Performance comparisons of the kicking of stationary and rolling balls

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Performance comparisons of the kicking of stationary and rolling balls in a futsal context Fabio A. Barbieri Sergio A. Cunha

a b

a

c

, Lilian T. B. Gobbi , Paulo R. P. Santiago &

d

a

UNESP – Posture and Gait Studies Lab, São Paulo State University at Rio Claro SP, Brazil b

UFSCAR – Federal University of São Carlos SP, Brazil

c

UFG – Federal University of Goiania GO, Brazil

d

UNICAMP – Biomechanical Instrumentation Lab, State University of Campinas SP, Campinas, Brazil Available online: 18 Mar 2010

To cite this article: Fabio A. Barbieri, Lilian T. B. Gobbi, Paulo R. P. Santiago & Sergio A. Cunha (2010): Performance comparisons of the kicking of stationary and rolling balls in a futsal context, Sports Biomechanics, 9:1, 1-15 To link to this article: http://dx.doi.org/10.1080/14763141003690211

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Sports Biomechanics March 2010; 9(1): 1–15

Performance comparisons of the kicking of stationary and rolling balls in a futsal context

Downloaded by [Vrije Universiteit Amsterdam] at 03:05 09 August 2011

FABIO A. BARBIERI1,2, LILIAN T. B. GOBBI1, PAULO R. P. SANTIAGO3, & SERGIO A. CUNHA4 1

UNESP – Posture and Gait Studies Lab, Saõ Paulo State University at Rio Claro SP, Brazil, UFSCAR – Federal University of Saõ Carlos SP, Brazil, 3UFG – Federal University of Goiania GO, Brazil, and 4UNICAMP – Biomechanical Instrumentation Lab, State University of Campinas SP, Campinas, Brazil 2

(Received 19 August 2009; revised 3 December 2009; accepted 8 December 2009)

Abstract Angular kinematics of the hip, knee, and ankle joints, as well as ball velocity and accuracy for stationary and rolling balls were compared in a futsal (Five-a-Side Indoor soccer) context. Ten futsal athletes performed five kicks each on stationary and rolling futsal balls. Six digital cameras (120 Hz) recorded the kicks. For both kick types, angles for hip, knee, and ankle joints were calculated using Euler angle conventions. Angular velocity, ball velocity, foot linear velocity, relative velocity, and accuracy also were analyzed. The kicking of both stationary and rolling balls showed similarities for ball velocity (24.2 ^ 2.2 m/s and 23.8 ^ 2.7 m/s, respectively), foot velocity (17.6 ^ 1.8 m/s and 17.2 ^ 2.2 m/s, respectively), and accuracy (26% and 24% target hits, respectively). We observed few differences in angular kinematics and angular joint velocities between kick types. Elite players can make online adjustments in the preparatory phase so that kicking a rolling ball is almost exactly like kicking a stationary ball.

Keywords: Angular kinematics, futsal, kicking a rolling ball, performance

Introduction Kicking performance is crucial for goal scoring in futsal. Performance varies according to whether the dominant or non-dominant limb is used; the height of ball travel (i.e. groundkick, half-volley or volley); the position of the ball: stationary (free kick) or rolling (in play); and whether the purpose of the kick is to deliver power or accuracy. A ball kicked while rolling close to the ground is the most frequent type of kick in futsal (Barbieri et al., 2008). Approximately 69% of goal shots are made from this approach, while players make goal shots by kicking a stationary ball 25% of the time (Ferreira, 1999). However, only 8% of rolling ball kicks results in a goal. Therefore, it is assumed that a better understanding of the differences between the kicking of stationary and rolling balls will lead to increased goal Correspondence: Lilian T. B. Gobbi, Saõ Paulo State University – UNESP – IB – Rio Claro Posture and Gait Studies Lab – Physical Education Department, Avenida 24-A, 1515 – Bela Vista–CEP: 13.506-900, Rio Claro/Saõ Paulo/Brazil. E-mail: [email protected] ISSN 1476-3141 print/ISSN 1752-6116 online q 2010 Taylor & Francis DOI: 10.1080/14763141003690211


