A Review of the Potential Effects of the World Athletics Stack Height Regulation on the Footwear Function and Running Performance

ms to synthesize and discuss the potential effects of a stack height modi - cation on the function of the different footwear features and their effects on running performance. Peer-reviewed studies were identi ed from electronic databases using a structured keyword search and a screening process. Complementary sources were used to illustrate and discuss the current racing footwear constructions. With regard to the shoe mass, it is suggested that a stack height difference of 20 mm could induce a meaningful effect on performance. With respect to the midsole properties, it seems

were identi ed from electronic databases using a structured keyword search and a screening process. Complementary sources were used to illustrate and discuss the current racing footwear constructions. With regard to the shoe mass, it is suggested that a stack height difference of 20 mm could induce a meaningful effect on performance. With respect to the midsole properties, it seems that reducing the stack height does not alter the energy returned, given that the lower midsole deformation is counteracted with an increased stiffness. However, it should be noted that this might affect the timing of the midsole deformation and restitution, which should be matched with the mid and propulsive stance phases. Lastly, the curved geometry of the forefoot sole needed to create the teeter-totter effect could be affected by the stack height reduction. However, current racing footwear designs have counteracted this modi cation by proximately placing the rocker axis and increasing the toe spring. Keywords:running performance; running biomechanics; footwear technology 1. Introduction The 2 h marathon barrier has been one of the greatest challenges in human physiology. Since 1950, the world record has been reduced by more than 16 min, with the improved athlete context as the potential cause of such progression (i.e., training programs, athletes monitoring, and socio-economic aspects) [1]. However, in 2016, the advances in footwear technology implemented for the breaking 2 h attempt marked a turning point in the natural progression of performance seen up to date [2]. Breaking the 2 h marathon barrier required athletes to maintain a metabolic steady- state on a 2:50 min:s pace over 42.195 m. Considering the different performance determi- nants in long-distance running (e.g., maximum oxygen consumption, running economy, critical velocity) [3–5], it seems that an extraordinary combination of them was required to achieve such demand. In this regard, Jones et al. [3] de ned the physiology of a group of top elite marathoners, displaying a peak oxygen uptake of 71.0 5.7 mL/kg/min Appl. Sci.2023,13, 11721.

required to achieve such demand. In this regard, Jones et al. [3] de ned the physiology of a group of top elite marathoners, displaying a peak oxygen uptake of 71.0 5.7 mL/kg/min Appl. Sci.2023,13, 11721.

Appl. Sci.2023,13, 11721 2 of 11 and relative oxygen consumption at 21 km/h of 95 5% of the peak oxygen uptake [3]. The metabolic steady state exhibited by the group was 20.2 0.6 km/h, their lactate turn point (4.6 1.3 mmol/L ), where 92 3% of the peak oxygen uptake was required with a cost of running of 188 20 mL/kg/km [3]. Due to the time limit at such running intensity, marathon times predictions from this evaluation were made by dividing the oxygen up- take (mL/kg/min) by the cost of running (mL/kg/km) at 96% of the lactate turn point [3]. A time prediction of 2:08:31 03:48 was found, with the best time being 2:00:01 [3]. At these performance levels, all the possible marginal improvements were contemplated for over- passing the 2 h target. In this regard, the marathon shoes released for this aim captured the attention of the running and scienti c community. It was later published that compared to a previous world-record marathon shoe model, the prototype provided by Nike improved by 4% the running economy [6]. Therefore, if an athlete of 2:03:40 displays an oxygen uptake of 65 mL/kg/min and a cost of running of 191 mL/kg/km (i.e., 60 65/191 = 20.4 km/h) at his marathon intensity (i.e., the lactate turn point) [3], the 4% improvement would have placed his marathon pace on 21.2 km/h (60 65/183.4 = 21.2 km/h) resulting in a nal time of 1:59:34. Likewise, other external factors such improvements in the course design [7] and pacing aids (i.e., shifting pacers, pacing car) were contemplated to increase the likelihood of success by improving the cost of running. After the implementation of these footwear innovations (i.e., lightweight materials, compliant and resilient midsole, increased longitudinal stiffness, curved forefoot geometry) in the athletic community and the proven performance improvements in long-distance road events [2,8], the controversy of “technology doping” emerged, leading the World Athletics to create a shoe regulation for the different events based on limiting the stack height (i.e., the amount of shoe material between the foot and the ground) (Table) [ 9]. This regulation is being

