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THE BIOMECHANICS OF KICKING IN SOCCER
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THE BIOMECHANICS OF KICKING IN SOCCER William R. Barfield, PhD
DEVELOPMENT OF KICKING SKILL
Of all the skills required to participate in soccer, none has received more attention from a biomechanical perspective than kicking. When skilled behavior, such as kicking, is evaluated there are two successive stages. Initially, movements are organized and simplified through powerful, temporal coupling between joint complexes so that the neuromuscular system becomes functionally useful for the learner. In the second stage, the learner's behavior becomes more efficient and organized as the active muscular and joint forces become more economical.'" Anderson and Sidaway3 suggested that the skilled movement of kicking will be enhanced and better organized with training, particularly if the objective is to maximize foot velocity, and in turn, resultant ball velocity. They believed that coordination, including summation of speed from the hip to knee seemed to be a great determinant in improved kicking and that unskilled players demonstrated less coordination than their skillful counterparts. As skill develops, linear foot velocity becomes greater through an increase in angular velocity at the knee without a concomitant increase in hip angular velocity. This suggests that practice provides economical organization for the learner by effectively reducing the numbers of degrees of f r e e d ~ m . ~ Kicking is a complex motor movement that follows generally predictable developmental stages. Although chronologic age is not necessarFrom the College of Charleston; and the Department of Orthopaedic Surgery, Medical University of South Carolina, Charleston, South Carolina
CLINICS IN SPORTS MEDICINE VOLUME 17 * NUMBER 4 OCTOBER 1998
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ily an accurate predictor of skill development, it appears that between the ages of 4 and 6 years, skill develops rapidly'l, 24 For boys and girls there appears to be a linear relationship between age and mature kicking skill between kindergarten and grade four.I5With skilled soccer players between the ages of 9 and 18 years, however, the timing pattern in kicking does not appear to differ ~ignificantly.~~ Differences between and within skill levels have been examined. When elite athletes are evaluated, the general consensus is that they will exhibit less mechanical variability and therefore will perform more consistently than less skilled athletes. However, Rodano and Tavana4'j found that even in highly skilled professional soccer players minimal variations in motor performances can negatively influence the characteristics of movement, when within subject variability is considered. Phill i p points ~ ~ ~ out that lack of variability is indicative of a more acutely refined motor program, whereby muscle contractions are temporally distinct. The alternative view is that when kicking, the variability and adaptability of the neuromuscular system and the mechanical interrelationships of skilled behavior are demonstrated best. When a skilled kicker was compared with a club player, the skilled kicker demonstrated less variability particularly with respect to angular positions during the kicking movement. Though there were no significant differences in resultant ball velocities between the skilled player and the club player, consistency of kicking by the skilled player was likely attributable to the two-step approach as opposed to the unrestricted inconsistent approach used by the club player, which subsequently led to greater standard deviations of kicking velocity.3y COMPONENTS OF KICKING
The process of ball kicking from a biomechanical perspective can be separated into components in various ways. This article defines kicking in soccer through six stages. They are: (1)approach angle, (2) plant foot forces, (3) swing limb loading, (4) flexion at the hip and extension at the knee, (5) foot contact with the ball, and (6) follow-through. Major differences between skilled and unskilled players are based on the relative percentage of movement that is devoted to each of the components. In unskilled players, ball kicking movement is generally poorly coordinated and dominated by the approach phase, whereas skilled players use approach movements, backswing, and forward swing movements to define the kicking movernent.'j*7, 36 Skilled athletes also appear to take longer strides as they approach the ball and the temporal proximity of selected events during the kicking movement (i.e., maximum hip extension and knee flexion) are more closely ass0ciated.l Approach Angle
When kicking, children between the ages of 3 and 5 years do not take any steps to the ball, and at earlier ages simply walk into the ball.
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As children mature chronologically a paced run-up is used, and the angle of the approach becomes more diagonal. Efficient segmental timing for the kick reveals that maximum angular velocity at the knee progressively increases with age during the preadolescent years, and the timing of maximum angular velocity at the knee corresponds more closely with ball contact. Though not all children follow traditional stages of development and there exists intragroup variability in similar age groups, there is a general developmental trend that most children tend to follow." Several authors have emphasized that the diagonal approach, as opposed to the straight approach, results in greater swing limb ~elocity.~, 29,31,40 P l a g e n h ~ f f believed ~~ that the two primary factors in determining ball velocity were (1) effective mass of the foot, and (2) foot velocity immediately before ball contact. His findings provided evidence that foot velocity before impact was similar with straight and angled approaches, yet the angled approach yielded greater ball velocity. This led to the conclusion that the primary differences in ball velocity were based on greater "effective mass of the foot" with an angled approach rather than foot velocity at ball contact.26,40 Isokawa and Leeszyshowed that peak ball velocity was greatest at an approach angle of 45 degrees although peak ankle velocity was greatest at a 30 degree approach angle. Peak velocity at the hip was greatest at a 15 degree approach angle, and peak knee velocity was greatest with the straight approach (0 degrees). The authors attribute these notable differences to increased striking mass and greater knee and ankle fixation at the 45 degree approach angle. In the straight-ahead approach, there is limited rotation of the leg about the vertical axis through the body. As the approach angle increases, however, the leg must rotate about a vertical axis to kick the ball straight. The resistance torque that is imparted to the vertical axis through the body by the ground reaction force (GRF) is reduced by the torque actively generated by the swing limb as the approach angle is increased. At an approach angle of 45 to 60 degrees, the player takes advantage of the active torque created by the swing limb, which completely balances the resistance torque generated by body motion, thereby increasing leg and foot mo38 mentum at ball contact.29, The position of the plant foot from a medio/lateral perspective with respect to the ball is well defined because if support foot position is located too far from the ball, the direction of the kick, and the kicker's body balance will be negatively compromised. Most research supports the belief that optimal support foot position is 5 to 10 cm to the left of the ball, assuming the kicker is kicking with the right foot.2hPlacement of the support foot alongside and adjacent the ball, perpendicular to an imaginary line drawn through the ball center appears to provide the most appropriate environment for a successful instep kick performance.' When skilled and unskilled players were compared, the skilled athletes placed the support foot alongside and closer the ball, whereas unskilled players tended to position the support foot behind the ball.y
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The position of the support foot from an anterior/posterior position is less well defined primarily because foot position will dictate ball flight. That is, the further behind the center of the ball the plant foot is placed the greater the likelihood that the ball will become flighted, which may or may not meet the kicker’s objective.26
Plant Foot Forces
The relationship between GRF on the support foot and predicted speed of a soccer kick has been investigated in skilled and unskilled populations. As expected, skilled players kicked faster than unskilled (25.9 ms-’ versus 23.4 ms-*) and the GRFs the skilled players exhibited vertically, anteriorly-posteriorly, and laterally were greater than among unskilled.zzDos Anjos and Adrian2zconcluded that some of the variance in greater speed of kicking among skilled players could be attributed to greater GRFs being generated by these subjects. Isokawa and Leesz9reported that as approach angle moved from 0 to 90 degrees peak vertical force varied minimally; however, peak frontal force and peak lateral force showed an anticipated inverse relationship, with frictional force oriented increasingly in a lateral direction as the approach angle neared 90 degrees. In a later study, Barfield8 found six medio/lateral force variables to be positively correlated with resultant ball velocity in dominant side kicking, yet none of the same variables, at the same approach angle, were correlated on the nondominant side. These variables included (1) maximum medio/lateral force, (2) time of maximum medio/lateral force, (3) medio/lateral force at ball contact, (4) mean medio/lateral force from support foot contact to ball contact, (5) mean medio/lateral force from support foot contact to maximum force, and (6) time between ball contact and maximum medio/lateral force. As demonstrated by the graphs in Figure 1, peak medio/lateral force occurred before peak anterior/posterior force, which is not an unexpected finding because body momentum will initially be blocked laterally owing to the angled approach. Medio/lateral forces reported by Barfield7(1.07 X body weight [BW]) and those reported by Rodano and Tavanag6(1.24 X BW) were similar in magnitude yet they found no significant correlations between GRFs and ball velocity. Subjects have been shown to exert greater forces vertically than medio/laterally or anterio/posteriorly, and in Abo-Abdo’s’ study, force exerted during movement in vertical and horizontal thrusting directions was significantly correlated with ”level of performance,” although level of performance was not quantified. The application point from these findings is that with an angled approach, when instep kicking for maximal effort, force that is generated from the approach path and plant foot position can and does influence the channeling of forces that eventually are used into propelling the ball.