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F. A. Barbieri et al.

production. More specifically, does the kicking of stationary and rolling balls rely on a particular motor skill that changes according to the ball’s movement features, or are different motor skills involved for each task? Kicking a rolling ball is a more complex task than kicking a stationary ball. Kicking a moving ball requires a high level of prospective control, and players must control the timing of the initiation of the movement and its speed, that is, perceptual skill and successful motor coordination (Egan et al., 2007). Kicking a rolling ball is a quick yet calculated response to a ball that is passed by a teammate. It requires the perception of surrounding environmental information in an effort to modulate motor system responses relative to the features of ball movement, such as velocity and position. In only two studies in the literature (Tol et al., 2002; Egan et al., 2007) have researchers compared stationary to rolling ball kicks. Tol et al. (2002) focused on the types of injuries sustained during kick performance rather than on the control of successful goal kicks. They found no differences in ball velocity between the kicking of stationary and rolling balls. Egan et al. (2007) investigated the effects of experience in the kicking of a stationary ball and a moving ball, as it related to accuracy. Results indicated that the experienced participants were more accurate than were their less experienced counterparts, whereas no differences were observed for maximum foot velocity between groups or between conditions. A proximodistal initiation sequence in the kicking leg was observed, which suggests that players can acquire this coordination pattern with relatively little structured practice. Further practice seems to improve players’ skills, possibly through the increased exploitation of passive dynamics, observed in rapid knee flexion during attempts to increase accuracy rather than velocity. The main purpose of this study was to analyze the spatial-temporal parameters of the two types of kicking that are typically used in the playing of futsal: the kicking of a stationary ball and the kicking of a rolling ball. Specifically, we compared ball velocity, accuracy, and angular kinematics between kick types. We expected to observe similarities for ball velocity and for foot velocity, as well as differences for angular kinematics, such as the leading differences in relative velocity and accuracy. On the other hand, if we could observe similar spatial-temporal parameters between kick types, we could assume that the same motor program is used to perform both types of kicks. Methods Participants Ten experienced futsal amateurs players (aged 21.8 ^ 2.2 years; body mass ¼ 73.3 ^ 4.1 kg; height ¼ 1.75 ^ 0.04 m) from a Brazilian state team volunteered to participate in the study. All participants had played in competitive matches for a minimum of 36 months (68.4 ^ 34.4 months). Participants trained for futsal four times per week and also played one or two competitive games per week during the season. Five players were preferred right-foot kickers and five were preferred left-foot kickers. Foot dominance was defined by a questionnaire (Coren, 1993) and confirmed by a practice test. All players were informed of the experimental protocol and each signed a written consent form to participate in the study. The protocol was approved by the Research Ethics Committee of Sao Paulo State University (UNESP), Rio Claro. Experimental procedures Before the experimental testing began, the participants followed a 20-minute warmup protocol (10-minute stretching exercises followed by 10-minute running and kicking


