COOPER, Ryan C.; BERTRAM, John E.A.; BUTCHER, Michael T.; Florida State Univ., Tallahassee: Keeping your feet firmly on the ground: influence of step length on required foot friction

Limbed locomotion is made possible by ground reaction forces (GRF) redirecting the center of mass (COM). In order to maintain static contact with the ground and the ability to redirect the COM there is a minimum, or required, coefficient of friction (RCOF) that must exist between the foot-ground interface. The RCOF can be determined at any point during a step by dividing the Horizontal GRF, or frictional force, by the Vertical GRF, or normal force. These GRFs vary dynamically throughout the step and can be effected by factors such as speed, traversing obstacles, bearing loads, and varying step length; all of which in turn effect the RCOF. Peak RCOF values are critical because they determine points where slips are most likely. In walking, even under widely different conditions, step length is a major determinant of the geometry of the interaction of the limb with the ground, and consequently the reaction forces generated during that contact. This study evaluated the relationship between the RCOF and step length while walking at a constant speed using a force plate level with the runway surface. Human subjects were used because training and motivational issues are minimal, though it is expected that the results of these studies can be applied to other terrestrial limb driven modes of locomotion. Six subjects walked using a series of constant step lengths ranging from 35-100 cm. A constant speed for all step lengths was maintained by controlling frequency using an electronic metronome. The peak RCOF for human walking occurs at approximately 15% of the contact time. Findings indicate that the RCOF changes linearly as step length is increased or decreased from that used in normal walking.