nuo
When the foot contacts the ground during a typical
running stride, the ground reaction force exceeds 2.5
times body weight (Cavanagh, 1990) and can be much
greater in more vigorous soccer movements. Boots
should have built into them materials designed to
reduce the eþ ect of these forces, but they often do not.
The shock force experienced by the player can increase
as a result of running speed or type of landing action
used and will be greater on hard as opposed to soft
surfaces. The shock force can be assessed by the use of
accelerometers placed on the lower tibia (Lafortune,
1991), which measures peak shank deceleration. Lees
and Jones (1994) investigated this characteristic for
soccer boots compared to running shoes when running
at a speed of 3.5 m s- 1 on a variety of surfaces. They
found that the mean peak shank deceleration when subjects
used soccer boots and ran on grass was 25.6 m s- 2;
when they used running shoes and ran on grass, it was
23.3 m s- 2 and on concrete it was 26.5 m s- 2. Running
on grass signi® cantly reduced the peak shank deceleration,
but wearing the less well-protected soccer boots
increased the peak shank deceleration by about 10%.
The bene® ts of the softer surface were lost when using
a boot that had no constructional midsole. Their view
was that boot construction, which included a midsole
element, would result in a reduction in impact severity.
Boot studs and cleats are important for providing
traction on a variety of surfaces. They have evolved from
a simple ridge on the sole through leather cleats to the
226 Lees and Nolan
modern plugs and spikes of various lengths (Segesser
and Pforringer, 1989). The grip provided is a function
of the depth of penetration of the stud and the ® rmness
of the turf . Very wet turf will mean that short studs fail
to penetrate into the ® rmer ground underneath and lead
to slipping. On the other hand, very hard turf will not
allow good penetration and lead to pressure areas on the
foot at the heel or forefoot of the boot. Studs of varying
length and diameter help to overcome some of these
problems. The amount of gr ip provided by a surface is
an important component of playing quality and has
been dealt with from the point of view of the surface,
but stud con® guration and type are also important.
Although Winterbottom (1985) found that sliding
resistance was aþ ected little by stud con® guration, he
did report large diþ erences in the coeYcient of sliding
fr iction (0.34± 0.84) between diþ erent stud types. The
greatest diþ erence between stud types was found for the
torsional traction coeYcients, which ranged from a high
of 2.5 to a low of 1.0: this clearly should be matched to
the pitch conditions. While players do this subjectively,
this is an area which deserves greater research attention.
Bowers and Martin (1975) reported large diþ erences in
the coeYcient of sliding fr iction between stud and cleat
types (range 1.16± 1.95), agreeing with Winterbottom,
although the values they reported are noticeably higher.
Boots should be able to distribute the force so that
it is not concentrated in certain areas, such as under
the heel or under the head of the ® rst metatarsal. The
positioning of studs is particularly critical, as well as
the method of attachment of the stud to the boot. Ring
(1995) demonstrated that wide screw studs lead to
lower foot temperature than the conventional narrow
screw stud, and this promoted foot comfort and
reduced the likelihood of blisters. The foot is susceptible
to knocking and treading by the feet of other players,
thus the material of the boot should provide protection
from this. The use of sound or padded leather is necessary.
All of these methods have been adopted by manufacturers
but no scienti® c data have been reported.
Novel design features appear from time to time which
purport to enhance performance. Two similar devices
relate to stud type. One is a non-symmetrical stud that
has an almost aerodynamic shape in cross-section,
which is placed in a circular pattern under the ball of the
foot. This device, known as `Spingrip’ , purports to
allow easier rotation on the foot (a reduction in torsional
traction) while still producing good sliding friction for
starting and stopping. The second device, known as
`Blades’ , is an asymmetrical cleat design that again
allows reduced torsional traction while maintaining and
enhancing linear motion. While these two devices have
appeared on commercially available boots, there have
been no published reports of any rigorous scienti® c
investigations.
Another novel approach to boot design is `Craig’ s
Boot’ (British Broadcasting Corporation, 1994). This
boot has a toe piece which has a high fr iction surface.
This provides better grip between the foot and the ball,
which results in a greater force and torque acting on the
ball. It is claimed that tests have demonstrated a 27%
increase in ball spin and a 7% increase in ball speed,
although these values have not been published in the
scienti® c literature or veri® ed by other investigators. If
the claims are true, the importance of boot design for
ball performance will be demonstrated.
Boot designers have acknowledged the need to provide
adequate forefoot ¯ exibility (Rodano et al., 1988).
This is achieved by providing a crease in the sole of the
boot along a line where the metatarsal heads would sit in
the boot. Although this provides a suitable construction
for ¯ exion of the joints, it has been noted by Asami and
Nolte (1983) that a major factor aþ ecting the success of
a kick is the stiþ ness of the dorsal surface of the foot,
which has to sustain forces in excess of 1000 N. This
stiþ ness is increased by the presence of the boot, but
reduced by the ¯ exibility crease created to promote
metatarsophalangeal joint ¯ exion. Lees (1993) has suggested
that these two requirements are incompatible,
and that it might be possible for the sole of the boot to
be designed with a hinge-locking mechanism providing
¯ exion in one direction but stiþ ness in the other. Such a
design has yet to be made commercially available.
Shin guards
Shin guards are used by soccer players to protect their
shins and ankles from the eþ ects of direct contact by an
opponent’ s boot. Their primary function is to protect
the skin, underlying soft tissues and bones of the shank
from external impacts. They prevent injury by means of
shock absorption, load spreading and by modifying the
energy absorption characteristics of the leg system. The
wearing of these shin guards is mandatory in soccer, yet
their eþ ectiveness in reducing the severity of impact has
only recently been evaluated, and no standard methods
for their evaluation exist.
The shin guard is constructed from a hard outer
casing and a softer inner layer. The material used for
the outer casing is usually thermoplastic moulded to the
curvature of the leg, with a shock-absorbing inner
material made of ethylene vinyl acetate (EVA) or some
other foam material. Lees and Cooper (1995) tried
to measure shin guard performance. They tested the
ability of ® ve types of contemporary shin guards to
reduce the impact from a direct blow. The methods they
used followed those for the testing of cricket pads
(British Standards Institution, 1981). This involved
dropping a 5 kg mass from a set height of 40 cm and
monitoring its deceleration on impact with the shin
The biomechanics of soccer: A review 227
guard, which was placed on a wooden shank form
supported by a rigid surface. From this, the dwell
time and energy return of the shin guard were determined.
