LAUSR

Following an editorial in the British Journal of Sports Medicine proposing that the governing body for track and field (World Athletics, formerly IAAF) should limit the midsole thickness of running shoes (Burns & Tam, 2019), Footwear Science’s Editor-in-Chief published a commentary addressing the lack of evidence of a performance advantage attributable to midsole thickness (Frederick, 2019). In that commentary, the author makes three major points. First, there is no evidence of a performance advantage attributable to midsole thickness. Second, the lack of a standard method for the measurement of midsole thickness, makes it hard to implement a rule limiting the maximum midsole thickness and makes enforcing that rule even harder. Third, the main take away message is that observations from head-to-head comparisons between shoes that differ in many aspects should not be treated as evidence of a more universal outcome of any single of those aspects. While few would argue against the second and third points, there is a lot more to say about the first point. Burns and Tam (2019) seem to propose limiting midsole thickness, not because increased midsole thickness by itself would improve running performance, but to provide some room for innovation and technological advances, without giving footwear companies a free pass to keep increasing midsole thickness more and more. This implicitly assumes that further increases in midsole thickness would provide more advantage. While Frederick (2019) argues that a thicker midsole by itself is not necessarily performance enhancing, Burns and Tam (2019) fear that not limiting midsole thickness will result in running shoes evolving into ‘some unrecognizable extension of the body’. The question is if indeed more is better regarding midsole thickness. Mercer, Stone, Young and Mercer (2018) did not observe differences in oxygen uptake during running in a thick midsole shoe and in a control shoe, but importantly multiple aspects of the test shoes differed, not just midsole thickness. Fortunately, Tung, Franz, and Kram (2014) have addressed this question with an innovative approach that allowed them to control all but one footwear aspect: midsole thickness. Rather than using any footwear to attach midsole foam to the foot, they attached midsole foam slats to the treadmill belt. While the exact metabolic savings they observed cannot be directly extrapolated to midsole foams other than the PhyliteVR -foam they used (60% PhylonVR and 40% rubber with an Asker Type C durometer reading of 52–58), their group data clearly showed that some midsole (10mm) is better than none, but that even more midsole (20mm) was not better. Another reason why more midsole thickness is not always better is related to frontal plane stability. Similar lateral forces at the level of the sole of the foot result in larger roll moments when midsole thickness (the effective moment arm) is increased. This is supported by both recent and classic studies where midsole thickness was varied while most other footwear aspects were kept constant. Hannigan and Pollard (2020) compared maximal (33/29mm) and traditional (22/18mm) shoes with identical uppers and observed increased eversion in the shoes with thicker midsole shoes, in line with the increased eversion they observed in a similar