Variable force spring shoes might be reminiscent of DIY mattress Variable force spring, kangaroo jumpers, or running prosthetics. If so, you're on the right track. Let's say you've attached mattress springs to the soles of your shoes and decide to go for a jog. Every time your foot hits the ground, the spring is compressed. The energy going into the compression is stored in the coil, so you don't lose as much energy as you normally would during a crash. Then, as you lift your foot, the spring stretches and this energy gives you a nice upward kick. The extra bounce is fun, but not for running. In running you want to run faster, not higher.
Running prosthetics and kangaroo jumping shoes are more sophisticated variants of mattress spring shoes, but they all have another fundamental limitation. They only use energy when you press the springs against the ground. If you really want to run faster, you should harness energy while your feet are in the air — which is where they spend most of their time when you run, about 80 percent, the researchers said.