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Time (sec) Figure 1. Representative trial depicting force platform data. F, = vertical, F, = medio/ lateral, F, = anterio/posterior. (Y = support foot contact; o = ball contact. (From Barfield WR: Biomechanics of kicking. In Garrett WE, Kirkendall DT (eds): Textbook of Sports Medicine. Baltimore, Williams 8, Wilkins, 1997, pp 86-94; with permission.)
Swing Limb Loading Consistent patterning of the lower limb segments from a temporal perspective seems to be an essential component for successful instep kicking motion.’ The initiation of forward movement begins with the kicking limb knee flexed and the kicking foot at approximately hip height. Swing limb motion and foot placement, during the loading phase, before flexion at the hip and extension at the knee is a function of age, skill level, ability, and the kicker’s distance and path to the As the plant foot strikes the ground adjacent the ball, the swing limb is concurrently involved in hip extension and knee flexion. During ”loading” of the swing limb the kicker’s eyes are focused on the ball and not the direction the ball will travel because ball direction is dictated by support foot position, and ultimately hip position at ball contact.’* The ”loading” of the swing limb allows the hip flexors and the knee extensors to be eccentrically stretched (loaded) in preparation for the forward movement of the limb into position to strike the ball. As demonstrated through Figure 2 from Robertson and Masher,@ the hip flexors and knee extensors show negative power between 0.00 and 0.07 s to halt the swing limb’s backswing. As the forward movement
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Figure 2. Hip (dashed line) and knee (solid line) angular velocity, net moment of force, and power during a soccer kick. FS = contralateral leg footstrike; HIT = ball contact; OFF = start of ball flight. (Modified from Robertson DGE, Mosher RE: Work and power of the leg muscles in soccer kicking. ln Winter DA, Norman RW, Wells RP, et al (eds): Biomechanics 1x43. Champaign, IL, Human Kinetics Publishers, 1985, pp 5 3 3 4 3 8 . )
of the kicking leg is initiated the activity of hip flexors and knee extensors becomes concentric changing the power vector from negative to positive.44 Before ball contact, a large extension torque at the knee (230 N-m) is produced to provide a rapid extension rate; however, the flexion torque (280 N m ) produced at, or immediately following ball contact is larger than the extensor torque. If flexion torque is initiated too early in the kicking movement the limb and kicking foot is slowed before contact, which subsequently decreases eventual ball velocity. Findings from Gainor et alZ5indicate that in kicking, 15% of the kinetic energy is
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transmitted to the ball, and the balance of energy must be expended by other mechanisms including activation of the knee flexors. Limiting the time available to dissipate stored energy, however, increases the muscle forces required to eventually slow the limb and places the kicker at risk for injury because of the large loads placed on the knee joint and soft tissues already under tension immediately following the When slow kicks (15.24 ms-*) were compared with medium (21.34 ms-') and fast kicks (27.42 ms-') knee extensor torque increased gradually with slow kicks and was maintained throughout knee extension. In medium and fast kicks, more effective coordination was required, and as one link decelerated a more distal link showed significant acceleration although knee extensor torque dropped quickly a few milliseconds before or at the initiation of knee e~tension.~'',51 Immediately before ball contact, the hamstrings (knee flexors) became dominant, which resulted in negative power as demonstrated by Figure 2 from Robertson and Mosher.44 Flexion at the Hip and Extension at the Knee
As the hip flexors forcefully contract to swing the thigh forward and downward, the leg (defined from knee to foot) and foot rotate as a unit. As thigh angular velocity decreases, the leg and foot begin to accelerate due to a momentum shift and support from elastic and contractile components in the knee extensors.26 The sequential segmental motion and concomitant decrease in angular velocity of the proximal segment while the distal segment increases 4f142 P l a g e n h ~ f fnoted ~ ~ in is thought to play a critical role in his research investigating angled versus straight approaches to the ball that kicks with lowest thigh deceleration showed greatest knee extension, which illustrates the strong influence of one segment on another. Generally, there is a significant positive relationship between linear foot velocity and resultant ball velocity.', 7, As seen in the two graphs on linear and angular velocity (Figs. 3 and 4, respectively),linear velocity at the ankle is close to maximum at ball contact as is angular velocity at the knee.8 Roberts and M e t ~ a l f efound ~ ~ that the primary factors that influence swing limb velocity are hip rotation followed by hip flexion and knee extension prior to impact. There appears to be an exchange in angular velocities between proximal and distal segments in kicking, which would indicate a transfer of momentum from the more massive thigh to the less massive legz7,43 Such claims, however, have not been confirmed, and there is evidence, based on segmental interaction analysis, that decreasing thigh angular velocity in kicking creates a less effective environment for an increase in leg angular velocity. The decrease in thigh angular velocity is thought by some to be the result of the leg's motion on the thigh.23,42 Putnam (1991) has shown that a decrease in thigh angular velocity results from a large hip flexor moment which counteracts the effect of the leg on the thigh, thereby limiting the loss of
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Figure 3. Representative trial depicting linear velocity of the toe (Ivt), knee, (Ivk), and hip (Ivh). Q = support foot contact; o = ball contact. (From Barfield WR: Biomechanics of kicking. ln Garrett WE, Kirkendall DT (eds): Textbook of Sports Medicine. Baltimore, Williams & Wilkins, 1997, pp 86-94; with permission.)
positive thigh angular velocity. The generally accepted thought that negative angular acceleration of a proximal segment leads to positive acceleration of an adjacent distal segment through the summation of speed principles cannot be supported by evidence from P ~ t n a mThese .~~ apparent contradictions concerning speed and moments relate to the fact
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that motion of a single segment within a linked system is nonlinear and cannot be attributed solely to muscle forces and moments working on that one segment, making quantification of separate segmental roles difficult to ascertain.42 DeProft and colleaguesz1examined the extent of agonist and antagonistic muscle function during a soccer kick. They reported that the extensors at the knee were likely to be most active during knee flexion, when they are antagonistic to the movement, and that skilled soccer player antagonistic muscle activity was greater when compared with nonskilled players. On the other hand, nonskilled players appear to have greater agonistic muscle action. Research concerning whether strength development improves per-
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formance has been well documented. The stretch-shortening cycle of strength training, when combined with soccer training, has been shown to result in greater functional kicking p e r f ~ r m a n c e . ~ ~ DeProft and associatesz0found that adult and youth (15.5 years of age) soccer players demonstrated significant increases in concentric hip flexor and eccentric hip extensor strength, eccentric hamstring strength (77%), and in concentric quadriceps strength (25%) in a study that evaluated the efficacy of a strength training program. Explosive muscle power tests (long jump, vertical jump, and single-legged triple jump) were also found to be significantly correlated with kick distance. These findings support a soccer training program that is enhanced with a strength program when the objective is to improve kick distance and functional power ability in youth and adult soccer players. When young adult male soccer players (mean = 22 years old) were compared with a control group, no statistically significant differences existed in hamstringlquadriceps torque when they were measured at 60°-s-', even though soccer players demonstrated values that were 5% to 10% greater than the controls. The concentric-eccentric ratio between quadricepslhamstrings was slightly lower for soccer players than for the controls, which would indicate that acceleration and deceleration demands of the game provide a suitable stimulus that allows for concurrent development of hamstring and quadriceps When decelerating from running, the hamstrings act eccentrically to slow extension at the knee and the quadriceps act eccentrically to control the lowering of body weight when players approach a stop. No differences existed between dominant and nondominant sides among soccer players indicating that training and matches generate sufficient bilateral exercise to prevent muscle imbalance and provide symmetry, which is an important aspect in soccer.1zBarfield: when testing skilled adult soccer players, found no significant differences between dominant and nondominant limbs in isokinetic testing at the hip and knee when subjects were evaluated at 180".s-'. These studies combined with another by Capranica and colleague^'^ seem to point to the importance of proper motor skill training and reasonable amounts of time spent developing strength. This two-level approach will provide the stimulus necessary to produce relatively equal levels of torque in preferred and nonpreferred limbs.17 Mognoni and colleagues34found that peak knee extension torque on the nondominant side was greater than on the dominant side. The authors attribute this seemingly paradoxical finding to the role the nondominant knee extensors play through eccentric support of body weight and torque generated by the swing limb. When prepubescent male (mean = 9.6 years of age) soccer players were compared with an untrained group for force and power, findings were similar to those reported by Brady and colleague^'^ and Capranica and colleague^.'^ Young soccer players performed, statistically, at a significantly higher level than the nonplayers. The authors surmised that soccer training enhances lower limb muscle function without strength
