Assessment of the Mechanics of the Shoe-Turf Release Phenomenon Over a Range of Normal Loads

Assessment of the Mechanics of the Shoe-Turf Release Phenomenon Over a Range of Normal Loads | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Assessment of the Mechanics of the Shoe-Turf Release Phenomenon Over a Range of Normal Loads Benjamin Koerber, Bronislaw Gepner, Mohan Jayathirtha, James Caldwell, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8904480/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The goal of this study was to develop a test methodology for use in studying cleat-turf mechanical interaction that permits laboratory testing, precision-controlled inputs, variation in normal load applied to the cleat, and imaging of the cleat-turf interaction. This new test method utilized a 6 DOF force/torque and position-controlled serial robotic test device to perform compress-then-shear tests between a cleated shoe surrogate and artificial turf samples. The test method proved to be highly repeatable in displacement-controlled inputs across variations in axial load. Force response data were also repeatable, but variations in shear response were indicative of inherent variability in turf construction. Precise identification of cleat-turf release was permitted through high-speed imaging and verified to occur at the time of peak horizontal force. A strong linear relationship was found between release shear force and normal force at time of release for both posterior and lateral shear test types under the loading regime investigated (R 2 = 0.96, R 2 = 0.97). This linear regression represents a “release traction” that can be used to help better understand how turf construction relates to cleat-turf interaction mechanics, which is necessary to optimize player safety and performance. artificial turf turf testing methodology shoe-turf interaction cleat release traction Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Introduction An athlete’s sporting shoe contributes to their interaction with a playing surface. Changes in shoe type and playing surface influence a player’s ability to accelerate, change direction, and stop [ 1 ]. Shoe-surface interaction is also a factor in the risk of injury to the lower extremities [ 2 – 15 ]. Foot “entrapment”, when the player’s foot ceases to move relative to the playing surface while the player’s body continues to move in either translation or rotation, has been postulated as a mechanism of some lower extremity injuries [ 16 – 18 ]. For example, lateral ankle ligaments can be injured by inversion of the ankle caused by foot entrapment [ 19 ]. The interaction between shoes and playing surfaces can be studied using human subject testing (e.g., [ 20 – 21 ]), though such testing is limited to non-injurious levels of load and confounded by inter- and intra-player repeatability, challenges in data collection, and difficulty in viewing the shoe-surface engagement. Mechanical testing methods have been developed to address some of these challenges. Torg et al. [ 16 ] performed one of the first, using a rotational motion test with combinations of cleated shoes and playing surfaces. Drag-type testing has also been performed where a shoe is pulled translationally across a playing surface [ 22 , 23 ]. Kent et al. [ 2 ] developed a test device (dubbed the ‘BEAST’), to test shoe-surface interactions in both translation and rotation at loads and rates relevant to elite levels of American football. The BEAST device has characterized different combinations of playing shoes and surface types in the laboratory and in situ [ 9 , 10 , 24 ]. In these tests, a weight was mounted to a shaft attached to a cleated foot form. This shaft was pulled by a cable attached to a pneumatic cylinder to generate either translational or rotational cleat motions. As the normal force is caused by gravity in this method, it cannot be actively controlled during the cleat’s movement. The cleat experiences a relatively constant normal force during tests, and a decrease in depth during shear motions of tests on some artificial surfaces, “ramping-out” of the turf (Fig. 1 ). These studies did not consider variations in the magnitude of normal weight used, nor high-speed imaging of the shoe-surface interaction to allow for visualization of the shoe release phenomena. Therefore, the goals of this study were to (1) develop a repeatable methodology for varying normal load while characterizing shoe-surface interaction mechanics at loads relevant to elite levels of American football, (2) capture high-speed video of shoe-turf interaction to document the entrapment/release phenomenon, and (3) quantify the relationship between normal load and release load for one surface with an idealized cleat configuration. The specific hypotheses are as follows: (1) the cleat release timing will coincide with the peak shear force; and (2) there is a positive correlation between normal force and shear release force. Materials and Methods Test Specimen Preparation Turf samples (1200 g/m 2 faceweight, slit-film carpet with 3-layer Styrene-Butadiene Rubber (SBR) and sand infill system) (FieldTurf, Montreal, Quebec, Canada) were constructed in custom built aluminum turf boxes (86 cm × 145 cm × 5.7 cm) (Fig. 2 ). A system for clamping turf trays down to the laboratory floor was designed and machined (Fig. 2 ). This system allowed for flexibility in locating the turf tray relative to the robot described below, which helped in finding the optimum position for maximizing robot speed and power output. Experimental Setup Turf characterization experiments were performed with a 6DOF force/torque and position-controlled robotic test system (Fig. 2 ). The system consists of a 6DOF serial industrial robot (KR300 R2500 Ultra, Kuka, Augsburg, Germany) controlled by a suite of purpose-built hardware/software tools (simVITRO, Cleveland Clinic). The robot is rated to a 300 kg payload throughout its entire range of motion with accelerations and decelerations of a maximum of 2g. A replica of a common football cleat forefoot used by professional athletes (Nike Vapor Jet) was machined out of aluminum and used as a cleat surrogate [ 24 ] for this testing (Fig. 2 ). Force and torque data generated by each test were measured by a 6 DOF strain-gauge load cell (Model 1914, R.A. Denton, Rochester Hills, MI, USA) mounted between the cleat surrogate and the end effector of the robot arm. Position data for the cleated foot form was recorded by the robot control software. High speed video of each test was captured at 1000 frames per second from two angles (top, side) and synchronized to test data collected using an electronic trigger signal. Test Matrix A total of 24 tests were performed using the turf construction and aluminum cleat surrogate described above (Table 1 ). Tests were performed in triplicate in the posterior shear and lateral shear conditions, at four different normal force preloads. The posterior shear condition involved an imposed motion displacing the cleatform rearward relative to the turf (such as the direction the cleat would move if a player slipped during forward pushoff). The lateral shear condition involved an imposed