School of Sport and Leisure Management
BSc (Hons) Sport and Exercise Science
Dissertation: 19-6244-00S
Ground reaction forces acting upon the support leg of a
soccer player during two kicking techniques
Lee Hole
May 2004
SHEFFIELD HALLAM UNIVERSITY
SCHOOL OF SPORT AND LEISURE MANAGEMENT
UNIT 19-6244-00S: DISSERTATION
Ground reaction forces acting upon the support leg of a
soccer player during two
kicking techniques
Lee Hole
Supervisor: Jon Wheat
In partial fulfilment of the requirements for the degree of Bachelor of Science in Sport
and Exercise Science.
May 2004
Acknowledgments
Firstly I would like to thank Mr Jon Wheat for his help and guidance throughout the completion
of the data collection, data analysis and other logistical issues of this study. I would also like to
express appreciation and thanks to the technical staff at Sheffield Hallam University and
especially to Steve for his assistance during particularly eventful data collection sessions. The
participants used for the study were exceptionally accommodating and I am extremely thankful
to them all. Finally I would like to say thank you to Andrew Gange and Kris Emerton who made,
otherwise boring data analysis sessions a little more bearable and I could not have done it
without them both.
ii
Abstract
Ground reaction forces of the support leg were studied in 10 male amateur soccer players whilst
performing a powerful kick and a kick for distance, in addition to ground reaction forces
approach speed and ball velocities were also recorded. Ground reaction force data were
collected on a Kistler 9281CA force platform (Alton, UK), approach speeds were monitored
using Brower infra-red timing gates (Draper, USA) and ball velocities were calculated using
SimiMotion software package (Unterschleissheim, Germany). Paired samples T-Tests revealed
no significant difference in vertical impact peak or loading rate and statistical analysis of the free
moment variable was prevented due to high levels of variability. Net anteroposterior impulse,
approach speeds and ball velocities were found to be significantly lower for the distance kick
than for the power kick. A high negative net anteroposterior impulse was suggested as a
possible reason for slower ball velocities in distance kicking, although a slower approach speed
was also highlighted as possible contributory factor.
A high level of inter-participant variation was present within data from all of the variables within
the study. Varying footwear, playing ability and playing position were some of the many factors
that were highlighted as methodological problems that may have caused the high interparticipant variation. It was suggested that future research would benefit by amending the
methodological flaws presented in this study and employing effect size statistics for data
analysis. Future research was also recommended into the comparison of other soccer specific
kicking techniques along with the analysis of female and elite players.
Key words: soccer, kicking, impact peak, loading rate, free moment, anteroposterior impulse.
iii
CONTENTS
Acknowledgments
ii
Abstract
iii
CONTENTS
iv
List of Figures
vi
List of Tables
vii
1.0. Introduction
1
1.1. Aim
3
2.0. Review of Literature
3
2.1. Injuries in soccer
3
2.1.1. Severity of injury
4
2.1.2. Location of injury
5
2.1.3. Cause of injury
5
2.2. Ground Reaction Forces
6
2.2.1. Running
6
2.2.2. Effects of extrinsic factors
7
2.3. Technique adaptations
8
2.4. Free Moment
9
2.5. Ground Reaction Forces and Soccer
10
2.5.1. Ground reaction forces and injuries
11
2.6. Aim and purpose of study
12
3.0. Methods
13
3.1. Participants
13
3.2. Data collection
14
3.3. Data analysis
16
3.3.1. Loading rates and identification of impact peak
16
3.3.2. Approach speeds and ball velocities
17
3.4. Statistical Analysis
17
4.0. Results
18
4.1. Max free moment
18
4.2. Impact peak
19
4.3. Loading rate
20
4.4. Anteroposterior impulse
21
4.5. Approach speed
22
4.6. Ball velocity
23
4.7. Force Time Curve
24
iv
5.0. Discussion
25
5.1. Discussion of results
25
5.2. Limitations
29
5.3. Recommendations for future research
32
6.0. Conclusion
35
7.0. References
37
8.0. Appendices
- 41 -
8.1. Appendix A
- 41 -
8.2. Appendix B
- 43 -
v
List of Figures
Figure 1 - Diagram of the layout of the lab during data collection. .....................................14
vi
List of Tables
Table 1 - Comparison of the impact peaks found by various authors using different
running velocities. .............................................................................................................7
Table 2 – Maximum free moment (NM) for the two types of kick. ........................................18
Table 3 – Mean impact peaks for each of the participants from both types of kick. ..........19
Table 4 – Mean loading rates for each of the participants from both types of kick. ..........20
Table 5 – Mean net anteroposterior impulses for each of the participants from both types
of kick. ..............................................................................................................................21
Table 6 – Comparison of mean approach speeds from the two kick types. .......................22
Table 7 – Comparison of mean ball velocities from the two kick types. .............................23
vii
1.0. Introduction
Soccer is reputed by many to be the most popular sport in the world with millions of individuals,
both male and female, participating across all age and ability spectrums (Inklaar, 1994a;
Lindenfeld et al., 1994; Lees and Nolan, 1998; Junge et al., 2002; Olsen et al., 2004).
Lindenfeld et al. (1994) and Putukian et al. (1996) both suggested that the presumed safety
from injury that accompanies soccer, in comparison to sports such as American football, may be
the reason for increased participant numbers. However, Hawkins et al. (2001) claimed that
soccer has a high risk of injury; Giza et al. (2003) also stated that injury occurs within soccer
every 0.8 to 2 matches and Rahnama et al. (2002) indicated that the cost of soccer related
injuries in Britain, due to treatment and loss of work, is estimated to be approximately £1 billion
per year and a similar distinction is made by Inklaar (1994a) who suggested that soccer injuries
cost health care funds, of countries where soccer is a popular sport, considerable amounts of
money.
Soccer has been the foundation for research in many areas such as injury (Fried and Lloyd,
1992; Inklaar, 1994a; Lindenfeld, et al., 1994; Putukian, et al., 1996; Hawkins et al., 2001;
Junge et al., 2002; Rahnama et al., 2002; Giza et al., 2003), the biomechanical aspects of the
game (Saggini and Vecchiet, 1994; Lees and Nolan, 1998; Barfield, 2000; Jensen and Barinotti,
2001; Dorge et al., 2002) and equipment development (Nike, 1997; Smith et al., 2002). The
increased knowledge of the game through research and the continuing rise in participation
figures has lead to the further development of soccer equipment, playing surfaces, the rules and
the way the game is coached.
The majority of ground reaction force research reported in the past has been largely concerned
with running and walking (Cavanagh and Lafortune, 1980; Cavanagh et al., 1985; Dickinson et
al., 1985; Williams, 1985; Frederick and Hagy, 1986; Munro et al., 1987; Nigg et al., 1987;
Whittle, 1997; Woodard et al., 1999). Research has also been completed into ground reaction
1
forces acting on individuals whilst performing various soccer related actions (Isokawa and Lees,
1988; Saggini and Vecchiet, 1994; Lees and Nolan, 1998; Barfield, 2000; Voloshin, 2000;
Jensen and Barinotti, 2001; Smith et al., 2002). However, the area that has attracted the most
attention in soccer is the analysis of the kicking action (Lees and Nolan, 1998). The vertical free
moment component of ground reaction force has received extremely limited amounts of
attention from previous researchers in all areas of sport. Holden and Cavanagh (1991)
investigated free moment and a possible relationship to pronation during running. Holden and
Cavanagh (1991) described vertical free moment as the force acting to resist toe abduction or
adduction, depending on direction. It was explained by Holden and Cavanagh (1991) that
excessive pronation may have a connection to certain injuries and that the use of free moment
to identify activities involving excessive pronation may aid in injury prevention.