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training.'3,17They also found no significant differences between preferred and nonpreferred limbs.13,14, l7 As would be expected, demonstration of muscle torque in soccer players increases with age with the largest gains occurring between the Differages of 16 and 17 years, particularly at low angular velo~ities.~~ ences between age groups appears to be mainly in magnitude of force production; however, among all age groups the pattern of skilled kicking movement appears to be similar in type and timing.33 Narici and c o - w ~ r k e r sfound ~ ~ when examining moment levels at the hip and knee at 180°.s-' and 3OO0-s-', respectively, that no differences existed between dominant and nondominant sides. There were significant differences, however, in ball velocities between the two sides which could be attributed to an increased ability of the dominant limb to ballistically produce higher torque.3s Cabri and associates1hexamined isokinetic strength at the hip and which corresponded with angular velocities that more knee at 210"~5-~, closely approximated the speed of dynamic limb movement in kicking, and correlated his findings with kicking a soccer ball for distance. Using a standardized three-step approach, soccer players and controls kicked a soccer ball as far as possible with the soccer players kicking the ball significantly further. Interestingly, there were not significant differences in the isokinetic concentric strength at the knee in the two populations. The authors concluded that soccer players have better ability to utilize their muscular system, thereby generating greater force in a shorter period of time, which is an asset when kicking for distance. Isokinetic findings from Barfield7 are similar to those found by Cabri and co-workers,16which indicated higher correlations between ball velocity and flexion/extension at the knee than with ball velocity and the same two isokinetic movements at the hip. Though TanP7 did not find a significant relationship between torque at the knee and hip and ball velocity, she did acknowledge that as the speed of isokinetic Cybex testing approached speed of limb movement a stronger correlational relationship was observed. Isokinetic values from Barfield7 were greater than those found in Cabri and associates.'h It should be noted that the speed of testing in Cabri and associates16was 210".s-' and Barfield7 tested at 180".s-'. As expected, peak torque will decrease linearly on a semilogarithmic scale as angular velocity increases.28 Barfield: in accordance with findings from Cabri and associates,'6 found that flexion and extension at the knee at 18Oo.s-' was correlated with maximal ball velocity on dominant and nondominant sides. Barfield6 also found that hip extension torque was correlated with ball velocity on the nondominant side, which would seem to provide evidence of greater variability and more randomness in movement on the nondominant side. When soccer players were compared with nonplayers, the electromyographical patterns of agonist and antagonist muscles involved in kicking showed remarkably similar patterns. The single greatest discriminator was that in nonplayers the agonists were always more active than
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the antagonists, which may have been indicative of a less coordinated, and less well defined movement. At ball contact, which is a critical point in the kicking movement, nonplayers demonstrated only 50% maximum voluntary contraction (MVC) in the antagonists, which was less than that seen in soccer players during the same phase of the movement. When the entire movement was considered, nonsoccer players showed more muscle activity than soccer players yet did not kick the ball as far. Therefore, soccer players kick the ball further with less muscle activity but greater eccentric antagonistic activity, which may be indicative of more synergistic control in skillful movement. The message here is that soccer players may need to train the knee extensors to act concentrically at ball contact and the flexors eccentrically for kicking motion to be fluid and well controlled.21 At the point of ball contact the knee extensors and hip flexors are agonists, and therefore need to be trained concentrically.'8 At other points during the kicking movement these same muscles act eccentrically, as antagonists and follow "the soccer paradox," which means that flexor activity is dominant during extension and extensor activity dominates during flexion. DeProft and colleaguesz1showed that quadriceps activity was greatest during the loading phase when they were antagonist to the movement and the hamstrings were most active during the forward swing, when they are antagonistic to the movement.21Robertson and MosheI"14 reported no knee extensor activity immediately prior to ball contact. In fact, eccentric activity from the knee flexors dominated the end of the kicking motion, which subsequently reduced angular velocity at the knee. The two possible explanations offered by the authors were (1)the hamstrings act eccentrically to reduce possible hyperextension at the knee, and (2) due to the velocity of knee extension (12OO0-s-'), concentric extensor activity is inhibited as a result of the force-velocity relationship.44 Based on EMG studies, peak activity in the hamstrings occurs near the time of ball contact, which will likely retard a strong Equilibrium and balance between the flexors and extensors is likely to reduce the incidence and frequency of injury, improve the neuromuscular kick pattern, and generally improve kick perf~rmance.'~ A weak and nonsignificant positive correlation between ball speed and angular velocity at the hip and knee46was similar to findings reported by Barfield7 who found that angular velocity at the knee was not significantly correlated with ball velocity. The maximum value for angular velocity at the knee, however, did occur in close proximity with ball contact. The close temporal relationship between maximum angular velocity and ball contact is desirable when the criterion is generation of greatest ball velocity. It has, however, been shown that the maximum extensor moment at the knee occurs very early in the kicking movement, before ball contact, and that the flexion moment at the knee is dominant as ball contact appro ache^.^^ Figure 4 demonstrates that although angular velocity at the knee is close to maximum at ball contact there is some slowing of knee extension, which is an expected finding.
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In summary, when EMG patterns of skilled and novice soccer players are compared in terms of muscular activity for all phases of the kicking movement, it can be deduced that skilled players demonstrate greater relaxation in the swing phase and less overall muscle activity than recreational players. For skilled players, peak quadriceps activity is seen at the end of the loading phase (flexion), which confirms the paradoxical muscular activity in soccer kicking. The fact that recreational players exhibit greater muscle activity throughout the kicking movement supports the concept that skilled players have more efficient use of their muscular system, thereby reinforcing the importance of development of technical motor skills.'* Foot Contact With the Ball
The kicker's support foot position and contact foot position, at the time the foot makes ball contact, is of critical importance in determining the final result. The time for foot/ball contact has been determined to be between 6 and 16 milliseconds ( m ~ ) 32, . ~40,, 43, 48 PlagenhofPO reported the lowest contact times (6 ms); however, inconsistency in results may be attributable to changes in the laws of soccer which allowed for the lowering of pressure in the ball in 1975 from 1.0 to 0.6 to 0.7 kgm-2. Lower ball pressure means that the ball will deform more, thereby increasing the time of foot/ball contact. At ball contact the knee is flexed and the foot is moving in a forward and upward arching direction. The foot and ball are in contact during the final few degrees of extension and angular velocity at the knee 15 ms before contact is between 1500" and ~OOO".S-'.~~ Estimated impact force is between 1.0 and 1.1 kN.5,48 The hip of the kicking limb is flexed as ball contact occurs; therefore, angular velocity of the thigh is minimal and hence provides a limited contribution to the kick at this point during the movement. Several investigators have examined the relationship between foot velocity and ensuing ball speed. Roberts and M e t ~ a l f ereported ~~ that foot velocity 15 ms before ball contact was 18 to 24, and the resultant ball velocity immediately following ball impact was 5 to 7 ms-' faster than foot speed. Asami and Nolte5 reported that foot speed decreased from 28.3 to 15.5 ms-', which differed markedly from PlagenhofPOwho earlier reported that deceleration of the foot, at contact was only 3 ms-' (24.1 to 21.0 ms-I). Differences may have been attributable to (1)ball inflation levels, which would have influenced the coefficient of restitution between balls used in the two projects, or (2) differences in technologies, such as differences in camera speed, used in the projects. Differences in foot deceleration, in turn, influenced striking mass with Asami and Nolte5 reporting a mean of 1.02 kg, which was lower than the striking mass reported by Plagenhoff (3.90 kg).40 Most authors have modeled the ball/foot impact as classical Newtonian mechanics. Although the time of impact is brief (6 to 16 ms),
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magnitude changes, displacement of the ball/foot collision, and the work accomplished by active muscle force must be considered when evaluating the ball/foot contact period during the kicking movement. Tsaousidis and Zatsiorsky48recently offered three points that led to the conclusion that the collision phase of kicking should not be analyzed as a classical elastic impact event during which conservation of momentum occurs. First, during contact there is substantial ball-foot displacement (26.0 k 2.3 cm). Second, at the instant of peak deformation, ball speed is 54% (13.4 ms-l) of ball speed when the foot is no longer in contact with the ball (24.9 ms-I). Therefore, greater than half of the resultant ball speed occurs without any contribution from the potential strain energy, which resulted from deformation of the ball. Finally, during the time that the ball regains its shape (recoil), foot deceleration does not occur despite the recoil force from the ball because of the compensatory work of the muscles.48 Greater foot velocity at ball impact does not correspond with greater resultant ball velocity in every case. Studies by Plagenhoff,4O Aitchison and Lees? and Rodano and Tavana4'j all found a poor relationship between foot speed and resultant ball speed. PlagenhofPO concluded that placement of the foot on the ball was a greater discriminator in attaining resultant ball velocity than maximum foot velocity. The indications of this discrepancy between foot speed and ball velocity, as PlagenhofPO suggested, are that other factors, such as (1) rigidity of the limb at impact and (2) position of the foot relative to the ball play meaningful roles that influence resultant ball velocity. This finding had been confirmed earlier by Ben-Sira9 when he evaluated ball velocity following ball contact among collegiate and professional soccer players (skilled athletes). He found that manner of contact between the player and ball appeared to be a major distinguishing factor between skilled and unskilled athletes and that no significant differences existed between the professionals and collegians when assessing the post-impact ball velocity. Ben-Sira9 found that skilled players contact the ball closer to the ankle and display less "give" at ball contact. Figure 5 from Barfields shows that forced plantar flexion at ball contact does occur even in a skilled population. Therefore, firmness of the foot at ball impact is an important factor that contributes to forceful kicking. Asami and Nolte5 found that change in the ankle joint angle did not negatively influence ball velocity, but the angle change at the metatarsophalangeal articulations correlated with decreased ball velocity. Follow-Through
The follow-through following execution of a kick, as in all ballistic movements, has two purposes. First, one of the primary objectives in kicking is for the performer to keep the contacting body part in touch with the ball for as long a period of time as possible. In kicking, the longer the foot can keep contact with the ball, the greater the possibility
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Figure 5. Representative trial depicting angular displacement (aas) of the ankle at ball impact. 01 = support foot contact; w = ball contact. (From Barfield WR: Biomechanics of kicking. In Garrett WE, Kirkendall DT (eds): Textbook of Sports Medicine. Baltimore, Williams & Wilkins, 1997, pp 86-94; with permission.)
