Function or Fashion? Design of sports shoes and directional energy return: Introducing a new concept of directional energy transfer.

Directional energy return in sports shoes aims at returning the energy put into the sole in the right direction, i.e. upward and forward before toe-off when running. The new concept of directional energy transfer introduced in this paper aims at transferring the energy, put into the sole in vertical direction, from vertical to horizontal, such that more energy can be taken out in horizontal direction than was put in. A linear two degree-of-freedom mechanism is presented which exemplifies this principle. The same principle applies to the Adidas Bounce tubes with a .-shaped cross section. In their original orientation at the shoe sole (with horizontal cross bar of the .) there is no energy transfer. The latter can be maximized by rotating the Bounce tubes (inclined cross bar of the .) provided that their compressive and shear stiffness are different. The correct direction of rotation, clockwise or counter clockwise depends on the ratio of shear stiffness to compressive stiffness, i.e. greater or smaller than one respectively.