88 131 o 2 cot mlkg 1 km 1 90 88 87 1922

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58.88 ± 1.31 O 2 COT (ml·kg −1 ·km −1 ) 196.7 ± 8.0 196.2 ± 9.0 201.0 ± 8.2 200.0 ± 8.8 194.6 ± 8.1 195.8 ± 8.7 192.2 ± 8.4 192.2 ± 8.5 201.6 ± 5.5 204.5 ± 3.7 199.0 ± 4.3 196.3 ± 4.4 Energetic cost (W·kg −1 ) 18.39 ± 0.87 18.60 ± 0.85 18.81 ± 0.91 18.88 ± 0.86 18.15 ± 0.91 18.43 ± 0.80 17.99 ± 0.88 18.24 ± 0.82 21.49 ± 1.36 21.74 ± 1.19 21.14 ± 1.21 20.86 ± 1.30
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337 Running Economy in Flats vs. Spikes There was a moderate correlation ( r = 0.45, 90% CL − 0.07 to 0.77) between the difference in rates of VO 2 and differences in contact time between NVF and ADI shoes at 15 km·h −1 . All other correlations between changes in VO 2 and changes in biomechanical variables were either trivial or small, but unclear. Using the multilinear regression model for the change scores in aerobic energy expenditure, and change scores in biomechanical variables, we determined, together, the percent changes in contact time, stride rate, and stride length explained 0.95% (± 90% CL, ± 0.83%) of the average 4.20% reduction in energetic cost between NVF and ADI shoes, while those same biomechanical variables only explained 0.17% (± 0.53%) of the average 2.60% reduction between the NVF and NZM shoes. 4 Discussion The aim of this study was to determine the magnitude of difference in the energetic cost of running between the NVF shoes and that of the ADI marathon racing shoes and NZM track spikes. The main ff ndings indicate that NVF improved running economy by 4.2% compared with ADI, and 2.6% compared with NZM. While our results are comparable to those reported by Hoogkamer et al. [ 7 ] and Low et al. [ 11 ] in regard to the differences between the NVF and ADI shoes, no study has examined the difference between the NVF and a track spike. Hoogkamer et al. [ 7 ] found an ~ 4% reduction in energetic cost when running at 14, 16, at 18 km·h −1 in a prototype version of the Nike Vaporffy shoe compared with the ADI shoe. In their study, Hoogkamer added 51 g of lead weights to the Vaporffy shoe to equalize to the greater mass of the ADI shoe. They suggested that unweighted Vaporffy shoes would likely save an average of ~ 4.4% versus the ADI shoes [ 7 ], assuming a conservative 0.8% savings per 100 g of shoe mass [ 8 , 27 ]. Using a similar methodology, Low et al. [ 11 ] found metabolic cost to be 2.8% lower in the same Nike Vaporffy shoe (unweighted) we used compared with ADI at 16 km·h −1 . Here, when matched by shoe weight, rates of VO 2 in the NVF + shoe were 2.9% less than ADI across all testing speeds. However, the Hoogkamer et al. [ 7 ] and Low et al. [ 11 ] studies were performed on force-measuring treadmills, which have rigid platforms compared with the Woodway ELG treadmill we used. Previous research has shown that treadmill platform compliance—the amount of compression that occurs when loaded with a certain force— itself affects running economy [ 28 30 ]. A less compliant (stiffer) treadmill platform elicits higher oxygen consump- tion (impaired running economy) [ 28 , 29 ]. The difference between our study and the other studies [ 7 , 11 ] might be Fig. 2 Rates of VO 2 at 14, 15, 16, and 18 km·h −1 in each of the four shoe conditions: Nike Zoom Matumbo 3 (NZM), Adidas Adios BOOST 3 (ADI), Nike Zoom Vaporffy 4% (NVF), and NVF plus weight to match the mass of ADI shoe condition (NVF +). Over the
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