that greater momentum can be imparted. Second, follow-through acts as a protective mechanism for the body, in particular the swinging limb. The muscle and elastic forces that have been generated during loading and the forward swing to the ball are dissipated during the followthrough.26The follow-through increases the time component of the impulse side of the impulse-momentum equation, thereby reducing injury possibility. Recent findings by Tsaousidis and Z a t s i o r ~ k yprovide ~~ convincing evidence for the recommendations that have been provided to athletes for many years to follow-through when completing an athletic movement, such as kicking. Based on their findings, follow-through increases the mechanical work the muscles provide for the ball, thereby improving the resultant ball velocity. Follow-through is best characterized initially by concentric hip flexor activity followed by eccentric knee flexor activity as the foot approaches ball contact. Toward the end of follow-through, concentric activity of the hip extensors dominates.44 CONCLUSION
Kicking will continue to be a topic that will require much discussion and research in the field of biomechanics because there continues to be a number of unresolved issues including the following: (1) the influence forces on the plant foot play in dictating ball velocity, (2) more definitive fractionization of moments and forces at the hip and knee, (3) the relative contributions each makes to kicking, and (4) the eccentric role of the hamstrings as a protective injury mechanism. With increased participation in soccer from every aspect of socieqyoung to old, men and women, and diverse ethnic groups-there will
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continue to be a need for active ongoing research to improve training, prevent injury, and assist with rehabilitation techniques. ACKNOWLEDGMENT The author gratefully acknowledges the able assistance of James DeMarco, MD, in the revision of this manuscript from an earlier edition.
References 1. Abo-Abdo HE: Kinematic and kinetic analysis of the soccer instep kick. Unpublished doctoral dissertation, Indiana University, IN, 1981 2. Aitchison I, Lees A A biomechanical analysis of place-kicking in rugby union. In Brodie DA, Bumie J, Eston RG, et a1 (eds): Proceedings of Sport and Science. United Kingdom, University of Liverpool, pp 1-7, 1983 3. Anderson DI, Sidaway 8: Coordination changes associated with practice of a soccer kick. Res Q Exerc Sport 65:9598, 1994 4. Asai T, Kobayashi K, Oshima Y Biomechanical analysis of instep kick in soccer. Proceedings of Japanese Physical Education, 139, 1980 5. Asami T, Nolte V Analysis of powerful ball kicking. In Matsui H, Kobayashi K (eds): Biomechanics VIII-B Champaign, 11, Human Kinetics, 1983, p p 695700 6. Barfield W R Effects of selected biomechanical variables on a coordinated human movement: Instep kicking with dominant and nondominant feet. Unpublished doctoral dissertation, Aubum University, AL, 1993 7. Barfield W R Effects of selected kinematic and kinetic variables on instep kicking with dominant and nondominant limbs. Joumal of Human Movement Studies 29:251-272, 1995 8. Barfield WR Biomechanics of kicking. In Garrett WE, Kirkendall DT (eds): Textbook of Sports Medicine. Baltimore, Williams & Wilkins, 1997, pp 86-94 9. Ben-Sira D A comparison of the mechanical characteristics of the instep kick between skilled soccer players and novices. Unpublished doctoral dissertation, 1980 10. Bemstein N: The Coordination and Regulation of Movement. New York, Pergamon, 1967 11. Bloomfield J, Elliott B, Davies C: Development of the soccer kick A cinematographical analysis. Joumal of Human Movement Studies 3:152-159, 1979 12. Bollens EC, DeProft E, Clarys J P The accuracy and muscle monitoring in soccer kicking. In Jonsson B (ed): Biomechanics X-A. Champaign, IL, Human Kinetics Publishers, 1987, pp 28S288 13. Brady EC, ORegan M, McCormack 8: Isokinetic assessment of uninjured soccer players. In Reilly T, Clarys J, Stibbe A (eds): Science and Football, vol. 2. New York, E & FN Spon, 1993, pp 351-356 14. Bumie J, Brodie DA: Isokinetic measurement in preadolescent males. Int J Sports Med 7,205209, 1986 15. Butterfield SA, Loovis E M Influence of age, sex, balance, and sport participation on development of kicking by children in grades K-8. Percept Mot Skills 79:691-697, 1994 16. Cabri J, DeProft E, Dufour W, et al: The relation between muscular strength an kick performance. In Reilly T, Lees A, Davids K, et a1 (eds): Science and Football. New York, E. & F. N. Spon, 1988, pp 186-193 17. Capranica L, Cama G, Fanton F, et al: Force and power of preferred and non-preferred leg in young soccer players. J Sports Med Phys Fitness 32:358-363, 1992 18. Chyzowych W The Official Soccer Book of the United States Soccer Federation. New York, Rand McNally, 1979 19. Clarys JP, Cabri J: Electromyography and study of sports movement: A review. J Sports Sci 11:37948, 1993 20. DeProft E, Cabri J, Dufour W, et al: Strength training and kick performance in soccer
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players. In Reilly T, Lees A, Davids K, et a1 (eds): Science and Football. New York, E & F. N. Spon, 1988, pp 108-113 DeProft E, Clarys JP, Bollens E, et al: Muscle activity in the soccer kick. In Reilly T, Lees A, Davids K, et a1 (eas): Science and Football. New York, E & FN Spon, 1988, pp 434440 Dos Anjos LA, Adrian MJ: Ground reaction forces during soccer kicks performed by skilled and unskilled subjects. Revista Brasileira de Cienias do Esporto (abstract). Sao Paulo, Brazil, 1986 Dunn EG, Putnam CA: The influence of lower leg motion on thigh deceleration in kicking. In deGroot G, Hollander AP, Huijing PA, et a1 (eds): Biomechanics XI-B. Amsterdam, Free University Press, 1988, pp 787-790 Elliott BC, Bloomfield J, Davies CM: Development of the punt kick: A cinematographical analysis. Journal of Human Movement Studies 6:142-150, 1980 Gainor BJ, Piotrowski G, Puhl JJ, et al: The kick Biomechanics and collision injury. Am J Sports Med 6185-193, 1978 Hay JG: The Biomechanics of Sports Techniques-Fourth Edition. Englewood Cliffs, NJ, Prentice Hall, 1996 Huang TC, Roberts EM, Youm Y: (1982). Biomechanics of kicking. In Ghista DJ (ed): Human Body Dynamics:Impact, Occupational, and Athletic Aspects. New York, Clarendon Press & Oxford University Press, 1982, pp 409-443 Ingemant-Hansen T, Halkjaer-Kristensen J: Force-velocity relationships in the human quadriceps muscles. Scand J Rehabil Med 11:85-89, 1979 Isokawa M, Lees A A biomechanical analysis of the instep kick motion in soccer. In Reilly T, Lees A, Davids K, et a1 (eds): Science and Football. New York, E & FN Spon, 1988, pp 449-455 Jelusic V, Jaric S, Kukolj M Effects of the stretch-shortening strength training on kicking performance in soccer players. Journal of Human Movement Studies 22:231-238, 1992 Kaufmann DA, Stanton DE, Updyke WF: Kinematical analysis of conventional-style and soccer style place kicking in football (abstr). Med Sci Sports Exerc 777-78, 1975 Lindbeck L: Impulse and moment of impulse in the leg joints by impact from kicking. J Biomech Eng 105:108-111, 1983 Luhtanen P:Kinematics and kinetics of maximal instep kicking in junior soccer players. In Reilly T, Lees A, Davids K, et a1 (eds): Science and Football. New York, E & FN Spon, 1988, pp 441-448 Mognoni P, Narici MV, Sirtori MD, et al: Isokinetic torques and kicking maximal ball velocity in young soccer players. J Sports Med Phys Fitness 34:357-361, 1994 Narici MV, Sirtori MD, Mognoni P: (1988). Maximal ball velocity and peak torques of hip flexor and knee extensor muscles. In Reilly T, Lees A, Davids K, et a1 (eds): Science and Football. New York, E & F. N. Spon, 1988, pp 429-433 Nishijima T, Tasaki E, Noda Y, et al: Development of principal motor movements controlling ball kicking performance (abstr). ACSM National Meeting Proceedings, 1996 Oberg 8, Moller M, Gillquist J, et al: Isokinetic torque levels for knee extensors and knee flexors in soccer players. Int J Sports Med 750-53, 1986 Olson JR, Hunter G R Anatomic and biomechanical analyses of the soccer style free kick. National Strength and Conditioning Association Journal 750-53, 1985 Phillips SJ: Invariance of elite kicking performance. In Winter DA, Norman RW, Wells RP, et a1 (eds): Biomechanics IX-B. Champaign, IL, Human Knetics Publishers, 1985, pp. 539-542 ’ Plagenhoff S: Patterns of Human Motion: A Cinematographic Analysis. Upper Saddle River, NT, Prentice-Hall, 1971 Putnam-CA: Interaction between segments during a kicking motion. In Matsui H, Kobayashi K (eds): Biomechanics VIII-B. Champaign, IL, Human Kinetics Publishers, 1983, pp 688-694 Putnam C A A segment interaction analysis of proximal-to-distal sequential segment motion patterns. Med Sci Sports Exerc 23:130-144, 1991 Roberts EM, Metcalfe A: Mechanical analysis of kicking. In Wartenweiler J, Jokl E, Hebbelinck M (eds): Biomechanics I. Baltimore, University Park Press, 1968, pp 315-319 -
