Shoe-surface Interface Research Articles

Biomechanical analysis of traction at the shoe-surface interface on third generation artificial turf

Knowledge of the relationship between artificial turf, traction, and shoe-surface interactions is based almost exclusively on mechanical experiments. Although rotational traction traditionally has received the most attention, movements inherent to soccer are predominantly translational. The aim of this study was to investigate whether different artificial turf systems exhibit unequal translational traction properties at the shoe-surface interface with various cleat configurations in vivo. Twenty-two male soccer players (mean ± SD age 23.1 ± 2.8 years, height 1.81 ± 0.1 m, body mass 77.5 ± 6.0kg) performed five short sprints with a 90¡ cut over a turf covered force plate for each combination of turf system (DU42 and DU60 top-level, DU50 recreational) and shoe (MT turf cleats, CM circular grass cleats, AP bladed grass cleats). There was no significant effect of turf system (F2, 706.48 = 0.44, p = 0.646) or shoe (F2, 706.46 = 2.61, p = 0.074) on traction coefficient. However, a significant effect of both turf system (F2, 176 = 4.63, p = 0.011) and shoe (F2, 176 = 12.63, p < 0.001) on peak traction force was found, showing it to be lower on DU60 than DU42 (p = 0.007) and DU50 (p = 0.027), and lower with AP than MT (p < 0.001) and CM (p = 0.003). Traction force was lower on long turf with more infill and with bladed cleats. Contrary to expectations, traction coefficients remained almost identical across turf systems and shoes, indicating that players adjusted for undesirable traction conditions.

Determination of dynamic ankle ligament strains from a computational model driven by motion analysis based kinematic data

External rotation of the foot has been implicated in high ankle sprains. Recent studies by this laboratory, and others, have suggested that torsional traction characteristics of the shoe–surface interface may play a role in ankle injury. While ankle injuries most often involve damage to ligaments due to excessive strains, the studies conducted by this laboratory and others have largely used surrogate models of the lower extremity to determine shoe–surface interface characteristics based on torque measures alone. The objective of this study was to develop a methodology that would integrate a motion analysis-based kinematic foot model with a computational model of the ankle to determine dynamic ankle ligament strains during external foot rotation. Six subjects performed single-legged, internal rotation of the body with a planted foot while a marker-based motion analysis was conducted to track the hindfoot motion relative to the tibia. These kinematic data were used to drive an established computational ankle model. Ankle ligament strains, as a function of time, were determined. The anterior tibiofibular ligament (ATiFL) experienced the highest strain at 9.2±1.1%, followed by the anterior deltoid ligament (ADL) at 7.8±0.7%, averaged over the six subjects. The peak ATiFL strain occurred prior to peak strain in the ADL in all subjects. This novel methodology may provide new insights into mechanisms of high ankle sprains and offer a basis for future evaluations of shoe–surface interface characteristics using human subjects rather than mechanical surrogate devices.

Science of synthetic turf surfaces: investigating traction behaviour

The traction forces produced between an athlete’s footwear and the playing surface are a crucial factor influencing a player’s performance. Four primary factors affecting traction have been identified from literature: the sports specific movement, the footwear, the playing surface, and the environment. Many authors have investigated traction behaviour mechanically, using a variety of shoe and surface types, concluding that the traction generated at the shoe–surface interface is dependent on each shoe–surface combination (see work by Gheluwe et al., Cawley et al., and Villwocket et al., details given in main text). There has been little attempt in the literature, however, to try and explain the behaviour of the surface from the traction resistance measurements that were observed, perhaps owing to the complex number of variables involved. Furthermore, the variety of methodologies used in past research makes it difficult directly to compare datasets. This paper presents datasets comparing the traction behaviour of several carefully prepared surface systems and states, using three mechanical test procedures, and investigates the factors influencing traction resistance. Results highlight that properties of synthetic turf carpets (fibres and tuft spacing) and the density state of the crumbed rubber infill component and the stud size and configuration influence the maximum traction forces generated at the shoe–surface interface. The magnitude of stud penetration under controlled vertical loading is also presented. The findings of this study further demonstrate the importance of understanding the detail of the surface system under test, and that the infill state has a measurable effect. The design and operation of mechanical traction measurement equipment also demonstrates influences on the traction values measured, and the largest influence is the normal load applied. The FIFA standard test is shown to be less sensitive to infill state than other tests. Recommendations are made for more robust testing methods for future research.

