A HIIT workout routine is a high intensity interval training which is a kind of highly intense cardio exercise. Normally the sessions will last about 20 to 30 minutes and that is sufficient enough to get you losing your body fats, your excessive weight and gaining more muscle than you would after session after session of the traditional steady style of workouts. HIIT is a very intense workout and is very short but it is more difficult than working out than doing a 90 minute workout in the gym. That is why, 24 hours after doing your workout, you still keep loosing weight with the increased oxygen intake and the boost to your metabolism. This means you could be asleep or chatting on the net but still losing weight. You should avoid muscle pains and damage by ensuring you get enough rest after the workouts. After all, you are still burning calories.