40. 41. 42. 43.
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44. Robertson DGE, Mosher RE: Work and power of the leg muscles in soccer kicking. In Winter DA, Norman RW, Wells RP, et a1 (eds): Biomechanics IX-8. Champaign, IL, Human Knetics Publishers, 1985, pp 533-538 45. Rochcongar P, Morvan R, Jan J, et al: Isokinetic investigation of knee extensors and knee flexors in young French soccer players. Int J Sports Med 9:448-450, 1988 46. Rodano R, Tavana R: Three-dimensional analysis of instep kick in professional soccer players. In Reilly T, Clarys J, Stibbe A (eds): Science and Football 11. New York, E & FN Spon, 1993, pp 357-361 47. Tant CL: Segmental interactions of a three-dimensional soccer instep kick. Unpublished doctoral dissertation. Texas Women’s University, Denton, TX, 1990 48. Tsaousidis N, Zatsiorsky V Two types of ball-effector interaction and their relative contribution to soccer kicking. Human Movement Science 15%-876, 1996 49. Wahrenberg H, Lindbeck L, Ekholm J: Knee muscular moment, tendon tension force and EMG during a vigrous movement in man. Scand J Rehabil Med 10:99-106, 1978 50. Zakas A, Mandroukas K, Vamvakoudis E, et al: Peak torque of quadriceps and hamstring muscles in basketball and soccer players of different divisions. J Sports Med Phys Fitness 35:199-205, 1995 51. Zernicke RF: Human lower extremity kinetic parameter relationships during systematic variation in resultant limb velocity. Unpublished doctoral dissertation. Madison, University of Wisconsin, 1974 52. Zernicke RF, Roberts EM (1976). Human lower extremity kinetic relationships during systematic variations in resultant limb velocity. In Komi PV (ed): Biomechanics V-B. Baltimore, University Park Press, 1976, pp 20-25
Address reprint requests to William R. Barfield, PhD Department of Orthopaedic Surgery Medical University of South Carolina 171 Ashley Avenue, 708CSB Charleston, SC 29425
SOCCER INJURIES
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THE BIOMECHANICS OF KICKING IN SOCCER William R. Barfield, PhD
DEVELOPMENT OF KICKING SKILL
Of all the skills required to participate in soccer, none has received more attention from a biomechanical perspective than kicking. When skilled behavior, such as kicking, is evaluated there are two successive stages. Initially, movements are organized and simplified through powerful, temporal coupling between joint complexes so that the neuromuscular system becomes functionally useful for the learner. In the second stage, the learner's behavior becomes more efficient and organized as the active muscular and joint forces become more economical.'" Anderson and Sidaway3 suggested that the skilled movement of kicking will be enhanced and better organized with training, particularly if the objective is to maximize foot velocity, and in turn, resultant ball velocity. They believed that coordination, including summation of speed from the hip to knee seemed to be a great determinant in improved kicking and that unskilled players demonstrated less coordination than their skillful counterparts. As skill develops, linear foot velocity becomes greater through an increase in angular velocity at the knee without a concomitant increase in hip angular velocity. This suggests that practice provides economical organization for the learner by effectively reducing the numbers of degrees of f r e e d ~ m . ~ Kicking is a complex motor movement that follows generally predictable developmental stages. Although chronologic age is not necessarFrom the College of Charleston; and the Department of Orthopaedic Surgery, Medical University of South Carolina, Charleston, South Carolina
CLINICS IN SPORTS MEDICINE VOLUME 17 * NUMBER 4 OCTOBER 1998
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ily an accurate predictor of skill development, it appears that between the ages of 4 and 6 years, skill develops rapidly'l, 24 For boys and girls there appears to be a linear relationship between age and mature kicking skill between kindergarten and grade four.I5With skilled soccer players between the ages of 9 and 18 years, however, the timing pattern in kicking does not appear to differ ~ignificantly.~~ Differences between and within skill levels have been examined. When elite athletes are evaluated, the general consensus is that they will exhibit less mechanical variability and therefore will perform more consistently than less skilled athletes. However, Rodano and Tavana4'j found that even in highly skilled professional soccer players minimal variations in motor performances can negatively influence the characteristics of movement, when within subject variability is considered. Phill i p points ~ ~ ~ out that lack of variability is indicative of a more acutely refined motor program, whereby muscle contractions are temporally distinct. The alternative view is that when kicking, the variability and adaptability of the neuromuscular system and the mechanical interrelationships of skilled behavior are demonstrated best. When a skilled kicker was compared with a club player, the skilled kicker demonstrated less variability particularly with respect to angular positions during the kicking movement. Though there were no significant differences in resultant ball velocities between the skilled player and the club player, consistency of kicking by the skilled player was likely attributable to the two-step approach as opposed to the unrestricted inconsistent approach used by the club player, which subsequently led to greater standard deviations of kicking velocity.3y COMPONENTS OF KICKING
The process of ball kicking from a biomechanical perspective can be separated into components in various ways. This article defines kicking in soccer through six stages. They are: (1)approach angle, (2) plant foot forces, (3) swing limb loading, (4) flexion at the hip and extension at the knee, (5) foot contact with the ball, and (6) follow-through. Major differences between skilled and unskilled players are based on the relative percentage of movement that is devoted to each of the components. In unskilled players, ball kicking movement is generally poorly coordinated and dominated by the approach phase, whereas skilled players use approach movements, backswing, and forward swing movements to define the kicking movernent.'j*7, 36 Skilled athletes also appear to take longer strides as they approach the ball and the temporal proximity of selected events during the kicking movement (i.e., maximum hip extension and knee flexion) are more closely ass0ciated.l Approach Angle
When kicking, children between the ages of 3 and 5 years do not take any steps to the ball, and at earlier ages simply walk into the ball.
