February 2013

Footwear properties and football injuries

Shutterstock.com #2799675

Shutterstock.com #2799675

Excessive rotational traction that occurs at the interface between the shoe and the playing surface, as well as shoe properties such as rotational stiffness, may have the potential to influence the high incidence of lower extremity injuries in athletes.

By Feng Wei, PhD, and Eric G. Meyer, PhD

American football is one of the most popular sports in the United States. In 2010 more than 1.1 million male high school athletes from more than 14,000 high schools1 and more than 66,000 male collegiate athletes2 played football. Participation in high school football has been continuously increasing, with more than 100,000 additional participants (a 12.2% increase) between 1997 and 2007.3 Football is also a leading cause of sports-related injuries. Out of all high school sports, football has the highest overall injury rate, almost twice that of basketball.4 Reports estimate that more than 300,000 high school athletes sustain football-related injuries annually.5,6

Injuries to the lower extremity are among the most frequent injuries for all levels of football, and injuries to the ankle and knee joints are by far the most widespread and costly.7,8 Ankle and knee injuries account for approximately 37% of all injuries in both high school and collegiate football.3 In professional football (the National Football League [NFL]), ankle and knee sprains combine to account for about 20% of all reported injuries.9

Within the knee, the medial collateral ligament (MCL), the patella/patellar tendon, and the anterior cruciate ligament (ACL) were the top three sites of injuries sustained by high school football players during 2010-2011.10 Foot and ankle injuries are also common in football, affecting 72% of elite collegiate football players.11 Approximately 85% of ankle sprains involve the lateral ankle ligaments, in particular, the anterior talofibular (ATaFL) and the calcaneofibular (CFL) ligaments.12 Two less common injuries, medial ankle sprain and high ankle sprain, involve the anterior deltoid ligament (ADL) and the anterior tibiofibular ligament (ATiFL), respectively.13,14 Although less frequent than lateral ankle sprains, medial and high ankle sprains are more problematic due to their potential for significantly greater time lost and subsequent chronic ankle dysfunction.15,16

Figure 1. Synthetic turfs and natural grass are both currently used as playing surfaces in football fields.

Figure 1. Synthetic turfs (left) and natural grass (right) are both currently used as playing surfaces in football fields.

Many of these injuries in football occur during contact between players, but more than a third of injuries are reported in noncontact situations,7,8 such as a rapid change in direction17 or with a combination of high compressive load and twist during a jump landing.18,19 Numerous studies have investigated the role of the shoe-surface interface and it is generally believed to be one of the primary contributors to noncontact injury, especially in football.20,21 Specifically, two epidemiological studies in 2012 showed that ACL injuries in the National Collegiate Athletic Association (NCAA)22 and the NFL23 occur more frequently in football played on in-fill surfaces than on grass surfaces. Eversion ankle sprains (medial ankle sprain, high ankle sprain, and injuries to the medial capsule) were also found to occur more frequently in the NFL on in-fill surfaces than on grass.23

There may also be a direct relationship between lower extremity injury risk and properties of the shoes themselves. The use of ankle braces has been shown to reduce lower extremity injury in high school football;24 however, the effects of shoe style, such as high top versus low top, have received limited epidemiological attention25 and football shoes have been evaluated only biomechanically.26 A recent biomechanical study indicates the rotational stiffness of football shoes may influence strain patterns developed in ankle ligaments, and therefore may affect the risk of injury and the type of injury that occurs during noncontact football scenarios.27

Shoe-surface interface as a risk factor

Figure 2. Representative shape of the torque vs shoe rotation data with (A) four regions: (1) initial breakaway, (2) build-up, (3) slippage, and (4) unloading; or (B) three regions: slippage (3) is omitted.

Figure 2. Representative shape of the torque vs shoe rotation data with (A) four regions: (1) initial breakaway, (2) build-up, (3) slippage, and (4) unloading; or (B) three regions: slippage (3) is omitted.

Traction is defined as the resistance to relative motion between a shoe outsole and a sports surface and does not necessarily obey classical (Coulomb) laws of friction.21 Traction characteristics between a shoe and a surface can be quantified for either linear or rotational motions.28 Although linear traction is necessary for high-level performance during any athletic contest, it is generally accepted that excessive rotational traction may precipitate ankle and knee injuries.29,30 FIFA (Fédération Internationale de Football Association) is currently the only major sports league that requires mechanical testing of installed artificial playing surfaces, such as determination of the rotational traction.

Torg and Quedenfeld31 were among the first to document the important role the interaction between a shoe and the playing surface has in noncontact sports injuries. They observed the number and size of cleats on a shoe are correlated with the occurrence of ankle and knee injuries in American football, with less aggressive cleats producing fewer injuries. In a follow-up study, Torg et al20 defined a release coefficient based on the peak torque developed at the shoe-surface interface to quantify the injury potential of specific shoe-surface combinations.

A later study in high school football players documented a significant relationship between cleat design, the amount of rotational traction, and the risk of ACL injury on grass.29 The edge-cleat design produced significantly higher rotational traction and was associated with an ACL injury rate 3.4 times higher than that of all other designs combined. In addition, a biomechanical study by Drakos et al32 used cadavers to investigate the effects of four shoe-surface combinations on development of ACL strains. The study concluded the cleat and natural grass combination produced less strain on the ACL than the other combinations, potentially resulting in fewer noncontact ACL injuries.

Livesay et al33 recently demonstrated that shoe-surface rotational stiffness, calculated from various shoe-surface combinations and defined as the rate at which moment is developed under shoe rotation on the surface, may be another risk factor for ankle and knee injuries, and therefore may provide a new criterion for the evaluation of shoe-surface interface. That study measured the rotational traction and shoe-surface rotational stiffness between five different playing surfaces and two types of shoes (10 shoe-surface combinations) under a compressive load.

