Volume 24, Issue 6 (November 1996)
Required Coefficient of Friction Versus Walking Speed: Potential Influences of Footwear and Walkway Surfaces
This paper presents data analysis for a representative subject from a pilot study where subjects walked at increasing speeds across a force plate in shoes with the top-piece/outsoles replaced by a variety of test materials, including test-foot materials commonly used in walkway-safety tribometry. The goal of this data analysis was to develop a methodology that can by used in a multi-subject experiment that combines force-plate and video data. More specifically, the objectives were: • To determine how force-plate assessment of required friction for subjects walking in shoes having soles and heels of various shoe-bottom or tribometric reference materials compares to tribometric testing of available friction using similar test-foot materials. • To explore the relationship between μr and walking speed as influenced by specific footwear and walkway surfaces. • To develop hypotheses to guide further data analysis and research.
The force plate was covered with polypropylene, a substance expected to cause slipping with some of the test materials. The walking speeds ranged from moderate to as fast as the subject could walk. Required coefficient of friction μr was calculated from force-plate data. Average available coefficient of friction ¯μa was measured using a portable articulated strut tribometer with the test materials used as test feet.
Computerized capture of sequential screens from time-coded videotape recordings of data stored on magneto-optical discs was used to visually assess the kinematics of walking and slipping. Observed slipping was studied in relation to the ratio of μr/¯μa. Profiles of μr versus time during stance were examined relative to top-piece/outsole materials and walking speed. Peak μr at heel contact versus walking speed was evaluated. Positive correlation was seen for certain test materials, while negative correlation was seen with others. The authors hypothesize that biomechanical adaptations of the subject to the footwear/floor-surface combinations account for observed differences. The paper discusses practical problems confounding use of the ratio μr/¯μa as a predictor of slip probability.