The Effect of Fatigue on Plantar Pressure in Soccer PlayersNovember 9, 2011

AbstractSoccer is the most popular team sport world-wide and a popular recreational activity. Unfortunately,

soccer players are at an elevated risk for developing certain injuries. Male soccer players have an elevated risk for 5th metatarsal stress fractures, a bone on the outside of the foot. Stress fractures occur when bones are subjected to repeated pressure and not given enough recovery time. Plantar pressure is the pressure between the bottom of the foot and the inside of the shoe, which may be measured with thin shoe insoles that transmit plantar pressure data through Bluetooth technology to a computer for analysis. This measurement quantifies the load experienced by different parts of the foot. Previous research has shown that fatigue changes the pressure distribution on the bottom of the foot during running and walking. Specifically, fatigued runners had greater peak pressure values. It has been suggested that people use their muscle fibers differently when fatigued and this translates into differences in plantar pressure. However, no studies have analyzed the effect of fatigue on sports played with cleats. The purpose of this study is to investigate the effect of fatigue on plantar pressure in male soccer players. This study will use 1-2 male soccer players. Their plantar pressure will be measured as they perform cutting maneuvers before and after they are fatigued. Fatigue will be validated by taking blood lactate measurements, since lactate accumulates in the blood during exercise. It is hypothesized that fatigue will cause an increase in plantar pressure. This data can be used to construct shoe inserts that mitigate the effects of fatigue by redistributing plantar pressure. It can also help coaches and trainers create safe training schedules, designed to give athletes enough recovery time to prevent stress fractures.

Introduction of Field

An Overview of the FieldEach year, Nike pays Michael Jordan $20 million dollars in endorsements. The retired basketball icon’s

prolific scoring and slam dunk abilities have earned him global recognition (Meyers, 1997). Few people reach this level of athletic performance and accordingly, Jordan’s journey to stardom was influenced by numerous factors. Two factors in particular, which contributed to Jordan’s rise to fame, were his technical skills and lack of serious injury. These facets form the core of the interdisciplinary field of biomechanics. Biomechanics is the scientific study of living things through mechanical principles (Bartlett, 1999; Bartlett, 2002; Besier, 2009; Boone, 2005; Hall, 1991). The term itself first emerged in the early 1970s (Hall, 1991). Broadly, this field combines knowledge of physics and anatomy to improve human performance and prevent injuries, a worthy goal for athletes at any level (Bartlett, 1999; Besier, 2009; Hall, 1991).

This section of the proposal will discuss the use of biomechanics as a tool for injury prevention in athletics. This can be accomplished through two different approaches (Boone, 2005; Hall, 1991). The quantitative approach utilizes numbers and measurements extensively (Boone, 2005). It could involve measuring the speed of an athlete’s foot through space or the joint angle of the elbow in a tennis swing. The use of numbers helps eliminate subjectivity and allows scientists to easily compare results. For this reason, published research tends to utilize this method. It is usually more predictive and publishable than the qualitative approach, which conversely, describes movement without the use of numbers (Boone, 2005). Qualitative data may include a “play-by-play” description of a baseball pitcher’s body position as he gets ready to throw the ball. This type of information may be especially useful for coaches trying to teach a new skill and is heavily reliant on observations. Biomechanists will choose an approach that matches the practicalities of their research experiments and fits the type of data they wish to collect.

Biomechanics research is limited by the capabilities of technology and the practice of using human participants. Researchers use instruments, like high speed cameras, force plates, and electromyography (machines that record the electrical activity of muscles), to heighten their own senses and to study characteristics outside the range of human detection. The tools used by biomechanists must be accurate, reliable and valid. They must yield

measurements that are precise and exact. The instruments must consistently yield those measurements. Lastly, biomechanics instruments must measure what they are designed to measure. Standard procedures and protocols are in place within the scientific community to test the accuracy, reliability, and validity of different instruments and methodologies. Another limiting factor biomechanists must contend with is the use of human subjects. While studying the capabilities of the human body, biomechanists must not exceed the bounds of what is ethical. They must remember to treat their human participants firstly as human beings, and not simply as a means to achieve the research goal. For example, when studying injury prevention, it would be unethical for a researcher to purposely injure an athlete to study how ankle sprains affect the gait cycle. Researchers are limited to naturally occurring injuries.

Tissues and InjuriesBefore looking into the prevalence of certain injuries and how they occur, it is important to understand

how a healthy, functioning body behaves. The human body is composed of several types of tissues that help carry out common bodily processes including movement. A tissue is a group of similar cells that perform a specialized function (Drake, Vogl, & Mitchell, 2010). There are four tissue types: epithelial, connective, muscle, and nervous tissue. Epithelial tissues line cavities and cover surfaces of the body. They are one main component of human skin. Connective tissues are a broad class of tissues that offer support and protection and are used as binding agents in the body. The cartilage that composes the human ear is an example of a connective tissue. Bone is another example of connective tissue. Muscle tissue forms the skeletal muscles that attach to bones, the cardiac muscle that forms the heart, and the smooth muscles that are found in organs. Lastly, nervous tissue form neurons, which transmit electrical signals from the brain to the body conveying sensory information and sending movement commands (Uno, 2010).