motion displacing the cleatform toward its lateral aspect relative to the turf (such as the motion that would result from a side-step cut). Table 1 Test Matrix Variable Total No Target Preload Normal Force Target Force Level n = 4 0.7kN, 1.4kN, 2.1kN, 2.8kN Cleat Motion Posterior Shear Lateral Shear n = 2 Turf Builds Single Turf Build n = 1 Repeats Three Tests per Condition n = 3 Test Matrix TOTAL n = 24 Table 2 Turf Build Parameters Component Parameters Value Infill Total Infill [kg/m 2 ] (lb/ft²) 44.92 (9.2) Total Infill height [mm] 44 Top Layer Constituent 100% 1.4 mm SBR Top Layer thickness [mm] 5 Middle Layer Constituent 67% 0.56 mm sand / 33% 0.72 mm SBR (by weight) Middle Layer Thickness [mm] 34 Bottom Layer Constituent 100% 0.56 mm sand Bottom Layer thickness [mm] 5 Fiber Material polyethylene Type slit-film Width [mm] 10.6 Thickness [mm] 0.141 Height [mm] 63.5 Fiber Denier 10861 Fibers/stitch 2 stitch rate [cm] (inch) 0.69342 (0.273) stitch gauge [cm] (inch) 1.905 (0.75) Faceweight [g/m 2 ] (oz/yd²) 1198.91 (35.36) Backing Layer Polypropylene backing coated with polyurethane (porous backing) E-Layer none Boundary and Loading Conditions Turf tests were performed in displacement control by compressing the turf to a target depth and dynamically shearing in either the posterior or lateral direction (Fig. 3 ). Zero depth for each test was defined as the vertical position of the cleat form when the bottom of the longest studs of the cleat just touched the top of the infill. Target vertical compression depths were tuned for each test individually based on a pre-test in load control to the target preload normal force. Each turf tray was divided into 12 test locations, and each test was performed in an unused location on the turf. A border of 20.32 cm was left untested on the edges of the turf sample. At t = 4 seconds, the target depth was reached, and the turf was allowed to relax for an additional 6 seconds until reaching a steady state. At t = 10 seconds, the shear test motion began. The cleat displaced 100 mm in the horizontal direction at a nominal rate of 0.3 m/s. A vertical displacement ramp out of the turf was included. A period of 5.75 seconds at the end of the test was included to allow the cleat and turf to settle before data recording was stopped. Distinct ramp-out trajectories were chosen for each test condition based on initial pre-testing with a goal of holding constant normal force throughout shear motions. These ramp-out trajectories were consistent across all three repeats of a test condition. Data Analysis Using the results of this study, the cleat release phenomenon was defined based on high-speed video and the measured force-time data. Shear and normal force for each test were extracted at the time of release. A linear regression analysis was performed to test for a relationship between normal force and shear force at the time of release. CORA analysis was performed on displacement data from repeated trials to quantify the repeatability of the test system [ 25 ]. All plotting and data analysis was performed using custom code (MATLAB ver. R2022b, MathWorks, Natick, MA, USA). Results Kinetic responses of the turf samples followed a consistent characteristic behavior for both posterior shear and lateral shear tests (Fig. 4 , Fig. 6 ). While normal force was applied first, a small (~ 50 N) shear force occurred (t = 4 sec, Fig. 4 ). The maximum normal force occurred at peak depth (Point 1). The normal force relaxed under constant depth from t = 4 sec. to t = 10 sec. Just after the application of horizontal displacement at t = 10 sec., shear force rose monotonically to a peak and then dropped off. Test System Repeatability Displacement time history data showed that the robotic test system was able to execute repeatable posterior and lateral shear motions regardless of the penetration depth and normal reaction load (Fig. 5 ). Displacement magnitudes varied by less than 6% and temporal variability was within 30 msec over the entire time history. CORA analysis performed on individual curves scored 0.97 or greater across all displacement time histories over the interval between 10.0 and 10.5 seconds. The normal preload, which is an output reflecting variability in a sample’s response to the foot form’s initial position, varied by approximately 100 N for each test condition (Fig. 6 ). Normal and shear force responses followed characteristic force-displacement paths. Normal forces between tests varied by less than 200 N during the time history up to peak shear force within similar test conditions, and shear forces varied by up to 900 N up to the peak shear force across all test conditions. Peak shear force and slope of the force displacement path increased as preload normal force condition increased (Fig. 6 ). Nominal 2800 N preload cases (average 2573 N preload) were comparable to BEAST shear test initial conditions (2800 N free weight load) [ 2 ]. Peak shear force responses in posterior tests in this study fell within the 1 standard deviation variation reported in peak posterior shear forces on artificial turf surfaces tested by the BEAST using the same cleated foot form (4.45 kN vs. 4.5 ± 0.3 kN) [ 9 ]. Release Force Timing Peak shear force was identified to coincide with the cleat position immediately before release of turf fibers from underneath the cleat (Fig. 7 ). The high-speed videos depicted a characteristic behavior across all test conditions. When the cleat started translating, a void space opened in the starting position of the cleat as fibers were trapped beneath the cleat. As the cleat continued to translate this void space grew and the surrounding turf structure deformed, stretching in the direction of the shear motion. The cleat released from the turf and started to translate relative to the turf. At that time, the fibers trapped beneath the cleat released and snapped back into the void space created. Release Force Analysis Release shear force increased linearly with peak shear normal force in both posterior shear and lateral shear tests for the range of normal forces investigated (R 2 = 0.96, R 2 = 0.97) (Fig. 8 ). Discussion Shoe-surface testing in this study represents a methodology that is comparable to that of the BEAST device [ 2 ]. A ramp-out in displacement was used in this study to replicate the cleat z-motion observed during prior BEAST testing on artificial turfs, and observed peak shear forces in 2800 N target normal preload cases fell within the range reported in BEAST testing on multiple on-site artificial turf surfaces [ 2 , 9 ]. The methodology described in this study expands upon that of the BEAST by including variations in normal force. The test setup described may also be used to program more complex test motions that more closely represent player cleat-surface interactions. Displacement controlled test inputs in this study proved to be highly repeatable, allowing for comparisons between tests to describe the variability exhibited by the turf response. One limitation of this work is that a metal cleat form was used, which does not represent the deformable cleats worn in real-world playing scenarios. Many previous studies have utilized full