Understanding the traction of tennis surfaces

The traction provided by a footwear-surface interaction can have an impact on player safety, performance and overall enjoyment of sport. Mechanical test methods used for the testing and categorisation of safe playing surfaces do not tend to simulate loads occurring during participation on the surface, and thus are unlikely to predict human response to the surface. For example, the pendulum system routinely employed by the International Tennis Federation (I.T.F.) utilises a standard rubber ‘foot’, rather than a shoe sole, and does not apply forces comparable to those in real play. There is a requirement for an improved scientific understanding of the tribological interactions at the shoe surface interface and the effects footwear and surface parameters have on the traction mechanism developed. The relationship between normal force and the coefficient of traction for the forefoot of a tennis shoe in contact with different tennis surfaces was examined using a bespoke traction rig. The effects of surface roughness were also examined. A power relationship was found between normal force and traction. As normal force increased differences in surface traction were found. The normal force, stiffness, and roughness of the surfaces affected the adhesive and hysteresis friction mechanisms that contribute to the overall traction force.

Improving the performance of soccer boots on artificial and natural soccer surfaces

An improved understanding of studded shoe-surface interactions is needed to optimise athletic performance in soccer. Soccer studs are required to penetrate the playing surface and provide traction to a soccer player. The translational traction at the shoesurface interface is important for soccer players performing dynamic accelerating movements. A study has been carried out to evaluate the performance of studded footwear on a natural surface and a third generation artificial surface. A mechanical traction test device was used to quantify the performance characteristics of soccer studs in terms of penetration and traction for both surfaces. Results found from testing an existing soccer stud were used as benchmark values in order to evaluate the performance of five other soccer studs. The effects of the surface properties and stud geometry on stud penetration and traction are discussed.

Playing Field Issues in Sports Medicine

The use of artificial turf on playing fields has increased in popularity. Advances in technology have allowed for the development of turf that closely mimics the properties of natural grass. Overall injury incidence does not differ between the two surfaces, but unique injury patterns are apparent between the two surfaces. Differences in shoe-surface interface, in-shoe foot loading patterns, and impact attenuation may provide insight into the different injury patterns. Player perceptions of artificial turf vary and may be related to different physiological demands between the two surfaces. Artificial turf has been implicated in skin infections, but concerns about other health consequences related to the synthetic materials have not been proven yet. Understanding the differences between artificial turf and natural grass will help physicians, athletic trainers, and coaches better care for and train their athletes.

The Effect of the Shoe-Surface Interface in the Development of Anterior Cruciate Ligament Strain

The shoe-surface interface has been implicated as a possible risk factor for anterior cruciate ligament (ACL) injuries. The purpose of this study is to develop a biomechanical, cadaveric model to evaluate the effect of various shoe-surface interfaces on ACL strain. There will be a significant difference in ACL strain between different shoe-surface combinations when a standardized rotational moment (a simulated cutting movement) is applied to an axially loaded lower extremity. The study design was a controlled laboratory study. Eight fresh-frozen cadaveric lower extremities were thawed and the femurs were potted with the knee in 30 deg of flexion. Each specimen was placed in a custom-made testing apparatus, which allowed axial loading and tibial rotation but prevented femoral rotation. For each specimen, a 500 N axial load and a 1.5 Nm internal rotation moment were placed for four different shoe-surface combinations: group I (AstroTurf-turf shoes), group II (modern playing turf-turf shoes), group III (modern playing turf-cleats), and group IV (natural grass-cleats). Maximum strain, initial axial force and moment, and maximum axial force and moment were calculated by a strain gauge and a six component force plate. The preliminary trials confirmed a linear relationship between strain and both the moment and the axial force for our testing configuration. In the experimental trials, the average maximum strain was 3.90, 3.19, 3.14, and 2.16 for groups I-IV, respectively. Group IV had significantly less maximum strain (p<0.05) than each of the other groups. This model can reproducibly create a detectable strain in the anteromedial bundle of the ACL in response to a given axial load and internal rotation moment. Within the elastic range of the stress-strain curve, the natural grass and cleat combination produced less strain in the ACL than the other combinations. The favorable biomechanical properties of the cleat-grass interface may result in fewer noncontact ACL injuries.