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As children mature chronologically a paced run-up is used, and the angle of the approach becomes more diagonal. Efficient segmental timing for the kick reveals that maximum angular velocity at the knee progressively increases with age during the preadolescent years, and the timing of maximum angular velocity at the knee corresponds more closely with ball contact. Though not all children follow traditional stages of development and there exists intragroup variability in similar age groups, there is a general developmental trend that most children tend to follow." Several authors have emphasized that the diagonal approach, as opposed to the straight approach, results in greater swing limb ~elocity.~, 29,31,40 P l a g e n h ~ f f believed ~~ that the two primary factors in determining ball velocity were (1) effective mass of the foot, and (2) foot velocity immediately before ball contact. His findings provided evidence that foot velocity before impact was similar with straight and angled approaches, yet the angled approach yielded greater ball velocity. This led to the conclusion that the primary differences in ball velocity were based on greater "effective mass of the foot" with an angled approach rather than foot velocity at ball contact.26,40 Isokawa and Leeszyshowed that peak ball velocity was greatest at an approach angle of 45 degrees although peak ankle velocity was greatest at a 30 degree approach angle. Peak velocity at the hip was greatest at a 15 degree approach angle, and peak knee velocity was greatest with the straight approach (0 degrees). The authors attribute these notable differences to increased striking mass and greater knee and ankle fixation at the 45 degree approach angle. In the straight-ahead approach, there is limited rotation of the leg about the vertical axis through the body. As the approach angle increases, however, the leg must rotate about a vertical axis to kick the ball straight. The resistance torque that is imparted to the vertical axis through the body by the ground reaction force (GRF) is reduced by the torque actively generated by the swing limb as the approach angle is increased. At an approach angle of 45 to 60 degrees, the player takes advantage of the active torque created by the swing limb, which completely balances the resistance torque generated by body motion, thereby increasing leg and foot mo38 mentum at ball contact.29, The position of the plant foot from a medio/lateral perspective with respect to the ball is well defined because if support foot position is located too far from the ball, the direction of the kick, and the kicker's body balance will be negatively compromised. Most research supports the belief that optimal support foot position is 5 to 10 cm to the left of the ball, assuming the kicker is kicking with the right foot.2hPlacement of the support foot alongside and adjacent the ball, perpendicular to an imaginary line drawn through the ball center appears to provide the most appropriate environment for a successful instep kick performance.' When skilled and unskilled players were compared, the skilled athletes placed the support foot alongside and closer the ball, whereas unskilled players tended to position the support foot behind the ball.y
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The position of the support foot from an anterior/posterior position is less well defined primarily because foot position will dictate ball flight. That is, the further behind the center of the ball the plant foot is placed the greater the likelihood that the ball will become flighted, which may or may not meet the kicker’s objective.26
Plant Foot Forces
The relationship between GRF on the support foot and predicted speed of a soccer kick has been investigated in skilled and unskilled populations. As expected, skilled players kicked faster than unskilled (25.9 ms-’ versus 23.4 ms-*) and the GRFs the skilled players exhibited vertically, anteriorly-posteriorly, and laterally were greater than among unskilled.zzDos Anjos and Adrian2zconcluded that some of the variance in greater speed of kicking among skilled players could be attributed to greater GRFs being generated by these subjects. Isokawa and Leesz9reported that as approach angle moved from 0 to 90 degrees peak vertical force varied minimally; however, peak frontal force and peak lateral force showed an anticipated inverse relationship, with frictional force oriented increasingly in a lateral direction as the approach angle neared 90 degrees. In a later study, Barfield8 found six medio/lateral force variables to be positively correlated with resultant ball velocity in dominant side kicking, yet none of the same variables, at the same approach angle, were correlated on the nondominant side. These variables included (1) maximum medio/lateral force, (2) time of maximum medio/lateral force, (3) medio/lateral force at ball contact, (4) mean medio/lateral force from support foot contact to ball contact, (5) mean medio/lateral force from support foot contact to maximum force, and (6) time between ball contact and maximum medio/lateral force. As demonstrated by the graphs in Figure 1, peak medio/lateral force occurred before peak anterior/posterior force, which is not an unexpected finding because body momentum will initially be blocked laterally owing to the angled approach. Medio/lateral forces reported by Barfield7(1.07 X body weight [BW]) and those reported by Rodano and Tavanag6(1.24 X BW) were similar in magnitude yet they found no significant correlations between GRFs and ball velocity. Subjects have been shown to exert greater forces vertically than medio/laterally or anterio/posteriorly, and in Abo-Abdo’s’ study, force exerted during movement in vertical and horizontal thrusting directions was significantly correlated with ”level of performance,” although level of performance was not quantified. The application point from these findings is that with an angled approach, when instep kicking for maximal effort, force that is generated from the approach path and plant foot position can and does influence the channeling of forces that eventually are used into propelling the ball.
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Swing Limb Loading Consistent patterning of the lower limb segments from a temporal perspective seems to be an essential component for successful instep kicking motion.’ The initiation of forward movement begins with the kicking limb knee flexed and the kicking foot at approximately hip height. Swing limb motion and foot placement, during the loading phase, before flexion at the hip and extension at the knee is a function of age, skill level, ability, and the kicker’s distance and path to the As the plant foot strikes the ground adjacent the ball, the swing limb is concurrently involved in hip extension and knee flexion. During ”loading” of the swing limb the kicker’s eyes are focused on the ball and not the direction the ball will travel because ball direction is dictated by support foot position, and ultimately hip position at ball contact.’* The ”loading” of the swing limb allows the hip flexors and the knee extensors to be eccentrically stretched (loaded) in preparation for the forward movement of the limb into position to strike the ball. As demonstrated through Figure 2 from Robertson and Masher,@ the hip flexors and knee extensors show negative power between 0.00 and 0.07 s to halt the swing limb’s backswing. As the forward movement
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Figure 2. Hip (dashed line) and knee (solid line) angular velocity, net moment of force, and power during a soccer kick. FS = contralateral leg footstrike; HIT = ball contact; OFF = start of ball flight. (Modified from Robertson DGE, Mosher RE: Work and power of the leg muscles in soccer kicking. ln Winter DA, Norman RW, Wells RP, et al (eds): Biomechanics 1x43. Champaign, IL, Human Kinetics Publishers, 1985, pp 5 3 3 4 3 8 . )
of the kicking leg is initiated the activity of hip flexors and knee extensors becomes concentric changing the power vector from negative to positive.44 Before ball contact, a large extension torque at the knee (230 N-m) is produced to provide a rapid extension rate; however, the flexion torque (280 N m ) produced at, or immediately following ball contact is larger than the extensor torque. If flexion torque is initiated too early in the kicking movement the limb and kicking foot is slowed before contact, which subsequently decreases eventual ball velocity. Findings from Gainor et alZ5indicate that in kicking, 15% of the kinetic energy is
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transmitted to the ball, and the balance of energy must be expended by other mechanisms including activation of the knee flexors. Limiting the time available to dissipate stored energy, however, increases the muscle forces required to eventually slow the limb and places the kicker at risk for injury because of the large loads placed on the knee joint and soft tissues already under tension immediately following the When slow kicks (15.24 ms-*) were compared with medium (21.34 ms-') and fast kicks (27.42 ms-') knee extensor torque increased gradually with slow kicks and was maintained throughout knee extension. In medium and fast kicks, more effective coordination was required, and as one link decelerated a more distal link showed significant acceleration although knee extensor torque dropped quickly a few milliseconds before or at the initiation of knee e~tension.~'',51 Immediately before ball contact, the hamstrings (knee flexors) became dominant, which resulted in negative power as demonstrated by Figure 2 from Robertson and Mosher.44 Flexion at the Hip and Extension at the Knee
As the hip flexors forcefully contract to swing the thigh forward and downward, the leg (defined from knee to foot) and foot rotate as a unit. As thigh angular velocity decreases, the leg and foot begin to accelerate due to a momentum shift and support from elastic and contractile components in the knee extensors.26 The sequential segmental motion and concomitant decrease in angular velocity of the proximal segment while the distal segment increases 4f142 P l a g e n h ~ f fnoted ~ ~ in is thought to play a critical role in his research investigating angled versus straight approaches to the ball that kicks with lowest thigh deceleration showed greatest knee extension, which illustrates the strong influence of one segment on another. Generally, there is a significant positive relationship between linear foot velocity and resultant ball velocity.', 7, As seen in the two graphs on linear and angular velocity (Figs. 3 and 4, respectively),linear velocity at the ankle is close to maximum at ball contact as is angular velocity at the knee.8 Roberts and M e t ~ a l f efound ~ ~ that the primary factors that influence swing limb velocity are hip rotation followed by hip flexion and knee extension prior to impact. There appears to be an exchange in angular velocities between proximal and distal segments in kicking, which would indicate a transfer of momentum from the more massive thigh to the less massive legz7,43 Such claims, however, have not been confirmed, and there is evidence, based on segmental interaction analysis, that decreasing thigh angular velocity in kicking creates a less effective environment for an increase in leg angular velocity. The decrease in thigh angular velocity is thought by some to be the result of the leg's motion on the thigh.23,42 Putnam (1991) has shown that a decrease in thigh angular velocity results from a large hip flexor moment which counteracts the effect of the leg on the thigh, thereby limiting the loss of
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Figure 3. Representative trial depicting linear velocity of the toe (Ivt), knee, (Ivk), and hip (Ivh). Q = support foot contact; o = ball contact. (From Barfield WR: Biomechanics of kicking. ln Garrett WE, Kirkendall DT (eds): Textbook of Sports Medicine. Baltimore, Williams & Wilkins, 1997, pp 86-94; with permission.)
