Effects of natural shoe wear on traction performance: a longitudinal study

Footwear outsole design is an important factor for shoe-floor friction and for preventing slipping. Shoes with small, uniformly-separated tread blocks (often included on slip-resistant shoes) have decreased slip risk due to their increased friction and better under-shoe fluid drainage. However, these traction performance metrics (friction and fluid drainage) diminish with wear. This study quantifies shoe traction performance in response to natural wear and compares the relationship between common wear metrics: time, distance walked, and worn region size (WRS). Participants wore two pairs of shoes in the workplace for up to 11 months and the distance walked was tracked with a pedometer. After each month of wear, traction performance and WRS of each shoe were measured. Traction performance was quantified by the under-shoe available coefficient of friction and fluid force during a simulated slip condition. Increased wear (months worn, distance walked, and WRS) was associated with decreased traction performance. A WRS of 800 mm² was associated with reductions in friction of 16-38% and increases in fluid force by 286-528%. Three and six months of wear were associated with WRS values of 251 mm² and 462 mm² and distances of 203 km and 519 km, respectively. A walking distance of 500 km was associated with a WRS of 406 mm². This study showed that all these wear metrics are good indicators of shoe traction performance loss. Thus, the most practical metric in a particular application can be selected. We argue that WRS may be the best indicator due to variations in wear rate from the user and environment. Therefore, tracking footwear usage and monitoring outsole wear can aid in shoe replacement recommendations to reduce slips and falls.