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Kinesthetic aftereffects induced by a weighted tool on movement correction in baseball batting
Human Movement Science 31 (2012) 1529–1540
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Kinesthetic aftereffects induced by a weighted tool on movement correction in baseball batting Hiroki Nakamoto a,⇑, Yasumitsu Ishii a, Sachi Ikudome b, Yoichi Ohta c a
Faculty of Physical Education, National Institute of Fitness and Sports in Kanoya, 1 Shiromizu, Kanoya, Kagoshima 891-2393, Japan Graduate School of Physical Education, Doctor’s Course, National Institute of Fitness and Sports in Kanoya, 1 Shiromizu, Kanoya, Kagoshima 891-2393, Japan c Faculty of Health and Medical Sciences, Department of Sports and Health Sciences, Aichi Shukutoku University, 9 Nagakutekatahira, Nagakute, Aichi, Aichi 480-1197, Japan b
a r t i c l e
i n f o
Article history: Available online 12 June 2012 PsycINFO classification: 2330 Keywords: Kinesthetic aftereffect Efference copy Perceptual illusion Baseball batting Movement correction
a b s t r a c t We investigated the kinesthetic aftereffects of a weighted tool on interceptive performance. Eight college baseball players performed three warm-ups before the interceptive task: a normal warm-up, a recalibrated warm-up with a standard 850-g bat and a 1200-g weighted bat, and a weighted warm-up with a 1200-g bat. For the interceptive task, subjects were asked to swing the standard bat coincident with the arrival and position of a moving target. After the warm-ups with the weighted bat, participants felt that the bat was lighter and swung faster. When participants needed to correct their swings to the target’s velocity change, larger timing errors were produced in the weighted than in the normal practice condition. These results indicate that warm-ups with a weighted tool create adverse effects for the movement (re)programming processes in interceptive action. This suggests that warm-ups with a weighted tool for an interceptive task affect the central nervous system and not the peripheral system. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction In many sports, athletes warm-up with heavier or lighter tools than those of a standard weight that are used in actual game situations. It is widely believed that this type of warm-up has positive effects on athletic performance (e.g., Lindeburg & Hewitt, 1965; Van Huss, Albrecht, Nelson, & Hagerman, 1962). For example, baseball players traditionally swing weighted bats in the on-deck circle to ⇑ Corresponding author. Tel.: +81 (994) 46 4975. E-mail address: [email protected] (H. Nakamoto). 0167-9457/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.humov.2012.04.005
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warm-up before stepping into the batter’s box. Previous studies of the acute effects of various weight warm-up conditions on bat swing velocity have shown that warm-ups using heavier and lighter bats produced greater swing velocities (Reyes & Dolny, 2009; DeRenne, Ho, Hetzler, & Chai, 1992; Southard & Groomer, 2003), smaller swing velocities (DeRenne et al., 1992; Otsuji, Abe, & Kinoshita, 2002; Southard & Groomer, 2003), and unaltered swing velocities (Szymanski et al., 2011) as compared to standard baseball bats. The acute effects of switching tool weight are not only physical but also psychological (Lindeburg & Hewitt, 1965; Nelson & Lambert, 1965; Nelson & Nofsinger, 1965; Stockholm & Nelson, 1965), a phenomenon referred to as the kinesthetic aftereffect (KA). The KA is defined as a perceived modification in the shape, size, or weight of an object or a perceptual distortion of limb position, movement, or intensity of muscular contractions as a result of an experience with a previous object (Sage, 1984). Otsuji et al. (2002) investigated the aftereffects of a weighted bat on the subjective perceptions of swing velocity and bat heaviness of batters. They found that the physical swing velocity just prior to contact significantly decreased when participants hit a static ball with a standard bat after hitting the ball with a weighted bat. Despite this negative mechanical effect, however, the participants subjectively felt that the bat was lighter and that they swung the bat faster in this situation. From these results, Otsuji and colleagues concluded that the advantage of warm-ups using a weighted bat was psychological rather than biomechanical. Although it has been reported that subjective feelings such as lighter and faster swings are advantageous to batting performance (DeRenne et al., 1992; Szymanski et al., 2011), the mismatch between subjective feelings and actual outcomes for bat swings may degrade the perceptual-motor control of batters. In the interceptive action, temporal anticipation is one of the critical factors in hitting a ball at the right moment, especially in brief interceptive action (Marinovic, Plooy, & Tresilian, 2008, 2010b). Poulton (1950, 1957, 1965) has described the various types of anticipation information necessary to successfully intercept a moving target. One is the receptor anticipation information obtained on the time of arrival of a moving target. Another is the effector anticipation information obtained on the duration of one’s own movement, such as that of a bat swing. Recent evidence has demonstrated that the prepared movement time based on the expectation of time-to-contact is a critical factor in hitting a ball with short movement times at the right moment (Marinovic, Plooy, & Tresilian, 2009, 2010a; Marinovic et al., 2008, 2010b; Tresilian, 2005; Zago, McIntyre, Senot, & Lacquanti, 2009). Tresilian (2005) emphasized that knowledge of one’s own movement time allows a performer to correctly initiate the interceptive action (see also Marinovic et al., 2008, 2009, 2010a, 2010b; Tyldesley & Whiting, 1975; Williams, Davids, & Williams, 1999), and also stated that the programmed MT depends on the internal state and advance information and expectation. Therefore, if the illusory subjective perception of swing velocity affects highly overlearned effector anticipation with a standard bat, the batter will alter his interceptive strategy, thereby degrading the coincident timing performance that permits a bat swing to arrive at the moving target at the right time (for example, if the subjective feeling is that of a faster swing, then the batter will exhibit a more delayed response strategy than usual, although his actual swing velocity is not slower). Bridgeman and Peery (1997) demonstrated the influence of perceptual illusions on motor control, showing that pointing movement direction was modified by the motion illusion of targets. Previous studies of the KA in batting, however, have investigated static conditions, such as tee batting (e.g., DeRenne et al., 1992; Otsuji et al., 2002; Southard & Groomer, 2003; Szymanski et al., 2011), rather than dynamic conditions that require perceptual-motor control in hitting a moving target. Scott and Gray (2010) have investigated changes in perceptual-motor control in response to switching to a bat with a different weight when performing a dynamic interceptive task with a 2D batting simulator (Gray, 2002, 2009). In this experiment, baseball batters used a standard bat to complete two experimental blocks of 15 swings. The next two blocks of trials differed among the three groups. Participants were randomly assigned to a lighter (i.e., bat weight reduced), heavier (i.e., bat weight increased), or control (i.e., no change in bat weight) group. In the two final blocks, they switched back to the standard bat. The researchers reported that the changes in perceptual-motor dynamics induced by switching the weight of the tool bat produced larger timing errors and decreased or increased swing velocity, depending on the conditions. The timing errors and swing velocity consisted of more early errors and faster velocity after the task was done with a heavier tool and vice versa. This result