Numerous authors have highlighted the fact that further research is required into the area of
soccer related injuries, with particular attention being paid to the identification of groups that
may be at risk or activities that may yield a higher injury risk than others (Inklaar, 1994a; Inklaar,
1994b; Anderson et al., 2000; Hawkins et al., 2001; Junge et al., 2002; Lilley et al., 2002;
Rahnama et al., 2002). Biomechanical research has also acknowledged the need for further
research surrounding soccer related activities. Lees and Nolan (1998) explained that although
there has been analysis of the biomechanical aspects of the kicking technique there are still
gaps in the research that warrants further work. Barfield (2000) explains that further research is
needed into the forces acting on the plant foot along with ball velocity and how the two affect the
nature of the kicking technique.
2
1.1. Aim
The aim of the current study was to investigate the ground reaction forces acting upon the
support foot during two different kicking techniques. The study used amateur standard male
soccer players and comparisons were made between a powerful instep kick, simulating a shot,
and a kick with the aim of gaining distance, which simulated a goalkick or defensive free kick.
Mean vertical impact peaks, loading rates and free moments, along with anteroposterior net
impulses were recorded as those variables have been highlighted as the most likely to be
associated with overuse related injuries in previous literature (Dickinson et al., 1985; Williams,
1985; Munro et al., 1987; Holden and Cavanagh, 1991), although other authors have expressed
conflicting views (Shorten and Wilmslow, 1992 and Nigg et al., 1987). Alongside ground
reaction force data, ball velocities and approach speeds were also recorded and differences
were expected between the two kicks in all of the three aforementioned factors. The powerful
kick was expected to exhibit greater vertical ground reaction force data and greater ball
velocities with the distance kick recording slower approach speeds and ball velocities with
greater anteroposterior impulses.
2.0. Review of Literature
2.1. Injuries in soccer
Many attempts have been made by previous authors to identify the frequency, type, severity
and mechanism of injuries in soccer to aid in the design of preventative programmes and
improve treatment of such injuries etc. Inklaar (1994a) reported on the incidence and severity
of injuries in soccer. When Inklaar (1994a) compared outdoor soccer to indoor soccer,
volleyball, jogging, field hockey and indoor tennis it was found that outdoor soccer contributed
29.1% of the total number of reported injuries, with indoor soccer the closest other sport with
7% of all injuries. However, Inklaar (1994a) suggests that the high injury rates may be a
3
reflection of the popularity of the game and the compared sports are all mainly non-contact
sports, which immediately reduces the injury risk of the game.
2.1.1. Severity of injury
Previous research has acknowledged numerous problems surrounding the definition of the
severity, type and occurrence of injury within soccer. Inklaar (1994a) suggests that problems
arise when using hospital or insurance company records to evaluate the incidence of soccer
injuries due to the fact the majority of injuries treated in hospitals or reported to insurance
companies are moderate to severe, which will result in milder and overuse injuries being
overlooked. Hawkins et al. (2001) used absence from training or competition to quantify the
severity of injury sustained and Rahnama et al. (2002) used the application of first aid for the
same reason. However, other authors have expressed concerns relating to both
methodologies.
Lindenfeld et al. (1994) and Putukian et al. (1996) criticised the use of time missing from
competition or training due to the fact that different players will have different pain thresholds
and variations in games and training sessions per week may lead to one player missing three
sessions in a single week when another player may only miss one session with a similar injury.
Inklaar (1994a) highlighted the problems with the use of treatment applied to grade injury
severity as this will depend on what type and standard of treatment is available and Putukian et
al. (1996) also suggested that players may feign injury or exaggerate contact and ask for the
trainer to influence a referee’s decision. With all of the problems above taken into account,
most authors have tended to use time missing from training and competition to grade the
severity of injury.
4
2.1.2. Location of injury
Unsurprisingly most research has shown that 60 to 90% of all soccer injuries are found in the
lower extremities (Inklaar, 1994a; Lindenfeld et al., 1994; Putukian et al., 1996; Hawkins et al.,
2001; Rahnama et al., 2002). Lindenfeld et al. (1994) found that the knee and ankle accounted
for 12 to 29% of all injuries in soccer; Putukian et al. (1996) also claimed that ankle and knee
ligament sprains accounted for 26.3% and 18.4% of all injuries respectively; both Rahnama et
al. (2002) and Inklaar (1994a) concurred that the ankle and the knee were the most susceptible
to injury during soccer.
Hawkins et al. (2001) reported the thigh to be the most frequently injured body part with 23% of
all injuries, although the knee and ankle were the next most frequent with both accounting for
17% of all injuries. Lilley et al. (2002) categorised the identification of injury very broadly, and
due to this, stress related injuries accounted for 12% of all injuries reported, other authors have
broken stress related injuries down into their diagnosed conditions such as stress fractures,
tendonitis etc. (Inklaar, 1994a); had this not been the case then earlier research may also have
ranked stress related injuries in the same way as in the study by Lilley et al. (2002).
2.1.3. Cause of injury
Soccer is a contact sport and although the contact is less intense than that involved in sports
such as rugby and American football the majority of injuries sustained within soccer are due to
direct contact. Giza et al. (2003) found that 67% of all foot and ankle injuries were due to direct
contact although Hawkins et al. (2001) and Putukian et al. (1996) reported slightly lower figures
with 38% and approx. 50% respectively. Lindenfeld et al. (1994) found that the most common
activity involving direct injuries in soccer was tackling.
Non-contact injuries such as overuse injuries have also received a great deal of attention within
previous literature. Lees and Nolan (1998) claimed that one third of all lower extremity injuries
5
in soccer are overuse injuries and many possibly explanations have been suggested to explain
the prevalence of this type of injury. Lees and Nolan (1998) highlighted the need for
investigation into playing surfaces and boots worn and the possible role that both may play in
overuse injuries in soccer. Ekstrand and Nigg (1989) suggested that a harder playing surface
may lead to more overuse injuries, however, it is also stated that it is more likely to be a
combination of poor footwear, poor surface and other anatomical problems such as muscle
stiffness and footfall misalignment etc.
2.2. Ground Reaction Forces
2.2.1. Running
The subject of ground reaction forces and the link between them and overuse injuries in
particular has received a lot of attention in previous research, although most previous work has
involved running and walking. Cavanagh and Lafortune (1980) reported the vertical ground
reaction force values at a running speed of 4.5m∙s-1 and impact peaks of 2.2 to 2.7BW were
found, although these values were dependent upon the participants’ running style. Two
differing running styles were highlighted by Cavanagh and Lafortune (1980) and a runner was
categorized as either a midfoot striker or a rearfoot striker, according to the area of the foot
making first contact with the ground. This type of classification has been widely used by many
other authors (Cavanagh et al., 1985; Dickinson et al., 1985; Williams, 1985; Munro et al., 1987;
Holden and Cavanagh, 1991).
Cavanagh et al. (1985) reported variations in the ground reaction forces acting upon the
participants’ left and right feet when running at 5.96m∙s-1. Vertical impact peaks of 2.7BW for
the left foot and 4.1BW for the right foot were recorded by Cavanagh et al. (1985), which are
greater than those found by Cavanagh and Lafortune (1980), although the increase in values
could simply be due to the increased running speed in the study by Cavanagh et al. (1985).
