Back kick
Taekwondo Back kick is the most powerful kick in both men and women (Pieter & Pieter, 1995); moreover, it can be used as an offensive or defensive technique (Lau, 2013). A back kick is also called a spinning back kick, horse kick, donkey kick, mule kick or the traditional Korean language the “Dwi-Chagi”. This action is performed by kicking backward onto the opponent behind you, like a horse, with the striking surface of the bottom of the foot, normally the heel part. Figure 1 shows four phases in a back kick. The rotation phase consists of facing the target first, and then the body pivots away from the target. During the contact phase, the kicking leg must be tight and close to the supporting leg, and strikes with the heel or the foot blade. The restoration phase is the pull back of the kick and back to kicking position. The turning motion provides a lot of power to the back kick, one should look over the shoulder when delivering the back kick to know where the target area is situated in and to maintain a firm balance. The balance can be broken off when kicking without proper procedure. The best kicking height of back kick is the kickers’ sternum (Estavan & Falco, 2013).
Figure 1. Step by step back kick model (Lee & Chen, 2008)
Back kick not only can be used as an offensive kick, but it is more often used as a counter attack kick, destroying the offensive rhythm of the opponent and can break opponents’ balance
6
(Eddie, 1989; Lee, 1996). Back kick requires right timing for speed and accuracy to take place in order for a valid point to be scored. Therefore, Taekwondo athletes needs best timing and enable them to guide their feet to future location of the target. Taekwondo uses various techniques, and these techniques have different effect and speed. It has been shown that the right and left legs of men and women had no significant difference in their kicking speed (Pieter
& Pieter, 1995).
Speed and Accuracy
The relation between movement speed and accuracy is one of the most robust phenomena in human movement performance. Common sense teaches us that when we perform movements faster, we make more mistakes or errors in terms of the goal we want to achieve. These situations support the adage "haste makes waste"; a long standing perspective about motor skills. Fitts (1954) proposed a mathematical model that was a critical step to describing formally, the speed-accuracy phenomenon and it has become one of the most significant laws in motor control. Fitts and Peterson (1964) asked participants moved their hands from a start location to the target as quickly and accurately as possible. He used two independent variables (A = amplitude; W = target width) to calculate the index of difficulty, ID=Log2(2A/W). The index of difficulty is the most important factor in Fitts's law to determine the average movement time. Fitts described this relationship between movement time, amplitude and target width as MT = a + b log2 (2A/W). The higher the value of ID, the longer the movement time, therefore, slower the speed of movement. Thus, Fitts’ law explains the speed-accuracy trade-off by implying an inverse relationship between the movement difficulty and the movement speed.
Fitts explained the relation between movement speed and accuracy in relation to the limited information processing capacity of the human system (Fitts 1954). When the number of stimulus response alternatives increases, the system needs more time to process this
7
information and resolve the uncertainty about alternatives (Fitts, 1954; Fitts & Peterson, 1964;
Schmidt & Lee, 2005). Fitts’s law can be applied to a variety of contexts of movements, including everyday activities. This law has also been conducted under different situations, such as using feet, arms, hands, movements conducted underwater, and different populations (young and old) (Goggin & Meeuwsen, 1992; Langolf, Chaffin, & Foulke, 1976; Newell &
Wade, 1978; Welford, 1969). Fitts’ law describes the speed-accuracy trade-off of limb movement toward a stationary target. Fitts’ Law does not apply to the situation where the aimed target is moving which is often occurred in many sport events.
Taekwondo kicks emphasize kicking accuracy as well as kicking speed. In Taekwondo, one needs to have the right speed to kick moving object. When the speed of the kick is too slow, the opponent might dodge the kick. Taekwondo athletes use electronic body protectors and headgears for scoring. Without accuracy in kicks, the foot will not land in the right target area on the sensors of the electronic protectors, therefore the points will not be scored. Kicking a stationary target is considered an easy task, but in Taekwondo competition, the opponent will move back and forth making it difficult to score points. Nevertheless, Taekwondo is a sport where your target is moving constantly. Temporal accuracy also needs to be taken into consideration.