forefoot geometry) in the athletic community and the proven performance improvements in long-distance road events [2,8], the controversy of “technology doping” emerged, leading the World Athletics to create a shoe regulation for the different events based on limiting the stack height (i.e., the amount of shoe material between the foot and the ground) (Table) [ 9]. This regulation is being widely deliberated in order to ensure that performance is achieved via the primacy of human effort rather than technology in running shoes [10–13]. On the one hand, Burns and Tam [10] suggested that limiting the stack height might be a solution as it would limit those footwear features that could enhance the running economy. Furthermore, this rule would also limit any future innovation that could jeopardize the integrity of athletics [10]. In response to such a proposal, Frederick [11] stated that there is a lack of evidence and consensus on the measurement of the stack height. Hoogkamer [12] supported these statements and mentioned that increasing the stack height results in no advantage in the running economy due to the increase in mass and instability. It was also added that the resilience property of the midsole (i.e., the capacity to store and to release the mechanical energy applied) of current midsole materials is not compromised by such limitation since the greatest deformations were found to be in 12 mm, needing just a few more mm to avoid the bottom out of the midsole, which could have affected the storing phase [12]. Nigg et al. [13] subsequently contributed to this debate by discussing the potential contribution of each shoe feature to the improvements in the running economy. The authors mentioned that the major effect of contemporary running shoes on running performance comes from the increased longitudinal stiffness and the forefoot rocker (i.e., the upward curvature of the forefoot sole) that generates the so-called “teeter-totter effect” [14]. Table 1. Maximum stack height established by the World Athletics from 2019 to 2024 for the different running events. Event 2019–2022 2022–2024 2024 <800 m 20 mm 20 mm 20 mm 800 m 25 mm 25

from the increased longitudinal stiffness and the forefoot rocker (i.e., the upward curvature of the forefoot sole) that generates the so-called “teeter-totter effect” [14]. Table 1. Maximum stack height established by the World Athletics from 2019 to 2024 for the different running events. Event 2019–2022 2022–2024 2024 <800 m 20 mm 20 mm 20 mm 800 m 25 mm 25 mm 20 mm Road 40 mm 40 mm 40 mm Cross Country 25 mm spike shoe or non-spike shoe 20 mm spike shoe or 40 mm non-spike shoe 20 mm spike shoe or 40 mm non-spike shoe Despite this stack height limitation, this rule was insuf cient to avoid the great improvement in long-distance events that began with the running shoe revolution in

Appl. Sci.2023,13, 11721 3 of 11 2017 [2,15], which resulted in stricter regulation, being the stack height further reduced (Table) [ 9]. Therefore, in order to clarify the potential effects of reducing the stack height, this scoping review aims to synthesize and discuss the potential effects of a stack height modi cation on the function of the different footwear features and their effects on run- ning performance. 2. Materials and Methods The footwear features of potential interest for this review were rst identi ed accord- ing to an expert consensus [16]. Subsequently, these were chosen based on their potential impact on running performance [13], with speci c attention to their susceptibility to being in uenced by a reduction in the stack height (i.e., shoe mass, midsole properties, teeter- totter effect items). The Pubmed and Web of Science electronic databases were used to identify studies from inception to the present. Each footwear feature term was combined with different running footwear and performance terms using “AND” and “OR” com- mands: (Shoe* OR Footwear OR Racing Flats OR Spikes) AND (Mass OR Midsole OR Longitudinal stiffness) AND (Economy OR Energy Cost OR Time OR Race OR Speed OR Pace). After removing duplicates and irrelevant articles, those with full-text availability were screened to identify if the footwear conditions compared in a running performance outcome (i.e., running economy, time trial, time to exhaustion) were created by a stack height reduction while maintaining the rest of the footwear features similarly. If not, those studies analyzing each footwear feature in isolation while controlling potential confounders on a running performance outcome were selected (Figure). The evidence provided by these studies was then discussed using complementary sources (see details in each section).Appl. Sci. 2023, 13, x FOR PEER REVIEW 4 of 12 Figure 1. Flow diagram of study selection. 3. Results 3.1. Effect of the Stack Height on Running Performance Two studies were identified that analyze the effect of varying the midsole thickness on a running performance outcome while maintaining the rest of the footwear features similarly [17,18]. This was performed by using the same footwear prototype with different