However, comparisons can only be made for the right foot as Cavanagh and Lafortune (1980)
6
did not report on the left foot. Dickinson et al. (1985) reported impact peaks of 1.4BW for the
left foot at a speed of 4.61m∙s-1 wearing trainers and 2.2BW when barefoot at the same speed.
2.2.2. Effects of extrinsic factors
Nigg et al. (1987) reported the effects of running velocity and midsole hardness on the ground
reaction forces during running. Participants ran over a force platform at four different running
speeds and as the running speeds increased by 1m∙s-1 from 3m∙s-1 to 6m∙s-1 Nigg et al. (1987)
found an increase in impact peak and in loading rate. Frederick and Hagy (1986) reported an
increase in impact peak with increases in running velocity, although the correlation coefficient
was found to be low (r=0.34), which Frederick and Hagy (1986) suggested could be due to the
variability of the impact peak magnitude. The relationship between velocity and impact peak
from various other authors can be seen in table 1.
Table 1 - Comparison of the impact peaks found by
various authors using different running velocities.
Speed
(m.s-1)
3.4
3.8
4.5
3.0
5.0
3.4
5.4
Author(s)
Frederick and Hagy (1986)
Munro et al . (1987)
Williams (1985)
Dickinson et al . (1985)
3.6 - 4.1
Cavanagh and Lafortune (1980)
4.5
Cavanagh et al . (1985)
6.0
Nigg et al . (1987)
3.0
6.0
Impact peak
(BW)
2.0
2.3
2.9
1.6
2.3
1.6 - 2.0
2.9
2.2
1.4
*
**
2.2 +
2.7++
2.7 L
4.1 R
1.33 KN
2.17 KN
* = Data from bare foot running. ** = Data from running wearing trainers.
+ = Data from rearfoot strikers. ++ = Data from midfoot strikers.
L = Data from left foot only.
R = Data from right foot only.
KN = Data shown in KN.
7
The effect of footwear on ground reaction forces during running has produce results that have
been somewhat uncertain (Williams, 1985). Research comparing barefoot running to running
whilst wearing trainers reported greater impact peaks during barefoot running. Whittle (1997)
found impact peaks of 757N during barefoot running, 411N in hard-soled shoes and 180N in
viscoelastic-soled shoes although Whittle (1997) used only one participant. Dickinson et al.
(1985) used data from six recreational runners and found a decrease in impact peak from
2.2BW at 3.6m∙s-1 – 4.1m∙s-1 whilst running barefoot to 1.4BW at the same speed when running
in their regular running shoes.
Nigg et al. (1987) and Luethi and Stacoff (1987) discussed the occurrence of a “bottoming out
effect” in shoes with soles that are too soft. Luethi and Stacoff (1987) stated that the common
assumption that a softer shoe sole will equate to a smaller impact peak is not necessarily the
case on account of the fact that if the shoe sole is too soft then it fails to slow the acceleration of
the heel rapidly enough at touch down, leading to subsequently high impact peak values (Nigg
et al., 1987). Nigg et al. (1987) does hypothesise that with softer soled shoes the subtalar joint
is exposed to greater loading and a harder mid sole has the same effect on the structures of the
medial ankle joint.
2.3. Technique adaptations
Previous research has suggested that excessive impact peaks and loading rates could play a
major role in the development of overuse injuries in distance runners and walkers (Dickinson et
al., 1985; Williams, 1985; Munro et al., 1987). Cavanagh and Lafortune (1980) claimed that
abnormalities in foot strike such as excessive pronation may predispose individuals to overuse
injuries through increased impact peaks and loading rates. However, Shorten and Wilmslow
(1992) suggested that various involuntary adjustments, such as an increase in knee flexion,
altered breathing patterns and a switch in running style from rearfoot running to midfoot running,
are all carried out in response to increased vertical ground reaction forces in an attempt to
8
decrease the shock of the foot strike impact on the human body. Other authors have also
observed an increase in knee flexion at foot contact in response to an increase in vertical
impact forces during running (Nigg et al., 1987) and drop jumping (Milgrom et al., 2000).
However, it may be questionable as to whether the adaptations mentioned here would be
possible during a kicking action.
2.4. Free Moment
There has been very little work done into the free moment component of ground reaction forces.
Holden and Cavanagh (1991) described vertical free moment as the shear forces between the
foot and the ground, which acts against either abduction or adduction of the toe depending on
the direction of the free moment. Holden and Cavanagh (1991) also described free moment as
the friction force between the foot and the ground.
Holden and Cavanagh (1991) investigated the relationship between pronation and free moment
using ten male runners, all of the subjects were classified as rear foot strikers and data were
collected while the participants were running at a speed of 4.5m∙s-1. Holden and Cavanagh
(1991) linked excessive pronation with running injuries and highlighted that footwear
modifications to reduce pronation had been successful in the treatment those injuries. It was
also stated that, secondary to the twisting moment that is experienced at the foot and ankle, a
subsequent rotational force at the knee is to be expected, which could also be assumed to have
injury implications.
9
Holden and Cavanagh (1991) found a relationship between pronation and an increase in the
free moment acting upon the foot and ankle in a positive direction (resisting toe abduction).
Therefore, the measurement of maximum free moment may aid in the identification of activities
with a high risk of pronation related injuries. However, very little research has been done in this
area and Holden and Cavanagh (1991) stated that further research into excessive pronation
and free moment and the possible injury implications related to both factors is required.
2.5. Ground Reaction Forces and Soccer
Research has been limited into the area of ground reaction forces and soccer in the past,
although more research has been done in recent years. The complexity and variety of the
different actions involved in soccer has lead to a diversity of studies into many of the skills and
movements performed during a game or training session, yet kicking has received the majority
of the attention (Lees and Nolan, 1998).
Smith et al. (2002) studied the ground reaction forces during a shot, a cruyff turn and a drag
back turn whilst wearing boots with both moulded studs and conventional screw in studs. The
highest ground reaction forces were recorded for shooting in comparison to the two turning
actions with vertical forces of 3.95BW and 3.74BW for the studded and moulded boots
respectively; a similar difference due to the sole configuration of the soccer boots was true for
all of the activities analysed by Smith et al. (2002). Lees and Nolan (1998) also reported on
kicking, although lower values of between 1.93BW and 2.67BW were found. Jensen and
Barinotti (2001) compared the ground reaction forces acting on the support leg of individuals
whilst kicking a stationary and a moving ball with the dominant and non-dominant foot. Jensen
and Barinotti (2001) reported greater ground reaction forces for the non-stationary condition; the
dominant foot ground reaction forces were also found to be greater for both stationary and nonstationary conditions.
10
When reporting on distance kicking, Barfield (2000) found that better performance resulted in
lower ground reaction forces acting upon the support leg. Barfield (2000) also reported that
skilled players kicked the ball faster and exerted greater ground reaction forces than the
unskilled players during powerful ball kicking, although it was concluded by Barfield (2000) that
the greater ball speed could have been as a result of the greater ground reaction forces. Asami
and Nolte (1983) recorded a mean vertical ground reaction force value of 2.4BW during
powerful ball kicking and Asami and Nolte (1983) also stated that there was no correlation
between vertical ground reaction forces and ball speed, which invalidates claims by Barfield
(2000) that ball velocity may be related to ground reaction forces.
2.5.1. Ground reaction forces and injuries
The relationship between ground reaction forces and stress related or overuse injuries has been
extensively covered in previous research. Voloshin (2000) stated that numerous soccer players
retire from the game due to osteoarthritis, which is in agreement with the 27% reported by
Drawer and Fuller (2001). Voloshin (2000) and Milgrom et al. (2000) suggested that the high
levels of impact and repetitive excessive loading of the lower limbs in soccer may play a
significant part in the development of arthritis, it is also suggested that poor footwear and/or
surface conditions along with certain activities may increase the risk of overuse injuries and
osteoarthritis.
