Author(s): PAGE, D., AUDET, A., LOCKE, C., MELVIN, A., LEPLEY, A., KOZLOFF, K., Institution: UNIVERSITY OF MICHIGAN , Country: UNITED STATES, Abstract-ID: 1258

INTRODUCTION:
Advances in running shoe technology focus on combining embedded carbon fiber plates with novel midsole foams to improve running performance. Variations in midsole stiffness and energy return (ER) across brands have been shown to influence running economy (RE) [1,2]. Differences in shoe last, foam composition, plate stiffness, and other specific design features make it difficult to attribute the importance and interaction between midsole stiffness and ER. The purpose of this study was to assess changes in RE in response to different midsole mechanical properties within a singular brand’s line.

METHODS:
This study examined three shoes of a singular brand’s line (M1, M2, M3) (Mass = 218, 210, 210g respectively [3.7% difference]), heel stack (42, 36, 33mm respectively [27.3% difference]) featuring nitrogen infused EVA foam midsoles. Only M1 contained a carbon fiber plate. Stack height was measured at the heel with sock liner intact under a load of 5N. The shoes were uniaxially loaded over 100 cycles (687N; 70kg body mass [BM] equivalent). Force was applied separately at the heel and midfoot regions, and stiffness and ER were obtained. 8 trained male runners (Age: 22.3 +/- 2.3yr, BM: 63.9 +/- 2.0kg, World Athletics Score: 904.9 +/- 159.0 pts) of the same shoe size (US9) completed a 10-min warm-up (12.39kph) followed by two, 3-min trials in each shoe (14.86kph). Shoes were tested in a randomized and mirrored order. Blood lactate was measured at rest, and after trials 1, 4, and 6 to ensure participants were below aerobic threshold (defined as an increase in blood lactate >1.5mmol/L above resting). RE was calculated using a 5 breath moving average of VO2 from the last minute of each trial and averaged between both trials run in the same shoe per subject. A repeated-measures ANOVA was used to assess differences between heel and midfoot stiffness and RE.

RESULTS:
Shoes differed in stiffness at the heel (M1=63.3; M2=64.3; M3=79.1 kN/m [25% max difference]) and midfoot (M1=54.4; M2=81.4; M3=72.0 kN/m [49.6% max difference]). ER was similar across all three models at the heel (M1=73.1; M2=72.8; M3=72.6% [.7% max difference]) and midfoot (M1=74.4; M2=72.2; M3=73.0% [3% max difference]). RE was not significantly different (p=0.057) between shoes, which differed up to 1.61% (M1=186.3 +/-13.1ml/kg/km, M2=188.3 +/- 10.9ml/kg/km, M3=189.3+/-11.9ml/kg/km).

CONCLUSION:
In the present sample of trained men of similar body mass, RE did not significantly differ in shoes whose midsole foams differed in stiffness but not ER. A more diverse subject pool and greater variations in ER between shoes may be needed to identify the magnitude of ER necessary for significant improvements in RE. Maximizing ER may be more impactful than altering midsole stiffness when determining foam properties that maximize distance running performance.

REFERENCES:
1. Burns et al, 2023 ECSS.
2. Joubert et al, 2023 Int J Sport Phys Perf.