Kicking of stationary and rolling balls

3

balls). Kicking order was randomly assigned for each participant. The kicks were performed at a distance of 10 m from the goal (kick-off point), with the objective of hitting a target (1 £ 1 m), which was centered between the goal posts. Participants were instructed to perform a maximal ball velocity instep kick. A FIFA-approved ball (mass ¼ 400 g; air pressure ¼ 0.5 atm) was used. Run-up was individualized for each participant, with no limitations on strides or the angle of approach. For stationary ball kicking, we placed the ball at the kick-off point. For the kicking of a rolling ball, the ball was rolled down a small ramp (height ¼ 0.40 m; length ¼ 0.70 m) in order to standardize its direction, position, and velocity. The ramp was placed at a 458 angle, 2.7 m from the kick-off place (Figure 1). The ball rolled toward the participant at a velocity of 2.2 m/s. The participants kicked the rolling ball when the ball crossed over the kick-off point. They performed two blocks of five kicks on stationary balls and five kicks on rolling futsal balls, without a rest interval between kicks. All participants rested between blocks, ranging from 30 seconds to two minutes. An official futsal pitch (FIFA standard) was used to solicit the kicks in order to simulate real game conditions. Six digital cameras ( JVCw GR9800u model; NTSC standard) at 120 Hz (shutter velocity: 1/250 s) were used to capture the passive markers (15 mm in diameter), which were placed externally on the participants’ lower limbs at the following anatomic landmarks: anterior superior iliac spine; posterior superior iliac spine; greater femoral trochanter; lateral and medial femoral epicondyle; head of fibula; lateral and medial malleolus; calcaneus; cuboid bone; and toe-distal end of the fifth metatarsal (Figure 2). One additional camera (60 Hz) was placed perpendicularly to the goal and it was used to analyze kick accuracy. A calibration frame (1 £ 1.5 £ 2.5 m) was used to calibrate the measurement area. The 3-D coordinates were expressed as right-handed, and the z-axis was vertical (pointing upward); the y-axis was horizontal and pointed toward the center of the goal (orthogonal to z-axis and the goal line); and the x-axis was the vector product of y and z. Before starting to record the kicks, the posture of each participant was recorded in an orthostatic position (anatomical) for several seconds in order to define the neutral positions of the hip, knee, and ankle joints.

Figure 1. Frontal view of ramp placement for the kicking of a rolling ball.


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Figure 2. Marker position. ASIS ¼ anterior superior iliac spine; PSIS ¼ posterior superior iliac spine; GFT ¼ greater femoral trochanter; LFE ¼ lateral femoral epicondyle; MFE ¼ medial femoral epicondyle; HF ¼ head of fibula; LM ¼ lateral malleolus; MM ¼ medial malleolus; CA ¼ calcaneus; CB ¼ cuboid bone; TEFM ¼ toe-distal end of the fifth metatarsal.

The kicking cycle was analyzed from toe-off of the kicking limb through the foot’s contact (swinging movement) with the ball (ball contact) (Figure 3). We recognized that ball impact can affect angular kinematics data. This is why the kinematics data were analyzed before ball contact. DVIDEOW software (Figueroa et al., 2003) was used for the synchronization, measurement, calibration, and 3-D reconstruction of the image sequences. The Direct Linear Transformation method (Abdel-Aziz and Karara, 1971) was integrated into this software and was used to obtain the 3-D coordinates of each marker. To evaluate experimental errors, an accuracy test was performed. The movements of a rigid rod were recorded to calculate accuracy: two 15 mm-diameter markers were placed at each extremity of a rigid rod at a known distance apart (direct measurement). As described previously, the 3-D coordinates of each marker were obtained. The Euclidean

Figure 3. Support phase (SP) (65% of the movement) – from toe-off of the kicking limb to full contact of the entire shoe sole with the ground; Contact phase (CP) (35% of the movement) from full contact of the entire shoe sole with the ground to the kicking limb’s point of contact with the ball.


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distance (Equation 1) between the markers was calculated for each time instant (measurement values), using the following equation: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 ð1Þ dðiÞ ¼ x2 ðiÞ 2 x1 ði Þ þ y2 ði Þ 2 y1 ði Þ þ z2 ði Þ 2 z1 ði Þ Where d(i ) is the Euclidean distance between the two markers: x1, y1, and z1 represent the first marker coordinates, and x2, y2 and z2 represent the second marker coordinates; i ¼ ranging from 1 to n, where n is the number of measurements performed in the calculated distance between the two markers. The study accuracy was calculated through systematic (bias) and random (precision) error, using equations 2, 3, and 4: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n 2 1 X dðiÞ 2 m P¼ ð2Þ n 2 1 i¼1 B¼m2m