Bartlett (1999) highlighted that high impact peaks are more of an injury issue during rearfoot
strikers in comparison to forefoot and midfoot strikers, which may have implications in kicking
due to the fact that most kicking will involve rearfoot striking with little possibility of alteration.
Voloshin (2000) did highlight the fact that loading of the human system can also have a positive
effect as it can stimulate strengthening of bone, cartilage and tendons within the lower
11
extremities; it is also suggested that further research is needed into the dynamic loading and
impact stresses effecting the support leg during kicking.
Milgrom et al. (2000) questioned whether high impact activities produce more of a strain on the
human body than running. Milgrom et al. (2000) suggests that fatigue during high impact
activities leads to a greater risk of injury due to the fact that the muscles of the body are less
able to dissipate the shock of impact. Gross and Nelson (1988) also found that impact shock
was reduced by 22% when heel contact was not present in drop jumping and that fore foot
primary contact also aided in dampening the impact forces. However, primary contact being
made by the heel may be unavoidable in kicking.
Holden and Cavanagh (1991) and Shorten and Wilmslow (1992) highlighted a major downfall
surrounding the use of a force platform to measure ground reaction forces acting upon the lower
limbs of an individual during any activity. Holden and Cavanagh (1991) and Shorten and
Wilmslow (1992) both explained that force platforms merely measure the acceleration of a
participant's centre of mass rather than the forces acting purely on the lower limbs. Therefore,
the ground reaction force data recorded by the force platform during different sporting activities
may not have been a true representation of the strains applied to the lower limbs and could
have been due to a shift in the centre of mass caused by the given action.
2.6. Aim and purpose of study
The need to identify actions associated with injury and "at risk" groups within soccer has been
identified by numerous authors (Inklaar, 1994a; Lilley et al., 2002; Rahnama et al., 2002) and
Lindenfeld et al. (1994) suggested that certain actions have a greater risk of injury associated
with them than others; therefore, the analysis of actions that are performed more frequently
12
according to position may aid in identifying "at risk" groups of players. Inklaar (1994) also
claimed that further research is needed into certain aspects of the game of soccer, including
kicking.
For the aforementioned reasons, the aim of the current study was to investigate the ground
reaction forces acting upon the support foot during two different kicking techniques in order to
identify whether either of the kicks exhibit high levels of the injury provoking forces. Mean
vertical impact peaks, loading rates and free moments were recorded along with anteroposterior
net impulses, as those variables have been highlighted as the most likely to lead to overuse
related injuries (Dickinson et al., 1985; Williams, 1985; Munro et al., 1987; Holden and
Cavanagh, 1991). The powerful kick was expected to exhibit greater vertical ground reaction
force data and greater ball velocities with the goal kick recording slower approach speeds and
ball velocities with greater anteroposterior impulses.
3.0. Methods
3.1. Participants
Ten sub-elite male soccer players with an average age of 21.4 (S.D 1.2) were used for this
study, all of the participants were regularly active in competitive soccer and played and trained
at least once per week. The sample size of ten participants was recommended by Bates et al.
(1992) as the minimum amount to ensure statistical power. The mean values of height and
weight for the participants were 181.5cm (S.D. 4.6) and 77.8Kg (S.D. 5.2) respectively; 8 of the
participants were right foot dominant and 2 were left foot dominant. All participants completed a
pre-test medical questionnaire and signed an inform consent form subsequent to reading the
participant information form (appendix A). The study was given ethical approval by the ethics
committee of the School of Sport and Leisure Management at Sheffield Hallam University prior
to the onset of testing and following the completion of a risk assessment form.
13
3.2. Data collection
A Kistler 9281CA force platform (Alton, UK) was located beneath the end of a 1.5m x 8m
artificial turf runway with Brower speed trap2 infrared timing gates (Draper, USA) set 1m apart,
at hip height at the end of the runway. Two Sony DCR-TRV950E Handycam digital cameras
(Japan) were used to capture footage of the ball as it was kicked for the purpose of ball velocity
calculation (see figure 1) and a Visual Analogue Scale (VAS) (Crichton, 2001; Rosier et al.,
2002) was used to rate the quality of the kick.
1meter
Figure 1 - Diagram of the layout of the lab during data collection.
Participants were asked to perform a maximum power kick, which would simulate a shot, and a
maximum distance kick, which would simulate a goal kick or defenders clearance kick. Ball
placement was used to ensure that the support foot landed cleanly on the platform and was at
the participants’ discretion due to the fact that Lees and Nolan (1998) explained variation in ball
placement between different individuals. The participants were allowed as many practice kicks
as needed to consistently hit the force platform with the support foot, ball placement was altered
if the support foot missed the platform and the artificial turf was marked using masking tape to
gauge where the foot missed and to quantify the required ball movement.
14
Once an optimum ball placement had been identified and the participant was comfortable with
the experimental requirements, data were collected from five acceptable shot trials followed by
five acceptable goal kick trials. Five trials performed by each of the ten participant was
recommended by Bates et al. (1992) as the sufficient amount for statistical power. For each
acceptable kick the participants’ approach speed was recorded according to the Brower timing
gates and vertical ground reaction force (Fz), vertical free moment (Mz) and anteroposterior
ground reaction force (Fy) data were recorded for 2 seconds using the Kistler 9281CA force
platform (Alton, UK) set to trigger at 0.01v with a 5% pre trigger; data were collected from the
force platform using the Bioware 3.21 software package (Kistler, Alton, UK). Ball velocity was
later calculated in the SimiMotion software package (Unterschleissheim, Germany) using the
footage recorded from the Sony DCR-TRV950E Handycam digital cameras (Japan). Following
each trial the participant was asked to replace the ball in preparation for the next trial and rate
the kick on a the visual analogue scale according to the foot to ball contact and how effective
the kick may have been in a game situation. An acceptable trial from the VAS was dependant
upon the kick being rated above eight out of ten, although this was not apparent to the
participant. If the VAS score, the data collection from the force platform and the timing gates, or
the digital camera footage was unsatisfactory then the trial was rejected.
Lees and Nolan (1998) highlighted the fact that if accuracy restraints are too high when
analysing powerful ball kicking then maximum power is sacrificed for increased accuracy.
Therefore, the participants were asked to hit a rough area around the lower portion of a vertical
line on a safety net suspended in front of the test area when performing the shot trials. Hughes
(1999) also explained that powerful ball kicking was optimised through striking the ball with the
laces of the shoe and the participants were therefore asked to abide by this criterion when
performing the shot trial. When performing the goal kick trials the participants were asked to
aim for the same vertical line area although the nature of the kick requires a loftier flight to the
ball, therefore the upper portion of the line was the target area for these trials.
15
3.3. Data analysis
Ground reaction force data from each of participant’s trials for the two kicking techniques were
stored and transferred to Excel (Microsoft Limited, UK) for analysis. Mean and standard
deviations for vertical impact peaks, loading rates, maximum vertical free moments and
anteroposterior impulses were all calculated using an Excel spreadsheet (Microsoft Limited,
UK).
