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// July 2013Kicking biomechanics: Importance ofbalance

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.

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.

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

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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. These effects highlight how sensitive all movements

are to variations in constraints related to the task, the performer, and the

environment.

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. 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. The linear velocity of the kicking foot is highly correlated with

the resultant ball velocity. 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,

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°/s and resultant ball velocities of 35 m/s have been recorded in

naturalistic settings.

Kicking leg kinetics

The kinetic features of the soccer kick are less well understood than the

kinematic features. Putnam’s 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 moments and the smallest moments are

associated with ankle plantar flexion. 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.

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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. 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, Anderson and

Sidaway 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.

More recent research has shown that coordination changes in the kicking leg are

task-specific and learner-specific. 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.

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.

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

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performance has been done in sports like pistol shooting and rifle shooting, in

which static balance is critical. Nevertheless, researchers are beginning to

pay greater attention to the importance of dynamic balance in a range of different

sports and some evidence suggests that highly skilled soccer players have

better general balance control than less-skilled players.

In one of the only studies to address an aspect of balance control during kicking,

Shan and Westerhoff 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, Shan and Westerhoff 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 colleagues 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.

An early study found no correlations between ground reaction forces on the

support leg and maximum kicking velocity. In contrast, Barfield 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 colleagues 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 colleagues 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.

To further examine the importance of the support leg in kicking, we 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. 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

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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. 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 force and when the bag was kicked with the same force but the kick

was initiated with the kicking foot on or off the ground. 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.

Conclusions

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

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