HIIT stands for High Intensity Interval Training and it's used to describe a type of training where you alternate short bursts of "all out" (high intensity) activity with longer bouts of "recovery" (low intensity) activity.
You should aim to make your bursts of "all out" activity something that you can keep up for no longer than 20-30 seconds, and bouts of "recovery" between 10-90 seconds. The great thing about HIIT workouts is that they are super-efficient. This means that a short work can pack a heart-pumpin', calorie-blastin' punch.
HIIT has also been shown to have a number of other health benefits including:
Increased strength and endurance
Better energy levels
Weight loss + fat loss
Improved blood pressure, insulin sensitivity, and other metabolic markers.
In fact, HIIT and interval weight training increase your normal metabolic rate. So even when you're not working out, you're burning calories faster than before! Pound for pound, the metabolic rate of lean tissue, specifically muscle tissue, is greater than the metabolic rate of fat tissue. So the more lean muscle tissue you have, the higher your metabolic rate. Studies done by the American Journal of Clinical Nutrition found that lean muscle tissue can account for up to 63% of differences in metabolic rate of people that are the same weight, age, height and gender.
So whether you use HIIT, machines, or free weights, strength training is an important part of the weight-loss equation. It will help reduce the amount of lean muscle tissue you lose, help you develop new muscle tissue, AND increase your metabolism to burn the calories you are eating! #twobirdsonestone