, Ballarat, Victoria, Australia b Hawthorn Football Club, Victoria, Australia Received 20 October 2009; received in revised form 3 June 2010; accepted 11 June 2010
Abstract Agility skill is important in Australian Rules football (ARF) as it enables an attacking player to successfully evade an opponent. To date, no research has examined offensive agility techniques in ARF. There were two purposes of this study: first, to compare the change of direction (COD) speed of three offensive agility techniques, and second, compare the reaction speed and accuracy when observing the same techniques from a defensive perspective. The techniques included the side-step, shuffle, and split-step. Seventeen players from an Australian Football League club were required to perform four trials of each technique. COD speed was expressed as a total time, and divided into entry, foot plant preparation, and exit time. In addition, nineteen players from the same club were assessed on a video-based reaction test. Players were required to respond by depressing a thumb switch to indicate whether the player on screen changed direction to the left or right. From an offensive perspective, the split-step foot plant preparation time (0.66 s) was significantly slower than both the side-step (0.7 s) and shuffle (0.75 s) (p < 0.05), but there were no significant differences in entry and exit times between techniques. From a defensive perspective, the players were significantly slower and less accurate when reacting to the player in the video performing the split-step (0.19 s) and shuffle (0.15 s) compared to the side-step (0.12 s) (p < 0.05). In a one-on-one situation in ARF, the split-step may be the most effective offensive technique. Not only was it slower and less accurate to react to, the exit speed following the change in direction was not significantly slower than the side-step. However, a performer would need to consider the cost of a slower foot plant preparation time versus the potential to deceive an opponent when under time stress. © 2010 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved. Keywords: Perception; Change of direction speed; AFL
1. Introduction In Australian Rules football (ARF), an attacker in possession of the ball may either kick or handball to a team mate in an attempt to move the ball forward and create a scoring opportunity. Alternatively, the offensive player may ‘take-on’ the opposition through the evasive action of rapidly changing direction or velocity. Examination of the physical factors which influence agility performance demonstrates that running technique can play a key role in maximising change of direction (COD) speed. Young et al.1 suggested that foot placement, trunk lean, stride adjustment and running posture are important technical components of agility technique. For example, it is ∗
Corresponding author. E-mail address: [email protected] (R.J. Bradshaw).
likely that a lateral foot placement would be ideal to generate a large medio-lateral ground reaction force. In addition, a forward lean may shift the centre of mass outside the base of support, allowing gravity to assist motion during the change of direction. This can only be speculated however, as specific performance based research on agility technique remains non-existent.2 To date, the major focus of research has been on injury prevention. In a one-on-one situation, an offensive player’s goal is to evade and outrun his opponent. As the opponent’s attempt to defend is reactive in nature, certain features of the offensive player’s movement will influence the defender’s action. Filmbased visual search and anticipation studies have shown that, coupled with superior pattern recognition and task specific knowledge, highly skilled athletes are able to successfully predict the action of an opponent before it is carried out.3 However, a major criticism of traditional perceptual test-
1440-2440/$ – see front matter © 2010 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jsams.2010.06.002
66
R.J. Bradshaw et al. / Journal of Science and Medicine in Sport 14 (2011) 65–69
ing has been the focus on observing consistent performance rather than enhancing uncertainty.4 Many of the display samples used may not accurately represent competitive situations where performers may attempt to disguise their intentions. Deception is a fundamental part of an offensive player’s movement in a one-on-one situation.5 An offensive agility technique that is fast to perform, and disguises a performer’s intention, may give the offensive player an advantage when performing under pressure from the opposition. Jackson et al.5 made the first attempt to compare non-deceptive and deceptive agility techniques from a defensive perspective. They found expert rugby player’s anticipation accuracy was equal when observing deceptive and non-deceptive sidesteps. Findings suggest that a deceptive side-step offers no advantage over a non-deceptive side-step. However, the results may have been a reflection of the closed-nature response required by players and familiarity with the agility technique. Further research examining unfamiliar techniques in a time-stressed situation may result in different findings. To date, no research has examined the COD speed and predictability of offensive agility techniques in ARF. As a result, there were two purposes of this research: first, to compare the COD speed of three offensive agility techniques, and second, to compare the reaction speed and accuracy when observing the techniques from a defensive perspective. The ultimate aim was to find the most effective offensive agility technique for a one-on-one situation in ARF (i.e. fastest speed exiting the change in direction, hardest to predict accurately and slowest to react to).