4 of 12 Figure 1. Flow diagram of study selection. 3. Results 3.1. Effect of the Stack Height on Running Performance Two studies were identified that analyze the effect of varying the midsole thickness on a running performance outcome while maintaining the rest of the footwear features similarly [17,18]. This was performed by using the same footwear prototype with different midsole thicknesses and adjusting the shoe weight by adding lead beads. 3.2. Effect of the Shoe Mass on Running Performance and Influence of the Stack Height Three studies were identified that analyze the effect of the shoe mass on a running performance outcome while maintaining the rest of the footwear features similarly [19– 21]. This was performed by adding lead beads to the same footwear model [19,21] or to the feet [20]. In order to illustrate the effects of reducing the stack height on shoe mass, a linear regression analysis was traced to the shoe mass and stack heights reported by Bar- rons et al. [17] in Figure 2. In addition, the midsole thickness and shoe mass of 15 common racing footwear models were searched on a specialized running footwear website [22] and reported in Figure 2. Figure 1.Flow diagram of study selection.

Appl. Sci.2023,13, 11721 4 of 11 3. Results 3.1. Effect of the Stack Height on Running Performance Two studies were identi ed that analyze the effect of varying the midsole thickness on a running performance outcome while maintaining the rest of the footwear features similarly [17,18]. This was performed by using the same footwear prototype with different midsole thicknesses and adjusting the shoe weight by adding lead beads. 3.2. Effect of the Shoe Mass on Running Performance and In uence of the Stack Height Three studies were identi ed that analyze the effect of the shoe mass on a running performance outcome while maintaining the rest of the footwear features similarly [19–21]. This was performed by adding lead beads to the same footwear model [19,21] or to the feet [20]. In order to illustrate the effects of reducing the stack height on shoe mass, a linear regression analysis was traced to the shoe mass and stack heights reported by Barrons et al. [17] in Figure. In addition, the midsole thickness and shoe mass of 15 common racing footwear models were searched on a specialized running footwear website [22] and reported in Figure.Appl. Sci. 2023, 13, x FOR PEER REVIEW 5 of 12 Figure 2. Rearfoot stack height and shoe mass relationship of the same footwear prototype [17] (Black dots: 35 mm, 214.5 g; 40 mm, 229.5 g; 45 mm, 241.5 g; 50 mm, 260.5 g) and from different racing footwear models retrieved from a specialized running footwear website [22]. White dots: Reebok Floatride Run Fast Pro (100 g, 16 mm), Nike Flyknit Racer (160 g, 24 mm), Saucony Fastwitch 9 (170 g, 19 mm), Adidas Adizero Takumi Sen 7 (170 g, 24 mm), Adidas Adizero Takumi Sen 9 (181 g, 33 mm), Nike Zoom Vaporfly 4% (184 g, 39 mm), Nike ZoomX Vaporfly Next% (187 g, 40 mm), PUMA Deviate Nitro Elite (190 g, 36 mm), ASICS MetaRacer (190 g, 24 mm), ASICS Metaspeed Edge (190 g, 29 mm), Adidas Adizero Takumi Sen 8 (193 g, 33 mm), Saucony Endorphin Pro+ (196 g, 35 mm), Nike ZoomX Vaporfly NEXT% 2

33 mm), Nike Zoom Vaporfly 4% (184 g, 39 mm), Nike ZoomX Vaporfly Next% (187 g, 40 mm), PUMA Deviate Nitro Elite (190 g, 36 mm), ASICS MetaRacer (190 g, 24 mm), ASICS Metaspeed Edge (190 g, 29 mm), Adidas Adizero Takumi Sen 8 (193 g, 33 mm), Saucony Endorphin Pro+ (196 g, 35 mm), Nike ZoomX Vaporfly NEXT% 2 (196 g, 39 mm), ASICS Metaspeed Sky (198 g, 33 mm) and PUMA Fast FWD Nitro Elite (198 g, 38 mm). 3.3. Effect of the Midsole Properties on Running Performance and Influence of the Stack Height Four studies were identified that analyze the effect of the midsole properties on a running performance outcome while maintaining the rest of the footwear features simi- larly [17,18,23,24]. This was performed by using the same footwear prototype with differ- ent midsole stack heights [17,18] or materials [24] and by running barefoot on a standard and cushioned surface [23]. 3.4. Effect of the Teeter-Totter Effect on Running Performance and Influence of the Stack Height A single study was identified that analyzed the teeter-totter effect on a running per- formance outcome while maintaining the rest of the footwear features similarly [25]. This was performed by comparing different racing footwear models with increased longitudi- nal stiffness of similar weights (<30 g) [25]. In order to further illustrate the potential im- plication of the stack height on the teeter-totter effect, the rocker and toe spring radius, as well as the rocker axis position have been analyzed in different current racing footwear models using the Kinovea free-access software [26] (Figures 3 and 4). Figure 2. Rearfoot stack height and shoe mass relationship of the same footwear prototype [17] (Black dots: 35 mm, 214.5 g; 40 mm, 229.5 g; 45 mm, 241.5 g; 50 mm, 260.5 g) and from different racing footwear models retrieved from a specialized running footwear website [22]. White dots: Reebok Floatride Run Fast Pro (100 g, 16 mm), Nike Flyknit Racer (160 g, 24 mm), Saucony Fastwitch 9 (170 g, 19 mm), Adidas Adizero Takumi Sen 7 (170 g, 24 mm), Adidas Adizero Takumi Sen 9