3.3.1. Loading rates and identification of impact peak
Loading rates were calculated by subtracting the highest vertical force recorded within the first
0.3seconds of foot contact (Fz1) (Bartlett, 1999) by the force value closest to, but greater than,
30N in the vertical direction (Fz2); the initial highest peak was also used as the impact peak
(Munro et al., 1987). The same subtraction was done with the times at each of the values (T1
and T2), the resultant values from these two calculations were divided, which results in the
loading rate: -
Loading rate =
Fz1  Fz 2 
T1  T2 
Woodard et al. (1999) evaluated four different methods of calculating loading rate and found
that any of the four methods provide acceptable data. Woodard et al. (1999) claimed that the
method used in the current study provided a more stable calculation of loading rate by utilizing
the most linear section of a ground reaction force curve; the same method was also adopted by
Munro et al. (1987).
The trapezium rule was used to calculate anteroposterior impulse by means of an Excel
spreadsheet (Microsoft Limited, UK). The trapezium rule can be seen below: -
Impulse =
h
2
y o  y n   2y 1  y 2  .....  y n1 
16
3.3.2. Approach speeds and ball velocities
Approach speeds were calculated by dividing one second by the time recorded by the Brower
Infra red timing gates (Draper, USA), finally, means and standard deviations for each participant
were calculated for both kicking techniques. SimiMotion (Unterschleissheim, Germany) was
used to calculate the ball velocities from the digital camera footage by digitising every available
frame from foot contact until the ball was out of view; SimiMotion then produced ball velocities in
m∙s-1.
3.4. Statistical Analysis
Data was checked to ensure normal distribution and any outliners within the data that
jeopardised normality were removed prior to further analysis. Once any outliners had been
removed a paired samples T-Test was done for the impact peaks and loading rates in the
vertical direction and the anteroposterior impulses. The same statistical analysis was done on
approach speed and ball velocities with the level of significance set at α = 0.05. Perneger
(1998) explained that, even though the use of the Bonferroni adjustment would aid in the
reduction of type one errors (incorrectly rejected the null) it would consequently inflate the
probability of executing a type two error (incorrectly accepting the null). Therefore, the
Bonferroni adjustment technique was not used in the current study.
17
4.0. Results
4.1. Max free moment
Maximum free moment results produced very high inter participant variability, which can be
seen in table 2. Mean values of 34.11NM for the distance kick and 32.34NM for the power kick
with standard deviations of 31.10NM and 31.32NM for distance kick and power kick respectively
confirms the high level of inter participant variability. The variability in vertical free moment
could be due to the connection with mediolateral ground reaction force, which has been found
to produce results with high variability in previous research (Williams, 1985). Due to the large
variability in maximum free moment the decision was made to conduct no further analysis of this
factor, as it was consider likely that no meaningful relationships would be found.
Table 2 – Maximum free moment (NM) for the
two types of kick.
Participant
Distance Kick
Power Kick
A
91.14
52.23
B
7.49
1.74
C
59.27
84.96
D
21.23
24.40
E
5.20
6.62
F
23.98
11.05
G
78.71
85.96
H
7.13
13.81
J
29.10
9.99
K
17.81
32.65
Mean
34.11
32.34
31.10
31.62
Standard
Deviation
18
4.2. Impact peak
Variability in impact peak was higher for the distance kick than it was for the power kick, which
can be seen in table 3. However, data from participant J for the distance kick was a lot higher
than that of the other participants and proved to jeopardise normality of the data and was
therefore removed. With the data from participant J removed, T-Test results showed that there
was no significant difference between the distance and power kicks (t(8) = 0.420; p =.686).
Table 3 – Mean impact peaks for each of the
participants from both types of kick.
Distance
Power
Impact peak
Impact peak
(BW)
(BW)
A
4.14
3.63
B
3.33
2.90
C
3.49
3.54
D
2.66
2.64
E
2.68
2.68
F
3.68
3.37
G
3.00
3.39
H
3.05
4.23
J
9.18
4.37
K
3.05
3.35
Mean
3.82
3.41
Standard Deviation
1.94
0.58
Participant
19
4.3. Loading rate
Standard deviations for both types of kick were high and, as with impact peaks participant J
displayed higher loading rates than most of the other participants for the distance kick.
However, participant F displayed higher values for both of the kicking types and participant G
displayed a low value for the distance kick (table 4). Following normal distribution testing the
data from participants J and F were removed and T-Test results showed no significant
differences between the two kick types (t(7) = 0.887; p =.405).
Table 4 – Mean loading rates for each of the
participants from both types of kick.
Distance
Power
Loading Rate
Loading Rate
(BW∙s-1)
(BW∙s-1)
A
182.53
202.37
B
228.92
179.51
C
131.55
148.42
D
183.70
173.01
E
184.19
168.19
F
458.23
417.95
G
79.20
105.49
H
161.32
249.50
J
428.41
183.21
K
119.28
145.24
Mean
215.73
197.29
127.07
86.23
Participant
Standard
Deviation
20
4.4. Anteroposterior impulse
Variability was also high for the net anteroposterior impulse and no singular outliners seemed to
be present. However, a number of participants for both of the kick types stood out as having
slightly higher impulses and participants D, F and H display higher values for both types of kick
(table 5). Once data from participant D were removed the net anteroposterior impulse did
provide significantly differing means at both the 95% and 99% confidence level (t(8) = 3.445; p
=.009) with distance kicking resulting in greater breaking forces than that of power kicking.
Table 5 – Mean net anteroposterior impulses for
each of the participants from both types of kick.
Distance
Power
Net Anteroposterior
Net Anteroposterior
Impulse (BWI)
Impulse (BWI)
A
-88.34
-82.31
B
-68.12
-56.70
C
-84.34
-78.40
D
-130.42
-120.97
E
-86.46
-88.51
F
-134.67
-121.48
G
-88.07
-84.13
H
-102.28
-102.96
J
-88.05
-105.35
K
-100.49
-95.17
Mean
-97.13
-72.53
20.86
65.48
Participant
Standard
Deviation
21
4.5. Approach speed
The T-Test of the approach speeds displayed a significant difference between the two types of
kick at the 95% confidence level (t(9) = -2.345; p =.044), which was acceptable for the current
study. Table 6 shows that the mean approach speed for the power kick (4.68m∙s-1) was greater
than that of the distance kick (3.05m∙s-1). Participant J exhibited the greatest difference in
speeds between the two kick types with 5.03m∙s-1 and 2.42m∙s-1 for the power and distance kick
respectively. Participants E and K were the only two participants to recorded faster approach
speeds for the distance kick than for the power kick, although participant E’s approach speed
for the distance kick was only 0.04s faster.
Table 6 – Comparison of mean approach
speeds from the two kick types.
Distance Kick
Power Kick
(m∙s-1)
(m∙s-1)
A
2.79
3.03
B
4.00
4.86
C
4.78
5.16
D
2.20
2.51
E
2.68
2.64
F
2.15
2.19
G
3.65
4.86
H
2.84
4.55
J
2.42
5.03
K
3.04
2.59
Mean
3.05
4.68
0.85
1.25
Participant
Standard
Deviation
22
4.6. Ball velocity
Ball velocities for the two kick types exhibited low levels of variance and were also significantly
different (t(9) = -3.212; p =.011). The power kick presented greater ball velocities with a slightly
higher standard deviation than the distance kick (table 7). Participant K recorded the fastest
ball velocity during the power kick, although the difference from the rest of the group was only
fractional. Participants G and D were the only participants to record slower ball velocities for the
power kick. However, participant G did record slower ball velocities than most for both of the
kicking techniques with values of 17.75m∙s-1 for the power kick and 18.21m∙s-1 for the distance
kick.
Table 7 – Comparison of mean ball
velocities from the two kick types.