2. Methods This study used a within subjects design to investigate two separate qualities (COD speed and perceptual skill). The three techniques were: 1. Side-step: Player runs straight ahead and in one step, places a leg to the side of the body to push off and change direction to the opposite side. 2. Shuffle: Player moves in a forward direction whilst performing a series of small ‘side-steps’ without any significant lateral displacement of the trunk. Final movement to change direction is similar to a side-step. 3. Split-step: Whilst moving forward, the player performs a small jump, landing with both feet approximately shoulder width apart and body weight evenly distributed. Upon landing, the foot opposite the intended direction is used to initiate a COD. Change of direction speed test: Seventeen players (mean (M) ± standard deviation (SD) age: 21.2 ± 2.7 years; height: 184.9 ± 6.7 cm; weight: 84.8 ± 6.5 kg) from an Australian Football League (AFL) club volunteered to participate in the COD speed test. Informed signed consent was obtained from each player before testing to comply with the University’s ethics committee.
Before testing, players were allocated five familiarisation sessions within their normal training schedule. Players were required to run around a stationary defender performing eight trials of each technique. Practice was equally distributed during the session to prevent a skill bias of the two less familiar techniques (shuffle and split-step). Testing was conducted on a carpeted indoor surface at the training venue of the club midway through the 2008 preseason (January). Players were tested individually, and were first required to warm-up under the supervision of the fitness staff. After the warm-up, players were required to perform four trials of each technique (two left and right) at maximal, game-like intensity. The order of technique and direction to change (left or right) was given to the players at the beginning of each trial. The test order was counterbalanced, and designed to prevent the order favouring any one technique. Each trial was separated by a 30 s interval which was considered adequate for full recovery following the physical exertion during the test (approximately 2 s). During the test, players were required to hold a football in both hands. Beginning with toes on the start line, players were instructed to accelerate forward towards a static defensive subject positioned 3.6 m from the start. The subject represented a physical obstacle similar to that encountered in a competitive situation. Once forward movement commenced, players triggered a pair of dual beam infrared timing lights (Swift, Lismore, Australia) and initiated the recording of COD speed (to the nearest 0.01 s). A second pair of timing lights positioned 1 m from the start measured entry speed. Players were instructed to use the defender to determine where to change direction so that they were not too close to be ‘tackled’. Once the change in direction was made, players accelerated for 3 m through a final pair of timing lights, positioned approximately 40–50◦ to the original running direction (left and right), completing a test trial. Previous research on player movement patterns in the AFL has shown that typical changes of direction during games are less than 90◦ .6 Each trial was recorded with a high speed digital camera (Redlake PCI 2000S) operating at 125 Hz. The camera was linked with the timing system so recording began once the first light gate had been triggered. COD speed was expressed as a total time (TTOT ). The video footage was used to divide TTOT into the following dependant variables for further analysis: Entry time (TENT ): Time between first and second pair of timing lights (1 m). Foot plant preparation time (TFP ): Time between the second pair of timing lights and initial ground contact of the foot used to change direction. This included the preparation of each technique prior to the COD, for example, the small jump of the split-step. Time to ground contact was determined by counting frames (to the nearest 0.008 s) from the beginning of the test. TENT was subtracted from this time to calculate TFP .