Injury to any of the above types of tissue is possible in all levels of sports though connective and muscle tissue injuries are particularly common. A scraped knee is an example of damaged epithelial tissue. A sprained ankle is an example of connective tissue detriment. Muscles may become sore or suffer more serious injury in the form of muscle strains. Nerve damage may result from a football tackle that culminates into paralysis.

Injuries from a biomechanists point of view are incidents where the load applied to a tissue exceeds the tolerance level of that tissue. Loads are simply the sum of all the forces acting on an object (Bartlett, 1999). Sports injuries result specifically from participation in a sport or exercise and cause a reduction in that activity or require medical advice or treatment (Bartlett, 1999). Often, sports injuries are characterized by how much time is required for recovery. Minor injuries require 1-7 days of recovery, moderate injuries require 8-21 days, and serious injuries necessitate over 21 days of recovery (Bartlett, 1999). In general, higher levels of competition result in more injuries. Team sports have higher injury incidence rates in matches, though individual sports have higher injury incidence rates during practice sessions (Bartlett, 1999). This knowledge can help team managers, athletic trainers, and coaches design schedules that enhance the health of their teams.

All sports injuries may be traced back to the failure of a biological material. The materials and structural properties of these bodily tissues are crucial to both the injury and recovery process. As stated earlier, injuries occur when the load applied to a tissue is beyond its tolerance level. When tissues are placed under stress they are deformed. The chemical bonds holding the tissue together are bent and stretched. These deformed bonds try to restore themselves, which causes a strain, and this strain can harm the tissue. The effect of the strain depends on how often it occurs (the frequency) and for how long (the duration). If the tissue is given sufficient time to recover, an intermittent strain may not cause any harm. If that load is applied frequently and the body does not have the chance to recover, an injury may occur (Bartlett, 1999).

Most athletic injuries manifest around joints and surrounding tissues. Injuries may either be traumatic or result from overuse. Traumatic injuries are caused by a single blow and have a rapid onset. Overuse injuries result from suffering repeated trauma. This leads to microscopic damage and the tissue does not get the chance to heal itself. Overuse injury is more likely during a sustained, strenuous activity since the muscles are tired and no longer able to neutralize the stress on the bone (Bartlett, 1999). Injuries may impact several structures in the body, or they may be confined to any single structure.

Specific Injury Mechanisms

The human body is comprised of 206 bones, which provide the framework for movement (Bartlett, 2002; Drake, Vogl, & Mitchell, 2010). Bones are relatively inelastic, though they undergo continuous remodeling to account for the changing stresses placed upon them (Weist, Eils, & Rosenbaum, 2004). The minerals in bones allow them to resist compression and provide the hardness and rigidity. Protein components allow for limited flexibility (Bartlett, 1991; Bartlett, 2002). These characteristics influence the rate of bone injuries, mainly dislocations and fractures, in sports. A bone fracture is simply a broken bone and may be classified based on the severity, shape, and position of the site of the fracture (Tortora & Derrickson, 2009). Fractures are the most common type of bone injury (Bartlett, 1991; Bartlett, 2002).

Trauma and overuse are both common causes of fractures. Fractures resulting from trauma tend to be more serious and involve a clean break in the bone. They are usually caused by an abnormally large force. Stress fractures result from overuse and are caused by microscopic fissures in a bone due to repetitive loads (Drake, Vogl, & Mitchell, 2010; Tortora & Derrickson, 2009). It requires more energy to break a bone in this manner, over a period of time. The characteristics of the bone also influence its resilience to injury. The body is composed of five types of bone which all have unique shapes. This may impact the load-bearing ability of any specific bone. If the magnitude of the force exceeds the strength of the bone, it will give way and break. Bone mass and density increase during adolescence. However, old age and diseases such as osteoporosis can decrease bone mass and density (Bartlett, 1999; Bartlett, 2002 Drake, Vogl, & Mitchell, 2010). It is thus important to consider characteristics of particular populations when studying bone injuries.

Muscles are another form of connective tissue, imperative for movement, and also frequently injured in athletic activities. Muscles are composed of many, many muscle fibers. These fibers receive nerve signals from the brain, then contract as a unit to produce movement. Muscles work in groups to move body segments and limbs and can produce many types of movement. Common muscular injuries include muscle strains and pulls. Essentially, the two result from tears in a muscle or a tendon (connective tissue that connects muscle to bone). Proper warm ups are the most effective preventative courses of action. Identifying which muscle groups have been affected and the extent of the tear are both critical elements to treating muscular injuries (Drake, Vogl, & Mitchell, 2010).