The results showed the differences in shoe-surface rotational stiffness across all combinations were greater than those of the rotational traction, implying that the shoe-surface rotational stiffness may be a more sensitive indicator of the mechanical interaction between different shoe-surface combinations than the peak moment (rotational traction) developed across the interface.

More recently, a mobile testing apparatus with a surrogate ankle was developed and used on site at actual surface installations to apply rotations and measure the traction and shoe-surface rotational stiffness between 10 football shoe models and four playing surfaces.21

Figure 3. Rotational stiffness of shoes was found to be significantly different between the Nike Air Zoom (A) and the Nike Flyposite (B).

Figure 3. Rotational stiffness of shoes was found to be significantly different between the Nike Air Zoom (A) and the Nike Flyposite (B).

The authors concluded the tested artificial surfaces yielded significantly higher peak moments and shoe-surface rotational stiffnesses than the natural grass surfaces (Figure 1). In addition, an interesting observation of the study was that, if the leading edge of a shoe was twisted and appeared to plow into the surface, as was the case with shoes with relatively pliable uppers, the plots of moment versus rotation of the shoe (Figure 2) often changed from the typical four regions (initial breakaway, buildup, slippage, unloading) to three regions only (the slippage phase was omitted).

The study therefore indicated that the materials used to construct a shoe’s upper can influence the shoe-surface rotational stiffness. As manufacturers continue to update football shoe and surface designs, researchers need to assess new evaluations of performance under simulated loading conditions to ensure that player performance needs are met while injury risks are minimized.

Footwear properties as risk factors

Lateral ankle sprains continue to be the most common injury sustained by athletes in all kinds of sports. Foot inversion is suspected in these cases, but internal rotation of the foot may also be a contributing factor to the mechanism of injury.34 A study on different playing surfaces used by the NFL has shown there is no significant difference in lateral ankle sprain rates sustained on grass versus in-fill surfaces.23 In the literature, however, little work has been conducted to investigate the effects of footwear restraint on lateral ankle sprain rates.

footwear-table1The mechanism of injury in medial and high ankle sprains is commonly ascribed to excessive internal rotation of the upper body while the foot is planted on the playing surface.17 Additionally, talus motion plays an important role in developing ankle ligament strains, especially under rotational loading,35 and therefore its motion is crucial in the study of potential mechanisms of ankle ligament sprain. A shoe with a pliable upper may allow the foot to pronate while the leg is internally rotated, therefore providing a degree of eversion at the talus. This loading scenario may result in rupture of the ATiFL or high ankle sprain, a severe time-loss injury.21,36

A study by Wannop et al37 measured linear and rotational tractions from two sports shoes and used them in an in vivo study involving athletes performing running V-cuts. Researchers collected kinematic and kinetic data for both shoes. Although the study used regular athletic shoes instead of football shoes, results showed that increased shoe tractions (both linear and rotational) significantly increased ankle and knee joint moments during a V-cut motion. However, the study was unable to measure bone motion in the shoes, which may be important in generating ligamentous injuries to the ankle joint.

A recent biomechanical study27 used surrogate and cadaver lower extremities to compare compared four football shoe designs in terms of rotational stiffness of the shoes (the rate at which moment is developed under foot rotation in the shoe). The shoe tests were conducted using a surrogate lower extremity and custom football cleat molds, therefore eliminating the effects of shoe-surface interaction. Rotational stiffnesses for the four football shoe designs were significantly different from one another, representing a unique property of those shoes that may be influenced by their uppers and shoe sole materials (Figure 3).

The most flexible and the most rigid shoe designs were then used for the cadaver tests. Twelve (six pairs) of male cadaver lower extremity limbs underwent external foot rotation of 30° using the two selected football shoe designs. Motion capture was performed to track the movement of the talus with a reflective marker array screwed into the bone. A computational ankle model was used to input talar motion for estimation of ankle ligament strains.38

Results of the study showed that talar axial rotation was greater in the rigid shoe, but that the flexible shoe generated more talar eversion (Table 1). These talus motions resulted in the same level of ADL strain (~5%) between shoes; however, there was significantly greater ATiFL strain (4.5 ± .4% vs 2.3 ± .3%) for the flexible shoe than the more rigid shoe design. This difference may be largely due to the increased talus eversion associated with the flexible shoe.

Although the study was limited to only a few shoe designs and was of a subfailure nature, it may provide some insight into the relationship between shoe design and ankle ligament strain patterns, and represents a first step in the attempt to understand the effects of football shoe design on the potential for ankle injury.

Interpretation

These studies indicate that a relationship may exist between lower extremity injuries and footwear properties due to the upper and cleat designs of football shoes in addition to the playing surface design. It is commonly believed that increases in rotational traction of shoe-surface interface lead to increased ankle and knee injuries. But it is also possible that the rotational stiffness of footwear could alter injury risk, especially for high ankle sprains. Therefore, footwear properties and shoe-surface interface properties may be two independent factors that could be modified to allow the development of safer footwear.

The risk of lower extremity injury during football is a very complex problem. However, as our understanding of each risk factor and injury mechanism increases through multidisciplinary research, we can create new solutions and safer designs so that athletes will minimize injury and maximize their enjoyment of sports.

Feng Wei, PhD, is a research scientist at the Rehabilitation Institute of Chicago and a joint researcher in the Department of Physical Medicine and Rehabilitation at Northwestern University in Chicago. Eric G. Meyer, PhD, is an assistant professor in the Biomedical Engineering Program and director of the Experimental Biomechanics Laboratory at Lawrence Technological University in Detroit, MI.

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