R.J. Bradshaw et al. / Journal of Science and Medicine in Sport 14 (2011) 65–69
Approach time (TENT + TFP ): Combination of TENT and TFP (represents first forward portion of COD speed). Exit time (TEX ): Time from initial ground contact of foot used to change direction to the instant the player passed through the final timing gate. This was calculated by subtracting TENT + TFP from TTOT . Video-based agility reaction test: Nineteen players (M ± SD age: 20.9 ± 2.7 years; height: 185 ± 6.6 cm; weight: 84.7 ± 7.2 kg) from the same club volunteered to participate in the reaction test (conducted 1–2 weeks following COD speed test). Players were tested individually in a closed off area to prevent others memorising the video sequence. A total of 21 clips were presented to the players. The first three served as practice trials to familiarise the players with the testing procedure. Each technique was presented equally during the sequence in a randomised order. The test video (edited using Windows Movie Maker version 5.1) included a sub-elite ARF player accelerating for 5 m then performing one of the three techniques to change running direction (approximately 40–50◦ ) either left or right. Footage was taken from a defensive perspective so when viewing the video the player was running towards the observer. The camera (JVC GR-DV 5000, recording at 50 Hz) was positioned approximately 170 cm from the ground to approximate eye level height of a defending player. The video was displayed on a large projection screen (4 m × 4 m) so the subject appeared approximately life-sized. A custom made film sequencer (Windows XP) was used to play the clips on a laptop computer linked with a digital projector. Players stood 5 m away facing the projection screen, holding custom made buttons in their left and right hands. The buttons were interfaced to the computer and linked with a digital timer which measured the player’s response to the nearest 0.01 s. At the beginning of the test, participants were instructed as follows: “You will see a player run towards you who will change direction to the left or right using either the side-step, shuffle or split-step techniques. If he runs to your right, press the right button and if he goes to your left, press the left button as fast and accurately as possible like in a game”. Players began each clip by activating one of the two hand held buttons with their thumb. The digital timer began when the first button had been depressed, and stopped when a decision and response had been made. Once the response time and accuracy was recorded for each trial, the next clip was manually loaded and the digital timer reset. Two response measures were used: Decision time (TDEC ): Interval between initial ground contact of foot used to change direction (stimulus onset) and the beginning of the player’s response. TDEC was calculated by subtracting the time to stimulus onset from the time recorded by the digital timer for each individual trial. Time to stimulus onset was found by counting frames from the beginning of the video (to the nearest 0.02 s). Decision accuracy: Comparison between the player’s response and the actual change of running direction in the
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clip. Incorrect responses were expressed as a total number, and a percentage of the total number of trials per technique. 2.1. Statistical analysis 2.1.1. Change of direction speed test First, descriptive statistics (M ± SD) for each dependant variable were calculated. Then, for the purpose of comparing COD speed, performance data from the group was entered into a technique (side, shuffle, split) × movement time (TTOT , TENT , TFP , TENT + TFP , TEX ) analysis of variance (ANOVA) with repeated measures, with the mean of four trials used to represent each technique. In the event of finding a significant difference, simple contrasts were performed to find which conditions were different. To determine the magnitude of difference between conditions, effect size (ES) calculations were performed using Hopkins (2002)7 thresholds: 0–0.2 trivial, 0.2–0.6 small, 0.6–1.2 moderate, 1.2–2.0 large, and 2.0–4.0 very large. To determine whether exit speed was related to approach time, the Pearson’s correlation coefficient was calculated. 2.1.2. Video-based agility reaction test First, descriptive statistics (M ± SD) for each dependant variable were calculated. Then, to compare the players TDEC , a repeated measures ANOVA was performed in which technique was the within-subjects factor. In the event of finding a significant difference, simple contrasts were performed to find which conditions were different. To determine the magnitude of difference between conditions, effect size (ES) calculations were performed using Hopkins7 thresholds. It should be noted that incorrect trials were found to be outlying values (i.e. either much faster or slower than the mean) and were removed during TDEC analysis. The mean calculated with the exclusion of these trials gave a better representation of the player’s decision time. For the purpose of comparing decision accuracy, a mean number of errors per player × technique ANOVA with repeated measures was performed. Significance was set at p < 0.05. All statistical analysis was performed on SPSS version 15 software.