positive thigh angular velocity. The generally accepted thought that negative angular acceleration of a proximal segment leads to positive acceleration of an adjacent distal segment through the summation of speed principles cannot be supported by evidence from P ~ t n a mThese .~~ apparent contradictions concerning speed and moments relate to the fact
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that motion of a single segment within a linked system is nonlinear and cannot be attributed solely to muscle forces and moments working on that one segment, making quantification of separate segmental roles difficult to ascertain.42 DeProft and colleaguesz1examined the extent of agonist and antagonistic muscle function during a soccer kick. They reported that the extensors at the knee were likely to be most active during knee flexion, when they are antagonistic to the movement, and that skilled soccer player antagonistic muscle activity was greater when compared with nonskilled players. On the other hand, nonskilled players appear to have greater agonistic muscle action. Research concerning whether strength development improves per-
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formance has been well documented. The stretch-shortening cycle of strength training, when combined with soccer training, has been shown to result in greater functional kicking p e r f ~ r m a n c e . ~ ~ DeProft and associatesz0found that adult and youth (15.5 years of age) soccer players demonstrated significant increases in concentric hip flexor and eccentric hip extensor strength, eccentric hamstring strength (77%), and in concentric quadriceps strength (25%) in a study that evaluated the efficacy of a strength training program. Explosive muscle power tests (long jump, vertical jump, and single-legged triple jump) were also found to be significantly correlated with kick distance. These findings support a soccer training program that is enhanced with a strength program when the objective is to improve kick distance and functional power ability in youth and adult soccer players. When young adult male soccer players (mean = 22 years old) were compared with a control group, no statistically significant differences existed in hamstringlquadriceps torque when they were measured at 60°-s-', even though soccer players demonstrated values that were 5% to 10% greater than the controls. The concentric-eccentric ratio between quadricepslhamstrings was slightly lower for soccer players than for the controls, which would indicate that acceleration and deceleration demands of the game provide a suitable stimulus that allows for concurrent development of hamstring and quadriceps When decelerating from running, the hamstrings act eccentrically to slow extension at the knee and the quadriceps act eccentrically to control the lowering of body weight when players approach a stop. No differences existed between dominant and nondominant sides among soccer players indicating that training and matches generate sufficient bilateral exercise to prevent muscle imbalance and provide symmetry, which is an important aspect in soccer.1zBarfield: when testing skilled adult soccer players, found no significant differences between dominant and nondominant limbs in isokinetic testing at the hip and knee when subjects were evaluated at 180".s-'. These studies combined with another by Capranica and colleague^'^ seem to point to the importance of proper motor skill training and reasonable amounts of time spent developing strength. This two-level approach will provide the stimulus necessary to produce relatively equal levels of torque in preferred and nonpreferred limbs.17 Mognoni and colleagues34found that peak knee extension torque on the nondominant side was greater than on the dominant side. The authors attribute this seemingly paradoxical finding to the role the nondominant knee extensors play through eccentric support of body weight and torque generated by the swing limb. When prepubescent male (mean = 9.6 years of age) soccer players were compared with an untrained group for force and power, findings were similar to those reported by Brady and colleague^'^ and Capranica and colleague^.'^ Young soccer players performed, statistically, at a significantly higher level than the nonplayers. The authors surmised that soccer training enhances lower limb muscle function without strength
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training.'3,17They also found no significant differences between preferred and nonpreferred limbs.13,14, l7 As would be expected, demonstration of muscle torque in soccer players increases with age with the largest gains occurring between the Differages of 16 and 17 years, particularly at low angular velo~ities.~~ ences between age groups appears to be mainly in magnitude of force production; however, among all age groups the pattern of skilled kicking movement appears to be similar in type and timing.33 Narici and c o - w ~ r k e r sfound ~ ~ when examining moment levels at the hip and knee at 180°.s-' and 3OO0-s-', respectively, that no differences existed between dominant and nondominant sides. There were significant differences, however, in ball velocities between the two sides which could be attributed to an increased ability of the dominant limb to ballistically produce higher torque.3s Cabri and associates1hexamined isokinetic strength at the hip and which corresponded with angular velocities that more knee at 210"~5-~, closely approximated the speed of dynamic limb movement in kicking, and correlated his findings with kicking a soccer ball for distance. Using a standardized three-step approach, soccer players and controls kicked a soccer ball as far as possible with the soccer players kicking the ball significantly further. Interestingly, there were not significant differences in the isokinetic concentric strength at the knee in the two populations. The authors concluded that soccer players have better ability to utilize their muscular system, thereby generating greater force in a shorter period of time, which is an asset when kicking for distance. Isokinetic findings from Barfield7 are similar to those found by Cabri and co-workers,16which indicated higher correlations between ball velocity and flexion/extension at the knee than with ball velocity and the same two isokinetic movements at the hip. Though TanP7 did not find a significant relationship between torque at the knee and hip and ball velocity, she did acknowledge that as the speed of isokinetic Cybex testing approached speed of limb movement a stronger correlational relationship was observed. Isokinetic values from Barfield7 were greater than those found in Cabri and associates.'h It should be noted that the speed of testing in Cabri and associates16was 210".s-' and Barfield7 tested at 180".s-'. As expected, peak torque will decrease linearly on a semilogarithmic scale as angular velocity increases.28 Barfield: in accordance with findings from Cabri and associates,'6 found that flexion and extension at the knee at 18Oo.s-' was correlated with maximal ball velocity on dominant and nondominant sides. Barfield6 also found that hip extension torque was correlated with ball velocity on the nondominant side, which would seem to provide evidence of greater variability and more randomness in movement on the nondominant side. When soccer players were compared with nonplayers, the electromyographical patterns of agonist and antagonist muscles involved in kicking showed remarkably similar patterns. The single greatest discriminator was that in nonplayers the agonists were always more active than
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the antagonists, which may have been indicative of a less coordinated, and less well defined movement. At ball contact, which is a critical point in the kicking movement, nonplayers demonstrated only 50% maximum voluntary contraction (MVC) in the antagonists, which was less than that seen in soccer players during the same phase of the movement. When the entire movement was considered, nonsoccer players showed more muscle activity than soccer players yet did not kick the ball as far. Therefore, soccer players kick the ball further with less muscle activity but greater eccentric antagonistic activity, which may be indicative of more synergistic control in skillful movement. The message here is that soccer players may need to train the knee extensors to act concentrically at ball contact and the flexors eccentrically for kicking motion to be fluid and well controlled.21 At the point of ball contact the knee extensors and hip flexors are agonists, and therefore need to be trained concentrically.'8 At other points during the kicking movement these same muscles act eccentrically, as antagonists and follow "the soccer paradox," which means that flexor activity is dominant during extension and extensor activity dominates during flexion. DeProft and colleaguesz1showed that quadriceps activity was greatest during the loading phase when they were antagonist to the movement and the hamstrings were most active during the forward swing, when they are antagonistic to the movement.21Robertson and MosheI"14 reported no knee extensor activity immediately prior to ball contact. In fact, eccentric activity from the knee flexors dominated the end of the kicking motion, which subsequently reduced angular velocity at the knee. The two possible explanations offered by the authors were (1)the hamstrings act eccentrically to reduce possible hyperextension at the knee, and (2) due to the velocity of knee extension (12OO0-s-'), concentric extensor activity is inhibited as a result of the force-velocity relationship.44 Based on EMG studies, peak activity in the hamstrings occurs near the time of ball contact, which will likely retard a strong Equilibrium and balance between the flexors and extensors is likely to reduce the incidence and frequency of injury, improve the neuromuscular kick pattern, and generally improve kick perf~rmance.'~ A weak and nonsignificant positive correlation between ball speed and angular velocity at the hip and knee46was similar to findings reported by Barfield7 who found that angular velocity at the knee was not significantly correlated with ball velocity. The maximum value for angular velocity at the knee, however, did occur in close proximity with ball contact. The close temporal relationship between maximum angular velocity and ball contact is desirable when the criterion is generation of greatest ball velocity. It has, however, been shown that the maximum extensor moment at the knee occurs very early in the kicking movement, before ball contact, and that the flexion moment at the knee is dominant as ball contact appro ache^.^^ Figure 4 demonstrates that although angular velocity at the knee is close to maximum at ball contact there is some slowing of knee extension, which is an expected finding.