Joints are the final component of skeletal movement, and are formed from several types of connective tissue. A joint is the site where two bones come together. The knee, elbow, and wrist are examples of joints. Joints allow for movement and flexibility since their principle components, bones, are too rigid to bend without snapping (Tortora & Derrickson, 2009). Flexible connective tissue holds bones together, while allowing certain movements. Ligaments, connective tissue that attaches bones to bones, are important components of joints. Ligaments may also be damaged in the course of sports performance. They stabilize joints and transmit loads. Sprains result from excessive joint motion and are classified as the forced twisting of a joint that stretches or tears a ligament without dislodging the bone (Bartlett, 1999; Tortora & Derrickson, 2009). Sprains occur when a load is placed on a joint that exceeds its capacity. Sprains also may damage the blood vessels, nerves, and muscles surrounding the joint (Tortora & Derrickson, 2009). Swelling is a common indication of a sprained joint, because of the chemicals released by damaged cells and blood vessels (Tortora & Derrickson, 2009). Sprains often occur when athletes bend and twist limbs far away from the center of their bodies. The ankle is the most commonly injured joint in sports (Bartlett, 1999).

The Role of MechanicsThese three individual elements, bones, muscles, and joints, provide a basic framework for human

motion. The mechanics component of biomechanics specifically analyzes how different forces impact these three elements individually and together as a whole. Mechanics is the branch of physics that studies the relationship between matter, energy, and force, and how these affect motion (Bartlett, 2002). These forces may either cause new movements or change the characteristics of existing motion. Biomechanists investigate the effect of external elements on the body, which includes analyzing how the environment influences human performance (Bartlett, 1999; Bartlett, 2002; Hall, 1991). External factors may include playing surface and sporting equipment (Bartlett, 1999). Generally, living systems are more complex than man-made ones, though mathematical approximations are used to apply mechanical principles to the body. Early in the 16th Century, Descartes first proposed that all living systems, including the body, acted as machines obeying the same laws of the universe (Humphrey, 2003).

External Factors that Influence InjurySince the body obeys these fundament laws of nature, it is also subject to injury when these laws are

disobeyed. Though many injuries result from chance, athletes can take certain measure to reduce their risk of injury by understanding how mechanical principles apply to the body and to the sporting environment. Factors such as the athlete’s fitness level, sporting equipment and training surfaces can also be monitored in order to prevent injuries.

Appropriate fitness levels are useful in practice and game situations. However, fatigue may be hard to avoid and exposes the body to risk in a variety of ways. Fatigued structures, such as bones, muscles, and joints, are at a higher risk for overused injury (Bartlett, 1999). Additionally, certain scientific studies have noted more complex complications in the body due to fatigue. These complications result from changing body mechanics. Fatigued soccer players change their balance strategies, meaning their brains recruited different muscle groupings subconsciously when fatigue set in. This subconscious use of different muscle group created a cascade of effects in the body. Joint stability decreased because of the utilization of different muscle groupings, which also affected the athletes’ coordination. The use of the different muscle activation patterns has been directly labeled an injury risk (Greig & Walker-Johnson, 2007). Other studies have indicated that fatigued runners will change their running technique automatically when their body senses a harmful load (Willson & Kernozek, 1999). Overall, a lack of cardiovascular fitness, combined with inflexibility, muscle weakness, unhealthy body compositions, and other components of physical fitness, may all contribute to overused injuries (Bartlett, 1999).

Playing surface is another factor of athletic performance that may contribute to injuries. Sports are played on a variety of surfaces, which must match the types of movement required for the sport. Since the interaction between the athlete and playing surface is usually large, many properties of the playing surface should be considered when evaluating the likelihood of injury. The amount of friction and traction (the force created by interlocking of objects in contact with each other) helps athletes remain stable. Friction is especially important in sports that require a lot of horizontal movement. Compliance, the inverse of stiffness, is also important. Compliant surfaces deform under the pressure of a load. There are several grades of compliance and too much compliance may tire out the athlete. Running on an overly-compliant surface would be like running on a semi-inflated moon bounce. Too little compliance is also harmful because a stiff surface increases the amount of impact force, though somewhat stiff surfaces are necessary for vertical jumping. Resilience, the amount of energy absorbed by a surface and then returned to the striking object, is also a characteristic of playing surfaces. Again, a lack of resilience may lead to fatigue. Fatigue changes muscle recruitment patterns, joint stability, and overall coordination (Greig & Walker-Johnson, 2007). Other factors, like inclination can cause injury by repeatedly stressing certain areas of the foot and certain leg muscles (Bartlett, 1999).