Participant
Distance Kick
Power Kick
A
23.66
23.92
B
21.46
22.89
C
21.50
24.77
D
20.14
19.55
E
21.91
23.69
F
20.79
22.71
G
18.21
17.75
H
19.04
23.68
J
22.16
25.42
K
21.35
26.86
Mean
21.02
23.12
1.57
2.69
Standard
Deviation
23
4.7. Force Time Curve
Figure 2 shows two force-time curves for the same participant for each of the kicking
techniques. Participants displayed similar traces for each of the kicking techniques although the
traces from each participant differed considerably from one another, which aids in
demonstrating the variability between the subjects.
B
3000
2500
2500
2000
2000
1500
1000
500
0
1
18 35 52 69 86 103 120 137 154 171 188 205 222 239 256 273 290 307 324 341 358 375 392 409
-500
Ground reaction force (N)
Ground reaction force (N)
A
1500
1000
500
0
1
18 35 52 69 86 103 120 137 154 171 188 205 222 239 256 273 290 307 324 341 358 375 392
-500
-1000
-1000
Time (s)
Time (s)
Figure 2 - Comparison of force-time curve for the two kicking techniques. A = Distance, B =
Power.
24
5.0. Discussion
The results of the current study provide information on the ground reaction forces acting upon a
soccer player when performing two different soccer specific kicks, a kick for power and a kick for
distance. The purpose of this was to identify whether there was a significant difference in the
vertical impact peak, loading rate and free moment along with the anteroposterior net impulse
due to the possible relationship between these factors and certain overuse injuries (Dickinson et
al., 1985; Williams, 1985; Munro et al., 1987; Holden and Cavanagh, 1991). Differences
between the two kick types would aid in the identification of at risk groups within soccer, as
players in certain positions would perform the two different kicks more often than others.
Therefore, any risk associated with one of the kicks would possibly result in a greater risk to any
player performing that kick more frequently than others.
5.1. Discussion of results
Free moment testing provided results with extremely high variability and was therefore deemed
exempt from further analysis. The high variability may have been due to the fact that
mediolateral ground reaction force data, among other things, are a component of free moment
and mediolateral ground reaction force data has been found to exhibit high levels of variability in
previous research (Williams, 1985). Table 2 shows the mean values for each of the participants
along with the mean and standard deviation for the group; the high standard deviation is
evidence of the variability. When reporting free moment in running Holden and Cavanagh
(1991) reported no such variation, although running is a more simplistic action in comparison to
kicking and future research into free moment and kicking should consider this.
25
The two kicking techniques resulted in differences in net anteroposterior impulse that were
significant at the 95% and the 99% level. The distance kicking presented significantly lower
mean net impulse of –97.13BWI than the –72.53BWI recorded for power kicking. This result
was in line with the hypothesis of the study and could be due to the lack of follow through in the
performance of a distance kick in comparison to the exaggerated follow through observed
during a powerful kick. Previous research into anteroposterior impulse during running has
resulted in values that were close to zero due to the fact that the participants were required to
maintain a constant velocity along the runway. In the current study the participants were
performing a kicking action on the force platform, which resulted in large negative values
signifying the breaking forces acting to slow the participants horizontal motion. No previous
work has been done into the net anteroposterior impulse during kicking, although Smith et al.
(2002) reported ground reaction force data for different soccer specific actions and found that
kicking exhibited the highest anteroposterior peak force. Future work is required into the net
impulses during kicking to back up or dispel the findings of the current study.
The approach speeds and ball velocities both proved to be significantly different at the 95%
level with distance kicking resulting in slower approach speeds of 3.05m∙s-1 in comparison to
4.68m∙s-1 for power kicking, ball velocities of 21.02m∙s-1 and 23.12m∙s-1 were also recorded for
distance kicking and power kicking respectively. The slower ball velocities and approach
speeds for distance kicking were hypothesised earlier in the study and the significant difference
in these two factors acts as an indication that the two kicking techniques were dissimilar and
that the participants were not simply reciprocating the same kick for both trials. The negative
net anteroposterior impulse was significantly lower for the power kick, which may have resulted
in the greater ball velocities, as the participants were not prohibiting horizontal motion with high
breaking impulses, although this was not the case during the distance kicking as negative
breaking forces were greater during these kicks. However, approach speeds may also have
played a contributory part in the difference between the ball speeds, as the mean approach
speed was significantly slower for distance kicking.
26
The impact peaks and loading rates that were recorded in the current study were not
significantly different for the two kick types, although they were higher than previously reported
and inter-participant variability was, again, considerably high. The mean impact peaks of
3.82BW and 3.41BW for distance kicking and power kicking respectively are both greater in
magnitude than the 2.67BW reported by Lees and Nolan (1998), the 2.4BW by Asami and Nolte
(1983) and considerably greater than the 1.48BW recorded by Saggini and Vecchiet (1994).
However, when investigating shooting Smith et al. (2002) reported values of 3.95BW when
wearing conventional studded boots and 3.74BW when wearing moulded stud boots, which are
higher than the values in the current study for power kicking but similar to those of the distance
kick. The difference in values according to the boots that where worn in the study by Smith et
al. (2002) highlights the influence of footwear on the vertical ground reaction forces. The high
impact peak and loading rate values that were recorded in the current study for both kicking
techniques may have overuse injury implications, the fact that no significant difference was
found does not discount the fact that both kicking techniques may carry equal risk. Further work
comparing the kicks of the current study to other variations of kicking technique may help to
answer this question.
The hypothesised theory that impact peak and loading rate would be higher for the power kick
than the distance kick was found to be inaccurate, although the values reported in the current
study were greater than those previously reported for kicking. Munro et al. (1987) suggested
that overuse injuries occur due to repetitive impact and loading of the lower limbs such as
during running. However, Milgrom et al. (2000) investigated whether high impact exercises
expose the lower limbs to higher strains than running. It was found that high impact activities
are more of a risk for injuries such as stress fractures etc. when fatigue occurs. This may be an
important factor for consideration as continuous kick practice during training may lead to a
potential for injury. Similarly, practicing kicking at the start of a training session rather than
practicing at the end when fatigue could be a factor may be a simple method of avoiding
overuse injury. However, performing the types of kicks mentioned in the current study when
27
fatigued may be unavoidable in some cases, e.g. towards the end of a game and Milgrom et al.
(2000) gave no explanation of the magnitude of a high impact activity so it is unknown as to
whether the kicking of the current study would be considered as a high impact activity by
Milgrom et al. (2000).
The relationship between the mean approach speeds for the two kicking techniques and the
resultant impact peaks and loading rates was contrary to previous running literature. Many
authors have reported a positive relationship between running speed and vertical ground
reaction force (Frederick and Hagy, 1986; Munro et al., 1987; Nigg et al., 1987). However, the
current study demonstrated similar impact peaks and loading rates at significantly differing
approach speeds. Power kicking resulted in impact peak and loading rate values of 3.41BW
and 197.29BW∙s-1 respectively at a speed of 4.68m∙s-1 and in contrast distance kicking resulted
in a mean impact peak value of 3.82BW and a mean loading rate value of 215.73BW∙s-1 at a
slower approach speed of 3.05m∙s-1.
