## 1 Introduction
The first laboratory experiments examining this emerging technology repeatedly demonstrated that AFT running shoes significantly improve the RE of elite and highly trained runners at speeds of 14-18 km/h by 2.8-4.4% compared to traditional racing flats [22; 23; 24]. Several laboratory experiments examining AFT models from either Nike, Adidas, or Saucony in recreational runners at slower running speeds of 9-15 km/h reported similar RE benefits of 1.6-5.0%, indicating that runners can benefit from AFT independent of running speed [25; 26; 27; 28; 29]. Notably, when comparing the same AFT model at different running speeds, RE benefits appear to increase with greater running speed [28; 29; 30]. As kinetic energy is partially stored in and returned from the midsole foam, and because the optimal midsole LBS has been shown to depend on running speed [20; 30], an association between running speed and RE benefit may be expected. Interestingly, this relationship appears to be present in both elite-level and amateur runners, with amateurs benefitting to a greater extent from AFT models [28]. This indicates that it is presumably not absolute running speed itself but rather relative physiological intensity that leads to greater RE benefits with increasing running speed. It is important to note that most studies evaluating running speed effects on the AFT response used a version of the Nike Vaporfly or Nike Alphafly. Different material properties of models from manufacturers other than Nike might imply that RE responses could be affected differently across a range of speeds. To date, only one study has compared RE benefits across commercially available AFT models from different manufacturers for the same cohort of athletes [31]. The researchers report that there are significant differences between RE benefits provided by different AFT models, ranging from no benefit in the Hoka Rocket X to a 3% benefit in the Nike Air Zoom Alphafly Next% when compared to a racing flat. They concluded that Nike's AFT racing shoes clearly confer the greatest degree of improvement in economy and that this might lead to a competitive advantage. However, the researchers utilized a specific, strictly homogenous group of highly trained male runners and tested RE at only one running speed (16 km/h). As both the type of runners and running speeds are known to influence RE benefits of AFT [28], these results may possibly not be generalized to other populations or running speeds. It is conceivable that some AFT shoes, for example, might simply be better suited for recreational athletes running at slower speeds. Further, [PERSON] et al. [30] focused on average group-level RE improvements, which neglects inter-individual variability and should thus not be misinterpreted that every runner could expect to gain this amount of RE benefit. A different approach recently proposed by [PERSON] et al. [32] addresses these issues by reporting the percentage of runners likely to receive a given amount of RE improvement. Those researchers demonstrated the feasibility of their approach by showing that only 25% of recreational runners could expect to experience the 4% RE benefit from Nike's Vaporfly 4% shoe reported by [PERSON] et al. [22]. In essence, an important finding of recent AFT research is that responses to AFT are highly individual, with large inter-individual variance in responses to different AFT models reported [25; 28; 32]. Individual RE responses to AFT range from no change to substantial improvement (\(\approx\)6%) between different individuals wearing the same shoes [24; 29]. This variability has led researchers to recommend an individualized approach to running shoe prescription [25; 28]. At this time, however, there is currently no scientifically justified strategy for determining optimal AFT properties for an individual runner. Factors that seemingly influence individual RE response are running speed, foot strike pattern, ground contact time, body mass, plantar flexor strength, foot arch stiffness, joint range of motion, and training level [33].