Sporting equipment is the last external factor that impacts the occurrence of injuries. Most sporting equipment utilizes materials designed to modify environmental factors. Work out clothing, protective gear, and racquets are examples of this. Of all the sporting equipment, footwear is the most important. The footwear-surface interface is a crucial factor to performance in many sports. This interface is a constant factor. Furthermore, changes to footwear may alter the force on the foot due to gravity and the motion of the body. This concept was demonstrated by athletes changing their technique or stride to compensate for this increase in force on the bottom surface of the foot (Willson & Kernozek, 1999). These compensations may lead to changes in muscle activation patterns and cause the changes in joint stability and coordination attributed to fatigue. (Bartlett, 1999; Greig & Walker-Johnson, 2007). To reduce this risk, sports shoes should be designed for specific sports and have qualities that are specific for a particular playing surface. This could include sneakers with high sides that support the ankle, like in basketball, or studded shoes that allow for gripping such as the cleats used in soccer and football. Shoe design may change the forces experienced in the foot by over 100% (Bartlett, 1999). Good shoe designs minimize the rotation of the foot and dissipate the energy of each foot strike (Bartlett, 1999).

Strategies for Injury PreventionSuccessful athletes must overcome and avoid injuries to reach their optimum levels of performance.

Injury prevention is one facet of the multidimensional field of biomechanics. Injuries may impact several tissues in the human body including bones, muscles and joints. Smart training techniques, like those of Michael Jordan, maintain appropriate fitness levels and use equipment to mitigate the harmful impact of the natural environment.

Through understanding the human body, and the ways in which it can be injured, biomechanists can provide training recommendations and design equipment, such as athletic shoes, that enhances the performance and health of athletes of all calibers.

Build to your QuestionImportance of the Foot

The foot is a critical link between the body and the environment. It is also a key component of human locomotion, necessary for completing every day activities and participation in athletics. In many sporting activities the body’s feet must provide a stable base of support and move the body through the environment. For these reasons, each foot must remain functional and free from injury. It controls muscular activity in the rest of the body, cushions and absorbs impacts from the ground, and transfers internal forces back to the ground to accelerate the body. The foot can also influence other parts of the body, including the behavior of the ankle, knee, hip, and back. Since the foot has an extensive role in daily functioning and athletic performance, various aspects of this appendage may be studied. Anatomical abnormalities in foot structure, muscle activation patterns, movement and posture analyses, and measurements of pressure variation across the foot all merit research (Rosenbaum, 1997). Changes in any of these factors may culminate in impaired movement, which could result in time away from school, work, athletics and leisure activities. The study of foot behavior and functioning can lead to knowledge of injury mechanisms. This information can be implemented in prevention strategies that keep people mobile.

Newton’s Third law of mechanics states that all actions have equal and opposite reactions. Such interactive forces between the ground and the body are present during human movement. The bottom of the foot, or plantar surface, acts as the interface between the ground and the body. Ground contact forces refer to the forces exerted by a person on a surface. These forces are due to gravity and the body’s own velocity. They may have vertical and horizontal components (Rosenbaum, 1997). Ground reactions forces are equal and opposite of ground contact forces. Ground reaction forces refer to the force exerted by the surface and on a person. Ground contact forces and ground reaction forces always occur together, as shown in Figure 1. The ground reaction force is labeled R. The running subject is experiencing the sum of force R. At the same time, the subject is exerting force F, the ground contact force, on the ground. Force R and force F are equal in magnitude. By measuring the ground reaction force, the forces to which the body is subjected during normal and extreme activities may be evaluated.

Figure 1: Force diagram of ground reaction forces and ground contact forces. Different vectors of a ground reaction force (R) and the ground contact force (F) are due to gravity and the motion of the body (Thompson, 2002).

The role of ground contact forces and ground reaction forces in daily movements and athletic activities has led to the development of technologies that measure the pressure beneath the foot. Plantar pressure is an

example of this type of objective measurement. It refers to force between the bottom of the foot and the inside of a shoe. Essentially, plantar pressure measures a component of the ground reaction force. Higher plantar pressure indicates that the ground reaction force has increased. Pressure is defined as the distribution of a force across a surface. Plantar pressure specifically analyzes how a force is distributed along the plantar surface of the foot.

Several systems are now available to measure plantar pressure. Force platforms and insole systems are two categories of such devices. Force platforms are commonly used since they have the ability to record ground reaction forces in multiple directions. These are essentially metal plates that quantify the ground reaction forces of static or dynamic bodies on top of them. Subjects either stand or perform movements on top of such plates, which then transmits pressure data to a computer. They provide accurate measurements for fast events because they have a high sampling frequency. However, force platforms are limited because they cannot yield information about the distribution of a load over the surface of a foot. Therefore, force platforms cannot be used to make inferences about foot anatomy and pathology. Force plates only provide information about the net effects of a ground reaction force on the body (Rosenbaum, 1997).

