If your goal is to lose fat quickly, it's necessary to work exceptionally hard at high intensity interval training and circuit training. The problem is knowing how hard should you push. With interval training and other methods, finding a workout intensity that is right for you is very tricky.
Below, Are the Methods I Suggest if You Are Trying to Maximize Fat Loss or Increase Your Muscle Definition
Usually, I try to divide the strength training part of my workout from the fat loss part. If you're going for muscle definition, I suggest heavy weights, combined with brief, low volume strength training. This will create firm defined muscles. Strength training is usually the easy part. The intensity comes into play when you are doing the fat loss, calorie burning portion of your workout. You should do a somewhat intense fat loss workout at least 3 times per week. This can be HIIT (High Intensity Interval Training), body weight training circuits, or any other strategic interval or circuit workout that makes sense.
A Low Intensity Workout Doesn't Create an "After Effect"
Most often, when I am at the gym, I see people doing a fat loss workout with no intensity in it. If you aren't resting or cooling down for the next part of your workout, walking on a treadmill is useless. When you do a cardio workout that is low intensity, you are not maximizing the amount of calories you could be burning.
High Intensity Workouts : Too High
If your workout is too hard and intense, you are very likely to "overcompensate" by resting more than normal after the workout. For the most effective fat loss workout, you should be able to go on without extra breaks as if you never worked out at all. Resting on the couch all day after a hard workout is a bad fat loss strategy - even if you're tired.
Is Your Fat Loss Workout the Right Intensity?
I look for a thing called the "HGH Flush". This is a good way to judge the effectiveness of your fat loss workout. The HGH Flush is when your skin becomes noticeably hot and slightly red, and when you are running short of breath post-workout. Remember your PE teacher in Junior High making you "run lines" or pushing you until you were out of breath and your skin felt like it was on fire? This is the HGH flush which is an indicator that your metabolism will be increased after your workout and that your body will release a bit more HGH than normal (your body's fat burning hormone).
Operate on the "Edge" of Your Comfort Zone
A common mistake I've witnessed is clients being pushed too hard, too soon by their personal trainers. I once hired a personal trainer who wanted to "destroy me" with each workout. That wasn't for me. This is what I call hitting the "Sweet Spot" of workout intensity. As your body gets in shape, you can push it a little harder every time. However, pay close attention and back off a bit if you find yourself overly tired after a workout.