Dickinson et al. (1985) reported a mean impact peak of 1.4BW at a speed of between 3.6 –
4.1m∙s-1 while distance kicking in the current study resulted in greater impact peaks (3.82BW) at
slower speeds (3.05m∙s-1) and power kicking presented greater impact peaks (3.41BW) at a
slightly higher speed (4.68m∙s-1). Cavanagh and Lafortune (1980), Cavanagh et al. (1985) and
Munro et al. (1987) have all reported impact peaks that were lower than those found in the
current study. Cavanagh and Lafortune (1980) found a mean impact peak of 2.2BW at 4.5m∙s-1
for rearfoot strikers, Cavanagh et al. (1985) recorded a mean impact peak of 2.7BW at a faster
speed of 5.96m∙s-1 and Munro et al. (1987) reported a mean impact peak of 2.3BW at 5m∙s-1; all
of which equate to lower impact peaks at higher speeds than those found in the current study.
This may suggest that both of the kicking techniques carry an equal or greater risk of overuse
injuries than running. However, there is still the question of whether the repetitive nature of the
lower impact forces acting on individuals during running outweighs the risk of the greater
magnitude of impact and loading rates during a less frequent kicking action.
28
Luhtanen (1988) reported on the same method of maximal instep kicking as the current study,
although junior soccer players were used as participants rather than senior players. Luhtanen
(1988) found ball velocities of 14.9 m∙s-1 for 9-11 year olds, 18.4m∙s-1 for 12-14 year olds and
22.2m∙s-1 for 15-18 year olds, which is close to values recorded in the current study for power
kicking with a similar standard deviation of 2.3. Asami and Nolte (1983) recorded vertical
ground reaction force and ball velocity from power based kicking and found no correlation
between vertical ground reaction forces and ball velocity, which concurs with the findings of
Luhtanen (1988) who made the same acknowledgement and with the current study, although no
statistical analysis of the correlation was undertaken.
5.2. Limitations
There were a number of limitations of the current study, which must be considered when
interpreting the results, recommendations and theories that have been presented. Primarily, the
fact that the study was carried out using only ten participants. Bates et al. (1992) states that ten
participants completing at least five trials is sufficient for statistical power. However, the interparticipant variability that was observed in the results for most of the variables may have been a
by-product of a small participant group and could possibly have been minimised with larger
participant numbers. The intention of the study was to use a larger participant group although
time constraints and the inability to locate soccer players of the required standard became a
restriction and the number identified by Bates et al. (1992) of ten became the bench mark and
was only just achieved.
As with all laboratory-based research the ecological validity of the study is questionable and
even though every effort was made to ensure that the performance of the kicks was as
authentic as possible no laboratory-based performance can be completely ecologically sound.
To complete the study outdoors would have required the use of electronic pressure sensitive
29
insoles (RS Scan, Belgium) and although this would have increased ecological validity via the
use of goal posts for the power based kick etc., the accuracy of the insoles in comparison to the
force platform was questionable. Other difficulties that the study would have faced had the
outdoor methodology been used would have been weather constraints, as the equipment would
have be protected from rain etc., finding suitable and available soccer fields during the
competitive season and transporting participants and equipment to the testing area once a
suitable field had been found. Due to the fact that timescale problems were already likely and
that data collection would be considerably more accurate, it was decided that laboratory-based
data collection would be the better option.
The current study made no attempt to establish constraints on the footwear worn by each of the
participants during data collection. It was decided that due to the data collection taking part
within the laboratory rather than outdoor, an attempt would be made to enhance ecological
validity wherever possible, therefore, the participants were asked to wear the shoes that would
be worn to compete in soccer when soccer boots weren’t possible, a similar methodology was
adopted by Munro et al. (1987). However, the variation in footwear could possibly have been a
major factor in the inter-participant variation that was present for all of the ground reaction force
data. Nigg et al. (1987) and Luethi and Stacoff (1987) both reported that varying shoe soles
altered the loading of the foot during running and it is also likely that the same theory is true
during kicking. Woodard et al. (1999) ensured that all participants wore the same footwear
during testing to discount the effect of footwear and any future research would benefit by
reproducing this method. However, it was impractical and unworkable for the same
methodology to be used in the current study. The most ecologically accurate footwear for
participants to wear during testing would be soccer boots. However, care must then be taken to
ensure that all boots are of the same sole configuration with either traditional aluminium studded
or moulded rubber studded boots being worn by all participants due to the fact that Jensen and
Barinotti (2001) observed a difference in ground reaction forces between the two styles of boot.
30
The participants of the current study had to meet certain criteria in order to qualify for testing.
All participants were male and above the age of 18 and were required to be currently playing
soccer to an amateur standard. However, no constraints were placed on the players according
to the position in which they regularly played and this may have had an effect on the results.
The problems that arise from participants playing in various positions are due to the fact that
certain participants will be more accustomed to performing certain kick types than others. For
example, a striker will consistently practice shooting in training and will perform that type of kick
regularly during a game; the specifications for performing a shot includes keeping the ball low
and therefore leaning over the ball at foot contact (Hughes, 1999). However, the criteria for a
distance kick would be exactly the opposite and strikers would probably be unfamiliar with the
performance of such a kick; which may become apparent within the results.
The abovementioned point may have been the reason for certain subjects within the current
study producing results that are contradictory to the rest of the participant group, e.g. participant
J displayed considerably higher anteroposterior impulse for power kicking with the opposite
being true for the majority of the rest of the group. The avoidance of this problem could have
been achieved through ensuring all participants played in the same position, although
participant recruitment for the current study was difficult with the current restraints and adding
more would have jeopardised reaching the target number of ten. Another option would have
been to have defenders performing distance kicks and strikers performing the power kicks,
although inter-participant group differences would have produced similar problems had this
been the case.
The current study aimed to evaluate players of the same playing ability and therefore amateur
players were the target group for participant recruitment. However, the structure of the modern
game means that players that are classified as amateurs can still vary considerably in playing
ability. Again, due to difficulties enlisting participants the problem of varying ability could not be
avoided. Barfield (2000) found that the performance of superior players resulted in faster ball
31
velocities and greater vertical and anteroposterior ground reaction forces. This issue may also
have been a contributing factor to the high levels of inter-participant variability. The use of
players from either the same amateur league or from the same team may help to reduce
inconsistencies due to differing playing abilities, although variation would still be present in both
of these groups the range of difference would hopefully be reduced. Another method of
reducing the variation between players due to ability levels would be to use elite soccer players,
although finding elite players to participate would have been near impossible for the current
study and may also be difficult for future research.
5.3. Recommendations for future research
The limitations of the current study that have already been highlighted bring about many
recommendations for future research to take into account. The use of only ten participants was
highlighted as a possible factor for the high inter-participant variability and could be
counteracted by simply repeating the study with a greater number of participants, although the
time scale of the study would have to be considered. Another possible modification that may be
put in place, were the study was to be re-examined, is the use of effect size statistical analysis
as described by Mullineaux et al. (2001). It was highlighted by Mullineaux et al. (2001) that
inferential statistics (the use of a significance level) leads only to a two way output, either to
reject or accept the null hypothesis depending on whether a given difference between two
factors is found to be significant at either the 99% or 95% confidence level. However,
Mullineaux et al. (2001) explained that effect size statistics provide a quantification of the
difference between two factors as small, moderate or large. Therefore, although inferential
statistics may provide non-significant differences between certain dependant variables effect
size statistics may identify moderate or small difference between the same variables.
32
The use of elite soccer players may provide results with greater stability and less interparticipant variability along with the fact that elite soccer players would hopefully be able to
perform the required kicking techniques with greater technical ability, which would result in a
more accurate ground reaction force representation of the different kicking techniques. Barfield
(2000) stated that skilled players exhibit greater ball velocities and ground reaction forces.
