Shoe Surface Research Articles

Effects of shoe bending stiffness and surface stiffness on lower extremity biomechanics during running

Increasing the forefoot stiffness of footwear has been proposed as a method of reducing athletic injury risk while increasing performance. In terms of injury, footwear with increased forefoot bendi...

Shoe surface traction and impact tester

Shoe surface traction and impact tester

Measurement of studded shoe–surface interaction metrics during in situ performance analysis

Interaction between studded footwear and performance surfaces plays an important role in sport. Discretising this interaction into quantifiable measurements can help optimise design of outsoles and identify parameters for performance testing in situ. Here, we describe the development and validity of an image-based three-dimensional (3D) measurement system to investigate shoe–surface interactions during locomotion performance in situ by eight skilled footballers. By calculating individual stud positions, results revealed that the 3D kinematic data could be distilled to a number of shoe–surface interaction metrics such as orientation, velocity, translation distance and location of the centre of rotation. Findings show how the measurement system and simple analysis methods can be used to provide informative shoe–surface interaction metrics from in situ performance capture for the footwear community.

A Method for Measuring Fluid Pressures in the Shoe–Floor–Fluid Interface: Application to Shoe Tread Evaluation

OCCUPATIONAL APPLICATIONS This study introduces a method for measuring fluid pressures in the shoe–floor interface. The novel method was then applied to shoes with varying tread depths. The rationale for this approach is that measuring fluid pressures can help to identify the reason for low friction and guide interventions for increasing slip resistance. High fluid pressures were observed in the absence of tread and the presence of high viscosity fluids. Fluid pressures were negligibly small when at least 1.5 mm of tread depth was present or when a low viscosity fluid was present. This study indicates that shoe tread is effective at channeling fluid out from the shoe–floor interface in the presence of highly viscous fluids. The presented methodology may be suitable for testing the performance of tread designs and establishing wear limits for shoe replacement.TECHNICAL ABSTRACT Background: Fluid contaminants cause slipping accidents by reducing shoe–floor friction. Fluid pressures in the shoe–floor interface reduce contact between the surfaces and, thus, reduce friction between the surfaces. A technological gap for measuring fluid pressures, however, has impeded improved understanding of what factors influence these pressures. Purpose: This study aimed to introduce a technique for measuring fluid pressures under the shoe and to demonstrate the utility of the technique by quantifying the effects of tread depth and fluid viscosity on fluid pressures for two different shoes. Methods: A fluid pressure sensor embedded in the floor surface was used to measure fluid pressures, while a robotic slip-tester traversed the shoe over the floor surface. Multiple scans were collected to develop 2D fluid pressure maps across the shoe surface. Two shoe tread types (an athletic shoe and a work shoe), two fluids (high-viscosity diluted glycerol and a low-viscosity detergent solution), and three tread depths (full tread, half tread, and no tread) were tested, while fluid pressures were measured. Results: Untreaded shoes combined with a high-viscosity fluid resulted in high fluid pressures, while treaded shoes or low-viscosity fluids resulted in low fluid pressures. The increased fluid pressures that were observed for the untreaded shoes are consistent with tribology theory and evidence from human slipping studies. Conclusions: The methods described here successfully measured fluid pressures and yielded results consistent with tribological theory and human slipping experiments. This approach offers significant potential in evaluating the slip-resistance of tread designs and determining wear limits for replacing shoes.

Shoe traction and surface compliance affect performance of soccer-related movements

Purpose: To determine how shoe-surface interaction, specifically traction and compliance, affects performance and biomechanics of soccer-related movements.Methods: Third generation artificial turf was installed in the laboratory to allow for kinetic and kinematic data collection both on the turf and on a laboratory surface (Pulastic sports surface). Twelve male athletes performed five 5 m sprint accelerations and five 180° sprint turns in three different shoe-surface conditions (indoor soccer shoe on the laboratory surface, indoor soccer shoe on the turf surface, soccer cleat on turf surface). Comparisons between the indoor shoe across surfaces indicated compliance effects and comparisons between the cleat and indoor shoe on turf indicated traction effects.Results: Performance increased for the sprint acceleration in the indoor shoe on the turf compared to the laboratory (1.04 s vs. 1.08 s); however, no further increase in acceleration performance occurred with the soccer cleat. For the turn movement, no change in performance occurred comparing the indoor shoe across surfaces however an increase in turn performance was seen when using the soccer cleat on turf compared to the indoor shoe (2.67 s vs. 2.56 s). The cleat had both increased utilised translational and rotational traction compared to the indoor shoe on turf for the turn movement. The cleat also resulted in increased ankle eversion moments as well as increased knee abduction and external rotation moments compared to the indoor shoe on the turf surface for the turn movement.Conclusion: Both compliance and traction shoe-surface characteristics affect performance; however, the effects of the different characteristics are different depending on the movement type.

