Boosting Energy Return Using 3D Printed Midsoles Designed With Compliant Constant Force Mechanisms

The enhancement of midsole compressive energy return is associated with improved running economy. Traditional midsole materials such as ethylene vinyl acetate (EVA), thermoplastic polyurethane (TPU), and polyether block amide (PEBA) foams typically exhibit hardening force–displacement characteristics. On the other hand, a midsole with softening properties, which can be achieved through compliant constant force mechanisms (CFMs), can provide significant benefits in terms of energy storage and return. This study presents the development of such a midsole, incorporating 3D printed TPU CFM designs derived through structural optimization. The mechanical properties under cyclic loading were evaluated and compared with those of commercially available running shoes with state-of-the-art PEBA foam midsoles, specifically the Nike ZoomX Vaporfly Next% 2 (NVP). Our custom midsole demonstrated promising mechanical performance. At similar deformation levels, the new design increased energy storage by 58.1% and energy return by 47.0%, while reducing the peak compressive force by 24.3%. As per our understanding, this is the first study to prove that the inclusion of CFMs in the structural design of 3D printed midsoles can significantly enhance energy return.