Therefore, the results of such a study would only be applicable to other players of the same
standard and even though the study of novice and amateur players would provide results that
would be more applicable to the general soccer playing population, the inter-participant variation
would probably be similar to that of the current study, which would raise similar data analysis
issues.
Female soccer players were not used in the current study as they were not as readily available
as male players and recruiting ten female players would have been extremely problematic.
However, with the constant increase in popularity of soccer throughout the world there has also
been an increase in female soccer players and for this reason further research should consider
female players. Saggini and Vecchiet (1994) previously compared male and female players
and reported that male players exhibited greater ground reaction forces during running and a
shot. However, only 15 female participants were used in the study by Saggini and Vecchiet
(1994) and further analysis of the ground reaction forces acting on female soccer players during
soccer specific actions is required.
The ground reaction force analysis of other kicking techniques from the game of soccer may
provide valuable information to gain an understanding of the cause of injury in soccer and how
possible preventative measures could be implemented. The execution of a corner kick or a free
kick with the aim of swerving the ball may develop greater free moment values than that of a
kick with simple linear aims. However, the inconsistency in this variable from the current study
may prove that free moment examination during kicking may not be possible and further
assessment of free moment data during kicking is required to establish whether the variability in
33
the current study was a methodology flaw or a generic problem. The previously mentioned
kicking actions are predominantly performed by the same person within a team, therefore any
risks associated with those kicking actions may predispose that player to injury due to repeating
the action during training and matches. A side foot or push pass is possibly the most frequently
executed kicking action within soccer and although the nature of the action may not be as
rigorous as that of the shot it is likely to be performed more regularly, which warrants future
analysis of the ground reaction forces acting on a player during this type of kick.
Further examination of the effects of varying playing surface on ground reaction force is another
area that requires attention in future work even though some research has been done (Ekstrand
and Nigg, 1989). The constant development of synthetic surfaces and the increased popularity
of indoor soccer as a means of keeping fit (Lindenfeld et al., 1994) suggests that ground
reaction force data from the two surfaces should be analysed by future studies, along with this,
the effect of weather on natural surfaces e.g. rain soaked and sun baked etc. may also require
attention in future research. Another area within soccer and biomechanics that has received
little attention is the effect of performing soccer specific activities with the dominant and nondominant foot. Dorge et al. (2002) did study the area of footedness of ground reaction forces
during kicking, although the variety of actions within soccer such as differing turning methods
and other kicking actions means that further research is possible and required.
Many authors have identified that certain adaptations to an individual’s running technique occur
in response to an increase in impact magnitude (Nigg et al., 1987; Shorten and Wilmslow, 1992;
Milgrom et al., 2000; Durward et al., 2001). Video footage of the lower extremities during a
kicking action followed by qualitative analysis of the ankle, knee and hip may aid in recognising
whether similar technique adaptations occur during kicking as a response to increased impact
magnitude and loading rate. The observation of technique from a qualitative stand point may
also aid in identifying differing technique factors that may attribute to the variation in ground
reaction force data between participants and why certain participants display greater ground
34
reaction force values than others, such as the impact peak value of 9.18BW displayed by
participant J in the current study.
The current study did not identify any significant difference between vertical ground reaction
force data for distance kicking and power kicking. However, the current study did establish that
the vertical forces acting upon soccer players during the two kicking actions are greater than
those previously reported for running. While the risk of overuse injury associated to high impact
activities is debatable it has been stated that repetitive impacts and loading of the lower
extremities does carry a risk of overuse injuries, especially if accompanied by an irregular
footfall or inadequate protection by footwear (Cavanagh and Lafortune, 1980). Therefore, it
would be beneficial for future research to be carried out in the field of match analysis to
establish the frequency of certain soccer related actions during a match situation. This data
could then be broken down to set apart the players that perform certain activities more
frequently and when used in conjunction with biomechanical data possible at risk groups of
players may be identified.
6.0. Conclusion
The purpose of the current study was to compare two kicking techniques in order to identify
differences in the ground reaction force data. The kicking techniques that were investigated
failed to provide significant differences in vertical impact peak and loading rate, although a
significant difference was observed for the net anteroposterior impulse of the two kicks along
with the approach speeds and ball velocities. The vertical free moment data that was collected
during the performance of the two kicking techniques displayed a large amount of variability and
no statistical analysis was therefore done with this variable. Even though no significant
difference was establish between the vertical ground reaction forces of the two kicking
techniques, the forces were found to be considerably greater than those previously recorded for
kicking, possibly questioning the overuse injury implications of performing the kicks.
35
The results of the current study and any assumptions made due to the results are questionable
due to the fact that the study contained many methodological flaws. The footwear worn during
testing, the position that the participant regularly plays in and the ability level of the participant
may all have had an effect on the recorded results of the study. With the methodological flaws
mentioned above in mind, the conclusions of the study were that distance kicking presented no
difference in vertical impact peak or loading rate, although distance kicking did display
significantly greater negative net anteroposterior impulse and significantly slower approach
speeds and ball velocities, yet these factors may have all be related.
The vertical ground reaction force data of the current study displayed no significant difference
depending upon the type of kick, though both kicks displayed high levels of vertical impact and
loading rate, which questions whether both kicking techniques carry a certain degree of over
use injury risk. Further research is required into the risks associated with high impact activities
that occur infrequently, in addition to this, investigation into all areas of soccer with attention
being paid to female players and other kicking techniques is also required to aid in improving
equipment and to promote awareness of injury issues etc.
36
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8.0. Appendices
8.1. Appendix A
School of Sport and Leisure Management
Research Ethics Committee
Participant Information Sheet
Project Title
Ground reaction forces acting upon the support leg of a
soccer player during two kicking techniques
Name of Participant
Supervisor/Director of Studies
Jon Wheat
Principal Investigator
Lee Hole
Purpose of Study and Brief Description of Procedures
(Not a legal explanation but a simple statement)
You will be asked to perform a soccer kick for distance (goal kick etc) and a kick for power (shot).
The forces that the ground exerts on your standing leg during the kicks will be measured using a
force platform that will be set within the floor of the lab. The none kicking foot will therefore be
required to land cleanly on the platform and any unacceptable trial will be repeated until a
satisfactory trial has been performed. You will be able to place the ball yourself to ensure you
make good contact with the force platform.
Once a kick has been performed you will be asked to rate the kick on a Visual Analogue Scale by
marking a 10cm line according to the quality of contact with the ball and how effective you felt the
kick would have been in a game situation e.g. a mark closer to the right hand side of the line
means contact was good and you feel that the kick would have been an effective one. The trials
will be recorded using a video camera but your ankle and foot will be the only part of your body in
view.
You will be asked to perform the two kicks continually until data has been collected from five
acceptable trials for each of the kicking techniques. Prior to the testing you will have sufficient
time for a warm and practice. You will also be asked to complete a pre test medical questionnaire
and an informed consent form. You will be free to ask questions at any time during testing and
you will be able to terminate the testing at any time for any reason.
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Lee Hole
The Centre of Sport and Exercise Science
School of Sport and Leisure Management
Collegiate Crescent Campus
Sheffield Hallam University
S10 2BP
Tel: 07984 608 052 or 01457 767 316
E-mail: holeymoley69@lycos.co.uk
It has been made clear to me that, should I feel that these Regulations are being infringed or that my
interests are otherwise being ignored, neglected or denied, I should inform Professor Edward Winter,
Chair of the School of Sport and Leisure Management Research Ethics Committee (Tel: 0114 225
4333) who will undertake to investigate my complaint.
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8.2. Appendix B
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