A method of determining in vivo dynamic human ankle stiffness during external foot rotation

This study presents a method to determine stiffness of the human ankle in internal–external rotation, inversion–eversion, and plantarflexion–dorsiflexion during an in vivo, single-legged, external foot rotation experiment. Five young male subjects stood on one foot and performed an internal rotation of the body (external rotation of the foot) on a force plate, while motions were captured with a motion capture system. Hindfoot rotations relative to the tibia and resistive moments in the above three directions were obtained using a commercial software package. The resistive moments were plotted against rotation during the loading portions of the curves. Linear approximations of these curves were used to determine stiffness of the ankle in directions of internal–external, inversion–eversion, and plantarflexion–dorsiflexion. Future data from the above experiment and other athletic motions may be useful for the design of surrogate devices used to evaluate the potential performance and ankle injury risk characteristics of various shoe–surface interfaces.

Fluid pressures at the shoe–floor–contaminant interface during slips: Effects of tread & implications on slip severity

Previous research on slip and fall accidents has suggested that pressurized fluid between the shoe and floor is responsible for initiating slips yet this effect has not been verified experimentally. This study aimed to (1) measure hydrodynamic pressures during slipping for treaded and untreaded conditions; (2) determine the effects of fluid pressure on slip severity; and (3) quantify how fluid pressures vary with instantaneous resultant slipping speed, position on the shoe surface, and throughout the progression of the slip. Eighteen subjects walked on known dry and unexpected slippery floors, while wearing treaded and untreaded shoes. Fluid pressure sensors, embedded in the floor, recorded hydrodynamic pressures during slipping. The maximum fluid pressures (mean+/−standard deviation) were significantly higher for the untreaded conditions (124+/−75kPa) than the treaded conditions (1.1+/−0.29kPa). Maximum fluid pressures were positively correlated with peak slipping speed (r=0.87), suggesting that higher fluid pressures, which are associated with untreaded conditions, resulted in more severe slips. Instantaneous resultant slipping speed and position of sensor relative to the shoe sole and walking direction explained 41% of the fluid pressure variability. Fluid pressures were primarily observed for untreaded conditions. This study confirms that fluid pressures are relevant to slipping events, consistent with fluid dynamics theory (i.e. the Reynolds equation), and can be modified with shoe tread design. The results suggest that the occurrence and severity of unexpected slips can be reduced by designing shoes/floors that reduce underfoot fluid pressures.

The effects of surface traction characteristics on frictional demand and kinematics in tennis

The interaction between footwear and surfaces influences the forces experienced by tennis players. The purpose of this study was to investigate traction demand and kinematic adaptation during tennis-specific movements with changes in traction characteristics of surfaces. We hypothesised that players would increase the utilised coefficient of friction (horizontal to vertical ground reaction force ratio) when the shoe surface combination had a high coefficient of friction and flex their knee after contact to facilitate braking. Eight participants performed two separate movements, side jump out of stance and running forehand. Ground reaction force was measured and three-dimensional kinematic data were recorded. Clay surface and cushioned acrylic hard court (low vs. high shoe–surface friction) were used. The peak utilised coefficient of friction was greater on clay than the hard court. The knee was less flexed at impact on clay ( − 5.6 ± 10.2°) and at peak flexion ( − 13.1 ± 12.0°) during the running forehand. Our results indicate that tennis players adapt the level of utilised friction according to the characteristics of the surface, and this adaptation favours sliding on the low friction surface. Less knee flexion facilitates sliding on clay, whereas greater knee flexion contributes to braking on the hard court.