Temporal accuracy
Taekwondo Athlete’s need to move as quickly and accurately as possible, and temporal accuracy is also a factor that needs to be taken into account for kicking moving object. By temporal accuracy, we refer to as the difference between the actual movement time and the criterion time of the task (Schmidt, 1969). Literature has shown that increasing movement velocity within the same criterion movement time results in a decreased timing error (Newell, Hoshizaki, Carlton, & Halbert, 1979; Schmidt, 1969). Many studies of the movement timing tasks suffered the limitation that the independent variables, such as manipulations of duration
8
and velocity, were confounded (e. g., Ellis 1969). In this case, movement time and movement velocity have rarely been manipulated independently. It is commonly the condition that short movement time has high velocity and long movement time has low velocity. The findings from early studies suggested that timing accuracy was affected by movement velocity but little was learned of time effect of timing accuracy.
Newell et al. (1979) examined systematically the movement speed and timing accuracy function over a range of movement velocity from 4 to 150cm/s. Their results suggested that the mean percentage of the movement time AE (absolute error) had little or no difference between different times at the same average velocity. In addition, timing accuracy decreased with longer movement times and slower average velocities. The velocities effect was independent of movement time, and suggested that average velocity is a main factor to determine the temporal accuracy in discrete timing movements. However, to interact with the environment (e.g., catch frisbee or bat hitting) one needs to have a specific movement toward a certain position at a certain time. Thus, the movements toward the incoming object have to be specified according to the temporal accuracy and spatial accuracy (Rieger, 2007). In Taekwondo competition, the opponent is always continuously moving back and forth. In this case, both focus on the spatial and temporal accuracy might give us a different prospective in movement accuracy.
Interception of moving target
Intercepting a moving object requires that the interceptive effector (hand or bat) get to the right place at the right time (Tresilian, Plooy, & Carroll, 2004). A large number of findings specified that the interception of moving object highlights the temporal aspect of the task, which uses the ratio of image size to its expansion rate that is approximately specifies time to contact between the upcoming object to the possible batter or catcher (Lee et al., 1983;
9
Savlsbergh et al., 1991). In addition, studies found that the participants anticipate the start of their activity when moving object reached a specific moment or criterion value (Michael et al., 2001). Although moving quickly increases the temporal accuracy, it decreases the spatial accuracy, so again a compromise must be found (Brouwer et al., 2000).
Most studies of interceptive tasks showed that the participants have a tendency to move their hand more quickly towards fast targets than towards slow ones (Bairstow, 1987; van Donkelaar & Lee 1994; Li 1996; Savelsbergh et al. 1992). In addition, Brenner and Smeets (1996) showed that the greater the speed of objects the shorter the movement time. Moreover, Mason and Carhan (1999) have also pointed out that the faster speeds were associated with shorter viewing time (VT, the time that is visible prior to the interception). More obvious advantage of making a faster, briefer movement in an interceptive task is that the briefer the movement, the more time you have to view the target (Breen 1967; Hay 1985). For instance, a longer VT will likely lead to improved perceptual estimates of quantities required for accurate control—the time remaining before the target reaches the person (its time-to-contact, TTC).
Certainly, viewing a moving target for a longer period can improve interceptive performance (Elliot et al. 1994; Sharp and Whiting 1974, 1975).
In addition to the temporal accuracy, no previous experiments have evaluated the specific effects of target’s height (spatial accuracy) on the performance of interceptive tasks.
Tresilian et al. (2004) reported that participants were constrained to move along a horizontal linear track to strike the target with different speed (140 cm/s and 220 cm/s) and different target heights (3 cm, 6 cm, and 12 cm) did not constrain the performance. In addition, movement time was unaffected by target heights but was systematically affected by lengths (4.1 cm, 9.1 cm, and 14.1 cm) (briefer movements to smaller length) and speed (briefer movements to faster targets). Moreover, participants were asked to move in a vertical plane normal to the target’s direction of motion. In this task, target height constrains the spatial accuracy required to contact the target. Three groups of eight participants struck targets of
10
different height but of constant length and speed, hence constant temporal accuracy demand (different for each group, one group struck stationary targets - no temporal accuracy demand).
On average, participants showed little or no systematic response to changes in spatial accuracy demand on any dependent measure (MT, Vmax, spatial variable error). Brouwer et al. (2005) reported different results using an interceptive task in which no additional constraints were imposed on how participants could move. It was found that when the target moved relatively slow, MTs were greater when the target was smaller; when it moved faster, there was no detectable effect of size on MT. Thus, in some conditions, it appears that requirements for greater spatial accuracy in interceptive tasks can lead to longer MT’s.
