Footwear Traction Research Articles

Biomechanical modeling of footwear-fluid-floor interaction during slips

Slips and falls are among the major concerns for public safety. Slipping risks can be reduced by ensuring adequate traction at the shoe-floor interface. The outsole design of footwear is a critical factor to maintain sufficient shoe-floor traction in the presence of slippery contaminants such as water or oil. While the role of floorings and contaminants on footwear traction has been studied widely, limited works have investigated the role of footwear outsole geometry and tread patterns on shoe-floor traction. In this work, eight footwear outsole designs and their traction performance were tested on a common flooring with water contamination, through the development of a novel fluid–structure interaction based computational framework. Induced fluid pressure, mass flow rates, and contact areas were quantified across the outsole patterns, and their effect on footwear friction was investigated. The study results were validated using mechanical slip testing experiments. The results indicated that the outsoles which had horizontal treads or untreaded heel regions can lead to drastic reduction of footwear friction. Also, contact area alone was quantified to be a poor choice in estimating the traction performance of footwear on water contaminated floorings. Such novel study results have not been reported to date, and are anticipated to provide important guidelines to footwear manufacturers to evaluate and optimize footwear tread parameters which would help in reducing the risk of slips.

Influence of Vertically Treaded Outsoles on Interfacial Fluid Pressure, Mass Flow Rate, and Shoe–Floor Traction during Slips

Accidental injuries due to slips and falls are considered serious threats to public safety. Sufficient friction at the footwear and flooring interface is essential to reduce slip-related risks. The presence of slippery fluidic contaminants, such as water, further reduces friction and increases the risks of slip-related accidents drastically. While the effect of floorings and contaminants on footwear traction has been measured extensively across a variety of footwear designs, only a few studies have explored the science of the outsole design and its role in providing sufficient traction. In this work, the tread design of a commonly encountered outsole pattern, i.e., with vertically oriented tread channels, was parametrically altered across its width and gap. Based on the impressions of an original footwear design, nine outsoles were fabricated. The induced fluid pressures, mass flow rates, and traction were quantified by using a computational fluid dynamics (CFD) framework and through slip testing experiments. Outsoles that had wide treads with small gaps decreased the overall slipping risk on dry floorings. As compared to the tread area, tread gaps were found to be a dominating parameter in providing adequate shoe–floor traction in wet slipping conditions. The methods, including the outcomes presented in this work, are anticipated to advance the understanding of the science behind footwear friction and help footwear manufacturers optimize outsole designs to reduce slip and fall risks.

A Novel Computational Model for Traction Performance Characterization of Footwear Outsoles with Horizontal Tread Channels

Slips and falls are among the most serious public safety hazards. Adequate friction at the shoe–floor contact is necessary to reduce these risks. In the presence of slippery fluids such as water or oil, the footwear outsole is crucial for ensuring appropriate shoe–floor traction. While the influence of flooring and contaminants on footwear traction has been extensively studied across several outsole surfaces, limited studies have investigated the science of outsole design and how it affects footwear traction performance. In this work, the tread channels of a commonly found outsole pattern, i.e., horizontally oriented treads, was varied parametrically across the widths (i.e., 2, 4, 6 mm) and gaps (i.e., 2, 3, 4 mm). Nine outsole designs were developed and their traction, fluid pressures, and fluid flow rates during slipping were estimated using a mechanical slip testing and a CFD-based computational framework. Outsoles which had wider tread (i.e., 6 mm) surfaces showed increased slip risks on wet flooring. Outsoles with large gaps (i.e., 4 mm) exhibited increased traction performance when slipped on wet flooring (R2 = 0.86). These novel results are anticipated to provide valuable insights into the science of footwear traction and provide important guidelines for the footwear manufacturers to optimize outsole surface design to reduce the risk of slips and falls. In addition to this, the presented CFD-based computational framework could help develop better outsole designs to further solve this problem.

