Understanding shoe-surface interactions in football

One of the key aims of modern football shoe manufacturers is to find the balance between developing a shoe that improves performance but also minimises the risk of injury. Traction properties of the outsole play an important part in reaching this balance; high levels of traction are necessary to enable players to accelerate and change direction without slipping, but excessive traction can lead to stud fixation, a potential cause of injuries. The ability to accurately measure and assess the traction properties is essential in the design of outsoles, but appropriate test parameters need to be used in order for the assessment to relate back to the intended use. The purpose of the study was to develop a method to identify how the shoe interacts with the surface during realistic football movements and then to use observations from data collected to recommend appropriate test parameters. A high-speed camera system was developed to capture the motion of the shoe in both a laboratory and natural turf environment. The cameras were calibrated using the checkerboard approach and filmed at 1000 Hz. Five markers positioned on the side of the shoe were tracked using a semi-automated algorithm developed using image processing techniques. Transposition matrices were used to identify the location of individual studs on the outsole of the shoe enabling the orientation, velocity and acceleration of the shoe to be calculated. Two data collection studies took place; firstly a single-participant study in the laboratory using a force-plate to relate kinematic results to kinetic information and secondly, a larger scale data collection outside on natural turf. Three movements representing scenarios requiring high levels of traction in football were assessed; acceleration, change in direction and braking. A representative trial for each movement was selected and full post-processing analysis was carried out. Information such as the orientation of the shoe on foot-strike, translation directions and centre of rotations during the transition phase and the number of studs in contact with the surface during push-off was obtained for each movement. The period at which the player was at greatest risk of slipping was identified for each movement. The motion of the shoe during this period was used to suggest appropriate test conditions for mechanical and computational traction testing methods. The influence of the shoe-surface interaction on outsole design was also considered; with the observed translation directions and centre of rotations being used to suggest a design aiming to enhance translational traction, but minimise rotational resistance.