Kicking is a whole-body movement that is responsive to a wide range of constraints related to the task, the environment, and the athlete. Preliminary research also suggests that balance control in the support leg plays a key role in athletes’ kicking performance.
By David I. Anderson, PhD, and Ben Sidaway, PT, PhD
Kicking, a fundamental motor skill usually acquired during childhood, can be adapted to accomplish a range of different task goals. Although it is most commonly associated with the sport of soccer (called football in most of the world), kicking is commonplace in martial arts, American football, Australian football, rugby union, rugby league, some contemporary fitness classes, and a variety of other sports. Consequently, a deeper understanding of kicking has implications for sports scientists seeking to improve kicking performance as well as for clinicians interested in rehabilitating lower limb function. Studying kicking tasks can also advance understanding of processes underlying the control and learning of complex multisegmented movements.1,2
Despite the prevalence of kicking in sports and the large body of research on kicking that has accumulated over the last 25 years, many gaps remain in our understanding of the motion. Although controlling whole-body balance and posture are critical to the expression of all skilled physical activity, most examinations of kicking have focused on the kicking leg, with few examining the role of the support leg in facilitating effective and efficient kicking motion of the opposite leg.3-6
The kicking leg
Kicking is a complex pattern of whole-body joint and segment motions that occur in multiple planes. Understandably, most of what we know about kicking has come from the work of sports scientists examining kicking in soccer. It is not clear to what extent research on the soccer kick can be generalized to kicking movements adapted for other purposes, however, it is well established that variations in the task goal of the soccer kick (e.g., for accuracy vs distance or passing vs scoring a goal) can have pronounced effects on movement kinematics and kinetics.7-9 These effects highlight how sensitive all movements are to variations in constraints related to the task, the performer, and the environment.10,11
Kicking leg kinematics
Many of the biomechanical descriptions of the soccer kick have been restricted to the two-dimensional motions of the kicking leg in the sagittal plane.9,12 This highlights the difficulty in accurately and reliably quantifying joint and segment motions occurring in the transverse plane around the long axes of limb segments and represents a major limitation in our understanding of segmental contributions to skilled kicking movements. Nevertheless, one of the most prominent features of the soccer kick is the proximal-to-distal sequencing of the segments of the kicking leg, and the unfolding of this sequence is most obvious in the sagittal plane.
A number of studies have highlighted the importance of the proximal-to-distal sequence of segmental angular velocities in generating a high linear velocity in the kicking foot.13-17 The linear velocity of the kicking foot is highly correlated with the resultant ball velocity.16,18-20 To generate linear velocity at the foot a skilled kicker will first rotate the hip backward into extension and flex the knee during the backswing phase of the kick. As the hip begins to flex, the knee continues to flex slightly, and then is held in this position for a brief period as the hip continues to flex. The knee begins to extend before the hip reaches maximal angular velocity,1 and, as the angular velocity of the hip declines, the knee velocity increases until the foot’s impact with the ball. Knee angular velocities can be as high as 1900°/s34 and resultant ball velocities of 35 m/s have been recorded in naturalistic settings.21
Kicking leg kinetics
The kinetic features of the soccer kick are less well understood than the kinematic features. Putnam’s24 seminal work demonstrated that the kicking movement is characterized by a complex blend of forces generated by muscle moments, motion-dependent moments that result from interactions among joints and segments, and gravitational forces. Hip flexion moments are nearly twice as large as knee extension moments14,17,22-26 and the smallest moments are associated with ankle plantar flexion.22 The most influential moments appear to be the extensor moment generated by the muscles that cross the knee joint and the moment associated with the angular velocity of the thigh.13,20,27
The specific time course of the moments at each joint during the kick has not yet been reliably established; various studies have reported different patterns.8,9 The values of the moments at each joint have also varied considerably, likely reflecting the range of methodologies that have been used to examine those moments, variations in data smoothing techniques, limitations in the assumptions of the inverse dynamic models used to estimate the moments, and differences in task constraints.
Coordination changes in the kicking leg
Considerable interest has been shown in how athletes acquire skilled coordination in the kicking leg. Drawing on the work of Bernstein,28 Anderson and Sidaway1 first described the learning curve for kicking as the process of freezing and then freeing degrees of freedom in the kicking leg. Novice kickers initially froze degrees of freedom by constraining the ranges of motion at the hip and knee joints. After 20 practice sessions, the kickers had significantly increased the range of motion at the hip and knee and had developed a qualitatively different pattern of coordination between the hip and knee joints, reflected primarily by an earlier onset of knee extension relative to the maximal angular velocity of the hip. Because the maximum linear velocity of the foot increased from prepractice to postpractice, without a concomitant increase in the maximum angular velocity of the hip, the release of degrees of freedom had presumably allowed a pattern of coordination to emerge that enabled the shank to exploit the momentum of the thigh. This conclusion is consistent with data showing the motion-dependent moment acting at the knee appears to compensate for the counterintuitive reversal of the muscle moment from extensor to flexor just prior to ball impact in skilled kickers.20
More recent research has shown that coordination changes in the kicking leg are task-specific and learner-specific.2,29-31 In some cases, degrees of freedom are constrained, then released, and then constrained again, consistent with the proposition that alternating reducing and increasing degrees of freedom is an ideal way to induce changes in coordination.32
Balance in kicking performance
Although balance control is presumed to be a fundamental constraint on the organization of skilled movement, it is surprising how few empirical studies have attempted to examine this presumption. Much of the work in this area has focused on acquisition of skills during the first year of life, when it is easier to see how limitations in infants’ ability to control their relationship to the environment constrain the expression of skilled activity. Many researchers have noted that control over balance and posture paces the emergence of all other skills, as skillful activity can occur only if infants can consistently regulate that relationship to the environment.3,5,6,33-35
Because kicking places considerable demands on postural control, it would seem to be an ideal task for studying the contribution that balance makes to skilled performance. Yet much of the research linking postural control to skilled performance has been done in sports like pistol shooting and rifle shooting, in which static balance is critical.36-38 Nevertheless, researchers are beginning to pay greater attention to the importance of dynamic balance in a range of different sports39 and some evidence suggests that highly skilled soccer players have better general balance control than less-skilled players.40
In one of the only studies to address an aspect of balance control during kicking, Shan and Westerhoff 41 examined the role of horizontal elevation of the arm on the nonkicking side on the maximal instep kick in skilled soccer players. While many researchers have assumed the arm plays a pivotal role in maintenance of balance during the kick,9 Shan and Westerhoff 41 argued that its primary function is to create a diagonal “tension arc” that helps generate velocity in the kicking leg by taking advantage of the stretch–shorten cycle in the hip flexors.
