Exploring interindividual running economy responses to advanced footwear technology shoes across a range of variables: A quantitative study

Abstract

Abstract In the past decade, the running landscape has changed due to advanced footwear technology (AFT) being imbedded in running shoes. While AFT shoes have been shown to improve running economy (RE) on average, little is known about the factors that explain the large interindividual variability in RE response. Therefore, this study aimed to investigate the interindividual variability in RE response to AFT and identify potential predictors of this variability using quantitative methods. This thesis considered participant characteristics, anthropometric measures, neuromuscular measures, and subjective perceptions as potential factors underpinning interindividual variability. This thesis is separated into three chapters. Chapter One is a brief review on AFT, their components, and their influence on RE and performance, as well as the observed interindividual variability in AFT shoe responses and the factors proposed to mediate this variability. AFT shoes have been designed to improve RE and include thick lightweight resilient and compliant foams, stiff elements embedded in the foam, and curved shoe geometry. Research has explored how these components may contribute to RE improvements and has proposed various factors that may explain the interindividual variability, such as running speed, training level, sex, leg length, body mass, shoe size, and plantarflexion muscle-tendon unit properties. Comparing the characteristics of responders to non-responders and exploring relationships between these characteristics and RE responses can assist in identifying runners more likely to benefit from running in AFT. Chapter Two presents an experimental study using a cross-sectional, repeated-measures design. A heterogenous sample of sixty-four participants (32 males, 32 females) of varying running levels, experience, age, and anthropometric characteristics completed two laboratory sessions between two and seven days apart. The first included baseline information and participant characteristics for anthropometric, neuromuscular measures, and maximal oxygen uptake. The second session served as the experimental phase, during which RE variables were evaluated in the two different shoe conditions: Salomon S/Lab Phantasm 2 (AFT) and Salomon Aero Glide 2 (CONTROL). RE was measured over four 6-minute trials in an AFT and a CONTROL shoe, allocated in a randomised, crossover order. Descriptive statistics (means ± SD), inferential comparisons (paired samples t-tests, Chi square, and Fishers exact), and Pearson’s correlation coefficients were calculated to compare between responders and non-responders to AFT shoes and explain AFT shoe response. The mean improvement in RE (expressed in terms of oxygen consumption) was 4.13 ± 1.6% when wearing AFT compared to CONTROL shoes, with individual responses ranging from -2.62% to 11.01%. From all the variables collected, only gastrocnemius medialis (GM) stiffness normalised to leg length was significantly different (p = 0.046) between groups, with responders exhibiting greater GM stiffness (329.8 ± 36.6 N/m²) compared to non-responders (296.4 ± 47.3 N/m²). No other between-group differences were statistically significant. Two variables were significantly and moderately correlated to RE response, where greater navicular drop (r = 0.31, p = 0.012) and greater GM stiffness (normalised to leg length) (r = 0.35, p = 0.005) were linked to greater RE benefits from running in AFT shoes. Additionally greater gear ratio (r = 0.26, p = 0.040) and lower standing plantarflexion isometric strength (normalised to body weight) (r = -0.26, p = 0.041) displayed significant and small correlations with improvements in RE when running in AFT shoes. A multiple linear regression including standing plantarflexion isometric strength, navicular drop, gear ratio, and GM stiffness (normalised to leg length) explained 27% of the variance in our AFT RE responses (R² = 0.2731, adjusted R² = 0.223), with GM stiffness being the most important factor (B = 0.014, p = 0.015). Lastly, Chapter Three summarises the findings, strengths, limitations, and recommendations for future research. This research provides further insight into interindividual variability in RE responses to AFT, showing not all runners benefit equally from AFT shoes. Therefore, runners should be aware that they may not improve their RE via AFT, however, individuals with greater GM stiffness, navicular drop, and static ankle gear ratio, and with lower standing plantarflexion isometric strength may benefit to a greater extent from AFT shoe wear. These factors may help guide more personalised footwear selection and prescription. Future research should investigate whether training interventions that modify these characteristics can enhance responses to AFT footwear and examine how other biomechanical variables, such as foot strike patterns or flight times, may interact with RE responses.