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Correspondence to Dr Benno M Nigg, Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Calgary, AB T2L 1N4, Canada; [email protected]
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 (perpendicular to the direction of the plate; figure 1).
If the curvature of the plate is designed correctly, the teeter-totter effect will result in a force during push-off that acts at the right location (heel of the foot), at the right time (during take-off) and with the right frequency (depending on the running velocity and the ground contact time somewhere between 2 and 4 Hz).2 This heel force likely contributes substantially to improve running economy.
To maximally improve performance with the curved carbon fibre plate requires three main characteristics:
The stiffness of the curved plate must be so that the resultant ground reaction force moves far enough anteriorly during the stance phase of running.
The pivot point (around which the teeter-totter effect takes place) should not be located too far anteriorly, allowing the shoe sole to act as a fulcrum.
The curvature of the shoe sole in the forefoot must be substantial3 but also not too extreme to allow for...