When runners impact the ground, they experience an external ground reaction force (GRF) up to 4x their bodyweight. Increased magnitude of the GRF is associated with lower limb injuries in runners. Midsoles cushion impact between the runner and the ground, reducing GRF. It is proposed that midsoles should be tunable with subject mass to minimize GRF and reduce risk of injury. Auxetics, structures possessing a negative Poisson’s ratio, demonstrate superior impact properties and tunability. Complex auxetic geometry demands additive manufacturing to fabricate. This work develops and simulates tunable auxetic midsoles against conventional midsoles. A thermoplastic elastomer (Hytrel 3D4100FL NC010) was selected for fused filament fabrication, and its process parameter-mechanical property relationship modelled to make predictions within 10% error. A 3D auxetic impact geometry was characterized for midsole application. Finally, the GRF of simulated auxetic midsoles were compared to conventional midsoles, finding tuned auxetic midsoles with subject mass may reduce GRF.