Barefoot-pedestrian tribometry: in vivo method of measurement of available friction between the human heel and the walkway.

We have developed an in vivo tribometer for characterizing the friction of a human heel against a planar test surface. The pedestrian steps down on an angled test surface and an observer determines if the person's heel slips. In the simplest variation, the subject simply steps down onto the test surface. The second variation has the standing subject's lower leg constrained to a vertically running carriage, forcing the pedestrian's leg to descend vertically. The third variation has the subject sitting, with an operator raising and lowering the carriage to which the leg is attached. The test surface was fixed at a given angle, a set of repeated tests was run, and the number of tests and slips were recorded. The test-surface angle was incremented through a range that varied from no slips to all slips. We analyzed the data using logistic regression. We found that the unconstrained test subject's logistic-regression curves varied greatly from subject to subject. The standing, constrained subjects were significantly closer to each other, but at the expense of a much higher spread of the angular range. The seated, constrained test subject's results were both extremely close and had extremely low angular spread.