High degrees of rotational traction associated with athletic footwear can increase the risk of noncontact lower extremity injury following an unexpected neuromuscular perturbation, possibly by increasing biomechanical joint loading at the ankle and knee.
By John W. Wannop, PhD; Ryan Madden, MSc; and Darren J. Stefanyshyn, PhD, PEng
Any athlete, whether recreational, competitive, or professional, is at risk of suffering an injury whenever they participate in sports. The majority of sports injuries (up to 78%) occur in the lower extremities,1 with ankle sprains being the most prevalent. Ankle sprains typically occur in sports that involve sudden stops and cutting maneuvers, such as football and basketball, and account for 15% to 30% of all injuries.2-4 Knee injuries are also common, with anterior cruciate ligament (ACL) injury being a frequent, severe injury of the knee joint. ACL injuries tend to be caused by sudden deceleration, cutting or pivoting, hyperextension or hyperflexion, or by a blow to the posterolateral aspect of the knee.5
Sports injuries are generally classified as either contact (58% of all sports injuries) or noncontact (up to 36.8% of all sports injuries [5.2% of injuries in these studies could not be determined as contact or noncontact]).6,7 One of the major variables associated with noncontact injury is the shoe-surface interaction, specifically footwear traction.8-10
Although footwear traction has been studied for many years, it remains an area of contention within the scientific community. One of the main difficulties in measuring traction arises from the fact that traction of footwear does not follow the laws of mechanical dry friction.
Dry friction occurs when two surfaces in contact move relative to one another, with Amonton’s laws governing the relationship that exists between the surfaces. Amonton’s laws state that: (1) the friction force is directly proportional to the applied load, and (2) the friction force is independent of the apparent area of contact of the surfaces. Due to the nonuniform topography and the viscoelastic properties of footwear and sports surfaces, footwear friction is not well characterized by Amonton’s laws.11 The boundary conditions observed when measuring footwear traction, such as the normal load, movement speed, temperature and moisture, will have a significant impact on the results.8,12-20
Commonly, when footwear traction is measured, it is divided into two components: translational traction and rotational traction. Translational traction is a coefficient calculated as the ratio of horizontal force to normal force. Rotational traction is described using the peak moment of rotation about the center of pressure,21 which refers to rotation of the foot around a point of contact on the shoe sole.22 Translational traction is thought to be necessary for athletes to start, stop, and run quickly, while rotational traction is important for cutting, pivoting, and rapid changes in direction.
Footwear traction depends on the interaction between the shoe and the sports surface. Although advancements in footwear design and technology are constantly occurring, the greatest changes during the past 40 years have occurred in sport surfaces, specifically artificial surfaces for field sports such as football and soccer (Figure 1).
Artificial turf was first used as an alternative to natural turf in 1966, with the first major installation occurring in the Astrodome in Houston, TX. These first-generation surfaces were made from fibers (usually nylon) densely packed with no shockpad or infill. Second-generation products began to appear in 1976, consisting of longer fibers with sand used to fill the spaces between the fibers, and shockpads incorporated under the surface. Third-generation surfaces were developed in the 1990s that were composed of less dense fibrillated fibers that closely mimicked natural grass due to the addition of an infill of rubber, sand particles, or both. Many facilities have begun to install third-generation infill surfaces due to their renowned durability allowing for year-round activity, as well as being relatively low maintenance compared with grass.23
Since the first iteration of artificial turf, researchers have sought to determine how comparable these new surfaces were to natural grass with respect to injury risk. Early studies concluded that first- and second-generation surfaces were associated with much higher injury rates than natural grass, with artificial turf injuries generally being more severe.8,9,12,13,24-26 Overall, lower extremity injury rates increased by 30% to 50%27 when sports were played on these first- and second-generation artificial surfaces.
Results have been less clear when comparing the injury rates associated with newer third-generation surfaces and those associated with natural grass. Some studies of American football have shown an increased risk of ACL injury on third-generation artificial surfaces,23,28 while others have found a slightly decreased risk of ACL injury29 or no difference in ACL injury risk between surfaces.30 Of particular interest is that two of the studies investigated the same population (National Collegiate Athletic Association athletes) during the same time period and reached different conclusions.23,30 This may have been due to the differences in the overall ACL injury incidences in the studies, and that only one specific type of artificial turf (FieldTurf) was investigated in one of the studies.30
When accounting for all injuries, a change in sports surface is associated with changes in the location, severity, and distribution of injuries sustained by athletes.29-31 Investigations into other sports, such as soccer, played on similar artificial surfaces have provided data as conflicting as those associated with American football. Some have observed increased rates of injury when soccer clubs install artificial turf at their home venue.32 Others found no overall differences in injury rates between the artificial and natural surfaces, but did observe differences in the location, magnitude, and distribution of injuries.33,34 Another study reported a lower incidence of total injury and lower substantial trauma on artificial turf (FieldTurf) compared with natural grass.35
Other sports in which surfaces have influenced injury include tennis, where in one study injury treatment during matches was required more often on grass and hard courts compared with clay courts;36 team handball, where the ACL injury rate in women was significantly higher on rubber-coated artificial floors than on wood floors;10 and women’s floorball, where players had a greater risk of injury on artificial floors.37 While these studies provided evidence that the sporting surface can influence athlete injury rates, the specific aspects of the surfaces that may increase the risk of injury remain unknown.