Quantitative methods that specifically assess foot pressure patterns are a fairly recent development. These methods rely on electromechanical sensors that act as force transducers. This technology is compiled in a shoe insert that collects pressure data and links it to specific regions of the foot. Pressure sensors are distributed across a thin, flexible insert, which is fitted into the shoe. The foot’s plantar pressure is determined by dividing the force experienced by a region by the area of that region. Electromechanical transducers change a mechanical event into an electrical signal, which is then stored for analysis. One system of interest, the Pedar System, uses a configuration of capacitive sensors in two electrically conductive surfaces. Data from these inserts is sent via Bluetooth technology to laptop computers for storage and analysis. These systems may be used for static and dynamic situations and may also provide valuable information about both normal and pathological populations. In-shoe systems can test the effect of different shoe constructions directly. These systems are often portable, so they may be used to test conditions outside of a laboratory environment (Rosenbaum, 1997).

Plantar Pressure and 5th Metatarsal Stress FracturesProfessional and recreation participation in sports are common, daily activities for many people. One

application of in-shoe pressure measurement systems is to study foot loading during athletic tasks. Foot loading patterns refer to the sequence of pressure fluctuations of certain regions throughout a movement. Pressure patterns resulting from athletic tasks are generally more extreme and place greater stress on the body (Rosenbaum, 1997). Loading patterns may provide insight into the occurrence and prevalence a variety of injuries, including stress fractures.

Stress fractures are microscopic breaks in the bone. According to Queen & Nunley, stress fractures are common in sports, accounting for 10% of all injuries due to overuse (2009). There are two prevailing theories that explain the development of stress fractures. One theory suggests that the activity of osteoblasts, cells responsible for bone formation, function more slowly than osteoclasts, cells that break down old bone tissue. Normally, certain chemicals trigger osteoclasts to remove areas of the bone that are micro-fractured. This theory maintains that over-active osteoclasts weaken areas of bone before osteoblasts have the chance to reinforce those areas. Another theory suggests that stress fractures occur when bones bend past their point of tolerance. Factors that influence the occurrence of stress fractures include gender, alignment of the lower extremity, bone density, poor training techniques, and improper footwear (Queen & Nunley, 2009).

Metatarsal stress fractures contribute to 25% of overuse injuries in the foot (Queen & Nunley, 2009; Weist, Eils, & Rosenbaum, 2004). Fifth metatarsal stress fractures are also known as Jones fractures and occur in the most distal metatarsal on the outside of the foot. Jones fractures are particularly difficult to heal and often result in re-fracturing. Although few studies have been conducted with a large number of participants with 5 th metatarsal fractures, literature indicates that this type of fracture is more common in men than in women (Queen & Nunley, 2009). Male soccer players are particularly predisposed for this injury. Bones that are subjected to greater stress repeatedly, including stress from increased plantar pressure, are more likely to develop stress fractures. Therefore, understanding factors that increase plantar pressure could help prevent stress fractures. Fatigue is one factor common in athletic activities that may influence plantar pressure.

Fatigue and its Impact on Plantar Pressure There is a growing body of research investigating the effects of fatigue on plantar pressure. Various

physiologic studies have indicated that muscle activation patterns change in response to both short-term and long-term exercise (Bisiaux & Morretta, 2008). Some studies have reported fatigue also impacts plantar pressure and other studies claim to have found no significant changes in plantar pressure. The studies reporting a relationship between fatigue and plantar pressure have also noted certain physiologic changes accompanying fatigue. These physiologic changes translate into biomechanical changes. Biomechanical changes influence the amount of force experienced by certain parts of the body. Specifically, biomechanical changes could alter the amount of pressure experienced by the foot. Plantar pressure could change when athletes use different postures and techniques during movements, through increased ground reaction forces. Inconsistencies in literature relating fatigue and plantar pressure may be attributed to the fact that exercise intensity and rest periods are not well defined throughout literature (Bisiaux & Morretta, 2008).

Several physiologic changes in athletic performance have been documented and attributed to fatigue. Therefore, several biomechanical changes may also be present during fatigue. Decreased neural input to muscles and less efficient contractile mechanisms was observed in marathon runners. Prolonged running has been shown to change the length and frequency of running strides. Mechanically, it is logical to presume that changes in gait pattern, such as shortened strides, induce changes in ground reaction forces experienced by the foot. Fatigue produces other biomechanical changes as well. It reduces muscular control, which may manifest as difficulty resisting eversion and inversion of the ankle. Average pressure, peak pressure, and regional contact area are all affected by the activation of different muscle groups in runners as well (Bisiaux & Morretta, 2008).