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suggests that the mismatch between subjective feelings and actual outcomes in bat swings degrade perceptual-motor control in dynamic conditions. However, the experiment offers limited information by which to understand the acute effect of the KA in interceptive action. The experiment created long inter-block intervals between the weighted condition and standard condition of the batters (i.e., 5 min), despite the fact that the KA is a relatively short-term effect (Otsuji et al., 2002; Sage, 1984). Also, the batters performed the simulation task with heavier and lighter bats before switching to the standard bat. This procedure can induce specific adaptations in perceptual-motor control unlike those of real situations. Moreover, the researchers did not estimate the subjective perceptions of batters with regard to swing velocity and weight. It is unclear whether perceptual distortion occurred with a standard bat. For these reasons, further information is needed to understand the acute effect of the KA in dynamic conditions. In baseball batting, batters often perform practice swings with a weighted bat followed by swinging with a standard bat before stepping into the batter’s box. This dynamic wielding with a standard tool may correct the perceptual distortion. Scott and Gray (2010) conducted a test to determine whether the perceptual-motor system can immediately reorganize a change in a tool’s weight on the basis of the dynamic wielding of the tool alone. They conclude that actors cannot recalibrate perceptual-motor controls solely on the basis of practice swings in interaction with moving objects because temporal swing errors persisted after dynamic wielding. In contrast to these results, previous studies have shown that after a brief period of dynamic wielding prior to use, the actor could rapidly adjust in response to a change in the tool (Carello, Thuot, Anderson, & Turvey, 1999; Turvey, 1996) and accurately control their actions to interact with stationary objects (Bongers, Smitsman, & Michaels, 2003; Bongers, Michaels, & Smitsman, 2004). These results suggest that dynamic wielding has no effects on motor control when hitting a moving target, but does have effects when hitting a static target. However, the experiment by Scott and Gray (2010) did not compare performance with and without dynamic wielding. It is unclear whether recalibration with dynamic wielding is totally ineffective or whether it has some effect in a dynamic task. Therefore, the main objective of this study was to investigate the influences of the subjectiveobjective mismatches in bat swings induced by the KA in dynamic interceptive performance. To accomplish this purpose, we used a batting simulator that, in addition to an unchanged moving target, enabled us to represent temporal and spatial changes in a moving target. In a dynamic condition such as baseball batting, pitchers often throw breaking balls in addition to fast balls. Therefore, batters have to correct their swing timing and position when the pitched ball velocity and trajectory change. If batters apply subjective information to anticipate and calibrate their movement times, we would expect effects on coincident timing errors in batting. Moreover, a second objective is the determination of whether the effect on performance of the weighted bat is preventable through dynamic wielding without dynamic interaction with the environment. 2. Methods 2.1. Participants Eight male college baseball players between 19 and 22 years of age participated in this study. All participants had normal or corrected-to-normal vision, belonged to the official college baseball team, and possessed 8 to 12 years of baseball experience. They generally spent 20 h per week on baseball training and participated regularly in matches. They were informed of the experimental procedures in advance and consented to take part in the experiment. In accordance with the Declaration of Helsinki, the present study was approved by the ethics committee of the National Institute of Fitness and Sports in Kanoya, Japan. 2.2. Experimental apparatus and task The experimental device consisted of a horizontal electronic trackway (4 m long and 60 cm above the ground), with 200 light-emitting diodes (LEDs) that simulated the linear motion of an object
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Fig. 1. Schematic representation of the experimental set-up. After a 3-s warning stimulus, the moving target displaced from one end of the trackway at a constant velocity (8 m/s); in one-third of the trials its velocity suddenly decreased (4 m/s) or its location suddenly shifted from the center to the outside of the trackway 250 ms after the target left the starting point. Participants were asked to swing their bats when the target arrived at the end of the trackway.
(Fig. 1) (AO-5N model; Applied Office Co. Ltd., Tokyo, Japan). The LEDs were quickly turned on and off in sequence. In this way, participants could clearly perceive the continuous motion of an approaching target. Target velocity was modified by changing instantaneously the turn on and off times of the LEDs. Target location was modified by shifting it from the center to the outside trackway, which was positioned on the center and the outside of a baseball home plate, respectively (Fig. 1). Participants performed a coincident timing task that consisted of swinging a standard weight bat at the moment of arrival of an apparently moving target, which ran on a straight trackway. The moving target displaced from one end of the trackway at a constant velocity (8 m/s) after the presentation of a 3-s warning stimulus. In one-third of the trials, its velocity suddenly decreased or its location suddenly shifted from the center to the outside trackway 250 ms after it left the starting point (Table 1). These manipulations of target velocity and location before its arrival at the interception position required that participants temporally or spatially correct their bat swings. The velocity change was set at 50% (8–4 m/s in the velocity decrease condition), which is above the threshold value that is generally found to be sufficient to allow the perception of a variation (Benguigui, Ripoll, & Broderick, 2003; Runigo, Benguigui, & Bardy, 2005). The total time of the target presentation creates a situation that a baseball batter generally faces in actual batting (Frohlich, 1984; Selin, 1959; Williams & Underwood, 1986). In order to induce the KA, we used a weighted bat (1200 g) for a practice swing before performing the same task with a standard bat (850 g). The standard bats weighed between 840 g and 900 g (M weight = 875 g), which are thought to be standard in collegiate baseball (DeRenne et al., 1992; Szymanski et al., 2011). The kinematic data of the bat were collected with a twentieth-camera digital three-dimensional motion analysis system (Motion Analysis Corporation, Santa Rosa, CA) working at a frame rate of 400 Hz. Calibration was performed by the wand calibration method, according to the manufacturer’s guidelines. The root mean square of positional error was less than 1.0 mm in threedimensional space. Reflective markers were attached to four markers placed on the bat. The distances from the distal end were 10 mm and 160 mm bilaterally. The three-dimensional coordinate system was defined with the y-axis as the batting direction, the z-axis as the vertical axis, and the x-axis as the right side of the home plate. An electronic pulse from a batting simulator was used to synchronize
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H. Nakamoto et al. / Human Movement Science 31 (2012) 1529–1540 Table 1 Summary of the stimulus velocity and time in each condition. Stimulus conditions
Initial velocity
Final velocity
Total time
Time before changing
Time after changing
Unchanged Velocity changed Location changed
8 m/s 8 m/s 8 m/s
8 m/s 4 m/s 8 m/s
500 ms 750 ms 500 ms
– 250 ms 250 ms
– 500 ms 250 ms