The interception of target can be completed in many different ways, such as grasping or hitting the object with hand or with implement. With a particular interception of a trajectory, the spatiotemporal accuracy and precision that is needed can still vary considerably. For instance, when the foot lands on the moving objects from kicking, a relatively high degree of temporal precision is required for the interception to take place. But if the moving object is coming toward the kicker less temporal precision may be needed: the kicking foot may not contact the moving object in an optimal timing but the kicking foot and the moving object will collide in their paths. Therefore the spatiotemporal constraint does not only depend on the speed of object and the chosen mode of interception such as kicking, but also on how the person moves and how they are positioned relative to the target paths of motion. It seems reasonable to say that when a person tries to execute a task in such a way to maximize the chances of success, they may try to adopt a strategy, which can lower the requirement of accuracy and precision (Brouwer et al., 2002; Tresilian & Longergan, 2002).
In Taekwondo, back kick consists of four phases, such as the rotation phase, contact phase, kick phase, and restoration phase and back kick plays a very important role in offensive and defensive technique. In addition, the speed and accuracy are two important components of the back kick performance. Based on the previous literature, the speed of
11
object was used to guide the hand to intercept moving target. Thus, the faster the moving object the quicker the hand will react. Nevertheless, there is no study that illustrates whether the lower limb task will follow the same phenomenon. Therefore we need to look at how the lower limb kick of Taekwondo can be guided by the moving target to aid participants to move faster with their feet to the moving target Brouwer, (2002) investigated the two different information needed for subject to hit the moving target, whether only the visual information was used or did it also include the information of the speed. He concluded that participants would only respond to the changes in the target position. However the object speed will influence the direction in which the hand moves indirectly. In a study conducted by Tresilian (2002) where he manipulated the target size and speed of the moving object, it was concluded that the target speed and target size had independent effects on the performance. . In this study we hope to find if the target speed is used in guiding the Taekwondo back kick..
12
CHAPTER III METHOD
Participant
Ten healthy elite Taekwondo male athletes were recruited from National Taiwan Normal University (age 20 ± 2 years; height 177.9±9 cm; weight 68.9±12.4 kg). All athletes had competition experience of more than 5 years and had trained at least 10 hours a week. They read and sign the informed consent before the experiment.
Apparatus
The Experimenter used a measuring tape (Stanley) to measure the distance between participant’s front foot to the sand bag and the leg length from the hip to the heel. Two high-speed cameras (Casio, 300 fps) were placed in a fixed position to record the experiment.
Camera 1 was placed 3.5m away from the line of the action in a sagittal view to record the movement time and the moment the participant kicks the sand bag. Camera 2 was placed 2.5m behind the participant to record the contact place of the foot on the sand bag. (See Figure. 2).
Taekwondo Sand Bag (28kg, Kwon) was used as the object for the participants to kick. It was hanged 67 cm away from the ceiling and 70 cm above the ground. A bull’s-eye target was attached to the center of the sand bag and the height of the target was adjusted to the height of the participant’s abdomen (See Figure 3). Two pieces of 1m x 2.5m cloths were hung from above the sand bag to cover the viewing of the sand bag when the sand bag was pulled backward into three different release angles (30°, 45° and 60°).
A white tape was strapped across the inner and outer arch of the foot for identifying the points scored when kicking the object target. A PC computer was used to run Kinovea video player software to register the temporal information of the kicking tasks
13
Figure 2. Schematics of the experimental setup.
Figure 3. Target area on the sand bag
14
Task
The experimental tasks were Taekwondo back kicks toward a stationary or a moving sand bag. There were 10 trials for the stationary sand bag condition (SP1) and 10 trials for each of the 3 moving speeds of the moving sandbag conditions. The speed of the moving sand bag was determined by dropping the sand bag from a release angle of 30°, 45°, and 60°
corresponding to 13.95 m/s (SP2), 27.9m/s (SP3), and 41.84 m/s (SP4), respectively, at the instance where the sand bag was passing through the 0° position (perpendicular to the floor).
Procedure
All the participants were informed of the general experimental procedures before they signed the consent form. The participants wore shorts and were bare feet in the experiment.
After the experimenter measured the height of the abdomen and the leg length for the participant, the inner and outer arch of the kicking foot of the participant was strapped with a piece of white tape.