The results showed that shin guards per se were
eþ ective in providing a substantial reduction in impact
deceleration of between 40 and 60% when compared
to striking the wooden shank form without shin guard
protection. They appeared to do this by increasing the
dwell time of the striker on the shin guard by 30± 40%.
The harder outer shell of the shin guards acted as an
area elastic surface (Nigg and Yeadon, 1987) and was
eþ ective in spreading the load. Thus it would appear
that a combination of peak force reduction and an
increase in impact area will reduce the localized
pressure, and therefore the likelihood for skin abrasion
and penetration from boots or studs. Although over 70%
of the impact energy was absorbed, shin guards do not
contain suYcient material to absorb large quantities
of energy, and so it is unlikely that they are capable of
preventing fractures from high-energy blows. The
results also showed that there were signi® cant diþ erences
between shin guards in their ability to reduce
peak deceleration. This reduction ranged from 28 to
56% of the impact deceleration obtained without a shin
guard. The shin guard which performed less well was
constructed of a thermoplastic outer casing with a foam
inner layer; the shin guard which performed best was of
a similar thermoplastic outer shell but with a 5 mm EVA
inner layer.
Summary
Many factors interact to aþ ect the response of the
equipment used within the game of soccer. The
equipment has mechanical characteristics which are
subject to variation, but which can be measured
reasonably well. The interaction between the player
and the equipment is also a source of variation, but this
is diYcult to quantify and makes the eYcacy of the
equipment more diYcult to predict. Although equipment
manufacturers will undoubtedly have conducted
extensive research, little of this is in the public domain.
Nevertheless, the examples have illustrated general
principles which can be applied across a range of soccer
equipment.
Biomechanical aspects of soccer injury
A third major area of concern in the biomechanics of
soccer is the causative mechanisms of injury; these will
help us to understand better the principles of injury
prevention. Injury generally originates from physical
causes and arises from forces greater than those which
can be tolerated by the biological structures. Injury
mechanisms may be due to player ® tness, training
errors, the rules of the game, the weather and equipment
(Ekstrand and Nigg, 1989). Here, we restrict ourselves
to an investigation of certain types of injury in soccer
associated with selected equipment, where the biomechanical
mechanisms can be identi® ed to provide an
understanding of causative factors.
Soccer is a contact sport and injuries to players are
frequent. In most injury surveys, soccer has been
reported to be the game with most injuries or injuries
per exposure (Sports Council, 1992; Arnason et al.,
1996). Lower extremity injuries are by far the most
prevalent, accounting for over three-quarters of
reported injuries (Ekstrand, 1994). Of these, about
one-third are overuse injuries and two-thirds are acute
injuries (Ekstrand and Nigg, 1989). It is relevant to
investigate whether the boots used and surfaces played
on are factors in these lower limb injuries. There is
growing concern over the long-term consequences of
minor injuries, which are often not re¯ ected in injury
statistics. To this extent, the impact received by the head
when heading the ball may lead to longer-term brain
damage and can be investigated biomechanically.
Boots
The soccer boot has many functions. It must be comfor
table, ® t the foot well and allow freedom of movement,
while providing protection against external
forces, spreading the pressures over the sole of the boot
and controlling foot movement, particularly of the rear
foot. The soccer boot was traditionally made with a
high ankle support. The advent of a faster running
game has led to a preference for the low-cut soccer-type
boot, which allows greater movement of the ankle and
subtalar joints. This lower-cut boot sacri® ces protection
for performance; consequently, more frequent and more
severe ankle injuries occur. The major ankle injury
resulting from foot instability is the ankle inversion
sprain. Ankle inversion injuries are reported to be
responsible for 9.6% of all soccer injuries (Kibler, 1993)
and are thought to be the most common injury in the
game (Surve et al., 1994), indicating that the soccer boot
performs its protective functions poorly.
The role of the boot in protecting the ankle joint was
investigated by Johnson et al. (1976). They investigated
the torsional stiþ ness about an anterior-posterior axis
through the ankle joint for diþ erent designs of boot
uppers. They modelled the shank and foot using a massspring-
dashpot system, which gives the joint its load
response characteristics (Fig. 8). The boot added
another restrictive layer to the outside of the ankle,
allowing the natural stiþ ness of the joint to be supplemented
by the properties of the boot. The low-cut boot
228 Lees and Nolan
Figure 8 A foot and ankle model based on a spring and dashpot system, with the eþ ect of additional stiþ ness from high-cut and
low-cut boots. Redrawn with permission from Johnson et al. (1976).
shank
talus
foot
C
effect of low
cut boot
shank
talus
foot
B
effect of high
cut boot
shank
talus
foot
A
protected the subtalar joint, whereas the higher-cut boot
protected both this joint and the ankle joint. The mean
angular stiþ ness of the foot in the high-cut boot was
14.6 N m rad- 1, whereas for the low-cut boot it was 9.6
N m rad- 1. A high-cut boot thus provides over 50% more
torsional stiþ ness than a low-cut boot. Johnson et al.
concluded that the loads carried by the collateral
ligaments in either an inversion or eversion injury would
be reduced when wearing high-cut boots compared to
low-cut boots. They also found that the torsional stiþ -
ness was aþ ected by the material used and the geometry
of boot construction. One interesting conclusion they
came to was that, if low-cut boots were to be worn, it
would be better for the material to be as soft as possible.
This is because the subtalar joint has a certain amount
of mobility, and if the ankle is turned in an inversion±
eversion mode, a low-cut boot would allow the subtalar
joint to accommodate most of the movement. If a
low-cut boot was of a stiþ construction, then the boot
would transfer some of the load away from the subtalar
joint to the ankle joint. As the latter joint does not have
any ¯ exibility in the inversion± eversion direction, the
additional load would be taken up by the collateral
ligaments, leading to a greater likelihood of ligament
damage. On the other hand, a high-cut boot should
be made with stiþ material because it already has a
protective function for the ankle joint and collateral
ligaments. The stiþ er the material, the more the load is
taken by the boot material rather than the ligaments. It
should be noted, however, that a high-cut boot with stiþ
material is only about twice as stiþ as a low-cut boot
constructed of low-stiþ ness material, and that for a
severe inversion movement, even a high-cut boot would
be insuYcient to prevent damage occurring.