the kinematic data. The electronic pulses were stored in a personal computer through an AD converter (NI PCI-6071E, National Instruments, USA) with a sampling frequency of 1200 Hz. The impact point was defined as the crossover point between the bat swing and the edge of the trackway above. Bat velocity at the impact point was y-axis component velocity. The bat position at the impact point examined the right (negative) and left (positive) position. 2.3. Experimental procedure The participants completed a coincident timing task in which a warning visual stimulus (lighting five LEDs) was followed by three, moving target stimulus conditions after an interval of 3000 ms. There was a constant interval (5 s) before the next warning visual stimulus (excepting the insufficient preparation of the participants). This experiment was conducted by three practice swing conditions that utilized different bat weights and procedures before the coincident timing task. The normal condition involved 3 practice swings with a standard bat. The weighted bat condition comprised 3 practice swings with a weighted bat. The recalibration condition involved three swings with weighted bat followed by three with the standard bat swing. All the test trials in the coincident timing task used a standard weight bat. The batters first stood beside a standard home plate that was placed on the floor just under the front edge of trackway. They familiarized themselves with the experimental procedure under all stimulus conditions (unchanged, temporal, and spatial changed conditions) until they felt natural. Subsequently, in the experimental task, the participants performed 30 coincident timing swings for each of the three practice swing conditions. These were divided into 6 simulation task swings and were performed with a maximal effort after 3 practice swings with standard or weighted bats. These blocks included three equivalent stimulus conditions. In each block, there was a 33% chance of velocity decrement or a spatial shift, a percentage that prevented the confounding of the effects of environmental change with predictions of differences in a priori probability or subjective expectancy by the participants. Therefore, the participants had to correct their expected responses in changed trials. After six swings in the simulation task, they were asked to make subjective judgments of the heaviness of the bat during the swing and the speed of the swing itself compared to those of typical weights and speeds. A 5-point scale was used for each judgment: apparently lighter (5), slightly lighter (4), equal (3), slightly heavier (2), and apparently heavier (1) for the bat weight; and apparently faster (5), slightly faster (4), equal (3), slightly slower (2), and apparently slower (1) for the speed (Otsuji et al., 2002). They were told the temporal error of each trial. The interval between practice swings and swings under the simulation task were set at within 5 s, and the block interval was approximately 3 to 5 min. 2.4. Behavioral measures and statistics Four dependent variables were used: subjective perception in a bat swing, bat swing velocity, and the absolute temporal (ATE) and spatial (ASE) errors. The subjective perception of bat speed and weight were subjected to a one-way ANOVA in practice conditions. The bat swing velocity was calculated at just the impact point (i.e., the end of the trackway). The mean values were subjected to a repeated measure two-way (Changing: unchanged, velocity changed, and location changed) 3 (Practice: standard, weighted, and recalibration) ANOVA. To compare the difference in interceptive performance in various bat weight conditions, we analyzed temporal and spatial errors. The temporal
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error is defined as the difference between the time the moving target arrived in front of the plate and the time when the bat crossed it. The spatial error is defined as the difference in the horizontal location of the center of percussion of the bat and final arrival location of the moving target when the bat crossed the front of the plate. Absolute values in temporal error (ATE) and spatial error (ASE) were analyzed as an index of the accuracy of interceptive performance. The comparisons of ATE among the three practice conditions were conducted in constant and velocity changed conditions that required the temporal correction in interceptive actions. Also, the ASE was compared for constant and changed location conditions that required spatial correction. The mean values of these dependent variables were subjected to a repeated measure two-way (Changing: unchanged and changed) 3 (Practice: standard, weighted, and recalibration) ANOVA. 3. Results 3.1. Subjective bat swing speed and weight The subjective perceptions in swing with a standard bat were differentially modulated depending on the practice swing conditions. Table 2 shows the perception of subjective bat swing speeds and weights. Two participants with less than 3 points in subjective scores (i.e., equal) during the weighted and recalibration conditions were excluded from all subsequent analysis. In the mean score of subjective feelings of swing speed, the ANOVA was significant: F(2, 17) = 17.93, p < .01. Tukey’s all-pairwise comparison test showed that the weighted and recalibration conditions had significantly higher scores than the normal condition. Also, the subjective feelings of bat weight revealed a significant difference: F(2, 17) = 7.93, p < .01. The post hoc multiple comparisons showed a significant difference between the normal and the other two conditions, that is, the participants felt that they swung faster and that the bat was lighter after the practice swing in the weighted and recalibration conditions. 3.2. Bat speed Fig. 2 shows the swing velocity at the interception position for the practice and stimulus conditions. The two-way ANOVA of the swing velocity showed significant main effects for stimulus, F(2, 10) = 7.20, p < .05, g2 = .59, and practice, F(2, 10) = 6.72, p < .05, g2 = .57. An analysis of the multiple comparisons of stimulus conditions indicated that the swing velocity in the unchanged condition was faster than those of the changed velocity and location conditions (p < .05). In the comparisons among practice swing conditions, there were marginally significant differences between the weighted and normal conditions (p < .10). 3.3. Coincident temporal and spatial errors Fig. 3A shows the ATE for each practice swing and stimulus condition. The trials with temporal errors lesser or greater than mean ± 2 SD were excluded from all subsequent analysis. The two-way ANOVA of the ATE showed a significant Stimulus Practice swing interaction: F(2, 10) = 3.98, p < .05, g2 = .44. An analysis of the simple main effects indicated that in the velocity unchanged condition, the ATE in the recalibration condition was smaller than those of the normal (p < .01) and weighted (p < .05) conditions; in the velocity changed condition, the ATE in weighted condition was larger than in the normal condition (p < .05). Moreover, in the recalibration condition, the ATE in
Table 2 Mean ratio of the perception of subjective bat swing speed and weight.
Swing speed Bat weight
Normal
Weighted
Recalibration
3.0 ± 0.0 3.0 ± 0.0
3.6 ± 0.3 3.6 ± 0.4
3.8 ± 0.4 3.7 ± 0.4
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Fig. 2. Mean bat velocity for each practice bat swing and stimulus condition. The bat velocity was defined as the velocity at the front edge of the home plate (i.e., impact point). Error bars are standard deviations.
Fig. 3. Mean absolute temporal error (Panel A) and absolute spatial error (Panel B) for each practice swing and stimulus condition.
the changed condition was larger than that in the unchanged condition (p < .05). Fig. 3B shows the ASE for each practice swing and stimulus condition. The two-way ANOVA of these data revealed no significant main effects or significant interactions.
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4. Discussion The main object of this study was to investigate the influence of the subjective-objective mismatch induced by the KA on interceptive performance. In order to confirm the occurrence of the mismatch that has been reported in a static interceptive task (Otsuji et al., 2002), we analyzed the subjective perception of bat weight and swing speed and the physical bat swing velocity in dynamic interceptive tasks. The participants in the simulation task felt that the bat was lighter after practice swings with the weighted bat than in the normal condition, despite using bats with the same weight. As for the perception of bat swing velocity, participants felt that the swing was slightly faster in the weighted condition than in the normal condition. In accordance with subjective perceptions, actual bat velocity at the impact point increased in the weighted condition. These results indicate that warm-ups with weighted bats increase velocity both physically and perceptually in a dynamic interceptive task. In other words, subjective-objective mismatches only occurred with bat weight but not with swing speed. Moreover, these subjective perceptions were not modulated by dynamic wielding (i.e., the recalibration condition). Although the subjective-objective mismatches of swing speed that we expected did not occur, warm-up with weighted bats affected interceptive timing performance. Interestingly, the ATE was larger in the weighted than in the normal condition only in the changed velocity task. In other words, the KA showed a selective effect of perceptual-motor control that