cleats attached to a test machine by fitting them around an artificial foot [ 16 , 23 , 26 – 30 ]. Such methods can be affected by differences in fit of the shoe on the artificial foot and deformation of the shoe upper and sole. The metal cleat chosen allowed for direct comparison to previous research [ 24 ] and provided a consistent boundary condition across tests. Further studies should investigate the effect that changes in friction or deformability of a cleat form have on the results of this study. The methodology described herein also differs from that of the BEAST in applied loading rate. The BEAST device applies translational motions at rates between 1.0 m/s and 1.5 m/s [ 2 ], which is 4–5 times greater than the testing performed in this study. The BEAST test rate is comparable to foot displacement rates applied by elite American football players performing a range of tasks [ 31 ]. Future research should investigate the rate dependency of artificial turf surfaces to help contextualize the differences in applied rates between these two test devices. Riley et al. [ 31 ] also described ground reaction forces (GRFs) from these athletic manoeuvres. Peak normal GRFs across these trials ranged from 170% to 220% of player body weight (1646 N to 2618 N based on player mass reported). The test methodology described in this study allows for evaluation of playing surfaces throughout this range. Previous studies have often employed the use of a ‘traction coefficient’ [ 22 , 23 ]. This traction coefficient is commonly defined as the ratio of normal force to shear force, deriving from the friction coefficient used in studying the forces generated when two relatively flat surfaces slide across each other. Some debate exists to the utility of this traction definition as it leads to the assumption that normal force and shear force are held at a constant ratio across test timing and inputs [ 2 ]. This study showed that a strong linear relationship exists between shear release forces and normal forces for the cleat surrogate and artificial turf tested within the range of normal forces investigated. However, force time histories recorded in this testing exhibit non-linearity at shear forces leading up to release (e.g. Figure 7 ). In this study, non-normalized values of normal force and shear force are reported, allowing for comparisons to be made to injury data for the lower extremity as well as testing of other shoe and surface combinations. The results of this study indicate that there is a threshold between shoe-surface shear force release and stick (or non-release) that is dependent on the normal force held beneath the player’s shoe (Fig. 9 ). Here we are using stick as opposed to entrapment, as this term does not imply injury. Any foot strike that does not cross this shear vs. normal force threshold will not release and allow a player to cut or change direction. Further, shoe orientation did influence the slope and intercept of this linear relationship but did not change the strength of the correlation (Fig. 8 ). This conclusion leads to the hypothesis that there may be a similar linear relationship existing for other cleat and surface pairings and this “release traction” may be characteristic of individual turf surfaces. However, the results of this study may be dependent upon the cleatform used paired with the artificial surface tested, and future testing should explore the linearity of release for other test conditions as well as outside the boundaries of the forces evaluated here, including down to low normal forces to better explore turf mechanical response. The ability to evaluate turf response across a range of normal forces allows for parametric evaluation of different turf characteristics. Turf construction can vary widely including fiber type (e.g., slit-film, monofilament, mixed), faceweight, infill materials and composition [ 12 ]. Future testing should address how release traction is affected by changes in turf parameters, as well as changes in cleat design (stud shape and spacing, and cleat materials). Environmental factors may also affect the release traction slope and should be considered in future work. The test method described allows for evaluating these variations under controlled experiments in a laboratory. Future evaluations of these different factors of turf construction could be useful to field managers and turf manufacturers. Conclusions A test methodology was developed to test shoe-surface interactions under varying axial load. High-speed imaging synced with force response data was used to identify a physical description of release force timing and this timing was concurrent with peak shear force, supporting our first hypothesis. Shear release forces proved to be highly linearly related to normal forces under the loading regime investigated which supported our second hypothesis. Shear data in this study gave similar results to other comparable on-field testing. Displacement input data and force response data proved to be highly repeatable, allowing for useful comparisons to be made across different test conditions. Declarations The research presented in this paper was made possible by a grant from the National Football League and the National Football League Players Association. The views expressed are solely those of the authors and do not represent those of the NFLPA, the NFL, or any of its affiliates. Ethics declaration: not applicable. References Gains GL, Swedenhjelm AN, Mayhew JL, Bird HM, Houser JJ (2010) Comparison of speed and agility performance of college football players on field turf and natural grass. J Strength Conditioning Res 24(10):2613–2617 Kent R, Crandall J, Forman J, Lessley D, Lau A, Garson C (2012) 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. 10.1080/14763141.2011.650188 Balazs GC, Pavey GJ, Brelin AM, Pickett A, Keblish DJ, Rue JH (2015) Risk of anterior cruciate ligament injury in athletes on synthetic playing surfaces. 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Am J Sports Med 37:518525. 10.1177/0363546508328108 Riley PO, Kent RW, Dierks TA, Lievers WB, Frimenko RE, Crandall JR (2013) Foot kinematics and loading of professional athletes in American football-specific tasks. Gait Posture 38(4):563–569. https://doi.org/10.1016/j.gaitpost.2012.03.034 Additional Declarations No competing interests reported. Supplementary Files Appendix.