mm, 241.5 g; 50 mm, 260.5 g) and from different racing footwear models retrieved from a specialized running footwear website [22]. White dots: Reebok Floatride Run Fast Pro (100 g, 16 mm), Nike Flyknit Racer (160 g, 24 mm), Saucony Fastwitch 9 (170 g, 19 mm), Adidas Adizero Takumi Sen 7 (170 g, 24 mm), Adidas Adizero Takumi Sen 9 (181 g, 33 mm), Nike Zoom Vapor y 4% (184 g, 39 mm), Nike ZoomX Vapor y Next% (187 g, 40 mm), PUMA Deviate Nitro Elite (190 g, 36 mm), ASICS MetaRacer (190 g, 24 mm), ASICS Metaspeed Edge (190 g, 29 mm), Adidas Adizero Takumi Sen 8 (193 g, 33 mm), Saucony Endorphin Pro+ (196 g, 35 mm), Nike ZoomX Vapor y NEXT% 2 (196 g, 39 mm), ASICS Metaspeed Sky (198 g, 33 mm) and PUMA Fast FWD Nitro Elite (198 g, 38 mm). 3.3. Effect of the Midsole Properties on Running Performance and In uence of the Stack Height Four studies were identi ed that analyze the effect of the midsole properties on a running performance outcome while maintaining the rest of the footwear features simi- larly [17,18,23,24]. This was performed by using the same footwear prototype with different midsole stack heights [17,18] or materials [24] and by running barefoot on a standard and cushioned surface [23].

Appl. Sci.2023,13, 11721 5 of 11 3.4. Effect of the Teeter-Totter Effect on Running Performance and In uence of the Stack Height A single study was identi ed that analyzed the teeter-totter effect on a running performance outcome while maintaining the rest of the footwear features similarly [25]. This was performed by comparing different racing footwear models with increased longitudinal stiffness of similar weights (<30 g) [25]. In order to further illustrate the potential implication of the stack height on the teeter-totter effect, the rocker and toe spring radius, as well as the rocker axis position have been analyzed in different current racing footwear models using the Kinovea free-access software [26] (Figures).Appl. Sci. 2023, 13, x FOR PEER REVIEW 6 of 12 Figure 3. Rocker axis position (i.e., the starting point of the forefoot upward curvature) and rocker radius of current footwear models of different brands with different stack heights. The measures were performed with the Kinovea free-access software using a sagittal image of each model placed in the same position. The rocker axis was placed at the center of the rocker radius circle and calcu- lated by the relative position to the total length (i.e., 28 cm for all models). The rocker radius indi- cates the degree of the upward forefoot curvature of the sole (i.e., a greater rocker radius indicates a lower upward curvature). (TOP): Adidas Adizero Prime X Strung (50 mm, 67%, 10.6 cm), (MID- DLE): Adidas Adizero Pro 3.0 (40 mm, 67%, 11.8 cm), (BOTTOM): Adidas Adizero Takumi Sen 9 (33 mm, 63.8%, 15.9 cm), (TOP): Asics Metaspeed Sky+ (39 mm, 65.8%, 11.2 cm), (MIDDLE): Asics Metaspeed Edge+ (39 mm, 64.7%, 12.8 cm), (BOTTOM): Asics Metaracer Tokyo (24 mm, 57%, 18.2 cm), (TOP): Nike Air Zoom Aplhafly Next% (39 mm, 71.1%, 10.5 cm), (MIDDLE): Nike ZoomX Vaporfly Next% 3 (40 mm, 66.4%, 11.7 cm), (BOTTOM): Nike ZoomX Vaporfly NEXT% 2 (39 mm, 66.4%, 13.0 cm). Note that the rocker radius is an approximation. Figure 3. Rocker axis position (i.e., the starting point of the forefoot upward curvature) and rocker radius of current footwear models of different

This review discusses the impact of stack height regulation on footwear and running performance.