Garmin Foot Pod SDM4 is small enough to attach to your shoelaces or fit in the mid-sole pocket of compatible shoes. It's always ready to use, and a small, replaceable watch battery powers the foot pod for a year of training. Unlike simple pedometers, this foot pod uses advanced MEMS inertial-sensor technology to analyze your movements and is responsive to stride-length changes to achieve 98% accuracy for speed and distance. Additionally, this sensor measures running cadence to help you optimize your training.