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8904480","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":593854143,"identity":"f0638726-cbdf-4870-9ffd-7e1624b8cae3","order_by":0,"name":"Benjamin Koerber","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA4klEQVRIiWNgGAWjYBACAwY2xgcMB2wY2ICcA0AuECQQ1MJswHAgjTQtbBIMBw4jixHQYi52LEHix5nziX3Sxy8eLiiwY+BnzzHAq8VydtoBw54btxPb+HIKDs8wSGaQ7HmDX4vB7fSGZIYPt43ZeHgSDvMYMDMY3CBgC0jLYYYP52Ba6hnsCWtJO9jMcOOAHBsP+wGglsMMBhKE/ZLM2HMmGaiFhwGo5TiPxJlnBXi1mEunmf/4ccyOR76H/fFnnj/VcvztyRvwakECPGD38BCrHATYH5CiehSMglEwCkYQAACbu0YKZtCKYQAAAABJRU5ErkJggg==","orcid":"","institution":"University of Virginia","correspondingAuthor":true,"prefix":"","firstName":"Benjamin","middleName":"","lastName":"Koerber","suffix":""},{"id":593854144,"identity":"57da64f5-0575-4db2-9762-b0879ed749ed","order_by":1,"name":"Bronislaw Gepner","email":"","orcid":"","institution":"University of Virginia","correspondingAuthor":false,"prefix":"","firstName":"Bronislaw","middleName":"","lastName":"Gepner","suffix":""},{"id":593854146,"identity":"a3fc7a36-dd77-4f8b-86c5-008b89c721ef","order_by":2,"name":"Mohan Jayathirtha","email":"","orcid":"","institution":"University of Virginia","correspondingAuthor":false,"prefix":"","firstName":"Mohan","middleName":"","lastName":"Jayathirtha","suffix":""},{"id":593854147,"identity":"740aa0f4-9499-430b-94b0-e10fac885b6b","order_by":3,"name":"James Caldwell","email":"","orcid":"","institution":"University of Virginia","correspondingAuthor":false,"prefix":"","firstName":"James","middleName":"","lastName":"Caldwell","suffix":""},{"id":593854148,"identity":"e8a71896-a5d8-466d-8790-5fa190feb1be","order_by":4,"name":"Cody O’Cain","email":"","orcid":"","institution":"Biomechanics Consulting and Research (Biocore) LLC","correspondingAuthor":false,"prefix":"","firstName":"Cody","middleName":"","lastName":"O’Cain","suffix":""},{"id":593854150,"identity":"450a8bee-24c6-4dc0-88e2-52c5a23523fc","order_by":5,"name":"E. Meade Spratley","email":"","orcid":"","institution":"Biomechanics Consulting and Research (Biocore) LLC","correspondingAuthor":false,"prefix":"","firstName":"E.","middleName":"Meade","lastName":"Spratley","suffix":""},{"id":593854153,"identity":"b5688296-83d1-480d-94e1-a5920c2c6036","order_by":6,"name":"Gwansik Park","email":"","orcid":"","institution":"Biomechanics Consulting and Research (Biocore) LLC","correspondingAuthor":false,"prefix":"","firstName":"Gwansik","middleName":"","lastName":"Park","suffix":""},{"id":593854157,"identity":"74e9bafa-0e90-48af-96ae-29f6caed6ffe","order_by":7,"name":"Richard Kent","email":"","orcid":"","institution":"University of Virginia","correspondingAuthor":false,"prefix":"","firstName":"Richard","middleName":"","lastName":"Kent","suffix":""},{"id":593854159,"identity":"f8288fa5-5f73-4844-b0ed-19aeb3c2fe32","order_by":8,"name":"Jason Kerrigan","email":"","orcid":"","institution":"University of Virginia","correspondingAuthor":false,"prefix":"","firstName":"Jason","middleName":"","lastName":"Kerrigan","suffix":""}],"badges":[],"createdAt":"2026-02-17 21:53:59","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8904480/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8904480/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":103216703,"identity":"7d516a34-2879-4cfc-a9da-f61c01a20685","added_by":"auto","created_at":"2026-02-23 09:37:45","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":337416,"visible":true,"origin":"","legend":"\u003cp\u003eExemplar \"ramp-out\" behavior from BEAST tests on artificial turf. Ramp-out occurs between 0 and 0.1 seconds (3.5 mm to 4 mm) and settles back into the turf at 0.15 seconds (negative displacement is out of the turf).\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8904480/v1/e9d89c0957198ee1419fa5f1.png"},{"id":103505216,"identity":"430f65e4-b76a-46c4-a80a-2d4fdbf43edd","added_by":"auto","created_at":"2026-02-26 13:27:50","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1116910,"visible":true,"origin":"","legend":"\u003cp\u003eDiagram of 6 DOF robotic test setup (left). Metal cleat surrogate with direction of imposed test motions labelled (right).\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8904480/v1/a9f1e5aea647d68756e1fd5a.png"},{"id":103505220,"identity":"6687e9ec-5e91-4bff-b98b-6764ea9e6e84","added_by":"auto","created_at":"2026-02-26 13:27:58","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":840381,"visible":true,"origin":"","legend":"\u003cp\u003eRepresentative displacement time history (left) and time window zoomed-in to time of shear event (right) depicting vertical displacement (top) and horizontal displacement (bottom).\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8904480/v1/65a63fbe19675ba2b9552572.png"},{"id":103216707,"identity":"53cdbf68-9b85-4fcc-ace8-b1956b98af2e","added_by":"auto","created_at":"2026-02-23 09:37:45","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":937285,"visible":true,"origin":"","legend":"\u003cp\u003eExample time history of a posterior shear test with notable normal force levels labelled. (1.) maximum normal force, (2.) preload normal force, (3.) \u0026nbsp;peak shear normal force, which is defined as the normal force at time of peak shear force. A. Full test time history. B. Zoomed-in to shear event timing.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8904480/v1/61e9594c815d6776a3773cee.png"},{"id":103216712,"identity":"8bf3c8d9-88fc-45e7-a26c-12297e37f8e1","added_by":"auto","created_at":"2026-02-23 09:37:45","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":692781,"visible":true,"origin":"","legend":"\u003cp\u003eCleat shear displacement time histories from posterior (left) and lateral (right) tests with variations in normal preload.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8904480/v1/66472bc9a249b52aa8578a99.png"},{"id":103216710,"identity":"7042e793-26e2-4b23-a376-9fe5a79d1205","added_by":"auto","created_at":"2026-02-23 09:37:45","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1591688,"visible":true,"origin":"","legend":"\u003cp\u003eNormal and shear force vs. displacement for three repeat tests (indicated by color) of the 2100 N nominal preload in the posterior shear condition (top left) and lateral shear condition (top right). Shear vs. displacement data for all posterior (top left) and lateral (top right) tests.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8904480/v1/38299254fd2398631e863519.png"},{"id":103216709,"identity":"ce22778c-96c0-48cc-addc-9ee114c9b069","added_by":"auto","created_at":"2026-02-23 09:37:45","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1434753,"visible":true,"origin":"","legend":"\u003cp\u003eExample time history of a posterior shear test with time points corresponding to video frames. (A.) Beginning of shear motion with no void space in front of the cleat, (B.) Cleat shear position immediately before release showing void space from trapped fibers beneath cleat, (C.) First fibers snapping out from beneath cleat after release, (D.) Void space in front of cleat filled again.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8904480/v1/57a33fc1b15ba1a89593c760.png"},{"id":103216711,"identity":"5b2e6194-bd02-4183-ad70-7065e0f28e9a","added_by":"auto","created_at":"2026-02-23 09:37:45","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":698516,"visible":true,"origin":"","legend":"\u003cp\u003eRelease shear force vs. peak shear normal force for all posterior shear and lateral shear tests.\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8904480/v1/bdb314f40d7c853f869a7c02.png"},{"id":103216708,"identity":"e1ae1bf9-255d-4520-966e-7cb063dd3da3","added_by":"auto","created_at":"2026-02-23 09:37:45","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":413541,"visible":true,"origin":"","legend":"\u003cp\u003eTheoretical release traction slope.