Torque prediction at the shoe–surface interface using insole pressure technology

Excessive rotational traction between the foot and the playing surface has been shown to increase the risk of athletic injuries to ligaments in the knee and ankle. It is not currently feasible to accurately determine the free torque generated at the shoe–surface interface utilizing live participants without an embedded force plate. The goal of the current research was to predict the free torques that developed at the shoe–surface interface during voluntary internal and external rotations of the body relative to a planted foot using an insole pressure measurement system. Six participants fitted with a shoe containing an insole pressure measurement device performed trials on an embedded force plate. A pressure sensor mask of specific sensors was determined based on the degree of linear correlation between sensor pressure and the free torque. Linear regression analyses were utilized to develop a General Linear Regression Model and Participant-Specific Linear Regression Models. The results of this study indicated that insole pressure technology, in conjunction with linear regression models, can be utilized to predict free torques generated at the shoe–surface interface during isolated rotational motions of the body with respect to a planted foot. Furthermore, Participant-Specific Linear Regression models may be more accurate in these predictions than a general model, based on a group of participants. When used with computational modeling, this technique might allow the investigation of strains produced in ligaments of the ankle and knee that are generated during internal and external rotations of the body with a planted foot on various turf surfaces outside a laboratory setting.

The effect of normal load force and roughness on the dynamic traction developed at the shoe–surface interface in tennis

During tennis-specific movements, such as accelerating and side stepping, the dynamic traction provided by the shoe–surface combination plays an important role in the injury risk and performance of the player. Acrylic hard court tennis surfaces have been reported to have increased injury occurrence, partly caused by increased traction that developed at the shoe–surface interface. Often mechanical test methods used for the testing and categorisation of playing surfaces do not tend to simulate loads occurring during participation on the surface, and thus are unlikely to predict the human response to the surface. A traction testing device, discussed in this paper, has been used to mechanically measure the dynamic traction force between the shoe and the surface under a range of normal loading conditions that are relevant to real-life play. Acrylic hard court tennis surfaces generally have a rough surface topography, due to their sand and acrylic paint mixed top coating. Surface micro-roughness will influence the friction mechanisms present during viscoelastic contacts, as found in footwear–surface interactions. This paper aims to further understand the influence micro-roughness and normal force has on the dynamic traction that develops at the shoe–surface interface on acrylic hard court tennis surfaces. The micro-roughness and traction of a controlled set of acrylic hard court tennis surfaces have been measured. The relationships between micro-roughness, normal force, and traction force are discussed.

Quantifying foot kinematics inside athletic footwear: a review

Movement of the foot and ankle are essential for human locomotion. Recent in vivo research using invasive techniques has highlighted the complexity of joint kinematics of the foot and ankle. Consequently, a detailed quantification of foot bone motion is often required. This presents challenges if non-invasive methods are used; even more so if footwear is incorporated into these investigations. A description of foot movement within footwear (in-shoe foot kinematics) is particularly valuable due to the plethora of potential applications in foot and ankle biomechanics. The objective of this review is to appraise existing literature and describe methods for quantifying foot kinematics inside athletic running type footwear and to provide a perspective on future directions for research. A number of methods were identified that have been implemented in an attempt to quantify in-shoe foot kinematics. Although varied, these all present threats to the validity of measurement in the form of either structural modification of footwear or surface marker application to the external shoe surface. Due to discrepancies in foot kinematics measured with markers applied on the external shoe surface compared to skin or bone mounted markers, it appears inappropriate to rely on external shoe markers to infer in-shoe foot motion. The impact of footwear modification on shoe structure and foot kinematics to gain access to anatomical landmarks of the foot requires further investigation. The recognition of the limitations imposed by these methods and the emergence of new non-invasive methods that allow for investigation during shod activity will facilitate more realistic representation and accurate investigation of foot motion inside the shoe.