Players’ preference for a low-cut design, with its consequential
inability to protect from serious ankle injury,
has led to the widespread use of various supplementary
methods for increasing ankle joint stiþ ness. Taping provides
an additional layer of support to the ligaments of
the joints and is a favoured prophylactic procedure,
although its eþ ectiveness may not be long-lasting. The
eYcacy of this procedure is illustrated by the work of
Surve et al. (1994), who reported a ® ve-fold reduction in
the incidence of ankle sprains when using a `Sport-
Stirrup’ semi-rigid orthosis in soccer players who had a
history of ankle sprains. This orthosis was constructed
as a stirrup of thermoplastic material placed around
the sole and medial and lateral sides of the ankle joint,
and contained two in¯ atable air cells on its inner surface
at the level of the malleoli. Even players who had no
previous history of ankle injury appeared to bene® t, as
the authors found a two-fold reduction in the incidence
of injury for these players. As well as reducing the incidence
of injury, the orthosis also reduced the severity of
injury. There was a ® ve-fold reduction in more severe
ankle sprains compared to mild sprains in the players
with a previous history of ankle sprains, although there
was no diþ erence between the incidence of mild and
severe sprains for players without a history of ankle
sprains. The orthosis did not lead to a greater incidence
of injury at other joints in the body, and the authors
concluded that the use of this particular semi-rigid
orthosis was to be recommended for the reduction of
ankle injuries in soccer players.
The biomechanics of soccer: A review 229
Surfaces
Ekstrand and Nigg (1989) suggested that 24% of the
injuries in soccer could be attributed to unsatisfactory
playing surfaces, but these often occurred in association
with one or more other factors, such as poor footwear,
muscle tightness or joint instability. The main precursor
to injury was thought to be the rapid changes between
diþ erent types of playing surfaces (during winter preseason
training) combined with inferior shoes. They
found no direct evidence that a harder (higher stiþ ness)
arti® cial surface produced more traumatic injuries than
a softer (lower stiþ ness) surface, although they did
speculate that the harder surface might lead to more
overuse injuries.
Winterbottom (1985) summarized the results of
studies concerned with injuries on natural and arti® cial
turf pitches. He reported that, in general, there was no
diþ erence in the number of injuries per exposure on
either type of surface; where a diþ erence was reported,
arti® cial turf tended to produce fewer injuries than
natural turf . In general, there are a greater number of
traumatic injuries on natural turf compared to arti® cial
turf pitches, while there are considerably more (up to
15 times) minor abrasions and friction burns on arti-
® cial turf compared to natural turf pitches. Subsequent
studies by Ekstrand and Nigg (1989) and the Football
League (1989) suppor t these ® ndings.
One concern regarding arti® cial turf is the possibility
of generating high translational and rotational friction
loads that could place a greater load on a player’ s knee.
In a report of American Football injuries (Zemper,
1984), the incidence of knee injuries was found to
be over twice as high on arti® cial turf surfaces. However,
it should be pointed out that American Football is a
collision sport and requires diþ erent skills from those of
soccer players; this could aþ ect the extrapolation of
these ® ndings to arti® cial soccer pitches. Other factors
that aþ ect the translational and rotational loads are stud
type, length, diameter and con® guration; these can be
manipulated to optimize translational and rotational
friction, and have been reviewed above.
The Football League (1989) acknowledged that there
was a `fear factor’ associated with playing on arti® cial
surfaces, which presumably refers to the likelihood of
sustaining abrasion or fr iction burn injuries. It was
noted that the incidence of fr iction burns decreased over
the duration of the investigation, and this was attributed
to two main factors. The ® rst was the change associated
with the wear of an arti® cial pitch; when new, its pile is
upright but drops with use, presumably as a result of
® bre fatigue. The second factor was that the players
developed a familiarity with the surface and they
changed their game accordingly. This latter fact was
supported by the assessment of referees, who remarked
that the game as played on arti® cial pitches was faster
and there were fewer hard challenges and sliding tackles.
The Football League also noted the lower incidence of
dislocations and fractures on arti® cial surfaces, agreeing
with Winterbottom (1985) and Ekstrand and Nigg
(1989); this was also attributed to the changes in the
way in which the game was played.
The adaptation of players to the surface is an important
factor in surface-related injuries. Ekstrand and
Gilquist (1983) repor ted that the risk of traumatic
injury increased when changing from one type of
surface to another. They suggested that it took about
six games for a player to adapt. This poses obvious
problems for players who change frequently from one
type of surface to another during the competitive
season; as a consequence, they are likely to be at a
greater risk of injury. Recently, in a study of soccer
injuries in Iceland, Arnason et al. (1996) found a 2.5
times greater incidence of injuries on arti® cial surfaces
compared to grass surfaces, with no clear diþ erences in
injury pro® les between the two and a higher overall
injury incidence than found in other studies. It may be
that the rapid change between surface types required by
the players in this study as a result of their playing
environment was a causative factor.
Arti® cial surfaces other than arti® cial turf are used
and these may lead to an even greater number of injuries.
Ekstrand (1994) reported injuries to be six times
more likely on gravel compared to arti® cial turf; in
contrast, Arnason et al. (1996) found that there were
fewer injuries on gravel than on grass, and three times
fewer on gravel than on ar ti® cial turf . Pitch size can also
have an eþ ect on injury rate. Hoþ and Martin (1986)
found that, in indoor soccer, the injury rate was six
times that of matches played on a full-size pitch. They
attributed this to the smaller playing area and con® ning
walls, which increased the intensity of play and thus the
risk of injury.
Heading of the ball and head injur ies
The possible injurious eþ ect of heading the ball has
been the subject of recent biomechanical investigations
as a result of potential legal cases over the misuse of
equipment for young players. The incidence of head
injury is more prevalent than is generally acknowledged.
Barnes et al. (1994) reported that, in a sample of 72
active players, 89% had experienced some kind of head
trauma. While these injuries were acute (loss of consciousness,
fractures, nose bleed, mouth lacerations),
there is a worry that the cumulative eþ ects of head
trauma can also produce a risk. The serious eþ ects of
accumulated head trauma have been reported by several
authors. Tysvaer and Storli (1981) found that, in a sample
of 128 active Norwegian players, 50% experienced
230 Lees and Nolan
symptoms associated with head impacts. In a follow-up
study on a sample of 37 former Norwegian players,
Tysvaer and Lochen (1991) reported that 81% demonstrated
some form of intellectual impairment, which
was attributed to cumulative trauma probably the result
of repeatedly heading the ball. Sortland et al. (1982)
reported that, in a sample of 43 former Norwegian
players, 21% complained of chronic neck problems,
with 58% showing a decreased range of motion and
radiographic abnormalities. However, Jordan et al.
(1996), in a comparison of the MRI scans of national
level US soccer players and elite track athletes, found no
evidence of accumulated trauma in either group and no
diþ erences between the groups. They concluded that
brain damage was more likely to result from acute
trauma and alcohol abuse rather than repetitive ball
heading. Despite this, it would appear that there is
suYcient evidence to suggest that intense involvement
in soccer might lead to severe long-term head and neck
problems and that, in part, these might be due to
heading the ball.
