Virtually all running shoes have midsoles made from vari ous foam materials

Virtually all running shoes have midsoles made from

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Virtually all running shoes have midsoles made from vari- ous foam materials that, to varying degrees, cushion impact, and store and return mechanical energy. The amount of energy stored by a foam material depends on its compli- ance, whereas the percentage of stored mechanical energy that is returned is called resilience [ 34 ]. Mechanical test- ing of the prototype version of the Nike Vaporffy and ADI shoes revealed that the Vaporffy shoe was two-times more compliant than the Adidas Adios Boost 2 shoe, deforming 11.9 mm versus 5.9 mm, and more resilient than the Adi- das Adios Boost 2 shoe, returning 87.0% of the potential energy compared with 75.9% [ 7 ]. Taken together, the Nike Vaporffy shoes can return more than double the mechan- ical energy per step of the ADI shoes (7.46 J vs. 3.56 J, respectively) [ 7 ]. For perspective, Ker et al. [ 35 ] calculated that the Achilles tendon and arch of the human foot return approximately 35 and 17 J of stored energy, respectively, while running at 4.5 m·s −1 (16.2 km·h −1 ) [ 35 ]. Some energy must also be stored and released in other tendons and liga- ments of the legs. However, unlike shoes, the energy stored in a tendon depends on the forces (muscles) that stretch the tendon, which require metabolic energy (adenosine triphos- phate [ATP]) [ 5 ]. Running shoes with compliant and resil- ient midsoles may actually lower the ATP cost of muscular contraction for the same force production and thus reduce the rate of oxygen demand at any given speed. Generally speaking, male runners in our study elicited greater improve- ment in running economy compared with female runners while wearing the NVF shoes (Table  4 ), which could be related to the compliance and resilience of the shoes. One would presume that the heavier male runners would have elicited greater ground reaction forces compared with the lighter female runners (Table  1 ) at any given running speed, resulting in higher mechanical energy storage in the mid- sole. Assuming the same percentage of stored mechanical
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340 K. R. Barnes, A. E. Kilding energy is returned (resilience), the shoes could return more mechanical energy for the heavier male runners than for the lighter female runners, potentially resulting in a reduction in VO 2 rates. The midsole of the NVF shoe also includes a relatively stiff carbon-ff ber plate inside the thick responsive foam midsole (Fig.  1 ), thereby increasing longitudinal bend- ing stiffness. These shoe properties change the mechani- cal advantage of the foot and leg by facilitating bringing the center of pressure from under the heal or middle por- tion of the foot during the contact period to the toes, lead- ing to a greater moment-arm from the ankle [ 36 ]. Previ- ous research has shown such plates can reduce the rate of oxygen demand (improve running economy) by ~ 1% [ 32 ] and improve sprinting performance [ 37 ]. The relationship between running economy and the amount of longitudinal bending stiffness in the midsole of a shoe has been described as a ‘U-shaped’ curve [ 32 ], suggesting there is an optimal
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