Medio-lateral and lateral edge friction in indoor sports shoes

It has previously been speculated that the occurrence and severity of lateral ankle sprain injuries is linked to excessive shoe-surface friction. Especially, the lateral parts of the shoe outsole are suggested to play an important role in such scenarios but have never been quantified in a systematic manner. Therefore, the purpose of this study was to investigate the variation of friction of indoor sport shoes with foot orientation and compare it to the traditional industry forefoot friction test standard. We modified the ISO:13287:2019 test for footwear slip resistance and positioned the shoe on its forefoot and lateral edge while replicating medio-lateral movements similar to previously reported ankle sprain incidents. All tests were conducted on an indoor vinyl/sport surface. The results from the modified setups were compared to those following the anterior-posterior orientated ISO standard. Medio-lateral friction was on average 17% lower and lateral edge friction 24% lower than anterior-posterior forefoot friction (p < .001). However, linear regression showed that the forefoot test could only explain 36% and 35% of the variation in medio-lateral and edge friction. This suggests that motion-specific tests are necessary to determine footwear friction properties meaningfully. These findings could have important implications for future research and product testing in the field of footwear friction, safety and injury prevention.