Brain damage can develop from: (1) direct impact
leading to excessive linear acceleration of the brain,
which causes compression waves and high internal
pressures; and (2) a glancing impact leading to rotational
accelerations of the brain, which cause shearing
between the brain and the skull (Levendusky et al.,
1988). The linear and rotational accelerations of the
head during impact can be determined, although it is
not known precisely what levels of acceleration are
thought to cause injury. For direct blows, a tolerance
level of about 80 g is thought to lead to a loss of consciousness.
This ® gure comes from the measured
acceleration of a professional boxer’ s punch (Atha et al.,
1985). The tolerance levels for rotational accelerations
are more diYcult to estimate. Holburn (1943) has suggested
that a rotational acceleration of 7500 rad s- 2 would
lead to a loss of consciousness, whereas Stalnaker et al.
(1977) suggested a ® gure of 5500 rad s- 2. More recently,
Schneider and Zernicke (1988) used a ® gure of 1800
rad s- 2 to indicate a tolerance threshold based on the
Head Injury Criterion used in vehicle accident research.
Burslem and Lees (1988) used a twin accelerometry
system to investigate the acceleration of the head when
heading the ball at a relatively low speed (ball speed
of about 7 m s- 1). They found that the accelerations
produced by the head on contact with the ball were
about 15 g, and the rotational accelerations were about
200 rad s- 2. Both of these ® gures are well below the
tolerance levels identi® ed above. Townend (1988) used
a mathematical model based on two spheres colliding as
a simulation of central impact. Using an initial ball
velocity of 10 m s- 1 and a head impact velocity of 3± 5
m s- 1, he found the impact acceleration of the head to be
about 20± 25 g. His simulation also predicted that the
impact acceleration would increase as the head± ball
mass ratio decreased. The impact acceleration was
found to increase as a linear function of ball mass, and
decrease with an increase in a player’ s body mass. The
heads of lightweight players therefore receive a proportionately
larger impact acceleration. In a more
detailed simulation, Schneider and Zernicke (1988)
estimated that, for a relative head± ball speed of 10 m s- 1
and a head± ball mass ratio of 10, the initial acceleration
of the head was about 19 g. This is in agreement with the
results of Burslem and Lees (1988) and Townend
(1988). It implies that there is no immediate danger
from heading the ball. However, these results suggest
that there might be a danger for small children. If
head± ball mass ratios drop to about 3 (typical for young
children), then heading a fast ball would put them close
to the tolerance threshold. The practical solution is
to reduce the mass of the ball for young players, and
ensure, by instruction and the rules of the game, that
they do not try to head fast-moving balls.
The results for rotational accelerations of the head are
also in general agreement. The 200 rad s- 2 repor ted by
Burslem and Lees (1988) is well below the tolerance
threshold of injury. Schneider and Zernicke (1988)
estimated that, for a header with a relative impact speed
of 10 m s- 1, the rotational acceleration is 366 rad s- 2, well
below their tolerance threshold. However, for children
using a full-size ball (head± ball mass ratio of about 3)
and trying to head a fast-moving ball (20 m s- 1 or more),
the rotational acceleration tolerance threshold is easily
reached. Their analysis suggested that the tolerance
threshold was reached more easily for rotational impacts
than for linear impacts. The general conclusion that can
be drawn is that, although heading a soccer ball appears
to be below the injury threshold, care needs to be taken,
particularly when dealing with young children during
the development of their skill. If the head± ball mass ratio
can be increased by using the muscles of the neck, the
eþ ect of the impact can be reduced. Skill training can
therefore play an important preventative role.
Summary
Soccer injuries are the result of many interrelating
factors, some of which can be isolated. For example,
the soccer boot has a poor protective function. Careful
boot design can have a minor in¯ uence on the severity
of inversion injuries. The inadequacy of the boot,
primarily determined by performance requirements, is
indicated by the need for, and success of, alternative
methods of providing ankle stability. Unlike the developments
in running shoe technology, little attention
has been paid to shock reduction or rear-foot control
characteristics of the soccer boot, which are often considered
aetiological factors in injury. Arti® cial surfaces
The biomechanics of soccer: A review 231
produce diþ erent injury pro® les than natural turf
pitches. There appears to be a tendency for fewer
serious injuries but more minor injuries on arti® cial
turf compared to natural turf pitches. It seems that the
type of surface may be indirectly responsible for a
change in injury pro® le by changing the nature of the
game. This change requires an adaptation period, and
players are more likely to be at risk if they change frequently
from one type of surface to another. Obtaining
clear evidence of speci® c pitch constructional characteristics
on injury is complicated by the interacting
in¯ uences of a number of factors. Long-term brain
damage in soccer would seem to be a possibility,
particularly for children, as a result of heading the ball.
Careful instruction and skill development, together
with correct equipment, is necessary for young players.
Tolerance thresholds are not well known, and although
simulation results suggest the importance of ball mass,
ball speed and player mass, there is still insuYcient
experimental data on head impact characteristics when
heading the ball. In particular, there is no information
regarding diþ erent methods of heading, the frequency
of occurrence of these methods, and the in¯ uence of
neck and shoulder muscles to increase the eþ ective mass
involved in the impact.
Conclusions
In this review, we have shown the diþ erent ways in
which biomechanics has been applied to soccer. We
focused on three main areas of application and showed
that the biomechanics of soccer is based on descriptive
experimental work that has covered a wide range of
topics, but there is little evidence of researchers taking
a systematic approach. There is much interest in
kicking as a skill, but there remain many gaps that biomechanists
can ® ll. As a consequence of these gaps,
experimental investigations have thrown up relatively
few contentious issues. Where these exist, it is more
likely to be the re¯ ection of subjects or the analytical
equipment used rather than a con¯ ict in understanding
the underlying mechanisms of performance. In some
examples, experimental work has given way to the use of
biomechanical modelling techniques. These have
helped both to investigate problems, in particular of an
injury-related nature where experimentation would be
diYcult to conduct, and to provide an understanding
of underlying mechanisms of performance. The multifactor
in¯ uences associated with many of the topics
considered are a limitation to our understanding, yet
this avenue of research must continue to be explored
if real progress is to be made.
In this review, we have shown that many features of
the game of soccer are amenable to biomechanical
treatment. There are still many opportunities for biomechanists
to have a role in the science of soccer, and it
is hoped that this review will help to direct future
investigations.