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In summary, when EMG patterns of skilled and novice soccer players are compared in terms of muscular activity for all phases of the kicking movement, it can be deduced that skilled players demonstrate greater relaxation in the swing phase and less overall muscle activity than recreational players. For skilled players, peak quadriceps activity is seen at the end of the loading phase (flexion), which confirms the paradoxical muscular activity in soccer kicking. The fact that recreational players exhibit greater muscle activity throughout the kicking movement supports the concept that skilled players have more efficient use of their muscular system, thereby reinforcing the importance of development of technical motor skills.'* Foot Contact With the Ball
The kicker's support foot position and contact foot position, at the time the foot makes ball contact, is of critical importance in determining the final result. The time for foot/ball contact has been determined to be between 6 and 16 milliseconds ( m ~ ) 32, . ~40,, 43, 48 PlagenhofPO reported the lowest contact times (6 ms); however, inconsistency in results may be attributable to changes in the laws of soccer which allowed for the lowering of pressure in the ball in 1975 from 1.0 to 0.6 to 0.7 kgm-2. Lower ball pressure means that the ball will deform more, thereby increasing the time of foot/ball contact. At ball contact the knee is flexed and the foot is moving in a forward and upward arching direction. The foot and ball are in contact during the final few degrees of extension and angular velocity at the knee 15 ms before contact is between 1500" and ~OOO".S-'.~~ Estimated impact force is between 1.0 and 1.1 kN.5,48 The hip of the kicking limb is flexed as ball contact occurs; therefore, angular velocity of the thigh is minimal and hence provides a limited contribution to the kick at this point during the movement. Several investigators have examined the relationship between foot velocity and ensuing ball speed. Roberts and M e t ~ a l f ereported ~~ that foot velocity 15 ms before ball contact was 18 to 24, and the resultant ball velocity immediately following ball impact was 5 to 7 ms-' faster than foot speed. Asami and Nolte5 reported that foot speed decreased from 28.3 to 15.5 ms-', which differed markedly from PlagenhofPOwho earlier reported that deceleration of the foot, at contact was only 3 ms-' (24.1 to 21.0 ms-I). Differences may have been attributable to (1)ball inflation levels, which would have influenced the coefficient of restitution between balls used in the two projects, or (2) differences in technologies, such as differences in camera speed, used in the projects. Differences in foot deceleration, in turn, influenced striking mass with Asami and Nolte5 reporting a mean of 1.02 kg, which was lower than the striking mass reported by Plagenhoff (3.90 kg).40 Most authors have modeled the ball/foot impact as classical Newtonian mechanics. Although the time of impact is brief (6 to 16 ms),
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magnitude changes, displacement of the ball/foot collision, and the work accomplished by active muscle force must be considered when evaluating the ball/foot contact period during the kicking movement. Tsaousidis and Zatsiorsky48recently offered three points that led to the conclusion that the collision phase of kicking should not be analyzed as a classical elastic impact event during which conservation of momentum occurs. First, during contact there is substantial ball-foot displacement (26.0 k 2.3 cm). Second, at the instant of peak deformation, ball speed is 54% (13.4 ms-l) of ball speed when the foot is no longer in contact with the ball (24.9 ms-I). Therefore, greater than half of the resultant ball speed occurs without any contribution from the potential strain energy, which resulted from deformation of the ball. Finally, during the time that the ball regains its shape (recoil), foot deceleration does not occur despite the recoil force from the ball because of the compensatory work of the muscles.48 Greater foot velocity at ball impact does not correspond with greater resultant ball velocity in every case. Studies by Plagenhoff,4O Aitchison and Lees? and Rodano and Tavana4'j all found a poor relationship between foot speed and resultant ball speed. PlagenhofPO concluded that placement of the foot on the ball was a greater discriminator in attaining resultant ball velocity than maximum foot velocity. The indications of this discrepancy between foot speed and ball velocity, as PlagenhofPO suggested, are that other factors, such as (1) rigidity of the limb at impact and (2) position of the foot relative to the ball play meaningful roles that influence resultant ball velocity. This finding had been confirmed earlier by Ben-Sira9 when he evaluated ball velocity following ball contact among collegiate and professional soccer players (skilled athletes). He found that manner of contact between the player and ball appeared to be a major distinguishing factor between skilled and unskilled athletes and that no significant differences existed between the professionals and collegians when assessing the post-impact ball velocity. Ben-Sira9 found that skilled players contact the ball closer to the ankle and display less "give" at ball contact. Figure 5 from Barfields shows that forced plantar flexion at ball contact does occur even in a skilled population. Therefore, firmness of the foot at ball impact is an important factor that contributes to forceful kicking. Asami and Nolte5 found that change in the ankle joint angle did not negatively influence ball velocity, but the angle change at the metatarsophalangeal articulations correlated with decreased ball velocity. Follow-Through
The follow-through following execution of a kick, as in all ballistic movements, has two purposes. First, one of the primary objectives in kicking is for the performer to keep the contacting body part in touch with the ball for as long a period of time as possible. In kicking, the longer the foot can keep contact with the ball, the greater the possibility
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w
60 55 50 h
m
g
45
u)
m
40 35 30
u
1.12
1.15
1.18
1.21
1.24
1.27
1.30
Figure 5. Representative trial depicting angular displacement (aas) of the ankle at ball impact. 01 = support foot contact; w = ball contact. (From Barfield WR: Biomechanics of kicking. In Garrett WE, Kirkendall DT (eds): Textbook of Sports Medicine. Baltimore, Williams & Wilkins, 1997, pp 86-94; with permission.)
that greater momentum can be imparted. Second, follow-through acts as a protective mechanism for the body, in particular the swinging limb. The muscle and elastic forces that have been generated during loading and the forward swing to the ball are dissipated during the followthrough.26The follow-through increases the time component of the impulse side of the impulse-momentum equation, thereby reducing injury possibility. Recent findings by Tsaousidis and Z a t s i o r ~ k yprovide ~~ convincing evidence for the recommendations that have been provided to athletes for many years to follow-through when completing an athletic movement, such as kicking. Based on their findings, follow-through increases the mechanical work the muscles provide for the ball, thereby improving the resultant ball velocity. Follow-through is best characterized initially by concentric hip flexor activity followed by eccentric knee flexor activity as the foot approaches ball contact. Toward the end of follow-through, concentric activity of the hip extensors dominates.44 CONCLUSION
Kicking will continue to be a topic that will require much discussion and research in the field of biomechanics because there continues to be a number of unresolved issues including the following: (1) the influence forces on the plant foot play in dictating ball velocity, (2) more definitive fractionization of moments and forces at the hip and knee, (3) the relative contributions each makes to kicking, and (4) the eccentric role of the hamstrings as a protective injury mechanism. With increased participation in soccer from every aspect of socieqyoung to old, men and women, and diverse ethnic groups-there will
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continue to be a need for active ongoing research to improve training, prevent injury, and assist with rehabilitation techniques. ACKNOWLEDGMENT The author gratefully acknowledges the able assistance of James DeMarco, MD, in the revision of this manuscript from an earlier edition.
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Address reprint requests to William R. Barfield, PhD Department of Orthopaedic Surgery Medical University of South Carolina 171 Ashley Avenue, 708CSB Charleston, SC 29425