The development of an apparatus to understand the traction developed at the shoe–surface interface in tennis

The traction developed at the shoe–surface interface can have a significant influence on a player’s injury risk and performance in tennis. The purpose of this study was to investigate shoe–surface traction on a dry acrylic hard court and two artificial clay court tennis surfaces in dry and wet conditions. A laboratory-based mechanical test rig was developed to measure the traction force developed at the shoe–surface interface. Linear regression analysis was used to examine the relationship between normal force and three measures of traction: initial stiffness, peak traction force and average dynamic traction force. The normal force did not significantly influence the initial stiffness for the shoe–surface system on the acrylic hard court but did on the artificial clay surfaces. The infill particle size and the addition of moisture influenced the traction developed on the artificial clay surfaces. Small, dry particles developed greater traction and with a sufficiently high applied normal force will provide traction comparable to that on an acrylic hard court. However, increased particle size and/or the presence of moisture generally reduced traction. Strong and significant positive linear relationships were found between peak traction force and average dynamic traction force for all surface types and conditions. This study improves the understanding of the influence surface characteristics have on shoe–surface traction mechanisms. Once traction mechanisms are understood, surface properties and/or footwear can be effectively changed to maximise performance and/or minimise injury risk.

The reliability, accuracy and minimal detectable difference of a multi-segment kinematic model of the foot–shoe complex

Kinematic models are commonly used to quantify foot and ankle kinematics, yet no marker sets or models have been proven reliable or accurate when wearing shoes. Further, the minimal detectable difference of a developed model is often not reported. We present a kinematic model that is reliable, accurate and sensitive to describe the kinematics of the foot–shoe complex and lower leg during walking gait. In order to achieve this, a new marker set was established, consisting of 25 markers applied on the shoe and skin surface, which informed a four segment kinematic model of the foot–shoe complex and lower leg. Three independent experiments were conducted to determine the reliability, accuracy and minimal detectable difference of the marker set and model. Inter-rater reliability of marker placement on the shoe was proven to be good to excellent (ICC=0.75–0.98) indicating that markers could be applied reliably between raters. Intra-rater reliability was better for the experienced rater (ICC=0.68–0.99) than the inexperienced rater (ICC=0.38–0.97). The accuracy of marker placement along each axis was <6.7mm for all markers studied. Minimal detectable difference (MDD90) thresholds were defined for each joint; tibiocalcaneal joint – MDD90=2.17–9.36°, tarsometatarsal joint – MDD90=1.03–9.29° and the metatarsophalangeal joint – MDD90=1.75–9.12°. These thresholds proposed are specific for the description of shod motion, and can be used in future research designed at comparing between different footwear.

A Novel Method for Evaluating the Effectiveness of Shoe-Tread Designs Relevant to Slip and Fall Accidents

Slip and falls account for a large share of occupational accidents. Slips are typically initiated when an insufficient amount of friction is present between the shoe and floor surfaces during walking. Shoe tread is thought to enhance the friction by channeling fluid contaminants away from the shoe and floor surface thus mitigating the fluid’s ability to lubricate the two surfaces and reduce friction. This study presents a novel method for evaluating the effectiveness of shoe tread by measuring fluid pressures during simulated slips. Sensors embedded into the floor measured fluid pressure while a robotic slip-tester simulated a human slip. A work shoe with three different tread depths (no, medium and full tread) was tested against a vinyl floor using a diluted (90%) glycerol and diluted detergent (2% detergent, 98% water) contaminant. Fluid pressures were high in the no tread condition but negligible in the other two tread depth conditions for the diluted glycerol and were negligible for all diluted detergent conditions. The no tread (COF: 0.005) also had lower friction coefficient values than treaded conditions (COF: 0.08-0.38). This study suggests that the effectiveness of tread to reduce the lubricating quality of the fluid can be directly measured using a robotic slip-tester and a fluid pressure sensor embedded in the floor. This method has the potential for developing tread depth recommendations and in evaluating the validity of slip-testers to simulate under-shoe conditions.