\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8904480/v1/3492d498ea01ba5a40ada557.png"},{"id":103509535,"identity":"ca82182b-5ec3-4e51-b9df-bd4f952ec927","added_by":"auto","created_at":"2026-02-26 13:59:43","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8634452,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8904480/v1/2b8453ee-683d-4e0d-a5c7-85e9686b1fa1.pdf"},{"id":103216704,"identity":"0875cb1f-b7e6-49c2-bc59-62faf5aebf35","added_by":"auto","created_at":"2026-02-23 09:37:45","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":16042,"visible":true,"origin":"","legend":"","description":"","filename":"Appendix.docx","url":"https://assets-eu.researchsquare.com/files/rs-8904480/v1/1857103521ef988507825096.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Assessment of the Mechanics of the Shoe-Turf Release Phenomenon Over a Range of Normal Loads","fulltext":[{"header":"Introduction","content":"\u003cp\u003eAn athlete\u0026rsquo;s sporting shoe contributes to their interaction with a playing surface. Changes in shoe type and playing surface influence a player\u0026rsquo;s ability to accelerate, change direction, and stop [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Shoe-surface interaction is also a factor in the risk of injury to the lower extremities [\u003cspan additionalcitationids=\"CR3 CR4 CR5 CR6 CR7 CR8 CR9 CR10 CR11 CR12 CR13 CR14\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Foot \u0026ldquo;entrapment\u0026rdquo;, when the player\u0026rsquo;s foot ceases to move relative to the playing surface while the player\u0026rsquo;s body continues to move in either translation or rotation, has been postulated as a mechanism of some lower extremity injuries [\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. For example, lateral ankle ligaments can be injured by inversion of the ankle caused by foot entrapment [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe interaction between shoes and playing surfaces can be studied using human subject testing (e.g., [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]), though such testing is limited to non-injurious levels of load and confounded by inter- and intra-player repeatability, challenges in data collection, and difficulty in viewing the shoe-surface engagement. Mechanical testing methods have been developed to address some of these challenges. Torg et al. [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] performed one of the first, using a rotational motion test with combinations of cleated shoes and playing surfaces. Drag-type testing has also been performed where a shoe is pulled translationally across a playing surface [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eKent et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] developed a test device (dubbed the \u0026lsquo;BEAST\u0026rsquo;), to test shoe-surface interactions in both translation and rotation at loads and rates relevant to elite levels of American football. The BEAST device has characterized different combinations of playing shoes and surface types in the laboratory and \u003cem\u003ein situ\u003c/em\u003e [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. In these tests, a weight was mounted to a shaft attached to a cleated foot form. This shaft was pulled by a cable attached to a pneumatic cylinder to generate either translational or rotational cleat motions. As the normal force is caused by gravity in this method, it cannot be actively controlled during the cleat\u0026rsquo;s movement. The cleat experiences a relatively constant normal force during tests, and a decrease in depth during shear motions of tests on some artificial surfaces, \u0026ldquo;ramping-out\u0026rdquo; of the turf (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). These studies did not consider variations in the magnitude of normal weight used, nor high-speed imaging of the shoe-surface interaction to allow for visualization of the shoe release phenomena.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTherefore, the goals of this study were to (1) develop a repeatable methodology for varying normal load while characterizing shoe-surface interaction mechanics at loads relevant to elite levels of American football, (2) capture high-speed video of shoe-turf interaction to document the entrapment/release phenomenon, and (3) quantify the relationship between normal load and release load for one surface with an idealized cleat configuration. The specific hypotheses are as follows: (1) the cleat release timing will coincide with the peak shear force; and (2) there is a positive correlation between normal force and shear release force.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eTest Specimen Preparation\u003c/h2\u003e \u003cp\u003eTurf samples (1200 g/m\u003csup\u003e2\u003c/sup\u003e faceweight, slit-film carpet with 3-layer Styrene-Butadiene Rubber (SBR) and sand infill system) (FieldTurf, Montreal, Quebec, Canada) were constructed in custom built aluminum turf boxes (86 cm \u0026times; 145 cm \u0026times; 5.7 cm) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). A system for clamping turf trays down to the laboratory floor was designed and machined (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). This system allowed for flexibility in locating the turf tray relative to the robot described below, which helped in finding the optimum position for maximizing robot speed and power output.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eExperimental Setup\u003c/h3\u003e\n\u003cp\u003eTurf characterization experiments were performed with a 6DOF force/torque and position-controlled robotic test system (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The system consists of a 6DOF serial industrial robot (KR300 R2500 Ultra, Kuka, Augsburg, Germany) controlled by a suite of purpose-built hardware/software tools (simVITRO, Cleveland Clinic). The robot is rated to a 300 kg payload throughout its entire range of motion with accelerations and decelerations of a maximum of 2g. A replica of a common football cleat forefoot used by professional athletes (Nike Vapor Jet) was machined out of aluminum and used as a cleat surrogate [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] for this testing (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Force and torque data generated by each test were measured by a 6 DOF strain-gauge load cell (Model 1914, R.A. Denton, Rochester Hills, MI, USA) mounted between the cleat surrogate and the end effector of the robot arm. Position data for the cleated foot form was recorded by the robot control software. High speed video of each test was captured at 1000 frames per second from two angles (top, side) and synchronized to test data collected using an electronic trigger signal.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eTest Matrix\u003c/h3\u003e\n\u003cp\u003eA total of 24 tests were performed using the turf construction and aluminum cleat surrogate described above (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Tests were performed in triplicate in the posterior shear and lateral shear conditions, at four different normal force preloads. The posterior shear condition involved an imposed motion displacing the cleatform rearward relative to the turf (such as the direction the cleat would move if a player slipped during forward pushoff). The lateral shear condition involved an imposed motion displacing the cleatform toward its lateral aspect relative to the turf (such as the motion that would result from a side-step cut).