There is no question that high-intensity interval training (HIIT) is ideal for fat loss while also preserving precious muscle tissue. There have been numerous studies, articles, blogs, etc., proclaiming the greatness of HIIT. It burns more calories, increases metabolism and EPOC (excess post-exercise oxygen consumption), preserves muscles, releases growth hormone, targets stubborn fat, reduces insulin resistance, increases endurance, all in a fraction of the time of long, slow cardio.

There are countless ways to implement HIIT into your workouts, whether through cardiovascular training, weight training or a combination of the two. The basic take-home point of interval training is to achieve a high heart rate in the zone of 80 to 95% of your estimated max heart rate for a short duration of time (from 10 seconds to three minutes). Follow this with a brief cool-down period, bringing your heart rate back into your cardiovascular zone, usually about 50 to 75% of your max heart rate.

Considerations

Everyone is individual and unique, so specific speeds for the following workouts are not provided. An extreme athlete may be able to sprint at 12 mph to achieve 90% of his or her estimated heart-rate max, while a deconditioned individual may achieve 90% of his or her heart-rate max at 6 mph. Instead, use a rate of perceived exertion (RPE) scale of 1 to 10 to designate exercise intensity, with 1 being zero effort/resting heart rate and 10 being all out max effort, equivalent to about 95% of your estimated max heart rate. As a reference point, moderate cardio is usually done at an RPE of 5 to 6.

Guidelines

1. Always start each program with a five-minute light warm-up.
2. Modify as necessary—these are merely guidelines, so push yourself, but don’t kill yourself. Work smart!
3. Get a heart-rate monitor. It makes tracking progress and getting to within your estimated max heart rate so much easier. To find your estimated max heart rate, subtract your age from 220. For example, a 30 year old has an estimated max heart rate of 190, so 90% of his or her max heart rate is 162 beats per minute.
4. Not all intervals are created equal. A short 10-second sprint is meant to max you out by 10 seconds, and a long 60-second sprint is meant to max you out by 60 seconds. So modify your speeds. You can sprint much faster for a 10-second sprint than a 60-second sprint. Know your limits!

Treadmill Gradual Uphill

-Start with a five-minute walk/jog to warm up.
-Set the treadmill to 5% incline and walk/jog at an intensity level of 8 to 9 for two minutes.
-Set the incline to 2% and recover for one minute.
-Increase the incline to 7% and walk/jog at an intensity level of 8 to 9 for two minutes
-Set incline back to 2% and repeat sequence until you get to 15%.
-If you walk, try not to hold onto the handles. You will get much more leg activation and your heart rate will be about 10 to 15 beats per minute higher by not holding on.

Treadmill Hill Sprints

-Start with a five-minute walk/jog warm-up.
-Stand on the edges of the treadmill and raise it to a 15% incline; next, set the speed to a fast walk or jog, about an intensity level of 9.
-Hop on the treadmill and walk/run for 30 seconds, then hop off and rest for 30 seconds.
-Repeat five rounds and then set the treadmill to flat and walk or jog for five minutes to recover. Repeat program if desired.

Mixed Treadmill Interval Workout

-Start with a five-minute warm-up.
-Set treadmill to 2% (a good “flat” that mimics outdoor running).
-Do a 60-second walk/jog (RPE 5-6) followed by a 60-second jog/run at (RPE 7-8).
-Slow the treadmill down to 2.0 to 3.0 mph and do a 30-second side shuffle to your right, and 30-second side shuffle to your left (RPE 5-6).
Cool down for one minute (RPE 2-3) and repeat three times. If you want, you can increase the incline by 2% each round to increase the intensity.

Other Flat Sprint/Rest Ratios

-Tabatas: 10-second all-out sprint (RPE 10), followed by a 20-second rest (jump off) for eight rounds (four minutes). Walk for five minutes and repeat. 
-8/12: Eight-second sprint (RPE 10), followed by a 12-second rest (jump off) for nine rounds (three minutes). Walk for five minutes and repeat.
-60/60: 60-second sprint (RPE 8), followed by a 60-second slow down (RPE 5) for 10 rounds.  

Read this Fit Fact for more information on high-intensity interval training.