Confounding factors related to artificial sports surfaces include the infill used,38 the fiber structure of the artificial surface,38 the maintenance or contamination of the surface,39 the surface hardness,40 infill compaction and surface wear,41 surface temperature,42 surface moisture,13 and the type of grass/soil comprising the natural grass surfaces. Any of these different surface properties may influence athlete injury rates; however, footwear traction is specifically mentioned by the majority of authors who have studied this topic as a mechanical property of the surface that contributes to the risk of noncontact injuries, with higher injury rates occurring on surfaces that provide greater footwear traction. However, in the majority of the previously mentioned studies, rarely were mechanical measurements of the playing surface or footwear reported.
In order to establish context and identify the potential mechanisms for injuries on these surfaces, the mechanical characteristics related to the confounding factors mentioned above must be determined. The need to quantify the mechanical properties of a playing surface is not limited to artificial surfaces, as the mechanical properties of natural grass surfaces can change vastly throughout the course of a season.43 While the majority of studies have focused on differences in injury risk between playing surfaces, mechanical testing must also be conducted to inform recommendations on how to manipulate the surfaces to reduce injury risk.
Footwear traction and injury risk
For the past 40 years, footwear traction has been implicated as a major cause of noncontact lower extremity injuries in sports. Foot fixation was first speculated to be affiliated with injury in 1969, when a study by Hanley44 found a significant decrease in knee and ankle injuries in varsity football players when the heel cleats were removed from typical football shoes. Subsequent studies attributed injuries not only to the heel cleats but also the forefoot cleats, with a reduction in forefoot cleat size being associated with a reduction in lower extremity injuries.45
In an effort to reduce injuries related to footwear traction, Cameron and Davis46 conducted an intervention study in which high school football athletes were given either a conventional football cleat (2373 athletes), or a swivel disc shoe (466 athletes) that replaced the typical forefoot cleats with a cleated turntable. The cleated turntable was designed to provide resistance to rotation of at least 10 Nm (the exact resistance changed depending on the normal load), after which the turntable was free to rotate. In addition, the heel cleats were replaced with a rigid plastic heel disc. The study found a lower injury rate in athletes wearing the swivel shoe (5.14% of swivel shoe athletes were injured, compared with 15.68% of control shoe athletes).
In a subset of these athletes, no difference was found in agility drill performance between the shoe conditions, indicating comparable performance between the cleats. While the results of the study appeared significant, failure to define and classify the severity of injury, including exposure rates, or subject their results to statistical analysis largely limits their utility. Furthermore, none of the studies mentioned previously directly measured traction associated with the footwear being tested.
Torg et al did the first study to quantify the traction of footwear and combine the results with a previous injury study. They observed a decrease in the incidence and severity of knee injuries as well as the number of injuries requiring surgery when high school football players wore “soccer-type” cleats with molded soles, rather than the conventional seven-cleat shoes.47 The authors followed this study by measuring the rotational traction of the soccer-type cleat and the typical football shoe, among other shoes models worn at the time.8 The results revealed that the conventional football shoe had higher rotational traction than the soccer-type cleat and further strengthened the link between footwear traction and lower extremity injury.
In a landmark three-year prospective study, Lambson et al9 examined the rotational resistance of modern football cleat design and the incidence of ACL tears in high school football players. Cleats with an “edge” design (longer irregular cleats placed at the peripheral margin of the sole with a number of smaller pointed cleats pointed interiorly) had the highest rotational traction and led to a significantly higher number of ACL tears compared with all other shoes common to high school athletes.
While this study was the first to examine the link between footwear traction and injury prospectively, there were some limitations to the interpretation of the results. Specifically, the actual playing surface and shoes that were used in the study did not undergo traction measurements; only representative sample surfaces and footwear were tested. In addition, the study looked only at injuries to the ACL, which may be the most expensive and traumatic noncontact injury but is not as prevalent as ankle injury, for example.