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running stride, the ground reaction force exceeds 2.5
times body weight (Cavanagh, 1990) and can be much
greater in more vigorous soccer movements. Boots
should have built into them materials designed to
reduce the eþ ect of these forces, but they often do not.
The shock force experienced by the player can increase
as a result of running speed or type of landing action
used and will be greater on hard as opposed to soft
surfaces. The shock force can be assessed by the use of
accelerometers placed on the lower tibia (Lafortune,
1991), which measures peak shank deceleration. Lees
and Jones (1994) investigated this characteristic for
soccer boots compared to running shoes when running
at a speed of 3.5 m s- 1 on a variety of surfaces. They
found that the mean peak shank deceleration when subjects
used soccer boots and ran on grass was 25.6 m s- 2;
when they used running shoes and ran on grass, it was
23.3 m s- 2 and on concrete it was 26.5 m s- 2. Running
on grass signi® cantly reduced the peak shank deceleration,
but wearing the less well-protected soccer boots
increased the peak shank deceleration by about 10%.
The bene® ts of the softer surface were lost when using
a boot that had no constructional midsole. Their view
was that boot construction, which included a midsole
element, would result in a reduction in impact severity.
Boot studs and cleats are important for providing
traction on a variety of surfaces. They have evolved from
a simple ridge on the sole through leather cleats to the
226 Lees and Nolan
modern plugs and spikes of various lengths (Segesser
and Pforringer, 1989). The grip provided is a function
of the depth of penetration of the stud and the ® rmness
of the turf . Very wet turf will mean that short studs fail
to penetrate into the ® rmer ground underneath and lead
to slipping. On the other hand, very hard turf will not
allow good penetration and lead to pressure areas on the
foot at the heel or forefoot of the boot. Studs of varying
length and diameter help to overcome some of these
problems. The amount of gr ip provided by a surface is
an important component of playing quality and has
been dealt with from the point of view of the surface,
but stud con® guration and type are also important.
Although Winterbottom (1985) found that sliding
resistance was aþ ected little by stud con® guration, he
did report large diþ erences in the coeYcient of sliding
fr iction (0.34± 0.84) between diþ erent stud types. The
greatest diþ erence between stud types was found for the
torsional traction coeYcients, which ranged from a high
of 2.5 to a low of 1.0: this clearly should be matched to
the pitch conditions. While players do this subjectively,
this is an area which deserves greater research attention.
Bowers and Martin (1975) reported large diþ erences in
the coeYcient of sliding fr iction between stud and cleat
types (range 1.16± 1.95), agreeing with Winterbottom,
although the values they reported are noticeably higher.
Boots should be able to distribute the force so that
it is not concentrated in certain areas, such as under
the heel or under the head of the ® rst metatarsal. The
positioning of studs is particularly critical, as well as
the method of attachment of the stud to the boot. Ring
(1995) demonstrated that wide screw studs lead to
lower foot temperature than the conventional narrow
screw stud, and this promoted foot comfort and
reduced the likelihood of blisters. The foot is susceptible
to knocking and treading by the feet of other players,
thus the material of the boot should provide protection
from this. The use of sound or padded leather is necessary.
All of these methods have been adopted by manufacturers
but no scienti® c data have been reported.
Novel design features appear from time to time which
purport to enhance performance. Two similar devices
relate to stud type. One is a non-symmetrical stud that
has an almost aerodynamic shape in cross-section,
which is placed in a circular pattern under the ball of the
foot. This device, known as `Spingrip’ , purports to
allow easier rotation on the foot (a reduction in torsional
traction) while still producing good sliding friction for
starting and stopping. The second device, known as
`Blades’ , is an asymmetrical cleat design that again
allows reduced torsional traction while maintaining and
enhancing linear motion. While these two devices have
appeared on commercially available boots, there have
been no published reports of any rigorous scienti® c
investigations.
Another novel approach to boot design is `Craig’ s
Boot’ (British Broadcasting Corporation, 1994). This
boot has a toe piece which has a high fr iction surface.
This provides better grip between the foot and the ball,
which results in a greater force and torque acting on the
ball. It is claimed that tests have demonstrated a 27%
increase in ball spin and a 7% increase in ball speed,
although these values have not been published in the
scienti® c literature or veri® ed by other investigators. If
the claims are true, the importance of boot design for
ball performance will be demonstrated.
Boot designers have acknowledged the need to provide
adequate forefoot ¯ exibility (Rodano et al., 1988).
This is achieved by providing a crease in the sole of the
boot along a line where the metatarsal heads would sit in
the boot. Although this provides a suitable construction
for ¯ exion of the joints, it has been noted by Asami and
Nolte (1983) that a major factor aþ ecting the success of
a kick is the stiþ ness of the dorsal surface of the foot,
which has to sustain forces in excess of 1000 N. This
stiþ ness is increased by the presence of the boot, but
reduced by the ¯ exibility crease created to promote
metatarsophalangeal joint ¯ exion. Lees (1993) has suggested
that these two requirements are incompatible,
and that it might be possible for the sole of the boot to
be designed with a hinge-locking mechanism providing
¯ exion in one direction but stiþ ness in the other. Such a
design has yet to be made commercially available.
Shin guards
Shin guards are used by soccer players to protect their
shins and ankles from the eþ ects of direct contact by an
opponent’ s boot. Their primary function is to protect
the skin, underlying soft tissues and bones of the shank
from external impacts. They prevent injury by means of
shock absorption, load spreading and by modifying the
energy absorption characteristics of the leg system. The
wearing of these shin guards is mandatory in soccer, yet
their eþ ectiveness in reducing the severity of impact has
only recently been evaluated, and no standard methods
for their evaluation exist.
The shin guard is constructed from a hard outer
casing and a softer inner layer. The material used for
the outer casing is usually thermoplastic moulded to the
curvature of the leg, with a shock-absorbing inner
material made of ethylene vinyl acetate (EVA) or some
other foam material. Lees and Cooper (1995) tried
to measure shin guard performance. They tested the
ability of ® ve types of contemporary shin guards to
reduce the impact from a direct blow. The methods they
used followed those for the testing of cricket pads
(British Standards Institution, 1981). This involved
dropping a 5 kg mass from a set height of 40 cm and
monitoring its deceleration on impact with the shin
The biomechanics of soccer: A review 227
guard, which was placed on a wooden shank form
supported by a rigid surface. From this, the dwell
time and energy return of the shin guard were determined.