3. Results Table 1 presents the descriptive statistics for the COD speed test and reaction test. ANOVA revealed a significant difference between the mean TTOT of each technique (p < 0.05). The side-step was performed fastest followed by the shuffle and split-step. Based on the effect size values, the greatest difference was between the side and split-step (1.34), with only a moderate and small difference between the side-step and shuffle (0.97), and shuffle and split-step (0.46) respectively. In its component parts, the side-step had the fastest TFP followed by the shuffle and split-step (p < 0.05). According to
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Table 1 Descriptive statistics for COD speed test and video-based agility reaction test. Side-step
Shuffle
Split-step
M
SD
M
SD
M
SD
COD speed test TTOT a TENT TFP a TENT + TFP a TEX
1.57 0.41 0.66 1.07 0.51
0.10 0.06 0.07 0.08 0.08
1.67 0.42 0.70 1.12 0.55
0.10 0.06 0.06 0.08 0.04
1.71 0.42 0.75 1.17 0.55
0.11 0.05 0.07 0.10 0.04
Reaction test TDEC a Decision accuracya
0.12 0.84b
0.03 0.77
0.15 0.32
0.05 0.67
0.19 0.05b
0.03 0.23
a b
Significant difference between means (p < 0.05). Significant difference between side-step and split-step only (p < 0.05).
the effect size values, there was a large difference between the side and split-step (1.33), and a moderate difference between the side-step and shuffle (0.6), and shuffle and split-step (0.84). As a result, the side-step had the fastest approach time (TENT + TFP ), followed by the shuffle and split-step (p < 0.05). No statistical difference was found between the TENT and TEX of the three techniques (p > 0.05). The relationships between TEX and TENT + TFP were low and not statistically significant (p > 0.05): side-step (r = −0.23), shuffle (r = 0.07), split-step (r = 0.06). Results from the reaction test revealed players made significantly slower decisions when observing the split-step followed by the shuffle and side-step (p < 0.05). Based on the effect size values, there was very large difference in TDEC between the side and split-step (2.35), and a moderate difference between the side-step and shuffle (0.78), and shuffle and split-step (0.91). Analysis of decision accuracy revealed more decision errors were made per player when reacting to the split-step compared to shuffle and side-step. However, the only statistically significant difference was between the side and split-step (p < 0.05). According to effect size values this was a large difference (1.4). When analysed collectively, a total number of 16 incorrect decisions out of 114 trials were made for the split-step (14.0%), 6 for the shuffle (5.3%), and 1 for the side-step (0.9%).
4. Discussion Change of direction speed test: A breakdown of total time found that differences in COD speed were a reflection of the foot plant preparation times, as no significant differences were found in entry and exit times. The differences in foot plant preparation times were likely due to the nature of footwork involved in the shuffle and split-step, which both require a greater change of running pattern compared to the side-step. If performing under no time pressure, differences in preparation or total time would be irrelevant. However, in a situation where an opponent was giving chase,
it may disadvantage a player if the movement speed during and following a COD was compromised for the purpose of technique preparation. Despite taking longer to perform, results showed that approach times were not significantly related to the player’s movement speed following the change in direction. Thus, there appeared to be no exit speed disadvantage associated with the slower foot plant preparation times. The largest difference in foot plant preparation times was between the side and split-step, followed by the side-step and shuffle. Compared to the side-step, the actual COD technique for the shuffle and split-step was similar. However, small variations in foot placement, trunk lean, and posture observed, likely reflect the specific foot plant preparation techniques. For example, the series of small side-steps performed during the shuffle results in only a small displacement of the centre of gravity. It is speculated that a low centre of gravity and trunk lean would be beneficial to maximise COD speed. Although foot plant preparation times were slower, results showed the approach time was not significantly related to the player’s movement speed following the shuffle. It is possible the small stretch-shortening cycle generated from right and left legs conserves running velocity, contributing to an effective impulse during the final foot plant. In contrast, the whole-body jump performed during the split-step generates a bilateral stretch-shortening cycle, which is likely to produce a greater braking force against the ground. Despite taking longer to perform, results show the approach time was not significantly related to the players exit speed. It should be noted that players began the test from a stationary start. The results from this study may not represent all situations during a game when players are running at various speeds. In addition, due to individual