The effect of shoe mid-sole construction on running performance was discussed with reference to the Nike Vaporfly 4%.1 Drs Burns and Tam described the mid-sole thickness as the major running shoe characteristic that contributes to changes in performance. Here, we highlight the role of the curved carbon fibre plate embedded in the mid-sole, and introduce a new effect on running mechanics, the `teeter-totter effect`. During ground contact in running, the point of application of the ground reaction force moves anteriorly during the second half of ground contact towards the front end of the curved carbon fibre plate. We suggest that, in this position, the ground reaction force produces a `reaction` force at the heel in upward direction.
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