Each participant stood in front of the sandbag/cloth screen in a kicking position, facing toward the direction of the kicking target with the pivot foot in front. The distance between the tip of the pivot foot and the projecting point to the floor from the bull’s eye of the stationary sandbag was measured to the leg length of the participant. After a 10-minute warm-up (jogging, stretching, and kicking drills, to avoid injury), the participant practiced 3 trials of the kicking tasks for each speed condition. After practice, participant began the back kick experiment and was instructed to kick as fast and as accurately as possible with the sand bag at the perpendicular position to the ground. The experiment included 40 trials of back kick tasks that were randomized from 10 trials by 4 speed-conditions (See figure 4).
15
Figure 4. Diagram showing the procedure
Data Analysis
MT was measured as the time between the rise of heel and the foot contact to the sand bag.. TE was measured using the time between the movement initiation of the participant and the moment where the target is perpendicular to the ground. Negative TE indicates the foot-sand bag contact moment was before the sand bag reached the 0° position.(See Figure 5.).
MT and TE were examined using the one-way repeated-measures ANOVA for 4 speed conditions. The Greenhouse-Geisser method would be used for the adjustment of the degrees of freedom if the test of sphericity was not satisfied. The p2 effect size was reported according to Cohen (1988) standard.
Preparation
• Read & Sign Form of consent
• Instrution given to participant
Arrangement
• Locate target area, abdomen to Sandbag Target
• Leg length = standing position away from sand bag
Warm up
• Tape white tape around the foot
• Warm up 10 minutes
Practice
• Practice each condition
Test
• Perform 10 trials for each condition (randomly )
• 5 minutes rest between 10 trials
16
TSA was recorded based on the contact position of the foot on the location of the target from the video replay of each trial. ASA was registered based on the angle of the sand bag when the foot contact to the sand bag was made. Two black belt Taekwondo international referees ranked the ASA from the perpendicular position (4) to the farthest away from the perpendicular position but closest to the participant (1) (See figure 5). The kappa values were used to examine the intra and inter rater reliability. All the kappa values were higher than 0.8 (Table 1).
The TSA and ASA were examined using Pearson Chi square test of independence for the 4 speed conditions. SPSS 23.0 was used for all statistical analyses where the 𝛼 level of significance was set at .05.
Figure 5. Four spatial accuracy levels of angular position at foot-target contact
Table 1 Kappa values of reliability for the 2 ASA raters
Rater T G
T .887* .840*
G - .954*
17
Chapter IV Results
Movement time
The average movement time for each participant in four different speed condition were as follows: 0.038±0.017 seconds in SP1, 0.032±0.009 seconds in SP2, 0.035±0.018 seconds in SP3 and 0.027±0.007 seconds in SP4. One way repeated measure ANOVA showed no significant main effect of the 4-speed condition to the movement time of the participant, F(1.98, 17.78) = 1.865, p = .184 with small effect size p2 = .174 (Figure 6).
Figure 6. Average movement time in four conditions
Temporal error
The average temporal error for each participant in three different speed condition were as follows: 0.003±0.010 seconds in SP2, -0.001±0.011 seconds in SP3 and -0.003±0.007 seconds in SP4. One way repeated measure ANOVA showed no significant effect of the
18
speed condition to the temporal error, F (1.82, 16.38) = 5.138, p = .21. with small effect size
p2 =.363 (Figure 6).
Figure 7. The average temporal error in 3 moving speed conditions
Spatial accuracy on target
The result of the Pearson Chi square test of independence showed that there were significant associations between the target speed and target accuracy, 2(12) =139.60, p < .05, Cramer’s V=.341 Examining the adjusted standardized residuals of each of the cells in the cross-tabulation (Table 2), high positive values form an implicit diagonal trend line from the top right to the bottom left (highlighted area in Table 2), indicating a strong association of high target accuracy for the stationary target, medium target accuracy for the medium speed condition, and poor target accuracy for the fast speed condition.
19
Table 2. Cross-tabulation of frequencies of TSA in 4 speed conditions
TSA Points Total Pearson Chi Square Asymptotic Significance (2-sided) .00**
*p < .05; **p < .01
Spatial accuracy of the sand bag angle
The result of the Pearson Chi square test of independence showed that there were significant associations between the speed and the spatial accuracy of the angular position of
The result of the Pearson Chi square test of independence showed that there were significant associations between the speed and the spatial accuracy of the angular position of