The results showed that shin guards per se were
eþ ective in providing a substantial reduction in impact
deceleration of between 40 and 60% when compared
to striking the wooden shank form without shin guard
protection. They appeared to do this by increasing the
dwell time of the striker on the shin guard by 30± 40%.
The harder outer shell of the shin guards acted as an
area elastic surface (Nigg and Yeadon, 1987) and was
eþ ective in spreading the load. Thus it would appear
that a combination of peak force reduction and an
increase in impact area will reduce the localized
pressure, and therefore the likelihood for skin abrasion
and penetration from boots or studs. Although over 70%
of the impact energy was absorbed, shin guards do not
contain suYcient material to absorb large quantities
of energy, and so it is unlikely that they are capable of
preventing fractures from high-energy blows. The
results also showed that there were signi® cant diþ erences
between shin guards in their ability to reduce
peak deceleration. This reduction ranged from 28 to
56% of the impact deceleration obtained without a shin
guard. The shin guard which performed less well was
constructed of a thermoplastic outer casing with a foam
inner layer; the shin guard which performed best was of
a similar thermoplastic outer shell but with a 5 mm EVA
inner layer.
Summary
Many factors interact to aþ ect the response of the
equipment used within the game of soccer. The
equipment has mechanical characteristics which are
subject to variation, but which can be measured
reasonably well. The interaction between the player
and the equipment is also a source of variation, but this
is diYcult to quantify and makes the eYcacy of the
equipment more diYcult to predict. Although equipment
manufacturers will undoubtedly have conducted
extensive research, little of this is in the public domain.
Nevertheless, the examples have illustrated general
principles which can be applied across a range of soccer
equipment.
Biomechanical aspects of soccer injury
A third major area of concern in the biomechanics of
soccer is the causative mechanisms of injury; these will
help us to understand better the principles of injury
prevention. Injury generally originates from physical
causes and arises from forces greater than those which
can be tolerated by the biological structures. Injury
mechanisms may be due to player ® tness, training
errors, the rules of the game, the weather and equipment
(Ekstrand and Nigg, 1989). Here, we restrict ourselves
to an investigation of certain types of injury in soccer
associated with selected equipment, where the biomechanical
mechanisms can be identi® ed to provide an
understanding of causative factors.
Soccer is a contact sport and injuries to players are
frequent. In most injury surveys, soccer has been
reported to be the game with most injuries or injuries
per exposure (Sports Council, 1992; Arnason et al.,
1996). Lower extremity injuries are by far the most
prevalent, accounting for over three-quarters of
reported injuries (Ekstrand, 1994). Of these, about
one-third are overuse injuries and two-thirds are acute
injuries (Ekstrand and Nigg, 1989). It is relevant to
investigate whether the boots used and surfaces played
on are factors in these lower limb injuries. There is
growing concern over the long-term consequences of
minor injuries, which are often not re¯ ected in injury
statistics. To this extent, the impact received by the head
when heading the ball may lead to longer-term brain
damage and can be investigated biomechanically.
Boots
The soccer boot has many functions. It must be comfor
table, ® t the foot well and allow freedom of movement,
while providing protection against external
forces, spreading the pressures over the sole of the boot
and controlling foot movement, particularly of the rear
foot. The soccer boot was traditionally made with a
high ankle support. The advent of a faster running
game has led to a preference for the low-cut soccer-type
boot, which allows greater movement of the ankle and
subtalar joints. This lower-cut boot sacri® ces protection
for performance; consequently, more frequent and more
severe ankle injuries occur. The major ankle injury
resulting from foot instability is the ankle inversion
sprain. Ankle inversion injuries are reported to be
responsible for 9.6% of all soccer injuries (Kibler, 1993)
and are thought to be the most common injury in the
game (Surve et al., 1994), indicating that the soccer boot
performs its protective functions poorly.
The role of the boot in protecting the ankle joint was
investigated by Johnson et al. (1976). They investigated
the torsional stiþ ness about an anterior-posterior axis
through the ankle joint for diþ erent designs of boot
uppers. They modelled the shank and foot using a massspring-
dashpot system, which gives the joint its load
response characteristics (Fig. 8). The boot added
another restrictive layer to the outside of the ankle,
allowing the natural stiþ ness of the joint to be supplemented
by the properties of the boot. The low-cut boot
228 Lees and Nolan
Figure 8 A foot and ankle model based on a spring and dashpot system, with the eþ ect of additional stiþ ness from high-cut and
low-cut boots. Redrawn with permission from Johnson et al. (1976).
shank
talus
foot
C
effect of low
cut boot
shank
talus
foot
B
effect of high
cut boot
shank
talus
foot
A
protected the subtalar joint, whereas the higher-cut boot
protected both this joint and the ankle joint. The mean
angular stiþ ness of the foot in the high-cut boot was
14.6 N m rad- 1, whereas for the low-cut boot it was 9.6
N m rad- 1. A high-cut boot thus provides over 50% more
torsional stiþ ness than a low-cut boot. Johnson et al.
concluded that the loads carried by the collateral
ligaments in either an inversion or eversion injury would
be reduced when wearing high-cut boots compared to
low-cut boots. They also found that the torsional stiþ -
ness was aþ ected by the material used and the geometry
of boot construction. One interesting conclusion they
came to was that, if low-cut boots were to be worn, it
would be better for the material to be as soft as possible.
This is because the subtalar joint has a certain amount
of mobility, and if the ankle is turned in an inversion±
eversion mode, a low-cut boot would allow the subtalar
joint to accommodate most of the movement. If a
low-cut boot was of a stiþ construction, then the boot
would transfer some of the load away from the subtalar
joint to the ankle joint. As the latter joint does not have
any ¯ exibility in the inversion± eversion direction, the
additional load would be taken up by the collateral
ligaments, leading to a greater likelihood of ligament
damage. On the other hand, a high-cut boot should
be made with stiþ material because it already has a
protective function for the ankle joint and collateral
ligaments. The stiþ er the material, the more the load is
taken by the boot material rather than the ligaments. It
should be noted, however, that a high-cut boot with stiþ
material is only about twice as stiþ as a low-cut boot
constructed of low-stiþ ness material, and that for a
severe inversion movement, even a high-cut boot would
be insuYcient to prevent damage occurring.