requires movement timing correction. These results were caused by the different swing velocities in each condition. The traveling time of the moving target was longer in the changed velocity condition than in the unchanged condition. Therefore, batters had to correct the arrival time of the bat to avoid early responses. In response to this assumption, the swing velocity under the changed task was significantly slowed compared to the unchanged task. Previous studies emphasize that briefer and faster interceptive action makes it difficult to correct movements after a motor pattern generator produces a final planned movement (Tresilian, 2005; Marinovic et al., 2008, 2009, 2010a). Marinovic et al. (2008, 2010b) reported that information about movement amplitude and direction in a brief interception must be provided at least 150– 200 ms and 250 ms prior to movement onset, respectively. Also, Teixeira, Lima, and Franzoni (2005) found that participants needed 300 ms to target arrival to correct movement timing to the moving target with unexpected velocity changes. Thus, correcting threshold was assumed to be around 150– 300 ms before the final ballistic movement onset. In the present study, the time to target arrival after changing is 500 ms in the temporal condition and 250 ms in the spatial condition, as shown in Table 1. These conditions allow enough time for batters to correct their movement. Moreover, this value can be reduced by motor preparation in advance (Marinovic et al., 2010a) and experts in fastball play show shorter visuomotor delays than novices (Runigo, Benguigui, & Bardy, 2010; Runigo et al., 2005). For these reasons, decreasing the swing velocity in the changed condition indicates that batters correct their motor plan before the motor pattern generator produces the final planned movement (final stage of motor preparation: Marinovic et al., 2008). On the other hand, in general, baseball batting is classified as a rapid interceptive action because the duration of the bat swing (from forward swing initiation to impact) is less than 160 ms. Taking these factors into account, batters could be modifying their motor plan up until the final decision is made to produce the planned movement (i.e., until forward swing initiation). However, the slower swing to the decreasing target appeared in the normal condition, not in the weighted condition. This finding indicates that after practice swings with weighted bats, baseball batters could not adjust their movement duration by slowing swing velocity up until the final decision. In fact, batters showed an early response tendency in the changed velocity condition, and it was evident in the weighted condition (normal: 102 ms, weighted: 120 ms). From these results, we conclude that the acute effect of the KA that selectively influences the movement timing correction process is caused by the failure to decrease swing velocity by altering the preprogrammed motor command. As in the case of subjective perception, temporal errors were not largely modulated by dynamic wielding (i.e., the recalibration condition). Previous studies of the effects on swing velocity of warm-ups with various weighted tools have reported that changing the latter results in alterations of muscle strength (e.g., Reyes & Dolny, 2009) and
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swing motor patterns (e.g., Southard & Groomer, 2003). Other, complex training procedures use various resistances to increase power output by improving muscle power (Baker, 2003; Gourgoulis, Aggeloussis, Kasimatis, Mavromatis, & Gara, 2003; Young, Jenner, & Griffiths, 1998). According to these inquiries, faster swings after the practice swing with weighted bats result in the changing of muscle force generation. The selective effect, however, does not sufficiently explain peripheral system adaptation because these changes affect swing velocity irrespective of stimulus conditions. Another possibility is that the selective effect is the result of central system influences. When substantial velocity changes occur in a moving target, the performer should considerably correct his swing in response. In this case, movement reprogramming that includes the motor inhibition of prepared response (Neubert, Mars, Buch, Olivier, & Rushworth, 2010; Sharp et al., 2010) and a re-specification for a new response (Larish & Stelmach, 1982; Leuthold & Jentzsch, 2002; Mars, Piekema, Coles, Hulstijn, & Toni, 2007) is needed for successful interceptive action (Teixeira, Chua, Nagelkerke, & Franks, 2006). Although the correction strategy with inhibition may not be effective due to difficulty and the additional time required for processing (Coxon, Stinear, & Byblow, 2007), previous studies have reported that inhibition is strongly associated with success in baseball batting (Gray, 2009; Nakamoto & Mori, 2008). The success or failure in inhibiting the motor response is related to the termination timing of the motor producing and inhibiting process. If the processing of motor command production reaches the point of no return before the processing of the inhibiting motor command, then the response cannot be successfully inhibited (Boucher, Palmeri, Logan, & Schall, 2007; Logan, Cowan, & Davis, 1984; Marinovic et al., 2009). Thus, earlier motor production is associated with a high probability of inhibiting failure. Scott and Gray (2010) reported that batters swing earlier and faster in response to a change in bat weight. Based on these findings, the failure to decrease swing velocity under velocity-changed tasks only in the weighted condition is explained by the inability of the motor inhibition required by movement correction. Scott and Gray (2010) have stated that higher swing velocity is induced by changes in a tool’s weight that negatively affect motor inhibition, thereby producing larger errors. This plausible explanation, however, is not supported by the spatial error under the weighted condition. If the KA induces the lack of motor inhibition, movement correction for spatial change would also occur. As for other possible explanations from the central system, the early response tendency under the weighted condition supports our hypothesis that the KA affects effector anticipation, and that therefore interceptive timing will change. From computational theory, sensory awareness is increased when the actual sensation (i.e., afferent information) mismatches predicted sensations (i.e., efference copy) and vice versa (e.g., Blakemore, Frith, & Wolpert, 1999; Blakemore, Wolpert, & Frith, 1998). According to this notion, the subjective perception of faster swings after swings in the weighted condition indicates that batters formulated the efference copy that predicted a slower swing than that of the actual swing velocity. In other words, the practice swing with a weighted bat distorted the formulation of the efference copy. Previous studies have shown that because of the inevitable delays associated with the use of actual sensory information for movement correction in fast ball sports (McLeod, 1987; Runigo et al., 2005), the performer adopts predictive mechanisms (i.e., internal model) to execute rapid movement correction of visual targets (De Azevedo Neto & Teixeira, 2009, 2011; Zago et al., 2009). In this case, such a prediction involves a continuous estimation of the actual state of the system by using an efference copy in relation to the desired one, an assessment achieved by using rapid, internal (central) feedback loops (Sabes, 2000). A comparator then makes comparisons between this predicted feedback and actual feedback, and these appraisals are used for online automatic movement adjustments, cancelling sensory reafference and improving movement prediction and planning (Leonard, Gritsenko, Ouckama, & Stapley, 2011; Nijhof, 2003; Wolpert, 1997; Wolpert & Kawato, 1998). Thus, movement correction is accomplished by efference copy information rather than actual information. These considerations indicate that batters cannot correctly modulate their interceptive timing under the changed task because of inadequate error detection that contributes to the distortion of efference copy in the internal comparator, thereby producing faster swings and larger errors. In contrast, under the unchanged task, they do not need to correct swings that rely heavily on the efference copy. On the other hand, although there were no significant differences among warm-up conditions in the unchanged task, temporal error decreased in the weighted and recalibration conditions.