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eTest Matrix\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVariable\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTotal No\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cb\u003eTarget Preload Normal Force\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eTarget Force Level\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cb\u003en\u0026thinsp;=\u0026thinsp;4\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e0.7kN, 1.4kN, 2.1kN, 2.8kN\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eCleat Motion\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePosterior Shear\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLateral\u0026nbsp;Shear\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003en\u0026thinsp;=\u0026thinsp;2\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eTurf Builds\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eSingle Turf Build\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003en\u0026thinsp;=\u0026thinsp;1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eRepeats\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eThree Tests per Condition\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003en\u0026thinsp;=\u0026thinsp;3\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eTest Matrix TOTAL\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003en\u0026thinsp;=\u0026thinsp;24\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eTurf Build Parameters\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eComponent\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eParameters\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eValue\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"7\" rowspan=\"8\"\u003e \u003cp\u003e\u003cb\u003eInfill\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTotal Infill [kg/m\u003csup\u003e2\u003c/sup\u003e] (lb/ft\u0026sup2;)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e44.92 (9.2)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTotal Infill height [mm]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e44\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTop Layer Constituent\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100% 1.4 mm SBR\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTop Layer thickness [mm]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMiddle Layer Constituent\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e67% 0.56 mm sand / 33% 0.72 mm SBR (by weight)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMiddle Layer Thickness [mm]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e34\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBottom Layer Constituent\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100% 0.56 mm sand\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBottom Layer thickness [mm]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"9\" rowspan=\"10\"\u003e \u003cp\u003e\u003cb\u003eFiber\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMaterial\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003epolyethylene\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eType\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eslit-film\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eWidth [mm]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eThickness [mm]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.141\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eHeight [mm]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e63.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFiber Denier\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10861\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFibers/stitch\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003estitch rate [cm] (inch)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.69342 (0.273)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003estitch gauge [cm] (inch)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.905 (0.75)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFaceweight [g/m\u003csup\u003e2\u003c/sup\u003e] (oz/yd\u0026sup2;)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1198.91 (35.36)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eBacking Layer\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003ePolypropylene backing coated with polyurethane (porous backing)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eE-Layer\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003enone\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e\n\u003ch3\u003eBoundary and Loading Conditions\u003c/h3\u003e\n\u003cp\u003eTurf tests were performed in displacement control by compressing the turf to a target depth and dynamically shearing in either the posterior or lateral direction (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Zero depth for each test was defined as the vertical position of the cleat form when the bottom of the longest studs of the cleat just touched the top of the infill. Target vertical compression depths were tuned for each test individually based on a pre-test in load control to the target preload normal force. Each turf tray was divided into 12 test locations, and each test was performed in an unused location on the turf. A border of 20.32 cm was left untested on the edges of the turf sample.\u003c/p\u003e \u003cp\u003eAt t\u0026thinsp;=\u0026thinsp;4 seconds, the target depth was reached, and the turf was allowed to relax for an additional 6 seconds until reaching a steady state. At t\u0026thinsp;=\u0026thinsp;10 seconds, the shear test motion began. The cleat displaced 100 mm in the horizontal direction at a nominal rate of 0.3 m/s. A vertical displacement ramp out of the turf was included. A period of 5.75 seconds at the end of the test was included to allow the cleat and turf to settle before data recording was stopped. Distinct ramp-out trajectories were chosen for each test condition based on initial pre-testing with a goal of holding constant normal force throughout shear motions. These ramp-out trajectories were consistent across all three repeats of a test condition.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eData Analysis\u003c/h2\u003e \u003cp\u003eUsing the results of this study, the cleat release phenomenon was defined based on high-speed video and the measured force-time data. Shear and normal force for each test were extracted at the time of release. A linear regression analysis was performed to test for a relationship between normal force and shear force at the time of release. CORA analysis was performed on displacement data from repeated trials to quantify the repeatability of the test system [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. All plotting and data analysis was performed using custom code (MATLAB ver. R2022b, MathWorks, Natick, MA, USA).