ð3Þ

A2 ¼ B2 þ P2

ð4Þ

In the equations above: d(i ) is the Euclidean distance between the two markers; i ¼ the range from 1 to n, in which n is the number of measurements performed in the calculated distance between the two markers; A is the accuracy (Equation 2); B is the bias (Equation 3); P is the precision (Equation 4); m is mean value of the n measurement; m is the real value of the distance between the two markers by direct measurement. The calculated accuracy value of this study was 7.9 mm ^ 1.3 mm; precision was 6.1 mm ^ 0.9 mm, and bias was 5.0 mm ^ 0.9 mm. Locally weighted scatter plot smoothing (LOESS – quadratic fit) nonparametric regression (Cleveland, 1979; Bracken and Holford, 2002; Cunha and Lima Filho, 2003; Barbieri et al., 2008) was used to filter the data. The value of the smoothing parameter (span ¼ 0.7) was chosen after a residual analysis of the results. The LOESS function was an appropriate procedure since it is sufficiently robust to identify outliers and to correct data weight (Cleveland, 1979). Also, it did not require a specific function to fit a model of all the data in the sample. The LOESS smooth was suitable because it assured the derivatives (velocity and acceleration) with minimal noise (Bracken and Holford, 2002). Data analysis The angular kinematics was analyzed in all kicks, independent of the target. Kinematic 3-D analysis requires modeling of the human body (Cappozzo et al., 1995). In our study, the lower limbs were considered to be rigid “bodies” linked by joints. To describe the orientation of the segment in space, at least three non-collinear points for each segment were necessary to reproduce the 3-D position. Thus, the pelvis, thigh, leg, and foot of the kicking limb were studied. To orientate the segments, an orthonormal basis, adapted from Cappozzo et al. (1995), was defined for each segment and determined by vectors i (transversal vector), j (sagittal vector), and k (longitudinal vector) (Levanon and Dapena, 1998). The joint angles were calculated by means of Euler angle convention. In this study, the three rotations occurred in relation to the three independent axes, which followed the sequence i, j’

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Table I. Direction of anatomically relevant references for hip, knee, and ankle joints. Joint

Positive values (þ)

Negative values (2)

Hip

Flexion Adduction Internal rotation

Extension Abduction External rotation

Knee

Extension Internal rotation

Flexion External rotation

Ankle

Dorsiflexion Adduction Supination

Plantar flexion Abduction Pronation

(already rotated initially), and k’’ (already rotated in the two previous operations). The three independent axes were used to calculate the angles, which allowed simultaneous measurement of three independent angular movements related to three spatially orientated axes of rotation (Chao, 1980). The joint angles were defined as rotations around the proximal segment, coinciding with the rotations in the distal segment (Chao, 1980; Zatsiorsky, 1998). The positive and negative values correspond to the clockwise and counterclockwise rotations, respectively (Table I). From the joint angles, the angular velocity was calculated as the first derivative angle as a function of time for the finite difference for each joint (Zatsiorsky, 1998), expressed in rads/s (Do¨rge et al., 2002). Also selected was the angular velocity for each joint motion axis at the point of ball contact for each kick. For ball velocity, 3-D coordinates were analyzed for the 10 frames immediately following ball contact. To calculate linear foot velocity, the 3-D coordinates were plotted for the toe-distal end of the fifth metatarsal marker throughout full movement. For ball velocity and foot velocity, the horizontal components (x- and y-axes) were calculated as the first derivative of linear regression lines, fitted to their non-filtered displacements. The vertical component (z-axis) of the ball was calculated as the first derivative of a quadratic regression line, with its second derivative set equal to 2 9.81 m/s and fitted to its non-filtered displacement in the available airborne frames. Therefore, we calculated the relative velocity between foot and ball at the ball contact point. For each kicking, the relative velocity resulted from the summation of the foot velocity and ball velocity at the ball contact moment (Greenwood, 1987). Accuracy was determined as a percentage of successful hits of the target. Statistical treatment The angular kinematics of the kicking of stationary and rolling balls was calculated from median and respective confidence intervals at the joint angle axes for each time instant. Median was used to prevent the outliers’ effect while the confidence interval was used to verify differences between kick types (MacGill et al., 1978). The parameter of confidence interval was 95%. Median and confidence interval were calculated from the each joint angle at each instant of time, since the kicks were normalized with respect to duration. Therefore, for example, for flexion and knee extension for kicking a stationary ball, at each instant of time, it was calculated the median and confidence interval of 50 kicks, creating a curve that represents the movement of players. Differences in angular kinematics were considered significant when the confidence intervals for kicking stationary and rolling balls overlapped. Angular joint velocity was also analyzed in this manner. A specific algorithm using Matlabw 6.5 software functions was written to analyze these variables for overlapping. The Shapiro-Wilk test confirmed the data’s normal distribution. A paired t-test was used to examine the differences between the kicking of a stationary ball and a rolling ball in ball