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Scott and Gray (2010) reported that the timing error was significantly larger after the simulation task with a weighted bat under nearly constant velocity conditions. The difference in the significance level between the present study and the study of Scott and Gray (2010) under unchanged conditions may be due to the exposure time of swinging with the weighted bat. Sage (1984) stated that the KA effect changes depending on the precedent handling time. Scott and Gray’s experiment had longer handling time with weighted tools than the present study and included direct interaction with the moving object before the standard bat condition. Therefore, the KA effect in our study was expected to be smaller than that reported previously by Scott and Gray (2010). The faster swing (i.e., short duration) in the present study produced smaller and larger errors in the unchanged and changed tasks, respectively, because participants often show late response errors and early response errors in unchanged and changed tasks, respectively. This tendency may reveal that the effect of handling weighted bats was simply to program faster movements that could not be adjusted online (Marinovic et al., 2008, 2009, 2010b). Taken together, the results suggest that the KA affects tasks that rely heavily on anticipation. That is, KA affects movement (re) programming because early response tendencies were induced after practice swings with a weighted bat. In conclusion, the KA adversely affects perceptual-motor control in dynamic situations that rely heavily on anticipation. This degradation of interceptive performance results from the interference in the central system based on efference copy and/or pre-programmed motor commands in the interceptive action process. This finding indicates that the adverse effect of the KA was not an intentional strategy change but rather an unintentional internal processing change. Previous studies that have investigated the relationship between the KA and athletic performance have reported that performance did not improve in basketball handling and shooting (Lindeburg & Hewitt, 1965), jumping (Stockholm & Nelson, 1965), and motor control (Nelson & Nofsinger, 1965). In addition to these studies, we demonstrate that the KA adversely affects the movement (re)programming processes. Moreover, we conclude that performers cannot adequately exert perceptual-motor control solely on the basis of practice swings in interacting with moving objects. Therefore, warm-ups with weighted tools are not recommended in actual athletic situations; however, the effects of lighter tools are not included in this finding. References Baker, D. (2003). 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Human Movement Science journal homepage: www.elsevier.com/locate/humov
Kinesthetic aftereffects induced by a weighted tool on movement correction in baseball batting Hiroki Nakamoto a,⇑, Yasumitsu Ishii a, Sachi Ikudome b, Yoichi Ohta c a
Faculty of Physical Education, National Institute of Fitness and Sports in Kanoya, 1 Shiromizu, Kanoya, Kagoshima 891-2393, Japan Graduate School of Physical Education, Doctor’s Course, National Institute of Fitness and Sports in Kanoya, 1 Shiromizu, Kanoya, Kagoshima 891-2393, Japan c Faculty of Health and Medical Sciences, Department of Sports and Health Sciences, Aichi Shukutoku University, 9 Nagakutekatahira, Nagakute, Aichi, Aichi 480-1197, Japan b
a r t i c l e
i n f o
Article history: Available online 12 June 2012 PsycINFO classification: 2330 Keywords: Kinesthetic aftereffect Efference copy Perceptual illusion Baseball batting Movement correction
a b s t r a c t We investigated the kinesthetic aftereffects of a weighted tool on interceptive performance. Eight college baseball players performed three warm-ups before the interceptive task: a normal warm-up, a recalibrated warm-up with a standard 850-g bat and a 1200-g weighted bat, and a weighted warm-up with a 1200-g bat. For the interceptive task, subjects were asked to swing the standard bat coincident with the arrival and position of a moving target. After the warm-ups with the weighted bat, participants felt that the bat was lighter and swung faster. When participants needed to correct their swings to the target’s velocity change, larger timing errors were produced in the weighted than in the normal practice condition. These results indicate that warm-ups with a weighted tool create adverse effects for the movement (re)programming processes in interceptive action. This suggests that warm-ups with a weighted tool for an interceptive task affect the central nervous system and not the peripheral system. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction In many sports, athletes warm-up with heavier or lighter tools than those of a standard weight that are used in actual game situations. It is widely believed that this type of warm-up has positive effects on athletic performance (e.g., Lindeburg & Hewitt, 1965; Van Huss, Albrecht, Nelson, & Hagerman, 1962). For example, baseball players traditionally swing weighted bats in the on-deck circle to ⇑ Corresponding author. Tel.: +81 (994) 46 4975. E-mail address: [email protected] (H. Nakamoto). 0167-9457/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.humov.2012.04.005
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warm-up before stepping into the batter’s box. Previous studies of the acute effects of various weight warm-up conditions on bat swing velocity have shown that warm-ups using heavier and lighter bats produced greater swing velocities (Reyes & Dolny, 2009; DeRenne, Ho, Hetzler, & Chai, 1992; Southard & Groomer, 2003), smaller swing velocities (DeRenne et al., 1992; Otsuji, Abe, & Kinoshita, 2002; Southard & Groomer, 2003), and unaltered swing velocities (Szymanski et al., 2011) as compared to standard baseball bats. The acute effects of switching tool weight are not only physical but also psychological (Lindeburg & Hewitt, 1965; Nelson & Lambert, 1965; Nelson & Nofsinger, 1965; Stockholm & Nelson, 1965), a phenomenon referred to as the kinesthetic aftereffect (KA). The KA is defined as a perceived modification in the shape, size, or weight of an object or a perceptual distortion of limb position, movement, or intensity of muscular contractions as a result of an experience with a previous object (Sage, 1984). Otsuji et al. (2002) investigated the aftereffects of a weighted bat on the subjective perceptions of swing velocity and bat heaviness of batters. They found that the physical swing velocity just prior to contact significantly decreased when participants hit a static ball with a standard bat after hitting the ball with a weighted bat. Despite this negative mechanical effect, however, the participants subjectively felt that the bat was lighter and that they swung the bat faster in this situation. From these results, Otsuji and colleagues concluded that the advantage of warm-ups using a weighted bat was psychological rather than biomechanical. Although it has been reported that subjective feelings such as lighter and faster swings are advantageous to batting performance (DeRenne et al., 1992; Szymanski et al., 2011), the mismatch between subjective feelings and actual outcomes for bat swings may degrade the perceptual-motor control of batters. In the interceptive action, temporal anticipation is one of the critical factors in hitting a ball at the right moment, especially in brief interceptive action (Marinovic, Plooy, & Tresilian, 2008, 2010b). Poulton (1950, 1957, 1965) has described the various types of anticipation information necessary to successfully intercept a moving target. One is the receptor anticipation information obtained on the time of arrival of a moving target. Another is the effector anticipation information obtained on the duration of one’s own movement, such as that of a bat swing. Recent evidence has demonstrated that the prepared movement time based on the expectation of time-to-contact is a critical factor in hitting a ball with short movement times at the right moment (Marinovic, Plooy, & Tresilian, 2009, 2010a; Marinovic et al., 2008, 2010b; Tresilian, 2005; Zago, McIntyre, Senot, & Lacquanti, 2009). Tresilian (2005) emphasized that knowledge of one’s own movement time allows a performer to correctly initiate the interceptive action (see also Marinovic et al., 2008, 2009, 2010a, 2010b; Tyldesley & Whiting, 1975; Williams, Davids, & Williams, 1999), and also stated that the programmed MT depends on the internal state and advance information and expectation. Therefore, if the illusory subjective perception of swing velocity affects highly overlearned effector anticipation with a standard bat, the batter will alter his interceptive strategy, thereby degrading the coincident timing performance that permits a bat swing to arrive at the moving target at the right time (for example, if the subjective feeling is that of a faster swing, then the batter will exhibit a more delayed response strategy than usual, although his actual swing velocity is not slower). Bridgeman and Peery (1997) demonstrated the influence of perceptual illusions on motor control, showing that pointing movement direction was modified by the motion illusion of targets. Previous studies of the KA in batting, however, have investigated static conditions, such as tee batting (e.g., DeRenne et al., 1992; Otsuji et al., 2002; Southard & Groomer, 2003; Szymanski et al., 2011), rather than dynamic conditions that require perceptual-motor control in hitting a moving target. Scott and Gray (2010) have investigated changes in perceptual-motor control in response to switching to a bat with a different weight when performing a dynamic interceptive task with a 2D batting simulator (Gray, 2002, 2009). In this experiment, baseball batters used a standard bat to complete two experimental blocks of 15 swings. The next two blocks of trials differed among the three groups. Participants were randomly assigned to a lighter (i.e., bat weight reduced), heavier (i.e., bat weight increased), or control (i.e., no change in bat weight) group. In the two final blocks, they switched back to the standard bat. The researchers reported that the changes in perceptual-motor dynamics induced by switching the weight of the tool bat produced larger timing errors and decreased or increased swing velocity, depending on the conditions. The timing errors and swing velocity consisted of more early errors and faster velocity after the task was done with a heavier tool and vice versa. This result
