Summary
High Intensity Interval Training (HIIT) is a cardio method used to aid in burning calories through boosting your metabolism. HIIT consists of alternating between sprinting and walking for 20 minutes, rather than the typical jogging cardio most people are sucked into. HIIT is highly effective because it increases your RMR (Resting Metabolic Rate) for up to 24 times as long as traditional, long aerobic workouts. Your body consumes much more oxygen because it attempts to adapt to the exercise you performed, hence increasing the number of calories you burn for 24-48 hours, whereas traditional aerobic workouts only increase your RMR for 1-2 hours. This boost in calories burned is perfect for lean muscle gain, because it shreds the fat--just make sure you are sticking to your weightlifting routine or you will get a skinny appearance rather than lean.
Here is an example of the HIIT workout that I perform 2-3 times a week on a treadmill:
The speeds at which I workout are based on me being 5'7. If you are taller, or shorter, I would suggest altering your speed according to what feels comfortable for your legs.
Workout Length: 20 minutes
Walk at 3.5 mph for 3 minutes (warm up)
Sprint at 7.5 mph for 2 minutes
[5 minutes]
Walk at 3.5 mph for 3 minutes
[8 minutes]
Sprint at 9.5 mph for 1 minute
Walk at 3.5 mph for 1.5 minutes
[10:30 minutes]
Sprint at 7.5 mph for 1 minute
Walk at 3.5 mph for 1.5 minutes
[13 minutes]
Sprint at 9.5 mph for 1 minute
Walk at 3.5 mph for 1.5 minutes
[15:30 minutes]
Sprint at 7.5 mph for 1 minute
Walk at 3.5 mph for 1.5 minutes
[18 minutes]
Sprint at 9.5 mph for 1 minute
Walk at 3.5 mph for 3 minutes (cool down)
[22 minutes]