As runners become fatigued, their ability to absorb shocks from ground reaction forces diminishes (Willson & Kernozek, 1999). Some researchers have suggested that fatigue of the plantar flexor muscle group during the heel-lift phase of the gait cycle may cause excessive bending of the metatarsals. Others have speculated that runners will automatically change their technique when their body perceives harmful loading patterns on their feet. Willson & Kernozek have shown that fatigued, recreational runners had significantly faster cadence patterns, decreased loading on the heel region, and increased loading under the first metatarsal (1999). As the body becomes more tired, it is unable to maintain its ideal level of functioning. Physiologic effects of fatigue require biomechanical adjustments. In this case, the stride length and foot loading patterns were altered.

Information regarding the body’s biomechanical and physiologic responses to fatigue is important because it could enhance the development of special training programs to strengthen and stabilize foot muscles.

Soccer and FatigueThe aforementioned studies were generally conducted on experienced or recreational runners. Thus, their

results may not necessarily apply to other sports and activities. The interface between the shoe and the ground may change with different shoe designs. Soccer players wear studded cleats, which could alter plantar pressure and muscle activation patterns significantly. Few studies have focused on fatigue and plantar pressure in cleated sports like soccer.

Soccer is the most popular team sport worldwide and it is played at a variety of levels. Overuse injuries, including stress fractures, are common at all levels of soccer. Information about the location and magnitude of loads acting on the foot could be used to design shoe inserts that decrease the occurrence of stress fractures in soccer players. Studies that have been designed to test plantar pressure in soccer players provide useful information about foot loading during certain soccer-specific movements. These movements include cutting, sprinting, and kicking. Observing plantar pressure during these movements ensures that significant findings may be applied to game-like conditions. Eils, Streyl, Linnenbecker, Thorwesten, Völker, & Rosenbaum have noted significant changes in loading patterns during such movements (2004). Therefore, certain areas of the foot may be more vulnerable to stress when players engage in these movements.

Muscular fatigue another component of soccer, particularly apparent during the closing minutes of both the first and second halves (Rahnama, Reilly, Lees, & Graham-Smith, 2003). The ability of the soccer players to perform at their maximum level is diminished, though they can continue to exercise at lower intensities. Fatigue is particularly evident during the closing stages of the 2nd half. One study found that players cover 5% more distance during the first half than the second. Determining exactly when fatigue begins to affect muscular output is

difficult, especially since soccer activity is highly intermittent (Eils et al., 2004). However, knowing the precise time fatigue-onset could help trainers manage their teams more effectively.

Since fatigue has clearly been linked to soccer, it would be prudent to investigate how fatigue may change factors such as muscle activation patterns and plantar pressure, which have been noted in other sports. These effects could very well carry over into soccer because soccer-induced fatigue has produced other physiological changes. Specifically, it caused decreases in force generation major leg muscles in both the dominant and non-dominant leg (Rahnama et al., 2003). Greig & Walker-Johnson demonstrated that soccer players use different groupings of muscles to maintain their balance after they became fatigued. These altered balance strategies meant that muscle recruitment patterns, joint stability, and overall coordination were affected. This study specifically stated that soccer players were more susceptible to injury when using these alternative balance strategies (2007). Since soccer players’ ability to generate muscular force diminishes with fatigue, they may have trouble maintaining proper form and technique as they become tired. Their muscles may not be able to continuously hold their bones in proper alignment. By extension, their tired muscles may also have difficulty absorbing the impact from ground reaction forces and transfer that extra force to the bones. Fatigued soccer players may also experience greater ground reaction forces as their techniques become less precise. These loads may be transferred to the skeletal bones from the tired muscles, increasing the likelihood of stress fracture development.

Fatigue has produced noticeable plantar pressure effects in runners wearing sneakers with flat soles, but it has not been studied thoroughly in cleated sports. Soccer-specific fatiguing protocols have been shown to reliably induce both the physiologic and biomechanical changes in players, which have the potential to translate into changes in plantar pressure (Greig & Walker-Johnson, 2007; Rahnama et al., 2003). This potential should be analyzed within the context of a soccer match to develop a greater understanding of possible metatarsal stress fracture mechanisms and design training protocols that safely control the amount of plantar pressure experienced by soccer players. Based off this information, the proposed research project seeks to investigate the following question:

How does fatigue affect planter pressure in soccer players?