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velocity, foot velocity, relative velocity, and in percentage of accuracy. Linear regressions were employed to verify the relationship between foot velocity and ball velocity and kick type. The Pearson correlation coefficient was used to verify the relation between angular velocity (at point of ball contact at each joint-motion axis), foot velocity, and ball velocity. An alpha level of 0.05 was used in all statistical analyses, which employed SPSS 10.0 (SPSS, Inc.) software.


Results Angular motion and velocity values of the hip joint were similar in flexion and extension and for internal and external rotation movements between the kicking of a stationary and a rolling ball. For the adduction and abduction movement, however, a significant difference (P , 0.05) was found (Figure 4). Angular motion was significantly different between kick types from 79% to 98% of the kicking cycle while angular velocity was significantly different from 52% to 82%. No differences were found in angular knee motion and velocity in flexion and extension between kick types. In the same way, the internal and external rotation movements were similar between the kicking of a stationary and a rolling ball (Figure 5). The angular ankle motion during the entire kicking cycle was similar in adduction and abduction and in internal and external rotation movements for both kick types. A significant difference (P , 0.05) was found only in the plantar flexion and dorsiflexion movements: 91% to 100% of the kicking cycle. For angular velocity, a difference in longitudinal plane was found: 1% to 13% and 81% to 100% of the cycle (Figure 6). Means, standard deviations, maximum and minimum values for ball velocity, foot velocity, and relative velocity between foot and ball at the point of ball contact are presented in Table II. A paired t-test did not reveal significant differences between kick types for ball velocity (t49 ¼ 1.65, P ¼ 0.10) and foot velocity (t49 ¼ 1.92, P ¼ 0.60). Relative velocity between ball and foot at the contact point was higher for the kicking of a rolling ball (t49 ¼ 2 9.9, P , 0.0001) than for the kicking of a stationary ball. No significant difference between kick types was found for accuracy (26% for kicking a stationary ball and 24% for kicking a rolling ball) in successful target hits (t49 ¼ 0.31, P ¼ 0.39). Furthermore, we found a relationship (Figure 7) between ball velocity and linear foot velocity for the kicking of both stationary (r ¼ 0.76; P , 0.001) and rolling balls (r ¼ 0.67; P , 0.001). However, we observed no significant correlations between ball velocity and the other dependent variables (Table III). Discussion and implications With regard to differences in relative velocity and accuracy, we expected to observe similarities for ball velocity and foot velocity, as well as differences for angular kinematics. Our results partially confirmed our expectations. There was no difference between kick types for ball velocity, linear foot velocity, and accuracy, which are in agreement with other studies (Tol et al., 2002; Egan et al., 2007). Few differences in angular kinematics and in angular joint velocities were observed between kick types. The relative velocity between foot and ball at the ball contact revealed a kick type effect. Few differences in angular kinematics can explain the slow relative velocity between foot and ball at the ball contact for the stationary ball condition. The differences occurred at the instant of ball contact, which is a decisive point during the kicking process. The joint position at ball impact is fundamental in guaranteeing the success of the kick (Barfield et al., 2002; Do¨rge et al., 2002). Small adjustments were observed in our participants’ movement patterns as they performed both



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Figure 4. Median and confidence intervals for hip angles (left column) and angular velocity (right column) during the kicking of a stationary and a rolling ball. The continuous line represents the kicking of a rolling ball, and the discontinuous line represents the kicking of a stationary ball. SP is the support phase, and CP is the contact phase. Vertical dashed line indicates the CP starts.



