7 who found contact times to be slightly longer and stride rate slower in a

7 who found contact times to be slightly longer and

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[ 7 ], who found contact times to be slightly longer and stride rate slower in a prototype version of the Nike Vaporffy shoe compared with the ADI shoe. Furthermore, Gonzalez et al. [ 13 ] that found no difference in contact times and longer stride lengths. Some of these discrepancies might again be related to differences in treadmill compliance and slip- periness discussed previously. Furthermore, in the current study, biomechanical parameters were based on high-speed video recordings versus force treadmill data in others [ 7 , 12 , 13 ]. Neither of these studies measured changes in the NVF shoe compared with a racing spike shoe. However, we agree with previous studies that the trivial to small bio- mechanical changes are not enough to explain the small to large reductions in rates of VO 2 [ 7 , 11 13 ]. Indeed, similar to other studies [ 7 , 12 , 13 ], changes in biomechanical char- acteristics (i.e. contact time, stride rate, and stride length) only explained a small amount of the variance in the changes in rates of VO 2 . Although the biomechanical characteris- tics measured in this study and previous studies showed little explanation for the changes in running economy [ 7 , 11 13 ], other kinematic and kinetic factors such as angular velocities of limb segments and joints, and ground reaction forces, have also demonstrated a relationship with running economy [ 5 ]. Ferris et al. [ 18 ] suggested that runners adjust the stiffness of their stance leg when running on compliant surfaces by reducing knee ffexion during the stance phase. This adjustment allows runners to maintain similar center of mass movement (e.g. contact time and stride rate) regardless of surface stiffness [ 18 ], while also improving the mechani- cal advantage of the muscles acting around the joints, which reduces the energetic cost of body-weight support [ 28 ]. We did not measure joint kinematics in the present study, how- ever Gonzalez et al. [ 13 ] observed no change in knee ffex- ion and an increase in center of mass vertical oscillation in the NVF shoe that is consistent with the greater stride lengths observed among the subjects in our study wearing the NVF shoe. Furthermore, in previous studies, vertical ground reaction forces measured while wearing the NVF shoe have been inconsistent with the higher, lower, and no
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341 Running Economy in Flats vs. Spikes differences observed compared with the ADI shoe [ 7 , 12 , 13 ]. This suggests a variety of individual characteristics, such as biomechanics, physiology, and psychology (i.e. com- fort), may all be contributing factors to the improvements in running economy while wearing the NVF shoes [ 32 , 37 ]. Although we did not directly set out to evaluate the effects the energetic saving of different racing shoes would have on running performance, nine of the participants (four male, ff ve female) in this study had competed in 3 or 5 km races in their own racing shoes (spikes for all nine aforementioned participants) before participating in this study, and then again over the same respective distance in NVF shoes. All
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