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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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