differences in strength, there may be different optimal approach speeds8 and jump heights9 for maximising force development during the stretch-shortening cycle. The true benefit of all the offensive agility techniques may be dependant on the approach speed and jump height used during a change in direction. A limitation of the COD speed test is that it measured movement speed only, and did not include an element of perceptual skill. In a game situation, an offensive player may change his strategy during a change in direction according to the movement of the opposition. Biomechanical analysis suggests that the temporal pressure experienced during reactive conditions may influence technique.10 Testing the shuffle and split-step under reactive conditions may provide further insight into the practicality of their use as offensive agility techniques. Video-based agility reaction test: The slower TFP times of the shuffle and split-step may not disadvantage the performer if the deceptive foot-pant preparation techniques mislead the observer and cause a slower reaction time. Results showed TDEC was significantly slower when observing the offensive player performing a split-step, followed by the shuffle and side-step (p < 0.05). It is likely the difference in decision time
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is related to the nature of the unfolding cues. For example, before a change in running direction is made, the performer must identify and interpret the relevant information, and select a response that fits the requirements demanded by the situation.3 An increase in decision time may result as the complexity of the stimulus increases (e.g. deceptive actions).11 During the side-step, the player’s intention may be more obvious as the goal is to move with speed rather than unpredictability. The small ‘side-steps’ of the shuffle exaggerate the kinematics of the COD making it difficult to identify the ‘genuine’ action. In contrast, the whole-body jump performed during the split-step delays any cues relating to the performers intention until ground contact has been made. Consequently, the observer cannot make an accurate judgement until after the performer has landed. Another explanation for the slower reactions to the shuffle and split-step may be the relatively little experience the players have had in observing these techniques, as they are not typically used in ARF. Players in this present study had to find a balance between decision speed and accuracy. Results showed that more decision errors were made when reacting to the split-step, followed by the shuffle, and side-step. This is interesting considering players took longer to respond to the shuffle and split-step (p < 0.05). The uncertainty created by the exaggerated kinematics of the shuffle, may have misled some players to perceive one of the fake movements as genuine. In addition, it appeared the combination of temporal pressure and a delay in cues resulted in some players being genuinely fooled by the split-step. Jackson et al.5 found expert rugby player’s anticipation accuracy was equal on deceptive and non-deceptive side-step trials. However, clips were separated by a 4 s interval during which participants were required to indicate their response. Different responses may have been triggered if the participants performed under temporal pressure. Previous research has shown that, as a by-product of superior task specific knowledge and experience, highly skilled performers are able to successfully anticipate the action of an opponent, and organise movements in advance.3 Our findings show that despite this, players were still susceptible to deceptive movements under time-stressed situations. A logical follow on from this study would be to examine visual search and the trainability of perceptual skill to deceptive changes of direction. It should be noted that responses were based on the movement pattern of one performer. It is not known to what extent individual differences in movement pattern of other performers may influence perceptual skill.
5. Conclusion Under the conditions of the present study, the split-step appeared to be the most effective offensive agility technique.
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Despite having a longer total time, the split-step was slower and less accurate to react to and the exit speed following the change in direction was not significantly slower than the side-step.
Practical implications • In a one-on-one situation in ARF, when the player has no better option, the split-step would be a good choice for ‘taking-on’ the opponent. • However, when surrounded by opposition players, the shuffle or split-step may not be appropriate because of the time and space needed to execute the preparatory movements. In addition, opposition players may come from all angles, making deceptive cues ineffective. Thus, the choice of techniques may be situational. • If running at, or close to full pace, a player would need to consider the cost of a loss in running velocity for purpose of technique preparation versus the benefit of deceiving an opponent.
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