HIIT workouts generally combine short bursts of intense exercise with periods of rest or lower-intensity exercise. At fitness studios and online, these workouts often mix aerobic and resistance training. To be clear, most of the interval workouts researchers have studied focus solely on aerobic exercise. Which means the scientific understanding of interval training is based on a more specific routine than what's appearing in most gyms, videos, and magazines. And the researchers' definition matters because when we're talking about the evidence of benefits, we need to be specific about the kinds of workouts that science was based on.