HIIT (high-intensity interval training) involves bouts of maximum or near-maximum effort with short periods of rest in between. The goal of HIIT is to alternate training speeds, never allowing the body to adjust to a given speed of movement.

Many studies have shown that HIIT is an effective training strategy, especially when it comes to improving metabolic function, VO2 max, and generally getting an effective cardio workout in a shorter amount of time – no more than 20 minutes or so.

Clearly, HIIT is effective, but that's not really the question. The question is whether HIIT is more effective than steady-state endurance training.

Continuous cardio training typically involves running (or using a thousand other pieces of cardio equipment) at a relatively constant pace for 30-60 minutes. Recent research suggests that a similar number of calories are burned during HIIT and continuous endurance training, so there doesn't seem to be an advantage either way in terms of energy expenditure.

In terms of fat-burning potential, a 2013 study noted only a negligible difference between the two training styles, so once again, there is no clear advantage either way.

Proponents of HIIT have argued that it requires more energy, creates a larger oxygen deficit (called EPOC), and therefore promotes increased metabolism of fat for a longer period following exercise when compared to continuous endurance training. But research has questioned this notion.

Several studies have shown that HIIT results in only slightly more EPOC than continuous cardio, and there's really no advantage either way in terms of EPOC or fat metabolism following training. The fact is that either training style will have positive results in these areas, and it doesn't much matter which style you choose. (Of course, HIIT may be a better option if you're short on time.)

Certain machines make more sense for certain people. It's mostly an individual choice. Sure, running is probably a more effective form of cardio than sitting on a stationary bicycle, but for the person who's choosing between pedaling on a stationary bike and sitting on the couch, the decision is obvious. (And larger strength athletes may prefer a bike to running, given their extra mass, for knee health.)

There isn't a ton of research in this area, but if you're looking for a good general rule:

treadmill > stair master > rowing machine > elliptical > stationary bicycle

If that seems a little oversimplified, it's because the emphasis should be on choosing movement over not moving.

Don't let your choice of cardio activity cause a lot of consternation. Get moving, raise your heart rate, and don't worry too much about how you're doing it.

1. Børsheim E et al. Effect of exercise intensity, duration and mod on post-exercise oxygen consumption. Sports Med. 2003;33(14):1037-60. PubMed.
2. Hazell TJ et al. Two minutes of sprint-interval exercise elicits 24-hr oxygen consumption similar to that of 30 min of continuous endurance exercise. Int J Sport Nutr Exerc Metab. 2012 Aug;22(4):276-83. PubMed.
3. Milanović Z et al. Effectiveness of high-intensity interval training (HIT) and continuous endurance training for VO2 max improvements: a systematic review and meta-analysis of controlled trials. Sports Med. 2015 Oct;45(10):1469-81. PubMed.
4. Skelly LE et al. High-intensity interval exercise induces 24-h energy expenditure similar to traditional endurance exercise despite reduced time commitment. Appl Physiol Nutr Metab. 2014 Jul;39(7):845-8. PubMed.
5. Williams CB et al. Changes in mechanisms proposed to mediate fat loss following an acute bout of high-intensity interval and endurance exercise. Appl Physiol Nutr Metab. 2013 Dec;38(12):1236-44. PubMed.