## 4 Discussion  
### Effect of the Stack Height on Running Performance  
Despite being a highly debated topic [10; 11; 12; 13], the effect of stack height on running performance has been scarily analyzed. The results provided by [PERSON] et al. [17] and [PERSON] et al. [18] are the unique published data regarding this topic (See references for further details). On the one hand, [PERSON] et al. [17] compared four identical models differing only in the misdsole thickness (i.e., 35, 40, 45, 50 mm). After matching shoe weights, no significant differences in running economy were observed. On the other hand, [PERSON] et al. [18] compared three stack height conditions (i.e., 30, 40, and 60 mm) on two footwear prototypes with two different misdsole materials (i.e., EVA and NITRO). After matching shoe weights, increasing the stack height in the EVA misdsole material resulted in a 0.4% running economy penalty per 10 mm added [18]. On the contrary, the NITRO material improved the running economy by 0.13% per 10 mm added [18].  
Two relevant factors of these results should also be noted. On the one hand, the running economy would be impaired if the added shoe mass was considered. According to  
Figure 4: Toe spring (**left**) and rocker radius (**right**) values, as well as rocker axis position of current track spikes of different brands for long, mid, and sprint events. (**TOP**): Adidas Adizero Avanti TYO (18.6 cm, 18.6 cm, 61.8%), (**MIDDLE**): Adidas Adizero Ambition (15.9 cm, 14.1 cm, 64%), (**BOTTOM**): Adidas Adizero Finesse (17.9 cm, 17.9 cm, 59.3%), (**TOP**): Nike ZoomX Dragonfly (19.4 cm, 19.4 cm, 54.4%), (**MIDDLE**): Nike Air Zoom Victory (18.9 cm, 18.9 cm, 62.3%), (**BOTTOM**): Nike Air Zoom MaxFly (16.7 cm, 15.5 cm, 59.3%). Note that the rocker and toe springs radius are an approximation.  
the 1% rule per 100 g added [20; 21], [PERSON] et al. [18] reported that the running economy deteriorated by 0.5% per 10 mm for the EVA midsole material tested and by 0.34% per 10 mm for the NITRO midsole material. Likewise, the conditions tested by [PERSON] et al. [17] impaired 0.33% per 10 mm added. Therefore, according to the largest stack height limitation established (i.e., 20 mm), running on an illegal shoe of 40 mm would result in a 0.7 to 1.0% detriment in running economy from the shoe mass standpoint. On the other hand, the contrasting differences reported between the EVA (i.e., +0.4%/10 mm) and the NITRO material (i.e., -0.13% per 10 mm) when matching weights also revealed an interesting factor of this footwear feature [18]. In this regard, [PERSON] et al. [17] analyzed the force-deformation profile of the different stack height conditions made with the same midsole material, observing an increased vertical midsole deformation and a decreased midsole stiffness when increasing the stack height, which resulted in similar energy returned. Therefore, the different trends observed on the midsole material used by [PERSON] et al. [18] could be caused by other factors, such as the timing of the force-deformation profile with the ground contact time displayed by the athletes at the running speed tested (i.e., 14 km/h) [18]. However, it seems that any potential improvement caused by altering the midsole properties due to an increased stack height would be counteracted by the shoe mass (i.e., \(-\)0.13% vs. 0.34% per 10 mm). The potential role of these footwear features on running performance and the effect of the stack height on it is further discussed in the following sections.