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
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
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
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.
The 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.
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.
- 2009-2010 High School Athletics Participation Survey. National Federation of State High School Associations website. Available at http://www.nfhs.org/content.aspx?id=3282. Accessed January 19, 2013.
- 1981-82 – 2009-10 NCAA Sports Sponsorship and Participation Rates Report. National Collegiate Athletic Association website. http://ncaapublications.com/p-4243-student-athlete-participation-1981-82-2010-11-ncaa-sports-sponsorship-and-participation-rates-report.aspx. Accessed January 19, 2013.
- Shankar PR, Fields SK, Collins CL, et al. Epidemiology of high school and collegiate football injuries in the United States, 2005-2006. Am J Sports Med 2007;35(8):1295-1303.
- Powell JW, Barber-Foss KD. Injury patterns in selected high school sports: a review of the 1995-1997 seasons. J Athl Train 1999;34(3):277-284.
- Adickes MS, Stuart MJ. Youth football injuries. Sports Med 2004;34(3):201-207.
- Ramirez M, Schaffer KB, Shen H, et al. Injuries to high school football athletes in California. Am J Sports Med 2006;34(7):1147-1158.
- Hootman JM, Dick R, Agel J. Epidemiology of collegiate injuries for 15 sports: summary and recommendations for injury prevention initiatives. J Athl Train 2007;42(2):311-319.
- Turbeville SD, Cowan LD, Owen WL, et al. Risk factors for injury in high school football players. Am J Sports Med 2003;31(6):974-980.
- Powell JW, Schootman M. A multivariate risk analysis of selected playing surfaces in the National Football League: 1980 to 1989. An epidemiologic study of knee injuries. Am J Sports Med 1992;20(6):686-694.
- McCullough KA, Phelps KD, Spindler KP, et al. Return to high school- and college-level football after anterior cruciate ligament reconstruction: A Multicenter Orthopaedic Outcomes Network (MOON) cohort study. Am J Sports Med 2012;40(11):2523-2539.
- Kaplan LD, Jost PW, Honkamp N, et al. Incidence and variance of foot and ankle injuries in elite college football players. Am J Orthop 2011;40(1):40-44.
- Beynnon BD, Murphy DF, Alosa DM. Predictive factors for lateral ankle sprains: a literature review. J Athl Train 2002;37(4):376-380.
- Fallat L, Grimm DJ, Saracco JA. Sprained ankle syndrome: prevalence and analysis of 639 acute injuries. J Foot Ankle Surg 1998;37(4):280-285.
- Gerber JP, Williams GN, Scoville CR, et al. Persistent disability associated with ankle sprains: a prospective examination of an athletic population. Foot Ankle Int 1998;19(10):653-660.
- Hopkinson WJ, St Pierre P, Ryan JB, Wheeler JH. Syndesmosis sprains of the ankle. Foot Ankle 1990;10(6):325-330.
- Waterman BR, Belmont PJ Jr, Cameron KL, et al. Risk factors for syndesmotic and medial ankle sprain: role of sex, sport, and level of competition. Am J Sports Med 2011;39(5):992-998.
- Guise ER. Rotational ligamentous injuries to the ankle in football. Am J Sports Med 1976;4(1):1-6.
- Arnold JA, Coker TP, Heaton LM, et al. Natural history of anterior cruciate tears. Am J Sports Med 1979;7(6):305-313.
- Faunø P, Wulff Jakobsen B. Mechanism of anterior cruciate ligament injuries in soccer. Int J Sports Med 2006;27(1):75-79.
- Torg JS, Quedenfeld TC, Landau S. The shoe-surface interface and its relationship to football knee injuries. J Sports Med 1974;2(5):261-269.
- Villwock MR, Meyer EG, Powell JW, et al. Football playing surface and shoe design affect rotational traction. Am J Sports Med 2009;37(3):518-525.
- Dragoo JL, Braun HJ, Harris AH. The effect of playing surface on the incidence of ACL injuries in National Collegiate Athletic Association American football. Knee 2012 Aug 21. [Epub ahead of print]
- Hershman EB, Anderson R, Bergfeld JA, et al. An analysis of specific lower extremity injury rates on grass and FieldTurf playing surfaces in National Football League games: 2000-2009 seasons. Am J Sports Med 2012;40(10):2200-2205.
- McGuine TA, Hetzel S, Wilson J, Brooks A. The effect of lace-up ankle braces on injury rates in high school football players. Am J Sports Med 2012;40(1):49-57.
- Barrett JR, Tanji JL, Drake C, et al. High- versus low-top shoes for the prevention of ankle sprains in basketball players. A prospective randomized study. Am J Sports Med 1993;21(4):582-585.
- Ricard MD, Schulties SS, Saret JJ. Effects of high-top and low-top shoes on ankle inversion. J Athl Train 2000;35(1):38-43.
- Wei F, Meyer EG, Braman JE, et al. Rotational stiffness of football shoes influences talus motion during external rotation of the foot. J Biomech Eng 2012;134(4):041002.
- Kent R, Crandall J, Forman J, et al. Development and assessment of a device and method for studying the mechanical interactions between shoes and playing surfaces in situ at loads and rates generated by elite athletes. Sports Biomech 2012;11(3):414-429.
- Lambson RB, Barnhill BS, Higgins RW. Football cleat design and its effect on anterior cruciate ligament injuries. A three-year prospective study. Am J Sports Med 1996;24(2):155-159.
- Nigg BM, Yeadon MR. Biomechanical aspects of playing surfaces. J Sports Sci 1987;5(2):117-145.
- Torg JS, Quedenfeld T. Effect of shoe type and cleat length on incidence and severity of knee injuries among high school football players. Res Q 1971;42(2):203-211.
- Drakos MC, Hillstrom H, Voos JE, et al. The effect of the shoe-surface interface in the development of anterior cruciate ligament strain. J Biomech Eng 2010;132(1):011003.
- Livesay GA, Reda DR, Nauman EA. Peak torque and rotational stiffness developed at the shoe-surface interface: the effect of shoe type and playing surface. Am J Sports Med 2006;34(3):415-422.
- Fong DTP, Wei F, Hong Y, et al. Ankle ligament strain during supination sprain injury – a computational biomechanics study. Portuguese J Sports Sci 2011;11(Suppl 2):655-658.
- Sarsam IM, Hughes SP. The role of the anterior tibio-fibular ligament in talar rotation: an anatomical study. Injury 1988;19(2):62-64.
- Wei F, Post JM, Braman JE, et al. Eversion during external rotation of the human cadaver foot produces high ankle sprains. J Orthop Res 2012;30(9):1423-1429.
- Wannop JW, Worobets JT, Stefanyshyn DJ. Footwear traction and lower extremity joint loading. Am J Sports Med 2010;38(6):1221-1228.
- Wei F, Braman JE, Weaver BT, Haut RC. Determination of dynamic ankle ligament strains from a computational model driven by motion analysis based kinematic data. J Biomech 2011;44(15):2636-2641.