Development of a Portable Device for Surface Traction Characterization at the Shoe–Floor Interface

Slip and fall accidents are widespread in workplaces and on walkways. Slipping is generally initiated by a sudden change in the flooring properties or due to a low available traction at the shoe–floor interface. To measure shoe-floor traction, mechanical slip and fall risk estimation devices are typically employed. However, to date, such existing devices are lab-based, bulky, and are unable to simulate realistic slip biomechanics and measure whole footwear traction in realistic contaminated floorings at the same time. Moreover, these devices are expensive and not available in low- or lower-middle-income countries with limited awareness regarding slip testing. To overcome these challenges, in this work, a biofidelic, portable, and low-cost slip testing device was developed. A strategic three-part subassembly was designed for the application of normal load, slipping speed, and heel strike angle for its modularity. The developed slip tester was extensively tested and validated for its performance using 10 formal footwears and two floorings, under dry and wet conditions. The results indicated that the slip tester was accurate, repeatable, and reliable in differentiating traction measurements across varying combinations of shoes, contaminants, and floorings. The instrumentation performance of the slip tester was found to also capture the differences between different shoe tread patterns in the presence of fluid films. The developed device is anticipated to significantly impact the clinical, industrial, and commercial performance testing of footwear traction in realistic slippery flooring conditions, especially in the low- or middle-income countries.

Effect of footwear material wear on slips and falls

Recurrent injuries related to slips and falls are common in workplaces, caused due to reduction of friction between the floor and footwear, due to material wear. However, due to lack of public awareness, such events are usually neglected and the role of footwear wear is underestimated. To date, a plethora of studies have tested footwear traction across floorings and contaminants, however the effect of shoe material wear on the friction has not been studied much. In this study, the outsole features of four common formal shoes were modelled and tested in their new and progressively worn conditions. The available coefficient of friction (ACOF) of the outsoles were estimated using a mechanical slip testing device across three common floorings (i.e., gloss, matt, and anti-skid), and in dry and slippery conditions such as with floor cleaner and canola oil. ACOF reductions in the range of 25–80% were observed in progressively worn outsoles. Outsoles with vertical tread orientation were found to generate higher ACOF in fluid contaminant conditions. Outsoles which had larger treads at the heel region showed consistent trend in the traction performance across all the conditions. The outsoles were found to produce generalizable results in the presence of canola oil as a contaminant. The results are anticipated to provide important guidelines for selection and design of footwear outsole features, and also in determining the replacement threshold of formal shoes.

A Biomechanical Investigation of Athletic Footwear Traction Performance: Integration of Gait Analysis with Computational Simulation

Evaluations are vital to quantify the functionalities of athletic footwear, such as the performance of slip resistance, shock absorption, and rebound. Computational technology has progressed to become a promising solution for accelerating product development time and providing customized products in order to keep up with the competitive contemporary footwear market. In this research, the effects of various tread pattern designs on traction performance in a normal gait were analyzed by employing an approach that integrated computational simulation and gait analysis. A state-of-the-art finite element (FE) model of a shoe was developed by digital sculpting technology. A dynamic plantar pressure distribution was automatically applied to interpret individualized subject conditions. The traction performance and real contact area between the shoe and the ground during the gait could be characterized and predicted. The results suggest that the real contact area and the structure of the outsole tread design influence the traction performance of the shoe in dry conditions. This computational process is more efficient than mechanical tests in terms of both cost and time, and it could bring a noticeable benefit to the footwear industry in the early design phases of product development.

Prediction of coefficient of friction based on footwear outsole features

Traction testing of footwear is expensive, which may create barriers for certain users to assess footwear. This study aimed to develop a statistical model that predicts available coefficient of friction (ACOF) under boundary lubrication conditions based on inexpensive measurements of footwear outsole features. Geometric and material hardness parameters were measured from fifty-eight footwear designs labeled as slip-resistant. A robotic friction measurement device was used to quantify ACOF with canola oil as the contaminant. Stepwise regression methods were used to develop models based on the outsole parameters and floor type to predict ACOF. The predictive ability of the regression models was tested using the k-fold cross-validation method. Results indicated that 87% of ACOF variation was explained by three shoe outsole parameters (tread surface area, heel shape, hardness) and floor type. This approach may provide an assessment tool for safety practitioners to assess footwear traction and improve workers’ safety.

Influence of averaging time-interval on shoe-floor-contaminant available coefficient of friction measurements

Available coefficient of friction (ACOF) is a common metric of footwear traction performance. ACOF is the ratio of friction to normal force, often averaged over a time-interval. The time-interval needed to achieve repeatable and valid ACOF is unknown. A post-hoc analysis was performed on nine shoe-floor-contaminant combinations to assess the repeatability and bias of data averaged across 4 time-intervals (2 ms, 50 ms, 100 ms, 200 ms) after the target normal force was reached. The ability to predict human slips was assessed for ACOF across these intervals. Differences in repeatability and validity across the four intervals were small. However, statistically significant differences were observed for the shortest compared with the longest interval (lower repeatability yet modestly improved predictive ability). Given the limited impact of time-interval on the results, a shorter interval of 50 ms is recommended to enable testing of smaller floor samples.