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suggests that the mismatch between subjective feelings and actual outcomes in bat swings degrade perceptual-motor control in dynamic conditions. However, the experiment offers limited information by which to understand the acute effect of the KA in interceptive action. The experiment created long inter-block intervals between the weighted condition and standard condition of the batters (i.e., 5 min), despite the fact that the KA is a relatively short-term effect (Otsuji et al., 2002; Sage, 1984). Also, the batters performed the simulation task with heavier and lighter bats before switching to the standard bat. This procedure can induce specific adaptations in perceptual-motor control unlike those of real situations. Moreover, the researchers did not estimate the subjective perceptions of batters with regard to swing velocity and weight. It is unclear whether perceptual distortion occurred with a standard bat. For these reasons, further information is needed to understand the acute effect of the KA in dynamic conditions. In baseball batting, batters often perform practice swings with a weighted bat followed by swinging with a standard bat before stepping into the batter’s box. This dynamic wielding with a standard tool may correct the perceptual distortion. Scott and Gray (2010) conducted a test to determine whether the perceptual-motor system can immediately reorganize a change in a tool’s weight on the basis of the dynamic wielding of the tool alone. They conclude that actors cannot recalibrate perceptual-motor controls solely on the basis of practice swings in interaction with moving objects because temporal swing errors persisted after dynamic wielding. In contrast to these results, previous studies have shown that after a brief period of dynamic wielding prior to use, the actor could rapidly adjust in response to a change in the tool (Carello, Thuot, Anderson, & Turvey, 1999; Turvey, 1996) and accurately control their actions to interact with stationary objects (Bongers, Smitsman, & Michaels, 2003; Bongers, Michaels, & Smitsman, 2004). These results suggest that dynamic wielding has no effects on motor control when hitting a moving target, but does have effects when hitting a static target. However, the experiment by Scott and Gray (2010) did not compare performance with and without dynamic wielding. It is unclear whether recalibration with dynamic wielding is totally ineffective or whether it has some effect in a dynamic task. Therefore, the main objective of this study was to investigate the influences of the subjectiveobjective mismatches in bat swings induced by the KA in dynamic interceptive performance. To accomplish this purpose, we used a batting simulator that, in addition to an unchanged moving target, enabled us to represent temporal and spatial changes in a moving target. In a dynamic condition such as baseball batting, pitchers often throw breaking balls in addition to fast balls. Therefore, batters have to correct their swing timing and position when the pitched ball velocity and trajectory change. If batters apply subjective information to anticipate and calibrate their movement times, we would expect effects on coincident timing errors in batting. Moreover, a second objective is the determination of whether the effect on performance of the weighted bat is preventable through dynamic wielding without dynamic interaction with the environment. 2. Methods 2.1. Participants Eight male college baseball players between 19 and 22 years of age participated in this study. All participants had normal or corrected-to-normal vision, belonged to the official college baseball team, and possessed 8 to 12 years of baseball experience. They generally spent 20 h per week on baseball training and participated regularly in matches. They were informed of the experimental procedures in advance and consented to take part in the experiment. In accordance with the Declaration of Helsinki, the present study was approved by the ethics committee of the National Institute of Fitness and Sports in Kanoya, Japan. 2.2. Experimental apparatus and task The experimental device consisted of a horizontal electronic trackway (4 m long and 60 cm above the ground), with 200 light-emitting diodes (LEDs) that simulated the linear motion of an object
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Fig. 1. Schematic representation of the experimental set-up. After a 3-s warning stimulus, the moving target displaced from one end of the trackway at a constant velocity (8 m/s); in one-third of the trials its velocity suddenly decreased (4 m/s) or its location suddenly shifted from the center to the outside of the trackway 250 ms after the target left the starting point. Participants were asked to swing their bats when the target arrived at the end of the trackway.
(Fig. 1) (AO-5N model; Applied Office Co. Ltd., Tokyo, Japan). The LEDs were quickly turned on and off in sequence. In this way, participants could clearly perceive the continuous motion of an approaching target. Target velocity was modified by changing instantaneously the turn on and off times of the LEDs. Target location was modified by shifting it from the center to the outside trackway, which was positioned on the center and the outside of a baseball home plate, respectively (Fig. 1). Participants performed a coincident timing task that consisted of swinging a standard weight bat at the moment of arrival of an apparently moving target, which ran on a straight trackway. The moving target displaced from one end of the trackway at a constant velocity (8 m/s) after the presentation of a 3-s warning stimulus. In one-third of the trials, its velocity suddenly decreased or its location suddenly shifted from the center to the outside trackway 250 ms after it left the starting point (Table 1). These manipulations of target velocity and location before its arrival at the interception position required that participants temporally or spatially correct their bat swings. The velocity change was set at 50% (8–4 m/s in the velocity decrease condition), which is above the threshold value that is generally found to be sufficient to allow the perception of a variation (Benguigui, Ripoll, & Broderick, 2003; Runigo, Benguigui, & Bardy, 2005). The total time of the target presentation creates a situation that a baseball batter generally faces in actual batting (Frohlich, 1984; Selin, 1959; Williams & Underwood, 1986). In order to induce the KA, we used a weighted bat (1200 g) for a practice swing before performing the same task with a standard bat (850 g). The standard bats weighed between 840 g and 900 g (M weight = 875 g), which are thought to be standard in collegiate baseball (DeRenne et al., 1992; Szymanski et al., 2011). The kinematic data of the bat were collected with a twentieth-camera digital three-dimensional motion analysis system (Motion Analysis Corporation, Santa Rosa, CA) working at a frame rate of 400 Hz. Calibration was performed by the wand calibration method, according to the manufacturer’s guidelines. The root mean square of positional error was less than 1.0 mm in threedimensional space. Reflective markers were attached to four markers placed on the bat. The distances from the distal end were 10 mm and 160 mm bilaterally. The three-dimensional coordinate system was defined with the y-axis as the batting direction, the z-axis as the vertical axis, and the x-axis as the right side of the home plate. An electronic pulse from a batting simulator was used to synchronize
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H. Nakamoto et al. / Human Movement Science 31 (2012) 1529–1540 Table 1 Summary of the stimulus velocity and time in each condition. Stimulus conditions
Initial velocity
Final velocity
Total time
Time before changing
Time after changing
Unchanged Velocity changed Location changed
8 m/s 8 m/s 8 m/s
8 m/s 4 m/s 8 m/s
500 ms 750 ms 500 ms
– 250 ms 250 ms
– 500 ms 250 ms