The support leg
Very little attention in the literature has been devoted to examining the role of the support leg in kicking performance. Lees and colleagues42 reported that skilled soccer players executing a maximal instep kick generated flexion/extension joint moments of 4, 3.2, and 2.2 Nm/kg for the hip, knee, and ankle joints, respectively. The support leg knee and ankle moments are much larger than those reported for the kicking leg.9
An early study found no correlations between ground reaction forces on the support leg and maximum kicking velocity.43 In contrast, Barfield19 reported significant correlations between mediolateral ground reaction forces and maximum kicking velocity on the dominant kicking leg but not the nondominant kicking leg in skilled soccer players. Similarly, Clagg and colleagues44 reported that female soccer players used greater pulling torques and smaller braking torques in the dominant than in the nondominant plant leg while kicking.
Surprisingly, though Orloff and colleagues45 found higher mediolateral ground reaction forces in female soccer players than male soccer players, no differences between men and women were seen in maximum kicking velocities. Despite these inconsistences, it is important to note that skilled soccer players have been shown to demonstrate superior unipedal balance and different unipedal balance control strategies than less-skilled players.46
To further examine the importance of the support leg in kicking, we47 provided unskilled kickers with external postural support by allowing the hand contralateral to the kicking leg to grasp a rigid support. The provision of this augmented support significantly increased ball velocity, suggesting that postural control over the support leg makes an important contribution to kicking performance.
More recently we attempted to quantify the role of the support leg in kicking performance through a correlation approach.48 We reasoned that single-leg balance on the support leg should predict kicking performance on the opposite leg if balance control was important for performance. Participants kicked a soccer ball with the right and left legs for maximum accuracy and velocity and performed single-leg balance on a force plate for 30 seconds with the right and left legs. Single-leg balance was significantly correlated with kicking accuracy, but not velocity. Dominant (right) leg kicking accuracy correlated more strongly with nondominant (left) leg balance than dominant (right) leg balance. (The right leg was the dominant leg in all study participants.) The same was not true, however, for nondominant (left) leg kicking accuracy, which was significantly correlated with nondominant (left) leg balance but not correlated with dominant (right) leg balance.
The asymmetrical nature of the results was interpreted as support for the dynamic dominance model of motor lateralization suggested by Sainburg and colleagues.49-51 This model proposes that each cerebral hemisphere/limb system becomes specialized for controlling different aspects of task performance. Although evidence in support of the model is confined to the upper extremity, if the dynamic dominance model holds for the lower extremities it would predict that the right leg/left hemisphere system would be specialized for trajectory control and the left leg/right hemisphere system for stability control in right-leg dominant kickers, consistent with what was found in our kicking study.
The specificity of balance
The lack of association between single-leg balance and kicking velocity ran counter to our prediction, suggesting the stability requirements associated with balancing on one leg are different from those required to support the body when swinging the kicking leg at maximal velocity. The finding may not be surprising given that substantial differences in the way posture is organized to facilitate movement have been documented for highly similar tasks. For example, the organization of anticipatory postural adjustments in a French kickboxing task was quite different when the boxers were required to kick a bag with minimal versus maximal force52 and when the bag was kicked with the same force but the kick was initiated with the kicking foot on or off the ground.53 Because there seems to be a high degree of task specificity in the way posture is organized to facilitate movement, it is likely that a more dynamic test of single-leg balance, such as hopping or swinging the free leg while standing on a force plate, would predict the capacity to generate maximum kicking velocity.
Much remains to be learned about how kicking is organized and how kicking performance might be improved. Researchers are increasingly realizing that kicking is a whole-body movement that is responsive to a wide range of constraints related to the task, the environment, and the performer. Recent research has confirmed that control of balance plays an important role in kicking performance, though clearly more work is needed in this area. Further studies on the relationship between balance and kicking can make broader contributions to our understanding of how complex skills are organized and acquired. Such understanding can in turn contribute to the development of strategies to facilitate the acquisition and reacquisition of movement skills.
David Anderson, PhD, is a professor of kinesiology at San Francisco State University. Ben Sidaway, PT, PhD, is a professor of physical therapy at Husson University in Bangor, ME.
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