US Patent 9668540 Footwear having a flex-spring sole

Footwear and soles for footwear are disclosed. The sole may include an energy transfer base. The energy transfer base may include top and bottom surfaces, and opposed first and second side surfaces. The energy transfer base may include at least one cavity. The at least one cavity may define upper and lower portions of the energy transfer base. The upper portion may be configured to move away from a nominal position and toward the lower portion when a force applied to the upper portion toward the lower portion has a magnitude that is above a predetermined value, and to move back to the nominal position when at least one of (1) the force is no longer applied or (2) the magnitude of the applied force is at or below the predetermined value.