(I'm going to cite the numbers from the classic University of Maryland study, because the volunteers for the study were average guys, not athletes, and they did not change their diets to restrict calories) this Means that if the average guy does a simple 3 month strength training program, gains 3 lbs of muscle and loses 4 lbs of fat (using only 3 sets of 12 each, of the 4 basic Olympic lifts, 3 times a week- approximately 30 minutes of exercise) he'll increase his resting metabolic rate by about 7%. Meaning just by sitting around all day he'll will burn 90-150 extra calories directly from his fat reserves. That's not even counting calories burned during exercise.
Meaning that by doing absolutely nothing more than just having 3 extra lbs of lean muscle, every 23-38 days or so, you will lose 1 lb of body fat (not water weight, not muscle, not your stomach & bowel contents, but pure fat). That's a whole pound of solid fat gone every month by doing NOTHING but maintaining the status quo.
Imagine at what rate that 3 lbs of extra muscle is burning fuel when you're not just sitting around. Now imagine if you took strength training more seriously and put on 5 or 10 extra lbs of lean muscle, and actually exercised instead of just sitting around all day. Your basal metabolic rate would skyrocket. If muscle at rest burns fat calories like a furnace, then muscle in motion burns energy like an atom bomb by comparison.
You may ask, why build muscle? Why not just go jogging or do other cardio/aerobic exercise for an hour to burn that extra 90-150 calories a day? I'll tell you: Aerobic exercise burns calories from the glycogen (sugar) supply in your blood. If you burn that sugar up, you need to replace it with more sugar. Blood sugar is the stuff that keeps you alive and kicking. That's what keeps your brain switched on. Meaning if you restrict calories and limit your exercise to a cardio/aerobic workout, your body will start screaming at you to fill it up with whatever sugar you can get your hands on because all you're doing is burning up your blood sugar.
If you want fat loss, you need to burn fat. If you want muscle definition, you need to burn fat. In short, if you want to see your abs, you need to burn fat. And guess which part of your body is responsible for burning stored body fat? Surprise, surprise- it's not your hair follicles. It's your old friends the muscles.
No I'm not suggesting that a women wanting to look fit start taking massive amounts of steroids, deadlifting hundreds of Kgs, eating 8000 kcal a day, and turn herself into the Incredible Hulk. Because that's what it would require for a woman to "bulk up" like that.
Strength training doesn't do that to women. It can't. They simply don't have enough testosterone in their bodies to grow bulky muscles. Developing lean muscle doesn't mean packing on bulk and looking like a linebacker. It just means replacing body fat with meat. It means that if you were to see a cross section of your leg and it looked liked fatty bacon, that strength training would make that leg look like lean steak.
I'm not talking about weight loss at all. The people in the study I referenced stayed the same weight, but they changed the content of their bodies- they cut out the fat and replaced it with muscle.
Most of my clients who tell me they want to lose weight don't actually need to lose weight. Most of them have a perfectly normal BMI. They need to lose body fat and grow muscle to replace it. Same weight, different body. But everybody is so fixated on the idea that they need to starve themselves, run on a treadmill like a hampster, do a few crunches and shrink to unrealistic proportions- and that somehow, that will transform them into a fitness model. No no no no no and no again, it won't.
Look at this stock picture of a magnificent beast of the jungle:
Now, take a look at this woman, Japanese professional MMA fighter Satoko Shinashi:
Why I am I pointing this out? because that woman is a professional athlete competing in one of the most physically demanding sports on the planet in terms of intensity, strength, endurance, and overall body conditioning- and she looks like a 105 lb slender little girl instead of a hulking monster like you might expect from someone who does as much strength training and eats as much as she does.
On the other hand, look at this guy:
Sorry, long tangent. I'm not trying to discourage anyone from doing abdominal exercises, cardio training, or aerobics classes. I'm just trying to emphasis the role of lean muscle in fat loss, and how fat loss is directly related to abdominal definition. You could have the weakest mid-section in the world, never do a single sit-up in your life, and you'd still have "abs" if you didn't have any fat on your belly. That's all a six pack means.