Development and evaluation of a surrogate ankle for use with a rotational traction measurement apparatus

Biofidelic devices are used in the automobile industry to assess injury risk during a vehicular accident. Similar biofidelic devices may have broad applicability in the field of sports injury prevention and could be used to enhance player safety. Ankle sprains constitute one of the most common sports injuries. Past studies have suggested that high rotational traction at the shoe—surface interface may increase the likelihood of lower-extremity injury. Researchers have assessed this risk by measuring the peak torque during an applied rotation. On the other hand, ankle sprains may be dependent upon the amount of strain developed in the ankle ligaments during rotation of the foot—ankle complex and not the magnitude of torque. The current study quantifies the torsional stiffness of the human foot—ankle complex based on cadaver experiments. The development of a surrogate foot—ankle complex is then detailed and compared with the human response. Finally, the results of a rotational traction study on a couple of football shoe—surface interfaces are presented using the surrogate ankle. The testing resulted in a new outcome variable, namely the peak twist of the ankle, that may allow assessment of the risk of injury to the ankle due to excessive rotational traction at the shoe—surface interface.

Football Playing Surface and Shoe Design Affect Rotational Traction

Background High rotational traction between football shoes and the playing surface may be a potential mechanism of injury for the lower extremity. Hypothesis Rotational traction at the shoe-surface interface depends on shoe design and surface type. Study Design Controlled laboratory study. Methods A mobile testing apparatus with a compliant ankle was used to apply rotations and measure the torque at the shoe-surface interface. The mechanical surrogate was used to compare 5 football cleat patterns (total of 10 shoe models) and 4 football surfaces (FieldTurf, AstroPlay, and 2 natural grass systems) on site at actual surface installations. Results Both artificial surfaces yielded significantly higher peak torque and rotational stiffness than the natural grass surfaces. The only cleat pattern that produced a peak torque significantly different than all others was the turf-style cleat, and it yielded the lowest torque. The model of shoe had a significant effect on rotational stiffness. Conclusion The infill artificial surfaces in this study exhibited greater rotational traction characteristics than natural grass. The cleat pattern did not predetermine a shoe's peak torque or rotational stiffness. A potential shoe design factor that may influence rotational stiffness is the material(s) used to construct the shoe's upper. Clinical Relevance The study provides data on the rotational traction of shoe-surface interfaces currently employed in football. As football shoe and surface designs continue to be updated, new evaluations of their performance must be assessed under simulated loading conditions to ensure that player performance needs are met while minimizing injury risk.

The effects of various infills, fibre structures, and shoe designs on generating rotational traction on an artificial surface

The purpose of this study was to investigate the role of infill material and fibre structure on the rotational traction associated with American football shoes on infill-based artificial surfaces. A mobile testing apparatus with a compliant ankle was used to apply rotations and measure the torque produced at the football shoe—surface interface. The mechanical surrogate was used to compare three infill materials in combination with three fibre structures, creating a total of nine unique surfaces. Infill material, fibre structure, and shoe design were all found to significantly affect rotational traction. The cryogenically processed styrene—butadiene rubber (SBR) infill yielded significantly higher peak torques than the ambient ground SBR and extruded thermoplastic elastomer (TPE) infills. An artificial surface with a nylon root zone yielded significantly lower peak torques than similar fibre surfaces without a nylon root zone. The size of infill particles and the presence of a nylon root zone may influence the compactness of the infill layer. These features may act to alter the amount of cleat contact with the infill, thereby influencing rotational traction. The amount of cleat contact with the surface may also be determined by the shoe design.