Vinyl Composite Tile Surrogate for Mechanical Slip Testing

OCCUPATIONAL APPLICATIONSThis study aimed to identify a durable laboratory-grade surrogate material for vinyl composite tile (VCT) suitable for footwear traction testing. To conduct this investigation, eight polymer tiles (cast nylon, polyethylene, polycarbonate, acetal, Delrin, PTFE, polypropylene, and nylon) were tested across different shoes and contaminants (water, sodium laurel sulfate, and oil), and the available coefficient of friction (ACOF) was measured and compared with that of two designs of VCT. Shoe ACOF performance on cast nylon tile with oil contamination was generally applicable to both VCTs in oil contaminated condition. Shoe ACOF performance for none of the eight candidate tiles were applicable to the VCTs for other contaminated conditions. Cast nylon is anticipated to serve as a durable VCT surrogate, for standard slip testing of footwear in oil-contaminated conditions.TECHNICAL ABSTRACT Background: Vinyl composite tile (VCT), which is a common flooring in workplaces, is sometimes utilized as the standard floor material for mechanical slip testing experiments. Unfortunately, VCT is a sub-optimal standard test material, since it changes over time and is difficult to manufacture consistently. Purpose: This study aimed to identify a durable laboratory-grade substitute flooring that could provide traction results that are representative of footwear performance on VCT. Methods: Eight polymer tiles (cast nylon, polyethylene, polycarbonate, acetal, Delrin, PTFE, polypropylene, and nylon) were tested and the available coefficient of friction (ACOF) was measured and compared with that of two VCT designs. First, a screening test was performed to identify good material candidates based on six shoes and two contaminants (water and oil). Two surrogate candidate tiles were then tested across 17 shoes and 3 contaminant conditions (water, sodium laurel sulfate, and oil). Results: Cast nylon tile was found to be the most generalizable VCT surrogate, exhibiting strong correlations with both VCTs for oil contamination. None of candidates were representative of the VCTs for other contaminants.Conclusions: Cast nylon may be a useful alternative for VCT for standard slip testing of footwear in oily conditions.

Generalizability of Footwear Traction Performance across Flooring and Contaminant Conditions

OCCUPATIONAL APPLICATIONSThis study investigates the correlations between available coefficients-of-friction (ACOF) of 17 work shoes across different flooring and contaminant conditions. Five floorings (two vinyl tiles and three quarry tiles) were tested with water, sodium laurel sulfate, and canola oil. Shoe ACOF performance on a single quarry surface with any of the three contaminants was generally applicable to all other quarry-contaminant conditions. Shoe ACOF performance for a vinyl tile was generally applicable to another vinyl flooring for the same contaminant. These findings are anticipated to reduce the need for redundant shoe ACOF testing and clarify the generalizability of traction testing results.

Biomechanics research and sport equipment development

Advances in sport equipment have revolutionized athletic competition with engineers developing equipment that can enhance performance. However, not all athletes are able to benefit from the new, ideal equipment, with some athletes performing worse. Although the engineering may be sound, the interaction between the piece of equipment, the athlete, and the action is missing. The purely mechanical system of the piece of equipment becomes a biomechanical system once it is interacting with the athlete. Research into the underlying mechanisms of performance in sport has relied heavily on biomechanical studies. The review of these studies has identified important performance and injury variables that can be influenced by sport equipment. This baseline information helps provide a fundamental understanding of human performance that guide equipment designers and developers. A flawlessly engineered mechanical piece of sport equipment can still fail if the athlete–equipment interaction is not properly addressed in the design process. How an athlete uses a piece of equipment and furthermore how an athlete may change or adapt to changes in properties of a piece of equipment must be taken into consideration. Using properties of footwear as an example, research has provided understanding of the biomechanical limiting factors regarding the influence of footwear traction on athletic performance. Data on other footwear properties such as cushioning and forefoot bending stiffness are limited. Intrinsic musculoskeletal properties, such as the force–length and force–velocity relationships of skeletal muscle, can be exploited through equipment design, as has been shown during cycling. Modifications to equipment parameters can shift the operating range of an athlete within these relationships to maximize force or power output, which was shown during the development of the clap skate. Additionally, these properties vary slightly from athlete to athlete and minor adjustments to a piece of sporting equipment can help optimize individual athletes according to their specific biomechanical characteristics.