\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cp\u003eKinetic responses of the turf samples followed a consistent characteristic behavior for both posterior shear and lateral shear tests (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). While normal force was applied first, a small (~\u0026thinsp;50 N) shear force occurred (t\u0026thinsp;=\u0026thinsp;4 sec, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The maximum normal force occurred at peak depth (Point 1). The normal force relaxed under constant depth from t\u0026thinsp;=\u0026thinsp;4 sec. to t\u0026thinsp;=\u0026thinsp;10 sec. Just after the application of horizontal displacement at t\u0026thinsp;=\u0026thinsp;10 sec., shear force rose monotonically to a peak and then dropped off.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eTest System Repeatability\u003c/h3\u003e\n\u003cp\u003eDisplacement time history data showed that the robotic test system was able to execute repeatable posterior and lateral shear motions regardless of the penetration depth and normal reaction load (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Displacement magnitudes varied by less than 6% and temporal variability was within 30 msec over the entire time history. CORA analysis performed on individual curves scored 0.97 or greater across all displacement time histories over the interval between 10.0 and 10.5 seconds.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe normal preload, which is an output reflecting variability in a sample\u0026rsquo;s response to the foot form\u0026rsquo;s initial position, varied by approximately 100 N for each test condition (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Normal and shear force responses followed characteristic force-displacement paths. Normal forces between tests varied by less than 200 N during the time history up to peak shear force within similar test conditions, and shear forces varied by up to 900 N up to the peak shear force across all test conditions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ePeak shear force and slope of the force displacement path increased as preload normal force condition increased (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Nominal 2800 N preload cases (average 2573 N preload) were comparable to BEAST shear test initial conditions (2800 N free weight load) [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Peak shear force responses in posterior tests in this study fell within the 1 standard deviation variation reported in peak posterior shear forces on artificial turf surfaces tested by the BEAST using the same cleated foot form (4.45 kN vs. 4.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3 kN) [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e\n\u003ch3\u003eRelease Force Timing\u003c/h3\u003e\n\u003cp\u003ePeak shear force was identified to coincide with the cleat position immediately before release of turf fibers from underneath the cleat (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). The high-speed videos depicted a characteristic behavior across all test conditions. When the cleat started translating, a void space opened in the starting position of the cleat as fibers were trapped beneath the cleat. As the cleat continued to translate this void space grew and the surrounding turf structure deformed, stretching in the direction of the shear motion. The cleat released from the turf and started to translate relative to the turf. At that time, the fibers trapped beneath the cleat released and snapped back into the void space created.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eRelease Force Analysis\u003c/h2\u003e \u003cp\u003eRelease shear force increased linearly with peak shear normal force in both posterior shear and lateral shear tests for the range of normal forces investigated (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.96, R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.97) (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eShoe-surface testing in this study represents a methodology that is comparable to that of the BEAST device [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. A ramp-out in displacement was used in this study to replicate the cleat z-motion observed during prior BEAST testing on artificial turfs, and observed peak shear forces in 2800 N target normal preload cases fell within the range reported in BEAST testing on multiple on-site artificial turf surfaces [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. The methodology described in this study expands upon that of the BEAST by including variations in normal force. The test setup described may also be used to program more complex test motions that more closely represent player cleat-surface interactions. Displacement controlled test inputs in this study proved to be highly repeatable, allowing for comparisons between tests to describe the variability exhibited by the turf response.\u003c/p\u003e \u003cp\u003eOne limitation of this work is that a metal cleat form was used, which does not represent the deformable cleats worn in real-world playing scenarios. Many previous studies have utilized full cleats attached to a test machine by fitting them around an artificial foot [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan additionalcitationids=\"CR27 CR28 CR29\" citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Such methods can be affected by differences in fit of the shoe on the artificial foot and deformation of the shoe upper and sole. The metal cleat chosen allowed for direct comparison to previous research [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] and provided a consistent boundary condition across tests. Further studies should investigate the effect that changes in friction or deformability of a cleat form have on the results of this study.\u003c/p\u003e \u003cp\u003eThe methodology described herein also differs from that of the BEAST in applied loading rate. The BEAST device applies translational motions at rates between 1.0 m/s and 1.5 m/s [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], which is 4\u0026ndash;5 times greater than the testing performed in this study. The BEAST test rate is comparable to foot displacement rates applied by elite American football players performing a range of tasks [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Future research should investigate the rate dependency of artificial turf surfaces to help contextualize the differences in applied rates between these two test devices. Riley et al. [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e] also described ground reaction forces (GRFs) from these athletic manoeuvres. Peak normal GRFs across these trials ranged from 170% to 220% of player body weight (1646 N to 2618 N based on player mass reported). The test methodology described in this study allows for evaluation of playing surfaces throughout this range.