Players’ preference for a low-cut design, with its consequential
inability to protect from serious ankle injury,
has led to the widespread use of various supplementary
methods for increasing ankle joint stiþ ness. Taping provides
an additional layer of support to the ligaments of
the joints and is a favoured prophylactic procedure,
although its eþ ectiveness may not be long-lasting. The
eYcacy of this procedure is illustrated by the work of
Surve et al. (1994), who reported a ® ve-fold reduction in
the incidence of ankle sprains when using a `Sport-
Stirrup’ semi-rigid orthosis in soccer players who had a
history of ankle sprains. This orthosis was constructed
as a stirrup of thermoplastic material placed around
the sole and medial and lateral sides of the ankle joint,
and contained two in¯ atable air cells on its inner surface
at the level of the malleoli. Even players who had no
previous history of ankle injury appeared to bene® t, as
the authors found a two-fold reduction in the incidence
of injury for these players. As well as reducing the incidence
of injury, the orthosis also reduced the severity of
injury. There was a ® ve-fold reduction in more severe
ankle sprains compared to mild sprains in the players
with a previous history of ankle sprains, although there
was no diþ erence between the incidence of mild and
severe sprains for players without a history of ankle
sprains. The orthosis did not lead to a greater incidence
of injury at other joints in the body, and the authors
concluded that the use of this particular semi-rigid
orthosis was to be recommended for the reduction of
ankle injuries in soccer players.
The biomechanics of soccer: A review 229
Surfaces
Ekstrand and Nigg (1989) suggested that 24% of the
injuries in soccer could be attributed to unsatisfactory
playing surfaces, but these often occurred in association
with one or more other factors, such as poor footwear,
muscle tightness or joint instability. The main precursor
to injury was thought to be the rapid changes between
diþ erent types of playing surfaces (during winter preseason
training) combined with inferior shoes. They
found no direct evidence that a harder (higher stiþ ness)
arti® cial surface produced more traumatic injuries than
a softer (lower stiþ ness) surface, although they did
speculate that the harder surface might lead to more
overuse injuries.
Winterbottom (1985) summarized the results of
studies concerned with injuries on natural and arti® cial
turf pitches. He reported that, in general, there was no
diþ erence in the number of injuries per exposure on
either type of surface; where a diþ erence was reported,
arti® cial turf tended to produce fewer injuries than
natural turf . In general, there are a greater number of
traumatic injuries on natural turf compared to arti® cial
turf pitches, while there are considerably more (up to
15 times) minor abrasions and friction burns on arti-
® cial turf compared to natural turf pitches. Subsequent
studies by Ekstrand and Nigg (1989) and the Football
League (1989) suppor t these ® ndings.
One concern regarding arti® cial turf is the possibility
of generating high translational and rotational friction
loads that could place a greater load on a player’ s knee.
In a report of American Football injuries (Zemper,
1984), the incidence of knee injuries was found to
be over twice as high on arti® cial turf surfaces. However,
it should be pointed out that American Football is a
collision sport and requires diþ erent skills from those of
soccer players; this could aþ ect the extrapolation of
these ® ndings to arti® cial soccer pitches. Other factors
that aþ ect the translational and rotational loads are stud
type, length, diameter and con® guration; these can be
manipulated to optimize translational and rotational
friction, and have been reviewed above.
The Football League (1989) acknowledged that there
was a `fear factor’ associated with playing on arti® cial
surfaces, which presumably refers to the likelihood of
sustaining abrasion or fr iction burn injuries. It was
noted that the incidence of fr iction burns decreased over
the duration of the investigation, and this was attributed
to two main factors. The ® rst was the change associated
with the wear of an arti® cial pitch; when new, its pile is
upright but drops with use, presumably as a result of
® bre fatigue. The second factor was that the players
developed a familiarity with the surface and they
changed their game accordingly. This latter fact was
supported by the assessment of referees, who remarked
that the game as played on arti® cial pitches was faster
and there were fewer hard challenges and sliding tackles.
The Football League also noted the lower incidence of
dislocations and fractures on arti® cial surfaces, agreeing
with Winterbottom (1985) and Ekstrand and Nigg
(1989); this was also attributed to the changes in the
way in which the game was played.
The adaptation of players to the surface is an important
factor in surface-related injuries. Ekstrand and
Gilquist (1983) repor ted that the risk of traumatic
injury increased when changing from one type of
surface to another. They suggested that it took about
six games for a player to adapt. This poses obvious
problems for players who change frequently from one
type of surface to another during the competitive
season; as a consequence, they are likely to be at a
greater risk of injury. Recently, in a study of soccer
injuries in Iceland, Arnason et al. (1996) found a 2.5
times greater incidence of injuries on arti® cial surfaces
compared to grass surfaces, with no clear diþ erences in
injury pro® les between the two and a higher overall
injury incidence than found in other studies. It may be
that the rapid change between surface types required by
the players in this study as a result of their playing
environment was a causative factor.
Arti® cial surfaces other than arti® cial turf are used
and these may lead to an even greater number of injuries.
Ekstrand (1994) reported injuries to be six times
more likely on gravel compared to arti® cial turf; in
contrast, Arnason et al. (1996) found that there were
fewer injuries on gravel than on grass, and three times
fewer on gravel than on ar ti® cial turf . Pitch size can also
have an eþ ect on injury rate. Hoþ and Martin (1986)
found that, in indoor soccer, the injury rate was six
times that of matches played on a full-size pitch. They
attributed this to the smaller playing area and con® ning
walls, which increased the intensity of play and thus the
risk of injury.
Heading of the ball and head injur ies
The possible injurious eþ ect of heading the ball has
been the subject of recent biomechanical investigations
as a result of potential legal cases over the misuse of
equipment for young players. The incidence of head
injury is more prevalent than is generally acknowledged.
Barnes et al. (1994) reported that, in a sample of 72
active players, 89% had experienced some kind of head
trauma. While these injuries were acute (loss of consciousness,
fractures, nose bleed, mouth lacerations),
there is a worry that the cumulative eþ ects of head
trauma can also produce a risk. The serious eþ ects of
accumulated head trauma have been reported by several
authors. Tysvaer and Storli (1981) found that, in a sample
of 128 active Norwegian players, 50% experienced
230 Lees and Nolan
symptoms associated with head impacts. In a follow-up
study on a sample of 37 former Norwegian players,
Tysvaer and Lochen (1991) reported that 81% demonstrated
some form of intellectual impairment, which
was attributed to cumulative trauma probably the result
of repeatedly heading the ball. Sortland et al. (1982)
reported that, in a sample of 43 former Norwegian
players, 21% complained of chronic neck problems,
with 58% showing a decreased range of motion and
radiographic abnormalities. However, Jordan et al.
(1996), in a comparison of the MRI scans of national
level US soccer players and elite track athletes, found no
evidence of accumulated trauma in either group and no
diþ erences between the groups. They concluded that
brain damage was more likely to result from acute
trauma and alcohol abuse rather than repetitive ball
heading. Despite this, it would appear that there is
suYcient evidence to suggest that intense involvement
in soccer might lead to severe long-term head and neck
problems and that, in part, these might be due to
heading the ball.
