Combo cardio is an extremely flexible training technique. You can do it almost anywhere and with any type of cardio exercise. It’s also very easy to design an effective workout based on your present level of fitness and how quickly you progress. The formula for combo cardio is a session of high-intensity intervals followed by a brief recovery period and then a session of low-intensity steady-state cardio.
Most of the key benefits of combo cardio have to do with burning stored fat, but it also has muscle-building benefits. First, the high-intensity intervals not only burn calories while you’re doing them, but they also set you up with long-range fat-burning abilities by stimulating a huge EPOC effect. Because high-intensity intervals are hugely anaerobic, your body consumes oxygen—and, therefore, energy—at a much higher rate for many hours after your workout. The research differs and is still ongoing, but the general consensus is that the EPOC effect continues for at least 48 hours and possibly as long as 72 hours after your workout. That means your metabolism works at a much higher rate for as long as three days after your workout.
The high-intensity interval cardio also creates a huge lipolytic effect in the body, as it creates enough oxidative stress to stimulate the release of stored fat from your fat cells via glucocorticoids like cortisol. In essence, the fat cells get the message to release stored fat to be turned back into glycogen and burned as fuel.
That’s where the steady-state cardio comes in. Low-intensity steady-state cardio boosts the body’s fatty acid oxidation so that it can take that fat and burn it as fuel. Because you’ve stimulated the process with your high-intensity intervals, your body is primed to let go of the fat and use it to power the steady-state segment.
That wouldn’t be true if you just did a longer session of steady-state cardio. With longer steady-state exercise you ordinarily burn carbs for the majority (if not the entirety) of your workout, and you also don’t generate the EPOC effect.