\u003c/p\u003e \u003cp\u003ePrevious studies have often employed the use of a \u0026lsquo;traction coefficient\u0026rsquo; [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. This traction coefficient is commonly defined as the ratio of normal force to shear force, deriving from the friction coefficient used in studying the forces generated when two relatively flat surfaces slide across each other. Some debate exists to the utility of this traction definition as it leads to the assumption that normal force and shear force are held at a constant ratio across test timing and inputs [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. This study showed that a strong linear relationship exists between shear release forces and normal forces for the cleat surrogate and artificial turf tested within the range of normal forces investigated. However, force time histories recorded in this testing exhibit non-linearity at shear forces leading up to release (e.g. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). In this study, non-normalized values of normal force and shear force are reported, allowing for comparisons to be made to injury data for the lower extremity as well as testing of other shoe and surface combinations.\u003c/p\u003e \u003cp\u003eThe results of this study indicate that there is a threshold between shoe-surface shear force release and stick (or non-release) that is dependent on the normal force held beneath the player\u0026rsquo;s shoe (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). Here we are using stick as opposed to entrapment, as this term does not imply injury. Any foot strike that does not cross this shear vs. normal force threshold will not release and allow a player to cut or change direction.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFurther, shoe orientation did influence the slope and intercept of this linear relationship but did not change the strength of the correlation (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). This conclusion leads to the hypothesis that there may be a similar linear relationship existing for other cleat and surface pairings and this \u0026ldquo;release traction\u0026rdquo; may be characteristic of individual turf surfaces. However, the results of this study may be dependent upon the cleatform used paired with the artificial surface tested, and future testing should explore the linearity of release for other test conditions as well as outside the boundaries of the forces evaluated here, including down to low normal forces to better explore turf mechanical response.\u003c/p\u003e \u003cp\u003eThe ability to evaluate turf response across a range of normal forces allows for parametric evaluation of different turf characteristics. Turf construction can vary widely including fiber type (e.g., slit-film, monofilament, mixed), faceweight, infill materials and composition [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Future testing should address how release traction is affected by changes in turf parameters, as well as changes in cleat design (stud shape and spacing, and cleat materials). Environmental factors may also affect the release traction slope and should be considered in future work. The test method described allows for evaluating these variations under controlled experiments in a laboratory. Future evaluations of these different factors of turf construction could be useful to field managers and turf manufacturers.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eA test methodology was developed to test shoe-surface interactions under varying axial load. High-speed imaging synced with force response data was used to identify a physical description of release force timing and this timing was concurrent with peak shear force, supporting our first hypothesis. Shear release forces proved to be highly linearly related to normal forces under the loading regime investigated which supported our second hypothesis. Shear data in this study gave similar results to other comparable on-field testing. Displacement input data and force response data proved to be highly repeatable, allowing for useful comparisons to be made across different test conditions.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eThe research presented in this paper was made possible by a grant from the National Football League and the National Football League Players Association. The views expressed are solely those of the authors and do not represent those of the\u0026nbsp;NFLPA, the\u0026nbsp;NFL, or any of its affiliates.\u003c/p\u003e\n\u003cp\u003eEthics declaration: not applicable.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eGains GL, Swedenhjelm AN, Mayhew JL, Bird HM, Houser JJ (2010) Comparison of speed and agility performance of college football players on field turf and natural grass. 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Gait Posture 38(4):563\u0026ndash;569. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.gaitpost.2012.03.034\u003c/span\u003e\u003cspan address=\"10.1016/j.gaitpost.2012.03.034\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"artificial turf, turf testing methodology, shoe-turf interaction, cleat release, traction","lastPublishedDoi":"10.21203/rs.3.rs-8904480/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8904480/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe goal of this study was to develop a test methodology for use in studying cleat-turf mechanical interaction that permits laboratory testing, precision-controlled inputs, variation in normal load applied to the cleat, and imaging of the cleat-turf interaction. This new test method utilized a 6 DOF force/torque and position-controlled serial robotic test device to perform compress-then-shear tests between a cleated shoe surrogate and artificial turf samples. The test method proved to be highly repeatable in displacement-controlled inputs across variations in axial load. Force response data were also repeatable, but variations in shear response were indicative of inherent variability in turf construction. Precise identification of cleat-turf release was permitted through high-speed imaging and verified to occur at the time of peak horizontal force. A strong linear relationship was found between release shear force and normal force at time of release for both posterior and lateral shear test types under the loading regime investigated (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.96, R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.97). This linear regression represents a \u0026ldquo;release traction\u0026rdquo; that can be used to help better understand how turf construction relates to cleat-turf interaction mechanics, which is necessary to optimize player safety and performance.\u003c/p\u003e","manuscriptTitle":"Assessment of the Mechanics of the Shoe-Turf Release Phenomenon Over a Range of Normal Loads","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-23 09:37:40","doi":"10.21203/rs.3.rs-8904480/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"cfddeace-e29c-48c9-b231-b2d14f65e6dc","owner":[],"postedDate":"February 23rd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-04-27T13:53:36+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-23 09:37:40","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8904480","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8904480","identity":"rs-8904480","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}