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Figure 5. Median and confidence intervals found for knee angles (left column) and angular velocity (right column) during the kicking of a stationary and a rolling ball. The continuous line represents the kicking of a rolling ball, and the discontinuous line represents the kicking of a stationary ball. SP is the support phase and CP is the contact phase. Vertical dashed line indicates the CP starts.

kicks. These adjustments may have been due to the imposed task constraints. Schmidt and Wrisberg (2000) claimed that a generalized motor program is used to perform both kick types; this appears to explain the similarities we found between the kicking of a stationary ball and a rolling ball. The kicks show an invariant temporal organization, which also confirm the absence of differences in accuracy. More specifically, regarding these behavioral aspects, since the kick is an open skill (Schmidt and Wrisberg, 2000) and a ballistic movement (Winter, 1990), fine adjustments are required in order to adapt the behavior according to environmental conditions, to improve accuracy (Barfield et al., 2002). Fine adjustments are exact and precise adaptations that take place during the skill performance (Patla, 2003). Participants seemed to adjust their behaviors in the hip and ankle joints. The hip motion in the sagittal plane assists the action of foot contact with the ball, facilitating the position of the adjacent segments in order for the proximal joint action to occur (Figure 4). The ankle motion in the transversal plane can increase contact between the foot and the ball with a larger plantar flexion and facilitate fine



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Figure 6. Median and confidence intervals found for ankle angles (left column) and angular velocity (right column) during the kicking of a stationary and a rolling ball. The continuous line represents kicking of a rolling ball, and the discontinuous line represents the kicking a stationary ball. SP is the support phase, and CP is the contact phase. Vertical dashed line indicates the CP starts.


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Table II. Means, standard deviations, maximum and minimum values for ball velocity, foot velocity, and relative velocity between foot and ball at the point of ball contact. Stationary ball

Rolling ball

Ball velocity Mean ^s (m/s) Minimum (m/s) Maximum (m/s)

24.2 ^ 2.2 19.5 29.7

23.8 ^ 2.7 16.3 29.5

Linear foot velocity Mean ^s (m/s) Minimum (m/s) Maximum (m/s)

17.6 ^ 1.8 13.5 21.3

17.2 ^ 2.2 13.7 21.6

Relative velocity Mean ^s (m/s) Minimum (m/s) Maximum (m/s)

17.6 ^ 1.8* 13.5 21.3

19.4 ^ 2.2 15.7 23.6

*Difference between stationary and rolling ball (P , 0.001).

Figure 7. Linear regression between ball velocity and foot velocity according to kick type. † kicking a stationary ball; A kicking a rolling ball; . . . . . . linear regression for the kicking of a stationary ball; – –- linear regression for the kicking of a rolling ball.

adjustments at this instant in the kicking process (Figure 6). Larger contact area between the foot and the ball improves the player performance (Barfield et al., 2002; Do¨rge et al., 2002). Our results are in agreement with this assertion, that is, different fine adjustments at the football contact were used for each kick in order to maintain the accuracy. Therefore, the football contact moment deserves attention from coaches for both teaching and correcting the player’s performance. Kicking a ball with maximum force is related to an individual’s capacity and ability to efficiently transfer forces throughout the linked segments of the body. For angular velocity, differences were found at the hip sagittal plane, beginning with the final support phase and continuing throughout approximately 80% of the kick cycle (Figure 4) and starting at the ankle longitudinal plane at the beginning of the support phase through the end of the contact phase (Figure 6). At moments where the differences between kick types were observed, the kicking of a rolling ball resulted in higher velocity than did the kicking of a stationary ball. The production of higher joint angular velocity was not advantageous with regard to

Ball velocity Foot velocity

Rolling ball

0.59 0.57 0.27 0.22

20.01 0.07 20.17 20.01

20.05 20.06

0.23 0.29

IEH

0.25 0.16

0.31 0.50

FEK

0.12 0.09

20.10 0.06

IEK

20.38 20.07

20.42 20.37

DFA

0.01 20.01

20.10 0.08

AAA

0.06 0.11

0.32 0.46

SPA

FEH, angular hip flexion and extension velocity; AAH, angular hip adduction and abduction velocity; IEH, angular hip internal and external rotation velocity; FEK, angular knee flexion and extension velocity; IEK, angular knee internal and external rotation velocity; DFA, angular ankle dorsiflexion and plantar flexion velocity; AAA, angular ankle adduction and abduction velocity; SPA, angular ankle supination and pronation velocity.

