Worn region size of shoe outsole impacts human slips: Testing a mechanistic model

Shoe outsole tread wear has been shown to increase slip risk by reducing
the tread’s ability to channel fluid away from the shoe-floor interface.
This study establishes a connection between geometric features of the worn
region size and slipping. A mechanistic pathway that describes the
relationship between the worn region size and slip risk is assessed.
Specifically, it is hypothesized that an increased worn region size leads
to an increase in under-shoe fluid pressure, which reduces friction, and
subsequently increases slipping. The worn region size, fluid pressure, and
slip outcome were recorded for 57 participants, who were exposed to an
unexpected slip condition. Shoes were collected from each participant and
the available coefficient of friction (ACOF) was measured using a
tribometer. A greater shoe worn region size was associated with increased
slip occurrence. Specifically, a 1 mm increase in the
characteristic length of the worn region (geometric mean of its width and
length) was associated with an increase in slip risk of ~10%. Fluid
pressure and ACOF results supported the mechanistic model: an increase in
worn region size correlated with an increase in peak fluid pressure; peak
fluid pressures negatively correlated with ACOF; and increased ACOF
correlated with decreased slip risk. This finding supports the use of worn
region size as a metric to assess the risk of slipping.