It sure would be nice to go for a run and never worry about your Fitbit dying. Researchers have previously come up with several strategies for harvesting the energy in our footfalls. Now, scientists have invented two lightweight devices that are small enough to actually fit in a shoe sole and still generate enough electricity power for wireless data transmissions while you walk, they report online today in Smart Materials and Structures. Both devices make use of the voltage induced in a stationary coil when a moving magnet’s magnetic field passes through it. The device shown in the above picture, known as the swing harvester, pushes a stack of 14 magnets through a set of coils when your legs shift during a stroll. The other, called the shock harvester, consists of a spring-loaded magnetic circuit that vibrates in the heel of a shoe when it strikes the ground. As of now, the devices can generate enough current during a fast walk to power a temperature sensor in a shoe that wirelessly transmits its readings over 10 meters. With further research, the scientists hope to boost the power output to recharge other wearable electronics.