Short bursts of intense cardio elevate heart rate and oxygen consumption close to their upper limit, usually above 85 or 90 percent of maximum rate. Athletes have long known that repeated surges of this intensity, termed high-intensity interval training, are an effective way to improve performance. HIIT usually involves short bouts, anywhere from ten to sixty seconds, of maximum effort that leaves one breathless (but not dangerously so) interspersed with periods of rest.