Expanding on the work of Lambson et al,9 Wannop et al48 performed a three-year study following 555 high school football athletes. The traction of the specific footwear to be used by the individual athletes during practices and games was measured at the start of the season on the actual field of play using a portable traction tester (Figure 2), while injuries suffered by the athletes during each season were recorded by certified athletic therapists.
The data showed a steady increase in injury rate as the rotational traction of the footwear worn by the athletes increased. Low rotational traction yielded an injury rate of 4.2 injuries per 1000 game exposures, while high rotational traction resulted in 19.2 injuries per 1000 game exposures. The study also found evidence that translational traction may be related to lower extremity injury, with a “midrange” of translational traction being associated with higher rates of injury than lower and higher levels of translational traction.
The previous studies that specifically investigated the influence of footwear traction on lower extremity joint injury offer strong evidence that high amounts of rotational traction increase an athlete’s risk of suffering a noncontact injury. There also appears to be a link between translational traction and injury, which has been largely ignored in past research. Although rotational traction and translational traction are thought to be highly correlated, current-generation footwear includes many directional traction elements that allow for increases in traction in specific directions or during specific movements, thus reducing the overall correlation between translational and rotational traction.48
While these previous studies have provided evidence that footwear traction is associated with athletic injury, research on the mechanisms underlying this relationship is lacking. One common theory regarding this mechanism is that, as footwear traction is increased, loading at the knee and ankle joints is also increased. Data indicating that increased joint loading is associated with an increased risk of injury has been published using cadavers,49,50 simulation studies,51,52 and active participants.53-55 This potential mechanism has been investigated in biomechanical studies in which joint loading at the knee and ankle was estimated by calculating the resultant joint moments (net twisting load on the joint) and angular impulse (cumulative loading experienced by the joint throughout stance phase) as athletes performed athletic movements in a laboratory setting.54,55 While joint moments and angular impulse do not determine the exact loading on an actual joint structure, they have been validated as predictors of the total load across a joint.56,57
Multiple biomechanical studies have shown that increased footwear traction is associated with increased joint loading during rapid changes of direction and cutting movements.58,59 These studies had athletes perform cutting movements in footwear with different mechanically measured levels of traction (both translational and rotational traction), while joint loading was calculated using data from a motion capture system and force plate. When performing movements in high traction shoes, athletes had higher joint moments and angular impulse at the knee and ankle joint than when they wore low traction shoes.58,59 Unfortunately, in these studies the high traction footwear was characterized by both high translational traction and high rotational traction, making it impossible to determine which aspect of traction was influencing the joint loading.
To determine how each aspect of traction influenced biomechanical joint loading, a follow-up study was performed with footwear conditions in which the translational traction and rotational traction were independently altered.60 Athletes performed cutting movements under different footwear conditions, and the knee and ankle joint moments and angular impulse were calculated using a motion capture system (Figure 3). Decreased rotational traction was associated with decreased loading in the transverse and frontal planes at the knee and ankle, while changing translational traction altered only frontal plane joint loading. These data indicate that both aspects of traction can influence joint loading, but that they influence joint loading in different ways.
Altering the mechanical properties of playing surfaces can influence the location, distribution, and severity of athletic injuries. One of the most prominent mechanical properties of playing surfaces—footwear traction—is linked with risk of athletic injury. Increases in rotational traction are associated with an increased risk of injury, which may be due to increased biomechanical joint loading at the ankle and knee. This joint loading, when coupled with an unexpected neuromuscular perturbation, may result in noncontact injury.
Further work is still required to determine the optimal range of footwear traction needed to reduce athletes’ risk of suffering a noncontact injury while maintaining their performance. Surprisingly, the majority of research within this area has been conducted in American football; therefore, more research is needed in other sports, including nonfield sports such as basketball. A recent study showed footwear traction has an influence on basketball-specific performance,61 but there are no data on how traction in basketball footwear influences injury risk.
Future epidemiological studies should strive not only to record all injury and exposure data from athletes, but also measure and record the surface properties and potentially the properties (or at least the model) of the footwear worn by study participants. While such a study would be lengthy and expensive, the data gathered would provide insight into the mechanical properties of the footwear and playing surfaces most often associated with injuries. This information will be vital for sports medicine professionals to effectively advise athletes about their footwear choices, and for footwear and sport surface manufacturers to alter the mechanical properties of their products in an attempt to reduce athletes’ injury risk.
John Wannop, PhD, is a postdoctoral scholar; Ryan Madden, MSc, is a researcher; and Darren Stefanyshyn, PhD, PEng, is the associate dean (graduate) and professor at the Human Performance Lab in the Faculty of Kinesiology at the University of Calgary in Canada.
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