Brain damage can develop from: (1) direct impact
leading to excessive linear acceleration of the brain,
which causes compression waves and high internal
pressures; and (2) a glancing impact leading to rotational
accelerations of the brain, which cause shearing
between the brain and the skull (Levendusky et al.,
1988). The linear and rotational accelerations of the
head during impact can be determined, although it is
not known precisely what levels of acceleration are
thought to cause injury. For direct blows, a tolerance
level of about 80 g is thought to lead to a loss of consciousness.
This ® gure comes from the measured
acceleration of a professional boxer’ s punch (Atha et al.,
1985). The tolerance levels for rotational accelerations
are more diYcult to estimate. Holburn (1943) has suggested
that a rotational acceleration of 7500 rad s- 2 would
lead to a loss of consciousness, whereas Stalnaker et al.
(1977) suggested a ® gure of 5500 rad s- 2. More recently,
Schneider and Zernicke (1988) used a ® gure of 1800
rad s- 2 to indicate a tolerance threshold based on the
Head Injury Criterion used in vehicle accident research.
Burslem and Lees (1988) used a twin accelerometry
system to investigate the acceleration of the head when
heading the ball at a relatively low speed (ball speed
of about 7 m s- 1). They found that the accelerations
produced by the head on contact with the ball were
about 15 g, and the rotational accelerations were about
200 rad s- 2. Both of these ® gures are well below the
tolerance levels identi® ed above. Townend (1988) used
a mathematical model based on two spheres colliding as
a simulation of central impact. Using an initial ball
velocity of 10 m s- 1 and a head impact velocity of 3± 5
m s- 1, he found the impact acceleration of the head to be
about 20± 25 g. His simulation also predicted that the
impact acceleration would increase as the head± ball
mass ratio decreased. The impact acceleration was
found to increase as a linear function of ball mass, and
decrease with an increase in a player’ s body mass. The
heads of lightweight players therefore receive a proportionately
larger impact acceleration. In a more
detailed simulation, Schneider and Zernicke (1988)
estimated that, for a relative head± ball speed of 10 m s- 1
and a head± ball mass ratio of 10, the initial acceleration
of the head was about 19 g. This is in agreement with the
results of Burslem and Lees (1988) and Townend
(1988). It implies that there is no immediate danger
from heading the ball. However, these results suggest
that there might be a danger for small children. If
head± ball mass ratios drop to about 3 (typical for young
children), then heading a fast ball would put them close
to the tolerance threshold. The practical solution is
to reduce the mass of the ball for young players, and
ensure, by instruction and the rules of the game, that
they do not try to head fast-moving balls.
The results for rotational accelerations of the head are
also in general agreement. The 200 rad s- 2 repor ted by
Burslem and Lees (1988) is well below the tolerance
threshold of injury. Schneider and Zernicke (1988)
estimated that, for a header with a relative impact speed
of 10 m s- 1, the rotational acceleration is 366 rad s- 2, well
below their tolerance threshold. However, for children
using a full-size ball (head± ball mass ratio of about 3)
and trying to head a fast-moving ball (20 m s- 1 or more),
the rotational acceleration tolerance threshold is easily
reached. Their analysis suggested that the tolerance
threshold was reached more easily for rotational impacts
than for linear impacts. The general conclusion that can
be drawn is that, although heading a soccer ball appears
to be below the injury threshold, care needs to be taken,
particularly when dealing with young children during
the development of their skill. If the head± ball mass ratio
can be increased by using the muscles of the neck, the
eþ ect of the impact can be reduced. Skill training can
therefore play an important preventative role.
Summary
Soccer injuries are the result of many interrelating
factors, some of which can be isolated. For example,
the soccer boot has a poor protective function. Careful
boot design can have a minor in¯ uence on the severity
of inversion injuries. The inadequacy of the boot,
primarily determined by performance requirements, is
indicated by the need for, and success of, alternative
methods of providing ankle stability. Unlike the developments
in running shoe technology, little attention
has been paid to shock reduction or rear-foot control
characteristics of the soccer boot, which are often considered
aetiological factors in injury. Arti® cial surfaces
The biomechanics of soccer: A review 231
produce diþ erent injury pro® les than natural turf
pitches. There appears to be a tendency for fewer
serious injuries but more minor injuries on arti® cial
turf compared to natural turf pitches. It seems that the
type of surface may be indirectly responsible for a
change in injury pro® le by changing the nature of the
game. This change requires an adaptation period, and
players are more likely to be at risk if they change frequently
from one type of surface to another. Obtaining
clear evidence of speci® c pitch constructional characteristics
on injury is complicated by the interacting
in¯ uences of a number of factors. Long-term brain
damage in soccer would seem to be a possibility,
particularly for children, as a result of heading the ball.
Careful instruction and skill development, together
with correct equipment, is necessary for young players.
Tolerance thresholds are not well known, and although
simulation results suggest the importance of ball mass,
ball speed and player mass, there is still insuYcient
experimental data on head impact characteristics when
heading the ball. In particular, there is no information
regarding diþ erent methods of heading, the frequency
of occurrence of these methods, and the in¯ uence of
neck and shoulder muscles to increase the eþ ective mass
involved in the impact.
Conclusions
In this review, we have shown the diþ erent ways in
which biomechanics has been applied to soccer. We
focused on three main areas of application and showed
that the biomechanics of soccer is based on descriptive
experimental work that has covered a wide range of
topics, but there is little evidence of researchers taking
a systematic approach. There is much interest in
kicking as a skill, but there remain many gaps that biomechanists
can ® ll. As a consequence of these gaps,
experimental investigations have thrown up relatively
few contentious issues. Where these exist, it is more
likely to be the re¯ ection of subjects or the analytical
equipment used rather than a con¯ ict in understanding
the underlying mechanisms of performance. In some
examples, experimental work has given way to the use of
biomechanical modelling techniques. These have
helped both to investigate problems, in particular of an
injury-related nature where experimentation would be
diYcult to conduct, and to provide an understanding
of underlying mechanisms of performance. The multifactor
in¯ uences associated with many of the topics
considered are a limitation to our understanding, yet
this avenue of research must continue to be explored
if real progress is to be made.
In this review, we have shown that many features of
the game of soccer are amenable to biomechanical
treatment. There are still many opportunities for biomechanists
to have a role in the science of soccer, and it
is hoped that this review will help to direct future
investigations.
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