We've certainly seen a lot of running shoes with shock-absorbing cushions in the soles, that are designed to absorb energy on the downstep and then release it on the upstep. The Enko shoe, however, takes that concept a step farther – it incorporates two actual coil-sprung shocks on each shoe, along with a hinged second sole.
In development by French runner Christian Freschi since 2002, the Enko's concept is fairly simple.
When the user steps down onto their heel, the shocks are compressed and the rear section of the sole folds up against the underside of the foot. As they stride forward and transfer their weight up to the ball of their foot, the shocks expand again. Since the ground blocks the rear part of the sole from moving down, all of the energy that's released by the coils instead goes into pushing the foot up and forward. That's the idea, at any rate.
Additionally, the setup also helps absorb potentially injury-causing impact every time the user's feet hit the ground.
A lever on the side of each shoe allows users to tweak the coil tension for use in walking or running. Additionally, users can choose between coils of varying stiffness, depending on their body weight. As an added bonus, the rubber studs on the bottom of the sole are replaceable, so users don't have to throw the shoes away as soon as their gripping surface wears down.
Freschi and his team are currently raising production funds for the Enko, on Indiegogo. A pledge of US$390 will get you a pair, when and if they're ready to go. Potential buyers might also want to check out Adidas' Springblade shoe.