The effect of translational and rotational traction on lower extremity joint loading

Previous studies have linked footwear traction to lower extremity non-contact injury; however, these studies mainly focussed on rotational traction exclusively. While studies have shown that increases in traction lead to increases in joint loading, represented by joint moments, these studies failed to determine how the individual components of rotational and translational traction affect joint loading. Therefore, this study investigated how each component of traction independently affects lower extremity joint loading. Traction testing was performed using a robotic testing machine on three shoes that had independent alterations of translational and rotational traction. All testing was conducted on a sample piece of artificial turf. Kinematic and kinetic data were then collected on 10 athletes performing two cutting movements in each shoe condition. As rotational and translational traction were independently altered, decreased rotational traction led to significant decreases in transverse and frontal plane joint loading at the ankle and knee joints, while increases in translational traction led to increases in frontal plane joint loading at the ankle and knee joints. Increases in joint loading in the transverse and frontal planes are one of the possible mechanisms of lower extremity non-contact injury. Both translational and rotational traction can independently alter the joint loading.

Footwear traction and three-dimensional kinematics of level, downhill, uphill and cross-slope walking

Outdoor activities are a popular form of recreation, with hiking being the most popular outdoor activity as well as being the most prevalent in terms of injury. Over the duration of a hike, trekkers will encounter many different sloped terrains. Not much is known about the required traction or foot-floor kinematics during locomotion on these sloped surfaces, therefore, the purpose was to determine the three-dimensional foot-floor kinematics and required traction during level, downhill, uphill and cross-slope walking. Ten participants performed level, uphill, downhill and cross-slope walking along a 19° inclined walkway. Ground reaction force data as well as 3D positions of retro reflective markers attached to the shoe were recorded using a Motion Analysis System. Peak traction coefficients and foot-floor kinematics during sloped walking were compared to level walking. When walking along different sloped surfaces, the required traction coefficients at touchdown were not different from level walking, therefore, the increased likelihood of heel slipping during hiking is potentially due to the presence of loose material (rocks, dirt) on hiking slopes, rather than the overall lack of traction. Differences in required traction were seen at takeoff, with uphill and cross-sloped walking requiring a greater amount of traction compared to level walking. Changes in sagittal plane, frontal plane and transverse plane foot-floor angles were seen while walking on the sloped surfaces. Rapid foot-floor eversion was observed during cross-slope walking which could place the hiker at risk of injury with a misstep or if there was a slight slip.

Footwear Traction and Lower Extremity Noncontact Injury

Football is the most popular high school sport; however, it has the highest rate of injury. Speculation has been prevalent that foot fixation due to high footwear traction contributes to injury risk. Therefore, the purpose of the study was to determine whether a relationship exists between the athlete's specific footwear traction (measured with their own shoes on the field of play) and lower extremity noncontact injury in high school football. For 3 yr, 555 high school football athletes had their footwear traction measured on the actual field of play at the start of the season, and any injury the athletes suffered during a game was recorded. Lower extremity noncontact injury rates, grouped based on the athlete's specific footwear traction (both translational and rotational), were compared. For translational traction, injury rate reached a peak of 23.3 injuries/1000 game exposures within the midrange of translational traction, before decreasing to 5.0 injuries/1000 game exposures in the high range of traction. For rotational traction, there was a steady increase in injury rate as footwear traction increased, starting at 4.2 injuries/1000 game exposures at low traction and reaching 19.2 injuries/1000 game exposures at high traction. A relationship exists between footwear traction and noncontact lower extremity injury, with increases in rotational traction leading to a greater injury rate and increases in translational traction leading to a decrease in injury. It is recommended that athletes consider selecting footwear with the lowest rotational traction values for which no detriment in performance results.

Footwear traction at different areas on artificial and natural grass fields

Footwear traction has been linked to lower-extremity injuries with the theory that higher traction leads to an increased incidence of injury. Recent studies have challenged this showing similar injury rates on artificial turf, which has higher traction than natural grass. This could suggest that the magnitude of traction may not be as relevant for injury as relative changes from one location to another, due to inconsistencies in the surface. The purpose of this study was to compare the traction at various locations on an in-fill artificial turf and natural grass surface. A portable traction tester measured the traction of three shoes at six locations on both surfaces. The results indicate that over the course of a season the traction on natural grass changes considerably, especially rotational traction. Surprisingly the artificial turf surface also had areas of high and low traction due to the movement of the in-fill material during play.