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This study was designed to characterized and compare plantar pressure during soccer-specific movements on two different surfaces. These researchers wanted to identify anatomical structures that experienced greater stress during these movements as they quantified the load underneath the foot. Subjects sprinted, cut, and took shots on goal on two different surfaces. Cutting maneuvers created greater loads in the lateral parts of the midfoot and forefoot and in the medial parts of the foot. Sprinting loads shifted to the lateral toes. While kicking, participants experienced greatest plantar pressure in the lateral heel and midfoot areas of the supporting foot. These transfers resulted in significantly higher peak pressure values in most of the areas named above. This research shows that soccer movements cause certain areas of the foot to experience greater stress. Therefore, plantar pressure studies should be consider they types of movements performed in different sports in addition to the specific types of footwear worn for different athletic activities.

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Thompson, D. (2002). [Image of ground reaction force and ground contact force vectors]. Retrieved from http://moon.ouhsc.edu/dthompso/gait/kinetics/GRFBKGND.HTM

Tortora, G. J., & Derrickson, B. (2009). Principles of anatomy and physiology, (12th Ed.). Hoboken, NJ: John Wiley & Sons, Inc.

Uno, J. K. (2010). Histology [PDF PowerPoint]. Retrieved from Blackboard’s online web site: https://blackboard.elon.edu/bbcswebdav/courses/PermBIO161Lab/Histology%20power%20point.pdf

Weist, R., Eils, E., & Rosenbaum, D. (2004). The influence of muscle fatigue on electromyogram and plantar pressure patterns as an explanation for incidence of metatarsal stress fractures. The American Journal of Sports Medicine, 32 (8), 1893-1898.

One of the primary roles of muscles in running activities is to minimize the impact force experienced by the bones. Decreased muscle force, due to bodily fatigue, can therefore cause increased loading in bones. Local muscle fatigue can be observed with electromyography (EMG) or recording the electrical activity of muscles. This study looked at the relationship between fatigue-related changes in muscle activity patterns and plantar loading differences during a fatiguing treadmill run, specifically to understand factors that may influence the development of stress fractures. This study found significant changes in foot-loading patterns and in muscle activity patterns in the calf muscles. Pressure distributions across the foot while subjects were fatigued had much higher peak pressure in the forefoot and midfoot regions. Thus, adaptational changes in forefoot and midfoot loading have been suggested as a mechanism in the development of stress fractures. These changes could disturb the remodeling process of the metatarsals. The increased loading under the midfoot may be due to increased pronation of the foot. Since soccer is a running-intensive sport, the same fatigue mechanisms apparent in runners could also apply to soccer players. Additionally, this paper used a similar methodology to the one I am proposing. The researchers induced fatigue, validated it through blood lactate measures, and observed changes in plantar pressure distribution.

Willson, J. D., & Kernozek, T. W. (1999). Plantar loading and cadence alterations with fatigue. Medicine & Science in Sports & Exercise, 31(12).

Methods/Approach

This study will be conducted using members of Elon University’s Men’s Club Soccer team. These individuals will be experienced soccer players who play at least twice a week. They will all be between ages 18-22. The proposed study is a pre/posttest design, meaning participants will be measured before and after an intervention to determine the effect of that intervention. The peak value of plantar pressure in the lateral aspect of the foot will be measured before and after the participants are purposely fatigued. Measures of perceived exertion and blood lactate will also be taken to ensure that participants are in fact fatigued from the fatiguing intervention.

Each participant will come to the Koury Athletic Center at Elon University and read the informed consent document outlining the experimental procedures. They will then be asked to sign the informed consent to continue involvement with the study. The shoe size, height and weight of each participant will be recorded. Participants will be asked to rate their perceived level of exertion using the Borg Ratings of Perceived Exertion scale. This scale uses numbers that corresponds to levels of exertion ranging from “no exertion at all” to “maximal exertion.” The participants’ blood lactate levels will be taken using a finger prick sampling method. This will be done with an Accutrend portable lactate analysis.

Following this, participants will complete a warm-up regimen of their choosing. Then they will watch as an agility course is explained to them. The agility course will consist of two common soccer maneuvers (side cuts and cross cuts) around a series of 4 cones, arranged in a zigzag pattern, spaced five meters apart, for a total distance of 20 meters.

Once the participants understand the agility course, they will complete it at a slow jogging pace. Then, the participants will be fitted with the shoe inserts manufactured by Pedar Mobile System (Novel, St. Paul, MN,

USA) as well as Pedar components that fit in a mobile pack, which will be born while participants complete the agility course. Once the participants feel comfortable with the equipment, they will run the agility course at full speed while they are recorded by digital video to provide pretest measures of plantar pressure. Pre-fatigue plantar pressure data will be collected at that time.