Ball velocity Foot velocity

Stationary ball

AAH

FEH

Table III. Correlation (r values) for ball velocity and foot velocity for each angular joint velocity at point of ball contact, by kick type.


12 F. A. Barbieri et al.



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accuracy. In this case, inter-segmental integration of the kicking limb is more important than is high joint angular velocity (Putnam, 1991). The combination of behavioral aspects and environmental variations can change kick performance. While the kicking of a rolling ball is more complex than is the kicking of a stationary ball, with regard to accuracy both tasks can be considered difficult for players. In the actual game and in training situations, a goalkeeper must be avoided in order for the player to score a goal. However, the objective in our experimental setting was simply for the player to hit the target. For the kicking of the stationary ball, the environmental variations in our experimental setting were minor; therefore the correspondent action, accordingly, was the planned action. In this case, the action control occurred in a closed-loop form (Rosenbaum, 1991; Shumway-Cook and Woollacott, 1995), and the subject was able to use visual feedback in order to make the necessary adjustments. For the kicking of the rolling ball, the player was required to predict the ball’s position and to then anticipate the required action accordingly, with the reference changing from moment to moment. This kick type was controlled by feedforward mechanisms (Rosenbaum, 1991; Shumway-Cook and Woollacott, 1995), where fine adjustments driven by sensory signs are crucial to a successful kick. The angular motion and angular velocity results revealed that the athletes used the feedforward mechanisms to perform the kicking of the rolling ball. The relevant information for this type of kick is the ball position. Coaches should teach players how to use this information to fine tune the effector system in order to improve their kick performance. Coupling between sensory information and motor action was necessary to improve kick performance (Savelsbergh et al., 2002). Our results allowed us to infer that the kicks were controlled in a closed-loop form, except at the moment of contact with the ball during the kicking of a rolling ball. Similarly, during adaptive locomotion (Patla, 1998; Mohagheghi et al., 2004), kicking movements are planned in the same way, with fine adjustments achieved at the hip and ankle joints during the moment of ball contact. The fine adjustments that resulted from sensorimotor coupling during the kicking of a rolling ball were reinforced by differences in the angular joint velocities, revealed in the results for the two kick types. The positive relationship between ball velocity and kick foot velocity in the kicking of the stationary ball is well documented in the literature (Nunome et al., 2002; Apriantono et al., 2006). To date this is the first study showing this relationship in the kicking of the rolling ball and it highlights the role of fine adjustments in this activity. However, the relationship between ball velocity and kick foot velocity appears to be stronger for the kicking of a stationary ball than for that of a rolling ball. This was expected, because during the kicking of a stationary ball, almost foot velocity is transferred to the ball, which is in agreement with the modeling results of Asai et al. (2005). In the kicking of a rolling ball, foot velocity must also counter-act the opposing motion. The purpose of fine adjustments in this more complex task may be the body’s attempt to control and maintain the transference of the foot velocity force to the rolling ball. Conclusions We concluded that there were no significant differences for ball velocity, linear foot velocity, or accuracy between the kicking of a stationary and a rolling ball. However, angular kinematics and the angular joint velocities during the kicking cycle were sensible to discriminate kick types. The relative velocity between foot and ball at ball contact was

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significant higher for the kicking of the rolling ball than for the kicking of the stationary ball. Our findings revealed a positive and high correlation between foot velocity and ball velocity for both types of kicks. Data analysis indicated that subjects made fine adjustments dependent on kick type. Elite players can make online adjustments in the preparatory phase so that kicking a rolling ball is almost exactly like kicking a stationary ball. Acknowledgements


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