Physiologic axial load, frictional resistance, and the football shoe-surface interface.

The purpose of this investigation was to report on the interaction between different types of athletic shoes and playing surfaces using physiologic loads of 40 and 220 lbs. This is a continuation of our previous report using a load of 25 lbs. Nine shoes by three manufacturers were characterized as turf, court, molded cleat, or traditional cleat and tested on both natural grass and synthetic turf. A specially designed pneumatic testing device was used in order to measure translational resistance and rotational torque of the shoe-surface interface. Measurements were acquired for 1) force-X describing translational loading, 2) moment-Y describing the torque generated by linear translation, and 3) moment-Z describing the moment generated by axial rotation, and data were analyzed using repeated measures analysis of variance and Tukey's post-hoc comparison. It was found that increased the axial loads from 40 to 220 lbs significantly increased the frictional resistance (p < .05) between the shoe and the artificial turf surface in a nonlinear fashion. Turf shoes demonstrated the most frictional resistance of any group for this condition. Increases in the forces generated in linear translation from the axial load of 40 to 220 lbs produced the most significant increases of any resistance test on the turf surface. The cleated shoes (both traditional and molded) generated the highest frictional and torsional resistance on the grass surface when compared to the other categories of shoes. Grass generated higher peak moments than turf for the cleated shoes. These results demonstrate the considerable differences between laboratory and physiologic conditions and that the increase in frictional resistance is nonlinear with increasing loads.