Participants will then change into running shoes (which will not be connected to the Pedar Mobile system) to complete the fatiguing protocol. The participants will be timed as they run two 20 meter sprints at their maximal speed. The fastest sprint time will be used to define the parameters for the fatigue protocol. The fatigue protocol chosen for this experiment was designed and validated by Nicholas, Nuttall, & Williams (2000) and called the Loughborough Intermittent Shuttle Test. It produces physiologic and metabolic responses similar to those generated by playing in a soccer game. A modified Loughborough Intermittent Shuttle Test will be performed outside Koury Athletic Center. It consists of two parts. During Part A, participants will go back and forth between two cones spaced 20 m apart. The researcher will call out the time parameter for each 20 m segment based on the participants’ maximal sprint speed to ensure the participants are running at the correct pace (walking, 55%, or 95% of maximal sprint speed). The participants will complete the exercise block five times. The participant will rest for 1 minute in between each part.

Part A Exercise Block 1x walk 1x 55%1x 95%

1x walk2x 55%2x 95%

1x walk3x 55%3x 95%

Part B consists of an exhaustive shuttle run during which participants will run 20 m alternating his speed between 55% and 95%. Again, the researcher will call out the time parameters. The test will end when the participant fails to run the 20 m in the allotted time.

Immediately following the fatiguing protocol (when the participant fails to complete the 20 m distance in the allotted time), the participant will rate his level of perceived exertion again using the Borg Rating of Perceived Exertion scale and have his blood lactate levels measured to validate a fatigued state. Participants will then put their cleats on and they will be reconnected to the Pedar Mobile system. Once this is completed, participants will run the agility course at full speed again to obtain posttest plantar pressure values. Digital video will be used to record the participants completing the agility course. After the participants have been disconnected from the Pedar system and they will perform a cool down of their choosing.

The ratings of perceived exertion and blood lactate levels of each participant will be compared to ensure each participant changes from a non-fatigued state to fatigued state after the fatiguing intervention. Peak pressure values in the lateral aspect of the foot will be statistically analyzed to determine whether the induced fatigue resulted in any significant changes in the pressure distribution along the plantar surface of the foot.

Nicholas, C. W., Nuttall, F. E., & Williams, C. (2000). The Loughborough Intermittent Shuttle Test: A field test that simulates the activity pattern of soccer. Journal of Sports Science, 18, 97-104.

TimelineFall 2011

Have well-defined, well-researched question at the end of the semester Submit and receive approval from Institutional Review Board Learn how to collect data with Pedar Mobile system Finish data collection Search for conferences for research presentation Submit project proposal to the College Fellows for review

Spring 2011 Enroll in 2 credit hours of independent research Analyze data at Duke’s Human Performance Laboratory Submit abstract for SURF poster session by Feb. 20 Prepare abstract, poster, results, and discussion for a SURF presentation on April 24 Submit 3rd year progress report before spring break Apply for reimbursement for money spent in Fall 2011

Fall 2012 Enroll in 2 credit hours of independent research Write paper to submit for publication Submit abstract to the 2013 Southeast chapter of the American College of Sports Medicine

(SEACSM) conference by October 1 Prepare for SEACSM conference Obtain appropriate signatures for 4th year progress report

Winter 2013 Finish up all Elon University graduation requirements

Spring 2013 Attend and present at SEACSM in Greenville, SC February 7-9, 2013 Submit 4th year Fellows Project Verification Form, Project Proposal, Project Product (a professional

research paper), and documentation of public presentation at the SEACSM

Budget/Budget Justification

Items Fellows *Other Total

Blood lactate measuring kit This measurement validates that participants become

fatigued after completing the modified Loughborough Intermittent Shuttle Test

$0 $300 $300

Gas money The primary equipment for this project (the Pedar Mobile

system) belongs to Duke University, so I must travel there to bring it back to Elon and to learn how to use it. I will also travel there to analyze my data.

$202 $0 $202

Rental of Pedar Mobile equipment from Duke University and consultation fees

Few research institutions have access to this technology. I have an agreement with Duke University to use and become trained on the Pedar Mobile system for my project at Elon.

$0 $1,500 $1,500

Participant reimbursement It is customary to offer some type of incentive to human

participants, especially if they will be performing difficult tasks.

$50 $0 $50

Presentation at the Southeast American College of Sports Medicine chapter conference.

This conference will be held in Greenville, SC. The Exercise Science department funds all related expenses for students who present at this conference.

$0 $ 500 $ 500

Total $252 $2,300 $2,552

* Additional funding provided by Elon University’s Exercise Science Department

Appendices

Borg Rating of Perceived Exertion

Borg GAV. Borg’s Rating of Perceived Exertion and Pain Scales. Champaign, IL: Human Kinetics, 1998.

Rating

Description

6 NO EXERTION AT ALL

7EXTREMELY LIGHT

8

9 VERY LIGHT

10

11 LIGHT

12

13 SOMEWHAT HARD

14

15 HARD (HEAVY)

16

17 VERY HARD

18

19 EXTREMELY HARD

20 MAXIMAL EXERTION