EFFECT OF STUD PARAMETERS ON TRACTION

Mechanical test devices have previously been used to investigate the influence of shoe and surface parameters on traction forces and moments at the studded shoe-surface interface, often using commercial footwear on natural and artificial sport surfaces [1] and [2]. In such studies trends between traction parameters and stud (cleat) parameters (stud length, shape and configuration) have been observed. However, the literature shows conflicting relationships regarding even the qualitative influence of a particular stud parameter's influence on traction. This can be attributed to multiple parameters being varied from test to test when commercial shoes are used. Barry and Milburn [3] modified their traction rig in order to test a variety of studs at different positions, providing insight into which stud positions have the most influence on tractive forces and moments

Peak Torque and Rotational Stiffness Developed at the Shoe-Surface Interface

Background Shoe-surface interactions have been implicated in the high number of noncontact knee injuries suffered by athletes at all levels. Purpose To examine shoe-surface interactions on newer field designs and compare these with more traditional shoe-surface combinations. The peak torque and rotational stiffness (the rate at which torque is developed under rotation) were determined. Study Design Controlled laboratory study. Methods A device was constructed to measure the torque versus applied rotation developed between different shoe-surface combinations. Data were collected on 5 different playing surfaces (natural grass, Astroturf, 2 types of Astroplay, and FieldTurf), using 2 types of shoes (grass and turf), under a compressive load of 333 N. Results The highest peak torques were developed by the grass shoe–FieldTurf tray and the turf shoe–Astroturf field combinations. The lowest peak torques were developed on the grass field. The turf shoe–Astroturf combination exhibited a rotational stiffness nearly double that of any other shoe-surface combinations. Conclusion The differences in the rotational stiffness across all 10 shoe-surface combinations were greater than those of the peak torques. It is possible that rotational stiffness may provide a new criterion for the evaluation of shoe-surface interface. Clinical Relevance An improved understanding of shoe-surface interactions remains a critical need to improve the design of shoe-surface combinations with the goal of meeting player needs while minimizing injury potential.

Shoe-surface interaction and injury prevention

One of the causes of injury in football is the development of excessive torque on the knee due to a combination of foot fixation and body rotation. Footwear fixation presents designers with a performance di lemma because of the need to develop sufficient translational friction and at the same time to minimise the development of rotational traction. While much research has been conducted on running and court shoes, little is known of the behaviour of field shoes in general and the traction at the shoe-surface interface in particular. The laws of friction between dry, solid, homogenous surfaces do not fully explain the mechanism of traction when one of the contacting surfaces is not smooth or solid but is granular and/or is wet, or when the footwear has cleats.

The effect of ambient temperature on the shoe-surface interface release coefficient.

Previous studies of the shoe-surface interface correlated foot fixation with cleat length, configuration, and material composition as well as turf type and surface conditions. Our study examined the effect of temperature on the rotational torsion resistance of artificial turf football shoes. Five football shoe models, a flat-soled basketball-style turf shoe, a natural grass soccer-style shoe, and three multistudded turf shoes, were studied on dry Astro Turf at five temperatures (range, 52 degrees F to 110 degrees F). An assay device, a prosthetic foot mounted on a loaded stainless steel shaft, was used to determine the force necessary to release a shoe from the turf's surface. We used a torque wrench to apply a rotational force so that each shoe was pivoted counterclockwise through an arc of 60 degrees. Our results indicated that release coefficients differ within and among the shoe models at various turf temperatures. We also found that an increase in turf temperature, in combination with cleat characteristics, affects shoe-surface interface friction and potentially places the athlete's knee and ankle at risk of injury. Based on an established risk criterion, which correlated shoe-surface interface combinations in the laboratory with documented clinical occurrences, only the flat-soled basketball-style turf shoe could be designated "safe" or "probably safe" at all five temperatures.

Foot-ground forces on sloping ground when lifting.

Slipping risk associated with manual material handling on inclined surfaces was investigated using a 24 factorial experimental design under controlled laboratory conditions. The reaction force data at the shoe sole-sloping surface interface, the box trajectories, and the subjects’ assessment of the risk involved in carrying out the prescribed tasks were obtained and analysed both analytically and statistically. The results of these analyses are presented in this paper.

Athletic Footwear and Orthotic Appliances

Four areas of concern regarding proper and improper footwear are taken into consideration, namely, the shoe-surface interface, the shoe components and construction, the shoe size and fit, and the shoe-player interface.

The shoe-surface interface and its relationship to football knee injuries.

The shoe-surface interface and its relationship to football knee injuries.