the kinematic data. The electronic pulses were stored in a personal computer through an AD converter (NI PCI-6071E, National Instruments, USA) with a sampling frequency of 1200 Hz. The impact point was defined as the crossover point between the bat swing and the edge of the trackway above. Bat velocity at the impact point was y-axis component velocity. The bat position at the impact point examined the right (negative) and left (positive) position. 2.3. Experimental procedure The participants completed a coincident timing task in which a warning visual stimulus (lighting five LEDs) was followed by three, moving target stimulus conditions after an interval of 3000 ms. There was a constant interval (5 s) before the next warning visual stimulus (excepting the insufficient preparation of the participants). This experiment was conducted by three practice swing conditions that utilized different bat weights and procedures before the coincident timing task. The normal condition involved 3 practice swings with a standard bat. The weighted bat condition comprised 3 practice swings with a weighted bat. The recalibration condition involved three swings with weighted bat followed by three with the standard bat swing. All the test trials in the coincident timing task used a standard weight bat. The batters first stood beside a standard home plate that was placed on the floor just under the front edge of trackway. They familiarized themselves with the experimental procedure under all stimulus conditions (unchanged, temporal, and spatial changed conditions) until they felt natural. Subsequently, in the experimental task, the participants performed 30 coincident timing swings for each of the three practice swing conditions. These were divided into 6 simulation task swings and were performed with a maximal effort after 3 practice swings with standard or weighted bats. These blocks included three equivalent stimulus conditions. In each block, there was a 33% chance of velocity decrement or a spatial shift, a percentage that prevented the confounding of the effects of environmental change with predictions of differences in a priori probability or subjective expectancy by the participants. Therefore, the participants had to correct their expected responses in changed trials. After six swings in the simulation task, they were asked to make subjective judgments of the heaviness of the bat during the swing and the speed of the swing itself compared to those of typical weights and speeds. A 5-point scale was used for each judgment: apparently lighter (5), slightly lighter (4), equal (3), slightly heavier (2), and apparently heavier (1) for the bat weight; and apparently faster (5), slightly faster (4), equal (3), slightly slower (2), and apparently slower (1) for the speed (Otsuji et al., 2002). They were told the temporal error of each trial. The interval between practice swings and swings under the simulation task were set at within 5 s, and the block interval was approximately 3 to 5 min. 2.4. Behavioral measures and statistics Four dependent variables were used: subjective perception in a bat swing, bat swing velocity, and the absolute temporal (ATE) and spatial (ASE) errors. The subjective perception of bat speed and weight were subjected to a one-way ANOVA in practice conditions. The bat swing velocity was calculated at just the impact point (i.e., the end of the trackway). The mean values were subjected to a repeated measure two-way (Changing: unchanged, velocity changed, and location changed) 3 (Practice: standard, weighted, and recalibration) ANOVA. To compare the difference in interceptive performance in various bat weight conditions, we analyzed temporal and spatial errors. The temporal
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error is defined as the difference between the time the moving target arrived in front of the plate and the time when the bat crossed it. The spatial error is defined as the difference in the horizontal location of the center of percussion of the bat and final arrival location of the moving target when the bat crossed the front of the plate. Absolute values in temporal error (ATE) and spatial error (ASE) were analyzed as an index of the accuracy of interceptive performance. The comparisons of ATE among the three practice conditions were conducted in constant and velocity changed conditions that required the temporal correction in interceptive actions. Also, the ASE was compared for constant and changed location conditions that required spatial correction. The mean values of these dependent variables were subjected to a repeated measure two-way (Changing: unchanged and changed) 3 (Practice: standard, weighted, and recalibration) ANOVA. 3. Results 3.1. Subjective bat swing speed and weight The subjective perceptions in swing with a standard bat were differentially modulated depending on the practice swing conditions. Table 2 shows the perception of subjective bat swing speeds and weights. Two participants with less than 3 points in subjective scores (i.e., equal) during the weighted and recalibration conditions were excluded from all subsequent analysis. In the mean score of subjective feelings of swing speed, the ANOVA was significant: F(2, 17) = 17.93, p < .01. Tukey’s all-pairwise comparison test showed that the weighted and recalibration conditions had significantly higher scores than the normal condition. Also, the subjective feelings of bat weight revealed a significant difference: F(2, 17) = 7.93, p < .01. The post hoc multiple comparisons showed a significant difference between the normal and the other two conditions, that is, the participants felt that they swung faster and that the bat was lighter after the practice swing in the weighted and recalibration conditions. 3.2. Bat speed Fig. 2 shows the swing velocity at the interception position for the practice and stimulus conditions. The two-way ANOVA of the swing velocity showed significant main effects for stimulus, F(2, 10) = 7.20, p < .05, g2 = .59, and practice, F(2, 10) = 6.72, p < .05, g2 = .57. An analysis of the multiple comparisons of stimulus conditions indicated that the swing velocity in the unchanged condition was faster than those of the changed velocity and location conditions (p < .05). In the comparisons among practice swing conditions, there were marginally significant differences between the weighted and normal conditions (p < .10). 3.3. Coincident temporal and spatial errors Fig. 3A shows the ATE for each practice swing and stimulus condition. The trials with temporal errors lesser or greater than mean ± 2 SD were excluded from all subsequent analysis. The two-way ANOVA of the ATE showed a significant Stimulus Practice swing interaction: F(2, 10) = 3.98, p < .05, g2 = .44. An analysis of the simple main effects indicated that in the velocity unchanged condition, the ATE in the recalibration condition was smaller than those of the normal (p < .01) and weighted (p < .05) conditions; in the velocity changed condition, the ATE in weighted condition was larger than in the normal condition (p < .05). Moreover, in the recalibration condition, the ATE in
Table 2 Mean ratio of the perception of subjective bat swing speed and weight.
Swing speed Bat weight
Normal
Weighted
Recalibration
3.0 ± 0.0 3.0 ± 0.0
3.6 ± 0.3 3.6 ± 0.4
3.8 ± 0.4 3.7 ± 0.4
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Fig. 2. Mean bat velocity for each practice bat swing and stimulus condition. The bat velocity was defined as the velocity at the front edge of the home plate (i.e., impact point). Error bars are standard deviations.
Fig. 3. Mean absolute temporal error (Panel A) and absolute spatial error (Panel B) for each practice swing and stimulus condition.
the changed condition was larger than that in the unchanged condition (p < .05). Fig. 3B shows the ASE for each practice swing and stimulus condition. The two-way ANOVA of these data revealed no significant main effects or significant interactions.
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4. Discussion The main object of this study was to investigate the influence of the subjective-objective mismatch induced by the KA on interceptive performance. In order to confirm the occurrence of the mismatch that has been reported in a static interceptive task (Otsuji et al., 2002), we analyzed the subjective perception of bat weight and swing speed and the physical bat swing velocity in dynamic interceptive tasks. The participants in the simulation task felt that the bat was lighter after practice swings with the weighted bat than in the normal condition, despite using bats with the same weight. As for the perception of bat swing velocity, participants felt that the swing was slightly faster in the weighted condition than in the normal condition. In accordance with subjective perceptions, actual bat velocity at the impact point increased in the weighted condition. These results indicate that warm-ups with weighted bats increase velocity both physically and perceptually in a dynamic interceptive task. In other words, subjective-objective mismatches only occurred with bat weight but not with swing speed. Moreover, these subjective perceptions were not modulated by dynamic wielding (i.e., the recalibration condition). Although the subjective-objective mismatches of swing speed that we expected did not occur, warm-up with weighted bats affected interceptive timing performance. Interestingly, the ATE was larger in the weighted than in the normal condition only in the changed velocity task. In other words, the KA showed a selective effect of perceptual-motor control that requires movement timing correction. These results were caused by the different swing velocities in each condition. The traveling time of the moving target was longer in the changed velocity condition than in the unchanged condition. Therefore, batters had to correct the arrival time of the bat to avoid early responses. In response to this assumption, the swing velocity under the changed task was significantly slowed compared to the unchanged task. Previous studies emphasize that briefer and faster interceptive action makes it difficult to correct movements after a motor pattern generator produces a final planned movement (Tresilian, 2005; Marinovic et al., 2008, 2009, 2010a). Marinovic et al. (2008, 2010b) reported that information about movement amplitude and direction in a brief interception must be provided at least 150– 200 ms and 250 ms prior to movement onset, respectively. Also, Teixeira, Lima, and Franzoni (2005) found that participants needed 300 ms to target arrival to correct movement timing to the moving target with unexpected velocity changes. Thus, correcting threshold was assumed to be around 150– 300 ms before the final ballistic movement onset. In the present study, the time to target arrival after changing is 500 ms in the temporal condition and 250 ms in the spatial condition, as shown in Table 1. These conditions allow enough time for batters to correct their movement. Moreover, this value