Identification of critical traction values for maximum athletic performance

The purpose of this study was to investigate the relationship between mechanically available footwear traction and performance in top-speed curved sprint running and maximum effort linear acceleration. Based on results from previous studies, it was hypothesized that performance would increase as available traction increased but only to a point after which performance would plateau and further increases in available traction would not affect performance. The goal of this study was to identify such critical traction values. Thirty-two recreational athletes performed maximum effort 2.3 m radius curve sprints and linear accelerations from a standing start using four identical mid-cut basketball shoes differing only in outsole traction. Available traction was modified by manipulating the outsole material. The traction coefficients of the test shoes, quantified with a portable traction tester on the actual test surface, were 0.26, 0.54, 0.82 and 1.13. Ground reaction forces and three-dimensional kinematics were quantified during the tests. Greater amounts of traction (both peak and average) were utilized as the mechanically available traction increased. Increases in available traction from 0.26 to 0.54 to 0.82 provided systematic performance advantages for both curved sprinting and linear acceleration. However, no further performance enhancements were detected when the available traction increased beyond 0.82. Increases in the use of available traction beyond a threshold of 0.82 were reflected in the peak but not the average utilized traction or overall ground reaction impulse generation.

The effect of normal load, speed and moisture on footwear traction

Background: Numerous studies have tested footwear traction using various normal loads and movement speeds, and on different surfaces. However, because of the divergence from Amontons’ laws of friction, the results of studies that tested footwear traction using different normal loads or movement speeds are difficult to compare. The purpose of this study was to investigate how alterations of normal load, speed of testing and surface moisture affect footwear traction. Method: Data were collected on three types of footwear using a portable traction tester on the field of play of a natural grass and an artificial turf football field. Testing was conducted with different normal loads (335, 433, 580, 678 and 776 N), movement speeds (50, 100, 150 and 200 mm s−1, and 30, 60, 90° s−1) and amounts of moisture added to the field to determine the effect each variable had on traction. Results: Surface moisture had a large effect on traction and was shoe and surface specific. Normal load had a linear effect on traction with the ranking of the footwear staying consistent, indicating that testing at small normal loads may be sufficient. Movement speed had a linear relationship for translational traction but was constant for rotational traction, with the ranking of the footwear staying consistent again, indicating that slow speeds may be sufficient for testing. Conclusions: The relationships found are only valid for the method of testing used in this study, and for the range of loads and speeds tested. The relationship may fail to remain linear at higher or lower loads or speeds. Future studies should continue to examine the effect that moisture has on footwear traction to better understand how shoes react to changing conditions.

Effect of normal load, speed and moisture on footwear traction

Effect of normal load, speed and moisture on footwear traction

Footwear traction and lower extremity non-contact injury

The knee and ankle joint are two of the most frequently injured joints in the body with most of these injuries occurring without contact between players (Malone et al. 1992). Recent studies have co...

Footwear Traction and Lower Extremity Joint Loading

Background: Traction is influenced by the sole architecture and playing surface, with increases in traction potentially leading to injury. The mechanism as to how or why increased traction could lead to injury remains unknown. Purpose: This study was undertaken to determine how shoes of different sole designs and traction influence knee and ankle joint moments. Study Design: Controlled laboratory study. Methods: Traction testing was performed on 2 shoes of varying sole designs (tread vs smooth) using a robotic testing machine. All testing was conducted on a 60-cm × 90-cm piece of sample track surface. Kinematic and kinetic data were then collected on 13 recreational athletes performing running V-cuts in the 2 different shoe conditions. Five trials per condition were collected with reflective markers placed on the right shank and shoe of each participant. Kinematic and kinetic data were collected using an 8–high-speed camera system and force plate. Results: The coefficient of translational traction and the peak moment of rotation were both significantly higher in the tread shoe compared with the smooth shoe (1.00 vs 0.87 and 23.87 N·m vs 16.12 N·m, respectively). The high-traction shoe had significantly higher peak ankle external rotation moments (89.58 N·m vs 80.17 N·m), peak knee external rotation moments (36.23 N·m vs 32.02 N·m), peak knee adduction moments (224.0 N·m vs 186.8 N·m), and knee adduction angular impulse (2.10 Nms vs 1.83 Nms) compared with the low-traction shoe. Conclusion: Increased shoe traction significantly increased ankle and knee joint moments during a V-cut. Despite the significant difference in traction, no difference in performance was observed. These changes could have an effect on ankle and knee joint injury. Clinical Relevance: Shoes with decreased traction could be used in sports to reduce the joint moments in the knee and ankle and potentially reduce injury without a loss in performance.