Next up is the glycolytic system, or medium energy system, which is pretty complicated. All you need to know is that this system powers your moderate-to-high intensity, short-duration activities, such as sprinting, lifting weights, playing a sport, or anything else that would feel like a workout. The medium energy system kicks in after about 10 seconds, when the fast energy system runs out, and works for slightly longer—although it still runs out fairly quickly.