can be reduced by motor preparation in advance (Marinovic et al., 2010a) and experts in fastball play show shorter visuomotor delays than novices (Runigo, Benguigui, & Bardy, 2010; Runigo et al., 2005). For these reasons, decreasing the swing velocity in the changed condition indicates that batters correct their motor plan before the motor pattern generator produces the final planned movement (final stage of motor preparation: Marinovic et al., 2008). On the other hand, in general, baseball batting is classified as a rapid interceptive action because the duration of the bat swing (from forward swing initiation to impact) is less than 160 ms. Taking these factors into account, batters could be modifying their motor plan up until the final decision is made to produce the planned movement (i.e., until forward swing initiation). However, the slower swing to the decreasing target appeared in the normal condition, not in the weighted condition. This finding indicates that after practice swings with weighted bats, baseball batters could not adjust their movement duration by slowing swing velocity up until the final decision. In fact, batters showed an early response tendency in the changed velocity condition, and it was evident in the weighted condition (normal: 102 ms, weighted: 120 ms). From these results, we conclude that the acute effect of the KA that selectively influences the movement timing correction process is caused by the failure to decrease swing velocity by altering the preprogrammed motor command. As in the case of subjective perception, temporal errors were not largely modulated by dynamic wielding (i.e., the recalibration condition). Previous studies of the effects on swing velocity of warm-ups with various weighted tools have reported that changing the latter results in alterations of muscle strength (e.g., Reyes & Dolny, 2009) and
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swing motor patterns (e.g., Southard & Groomer, 2003). Other, complex training procedures use various resistances to increase power output by improving muscle power (Baker, 2003; Gourgoulis, Aggeloussis, Kasimatis, Mavromatis, & Gara, 2003; Young, Jenner, & Griffiths, 1998). According to these inquiries, faster swings after the practice swing with weighted bats result in the changing of muscle force generation. The selective effect, however, does not sufficiently explain peripheral system adaptation because these changes affect swing velocity irrespective of stimulus conditions. Another possibility is that the selective effect is the result of central system influences. When substantial velocity changes occur in a moving target, the performer should considerably correct his swing in response. In this case, movement reprogramming that includes the motor inhibition of prepared response (Neubert, Mars, Buch, Olivier, & Rushworth, 2010; Sharp et al., 2010) and a re-specification for a new response (Larish & Stelmach, 1982; Leuthold & Jentzsch, 2002; Mars, Piekema, Coles, Hulstijn, & Toni, 2007) is needed for successful interceptive action (Teixeira, Chua, Nagelkerke, & Franks, 2006). Although the correction strategy with inhibition may not be effective due to difficulty and the additional time required for processing (Coxon, Stinear, & Byblow, 2007), previous studies have reported that inhibition is strongly associated with success in baseball batting (Gray, 2009; Nakamoto & Mori, 2008). The success or failure in inhibiting the motor response is related to the termination timing of the motor producing and inhibiting process. If the processing of motor command production reaches the point of no return before the processing of the inhibiting motor command, then the response cannot be successfully inhibited (Boucher, Palmeri, Logan, & Schall, 2007; Logan, Cowan, & Davis, 1984; Marinovic et al., 2009). Thus, earlier motor production is associated with a high probability of inhibiting failure. Scott and Gray (2010) reported that batters swing earlier and faster in response to a change in bat weight. Based on these findings, the failure to decrease swing velocity under velocity-changed tasks only in the weighted condition is explained by the inability of the motor inhibition required by movement correction. Scott and Gray (2010) have stated that higher swing velocity is induced by changes in a tool’s weight that negatively affect motor inhibition, thereby producing larger errors. This plausible explanation, however, is not supported by the spatial error under the weighted condition. If the KA induces the lack of motor inhibition, movement correction for spatial change would also occur. As for other possible explanations from the central system, the early response tendency under the weighted condition supports our hypothesis that the KA affects effector anticipation, and that therefore interceptive timing will change. From computational theory, sensory awareness is increased when the actual sensation (i.e., afferent information) mismatches predicted sensations (i.e., efference copy) and vice versa (e.g., Blakemore, Frith, & Wolpert, 1999; Blakemore, Wolpert, & Frith, 1998). According to this notion, the subjective perception of faster swings after swings in the weighted condition indicates that batters formulated the efference copy that predicted a slower swing than that of the actual swing velocity. In other words, the practice swing with a weighted bat distorted the formulation of the efference copy. Previous studies have shown that because of the inevitable delays associated with the use of actual sensory information for movement correction in fast ball sports (McLeod, 1987; Runigo et al., 2005), the performer adopts predictive mechanisms (i.e., internal model) to execute rapid movement correction of visual targets (De Azevedo Neto & Teixeira, 2009, 2011; Zago et al., 2009). In this case, such a prediction involves a continuous estimation of the actual state of the system by using an efference copy in relation to the desired one, an assessment achieved by using rapid, internal (central) feedback loops (Sabes, 2000). A comparator then makes comparisons between this predicted feedback and actual feedback, and these appraisals are used for online automatic movement adjustments, cancelling sensory reafference and improving movement prediction and planning (Leonard, Gritsenko, Ouckama, & Stapley, 2011; Nijhof, 2003; Wolpert, 1997; Wolpert & Kawato, 1998). Thus, movement correction is accomplished by efference copy information rather than actual information. These considerations indicate that batters cannot correctly modulate their interceptive timing under the changed task because of inadequate error detection that contributes to the distortion of efference copy in the internal comparator, thereby producing faster swings and larger errors. In contrast, under the unchanged task, they do not need to correct swings that rely heavily on the efference copy. On the other hand, although there were no significant differences among warm-up conditions in the unchanged task, temporal error decreased in the weighted and recalibration conditions.
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Scott and Gray (2010) reported that the timing error was significantly larger after the simulation task with a weighted bat under nearly constant velocity conditions. The difference in the significance level between the present study and the study of Scott and Gray (2010) under unchanged conditions may be due to the exposure time of swinging with the weighted bat. Sage (1984) stated that the KA effect changes depending on the precedent handling time. Scott and Gray’s experiment had longer handling time with weighted tools than the present study and included direct interaction with the moving object before the standard bat condition. Therefore, the KA effect in our study was expected to be smaller than that reported previously by Scott and Gray (2010). The faster swing (i.e., short duration) in the present study produced smaller and larger errors in the unchanged and changed tasks, respectively, because participants often show late response errors and early response errors in unchanged and changed tasks, respectively. This tendency may reveal that the effect of handling weighted bats was simply to program faster movements that could not be adjusted online (Marinovic et al., 2008, 2009, 2010b). Taken together, the results suggest that the KA affects tasks that rely heavily on anticipation. That is, KA affects movement (re) programming because early response tendencies were induced after practice swings with a weighted bat. In conclusion, the KA adversely affects perceptual-motor control in dynamic situations that rely heavily on anticipation. This degradation of interceptive performance results from the interference in the central system based on efference copy and/or pre-programmed motor commands in the interceptive action process. This finding indicates that the adverse effect of the KA was not an intentional strategy change but rather an unintentional internal processing change. Previous studies that have investigated the relationship between the KA and athletic performance have reported that performance did not improve in basketball handling and shooting (Lindeburg & Hewitt, 1965), jumping (Stockholm & Nelson, 1965), and motor control (Nelson & Nofsinger, 1965). In addition to these studies, we demonstrate that the KA adversely affects the movement (re)programming processes. Moreover, we conclude that performers cannot adequately exert perceptual-motor control solely on the basis of practice swings in interacting with moving objects. Therefore, warm-ups with weighted tools are not recommended in actual athletic situations; however, the effects of lighter tools are not included in this finding. References Baker, D. (2003). 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