Slipperiness of the shoe-floor interface: comparison of objective and subjective assessments

Tribollogically and biomechanically valid slip-resistance measurement methods are needed urgently to facilitate new developments for improving the safety of footwear and floor coverings. Objective coefficient-of-friction measurements and subjective walking experiments were compared to scrutinize and validate anti-slip assessments of the shoe-floor interface. Four shoe types were tested on a smooth stainless steel floor contaminated with viscous glycerol. The friction utilization ratio during one step, the sliding distance, and the subjective rating of slipperiness were evaluated by seven test subjects in 420 walking trials. All subjects assessed the footwear in the same rank of slipperiness. The friction utilization ratios during the walking trial were a poor indicator for anti-slip assessments. The apparatus-based kinetic friction values mu(k1), however, were more valid and, in particular, more reproducible. The safe level of mu(k1) was approximately 0.22, which is in accordance with previous slip-resistance clasification of shoes and floors. Futher trials in other shoe-contaminant-floor conditions and walking tasks are needed to verify the findings in this study.