Unless I'm totally misunderstanding what people here are saying, according to almost everything I've read here, you can't increase your muscle mass while trying to loss weight, since the first requires a calorie surplus and the other a calorie deficit. But I'm still confused about what goes on physiologically while losing weight through diet and cardio exercise. If I start doing a lot of cardio (running and rowing, in particular) and, through that exercise and a careful diet, maintain a calorie deficit, I should be able to lose fat while also becoming a better, faster runner. So, I'm wondering---if I'm maintaining, say, a 500 Cal/day deficit and losing about a pound of fat a week and am running, on average, 4 miles a day (which should be about 500 Cal/day) and am improving my athletic performance, what is really going on if I'm not building up muscle anywhere due to my calorie deficit? Am I really not gaining muscle mass anywhere, either in my legs or my myocardium? What, physiologically, is improving in my body if I'm not gaining leg or heart muscle? Is the improvement entirely due to improvements in blood vessels and the way the body delivers and uses energy?

Also, is it true for everyone that you can't gain muscle and lose fat simultaneously, or is it just true for people who are already high-muscle, low-fat? I know during the first spring that I ran track in high school, I had lost weight, but my calf muscles had certainly gotten bigger. And back then, I ate a lot of junk all the time.

By running and working your aerobic machine, several things will happen, all of which are good:

1. Your muscles will learn to use the oxygen they get more efficiently
2. Your heart and lungs will be able to deliver the oxygen more efficiently and quickly
3. Your leg muscles will increase vascularization and increase the number of mitochondria, to a degree, but this is more effective on runs of at least 45 minutes
4. You will become psychologically stronger, i.e., better able to handle the pain mentally
5. I don't think 4 miles / day is enough to increase the size of your heart muscle and create what is referred to as an "athlete's heart". I think that takes more than 4 miles / day.

I doubt your legs will get bigger, per se, but certainly a lot more defined. Running is not an effective way to add leg mass.

I am not convinced of the "you can't add muscle mass while in a caloric deficit" from a scientific standpoint or my own personal observations. But, one caveat that is oft-repeated is that you can get newby gains, i.e, muscle gain concurrent with fat loss. Maybe that is what I have done? I dunno.

That would make sense to me. If someone has a lot of fat and not much muscle, it would seem to me easier for the body to remove fat from the body and gain muscle tissue. When one is near the limits of their body's potential, I imagine that's when tradeoffs start happening, like you can't build muscle mass and lose fat at the same time or like your fast-twitch and slow-twitch muscle development are competing with each other. Is that fair?

Now, as far as running and building muscles, I'm wondering if this general idea of mine is correct---muscle mass increases due to a need to perform the demands that are placed on it. With someone who has a minimal amount of leg muscle, running would cause that person to increase muscle mass in their legs (and, to a lesser extent, elsewhere in the body) because the muscle they had prior to becoming a runner was insufficient for the strength and power demands of running. Once those muscles become sufficiently large for running, further athletic conditioning of the whole body and toning of those specific muscles would occur, but the leg muscles wouldn't be gaining mass because there wouldn't be a need for it. At this point, for the runner to gain muscle mass, he would need to place additional demands on the legs, e.g., weight-lifting. In my specific case, when I first joined track, my leg muscles were almost none existent. After my first spring runnng track, my calves were noticably larger. But after high school, my leg muscles stopped getting bigger. As it was by high school graduation, and as it is now, my leg muscles are capable of delivering the power I need for running, especially if we aren't talking about all-out sprinting. Therefore, long-distance runs won't build muscle because the body knows there's no need for more leg muscle, and I would need to start adding additional resistance, such as leg press, to gain leg muscle mass. Do I have the right idea here?

If all of this is right, then would it be fair to say that my performance in long-distance runs (e.g., the half-marathon I'll run in a few months) wouldn't really improve with further gains in leg muscle mass, but rather would improve mostly due to increased cardio performance? But, on the other hand, increased leg muscle mass would improve my sprinting and jumping capability, right?

I agree with just about everything. But, I would add that running performance will increase with more running not from added muscle mass or just from cardio, as you mentioned, but also from your body producing gazillions more mitochondion organelles (the energy producers) and the increased vascularization to deliver red blood cells and oxygen to the muscle cells.

That would seem to make some sense.

I would think that the primary ' adaptation ' you want from your leg muscles to develop from long-distance runs in to improve the stamina / endurance of your leg muscles. So, the more you progressively overload your leg muscles to [/QUOTE]work longer and longer, the better endurance ' adaptation ' you'll see in your legs. Simply adding more muscle mass in your legs won't translate to an optimum endurance adaptation you're looking for I wouldn't think.

Again, I think you're right.

If your goal is to add mass, you want to create the sort of progressive overload, via resistance training, such that your muscles adapt over time by becoming stronger and bigger in order to work harder at lifting more weight.

I'd agree...I'd train for the demands of the event you want to excel at ...and in this case, for an endurance event like a half-marathon....train for muscle endurance and cardiovascular endurance.....not muscle mass / strength / power.