Understanding the influence of surface roughness on the tribological interactions at the shoe–surface interface in tennis

The traction provided by shoe–surface interactions in tennis can have an impact on player safety, performance and overall enjoyment of the sport. There is a requirement for an improved scientific understanding of the tribological interactions at the shoe–surface interface and the effects that footwear and surface characteristics have on the traction developed. The aim of this study was to investigate the influence surface roughness has on traction present during a sliding contact between a shoe and an acrylic hard court tennis surface. The substrate of acrylic hard court tennis surfaces are formed from a coating of a silica sand and acrylic paint mix, giving them variations in surface roughness characteristics. Mechanical traction tests were performed on five surfaces of varying roughness at normal forces of between 400 N and 800 N with a commercially available acrylic hard court tennis shoe (with a herringbone outsole pattern) and a shoe with a flat outsole. Significant linear relationships were found between dynamic traction force and normal force. A trend for decreased dynamic traction with increased roughness was found. When examined under a microscope after testing evidence of contact between the shoe and the surface decreasing with increased surface roughness was found. With the flat shoe, surface roughness affected the relationship between the coefficient of traction and the normal load. As roughness increased the coefficient of dynamic traction becomes less dependent on normal force. Evidence of a sudden increase in traction with the flat shoe when tested on the roughest surface suggests other friction mechanisms caused by the outsole overcoming the roughness of the surface, such as abrasive wear, had a significant effect on dynamic traction force.

Augmented reality-based design customization of footwear for children

This paper presents a systematic framework for design customization of footwear for children and identifies three modules related to shoe styling: shoe surface, shoe bottom, and accessory. A new module, shoe cloth, is created to allow a quick change of shoe appearance. Consumers can specify various design attributes in each module, including color, texture, embroidery, and shape. A prototype design system is implemented using augmented reality and sensing technologies based on the framework. This system consists of novel functions that support customization, design evaluation, and pattern development. The user can virtually put a customized shoe model on his/her foot in a video stream. The proposed framework not only facilitates evaluating products that are highly interactive with users, but also helps engage them in the design process. This work realizes the concept of human-centric design for mass customization.

The influence of surface characteristics on the tribological interactions at the shoe-surface interface in tennis

During dynamic tennis specific movements, such as accelerating and side stepping, the traction provided by a shoe-surface combination plays an important role in the injury risk and performance of the player. Acrylic hard court tennis surfaces have been reported to have increased injury occurrence due to an increased traction coefficient. 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 friction mechanism developed. Often mechanical test methods used for the testing and categorisation of 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. A new traction testing device, discussed in this paper, has been developed to mechanically measure the traction force between the shoe and the surface under appropriate loading conditions. Acrylic Harcourt tennis surfaces generally have a rough surface topography, due to a sand and acrylic paint mixed top coating, and have a deformable under layer to provide impact attenuation. Surface micro-roughness has been found to influence the friction mechanisms presents during viscoelastic contacts, as found in footwear-surface interactions. This paper aims to further understand the influence of micro-roughness on tennis surfaces. The micro-roughness and traction of a controlled set of acrylic hard court tennis surfaces have been measured. The influence of roughness on tennis surfaces traction is discussed.

A radiological method to determine the accuracy of motion capture marker placement on palpable anatomical landmarks through a shoe

The accuracy of marker placement on palpable surface anatomical landmarks is an important consideration in biomechanics. Although marker placement reliability has been studied in some depth, it remains unclear whether or not the markers are accurately positioned over the intended landmark in order to define the static position and orientation of the segment. A novel method using commonly available X-ray imaging was developed to identify the accuracy of markers placed on the shoe surface by palpating landmarks through the shoe. An anterior–posterior and lateral–medial X-ray was taken on 24 participants with a newly developed marker set applied to both the skin and shoe. The vector magnitude of both skin- and shoe-mounted markers from the anatomical landmark was calculated, as well as the mean marker offset between skin- and shoe-mounted markers. The accuracy of placing markers on the shoe relative to the skin-mounted markers, accounting for shoe thickness, was less than 5 mm for all markers studied. Further, when using the developed guidelines provided in this study, the method was deemed reliable (Intra-rater ICCs = 0.50–0.92). In conclusion, the method proposed here can reliably assess marker placement accuracy on the shoe surface relative to chosen anatomical landmarks beneath the skin.

Center of Pressure Variations in High-Heeled Shoes

The way in which a person’s foot contacts a shoe surface can have short-term and long-term effects on the whole body. Most previous research on footwear has been carried out using commercially available shoes, and as a result the surface of the shoe with which the foot interacts has not been systematically investigated. A recent invention has allowed the design parameters of a shoe to be investigated. The wedge angle and midfoot conditions were varied, and their effects were modeled with psychophysical relationships. The results show that perceived feelings are closely related to the location of the center of pressure.

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.