"Just" Cardio

In the first 20 minutes of exercise, your body uses ATP as it's first energy source. ATP only lasts about 20 minutes of use before the source is depleted. Afterwards, the body uses the fats and carbohydrates that were stored in the body from previous meals. The thing is, after your fat and carb sources are depleted, your body begins to burn fat (stored in the body -- the fat that makes us look -- well, fat), but for runners, they usually don't have this stored fat, so they have to use muscle stored in the body, which is why THIS kind of intense cardio burns muscle.

If you're a new runner (I'm currently 138lbs and been running since May this year) how would this logic apply to me? As in, I can definitely feel a layer of fat still on my legs, arms, back and stomach. Running seems to be slowly reducing this but it's still there. Am I right in assuming that at the stage of running and body composition I'm at currently, that I will continue to burn the fat off rather than muscle when doing runs of 20 minutes and over?


There was also a comment (page 3 or somewhere!) about the lactic acid build up in the muscles which means the muscle is overloaded and that the "burn" will make you stop lifting, and how you don't get this running. I get this when I run and when I cycle (been cycling for over 2 years and I still get it going up hills!!!) so does this mean that my muscles are being overloaded and if so what does this mean in terms of what my muscles are doing?

Very interesting thread BTW I've enjoyed reading it!

Of course cardio builds muscle, because as the OP pointed out you have to use muscle to do cardio. I think the disconnect is that just doing cardio will at some point stop building muscle and just maintain that muscle. Once you have enough muscle to efficiently run 3 miles your legs will not keep building bigger and bigger muscles (thank God!) if that's all you do because it's not needed. But those muscles you originally built will remain as long as you continue to run 3 miles regularly. And you'll continue to burn calories, which I why I love cardio.

You didn't build them is the point..................they were already there. You have back muscles. You have shoulder muscles. You have butt muscles. Reducing fat exposes them, and the misconception is that muscle was "built". If you weighed 115lbs and skinny, then later on weighed 150lbs and had much bigger muscles, you built muscles. Going the opposite direction doesn't build muscle.
You don't build muscle on CALORIE DEFICITS. This is scientifically proven.

Built them up. Made them bigger/stonger/toned. Semantics. The muscles are bigger and stronger after beginning cardio than they were when sitting on the couch all day.

FOR THE LOVE OF GOD! NO THEY ARE NOT. Unless you've been doing some sort of lifting.

I bet if you would have measured your legs at the thigh and calf before and did it again now they'd be smaller.

Of course they are smaller because I lost fat.

Yes, of course but your're probably not building muscle.

Why do you think you're right when this is something agreed upon by almost every expert in the sports nutrition and fitness world?

Show me a runner who does not lift weights and has bigger than normal legs and then we can debate.

Why do you say "probably not building muscle" then isist there is no possibilty that I built muscle? I know my legs.

If you are trying to burn any amount of fat, you need to do cardio quite a bit.

Please, don't tell MY body this.

I don't do cardio...period. I've lost 37lbs of FAT in what amounts to less than 3mos.

This debate is so old. The research is out there. The results of both strength training and cardio, are hormonal, and opposite. To very much simplify, one tells your body to lighten the load in order to make your work easier...via burning off muscle, the other, to increase your strength for the same reason.

/sigh...and to say you have to do cardio to burn fat Is just misleading and completely inaccurate.

You admitted your legs are smaller, that proves you didn't build muscle. The muscle was already there, the fat burned off and revealed it. It's really not that complex. If I maintain my current muscle mass, an just lower my body fat to 4 or 5% I'll look completely shredded, but I will not have built up any muscle.

Interesting. So, if someone just does weights and their legs get smaller, then they've not gained any muscle either??

Nope. They haven't. If you gain muscle, the muscle gets bigger, not smaller. Muscles can get stronger by being trained, which means they work more efficiently, which has nothing to do with the size. Building muscle mass and actually gaining pounds of muscle is very difficult and takes eating well over maintenance calorie levels to achieve. If you are eating at a calorie deficit to lose weight, even if you're doing nothing but weights, you aren't gaining muscle. Your current muscle might be getting stronger, but it isn't growing. Your body still burns fat during weight training, just less efficiently.

I cannot believe this debate is still going on 5 pages later. Or that the original premise was even proposed in the 1st place.

I have been both a serious weight lifter and a serious runner at various times in my life.

Would anybody like to guess at which point I had lots of muscles, and at which point in my life they quickly faded away.

If you're a new runner (I'm currently 138lbs and been running since May this year) how would this logic apply to me? As in, I can definitely feel a layer of fat still on my legs, arms, back and stomach. Running seems to be slowly reducing this but it's still there. Am I right in assuming that at the stage of running and body composition I'm at currently, that I will continue to burn the fat off rather than muscle when doing runs of 20 minutes and over?

Get a body fat measurement and find out what you lean mass is. In a week do it again and see if there's a significant loss in either fat or lean mass. I had a client who had the same issue. Found out she wasn't losing as much fat as lean mass, so I switched her program up (she was running too much). Last month she was "hard" and "lean" enough she ended up doing a Fitness show.

There was also a comment (page 3 or somewhere!) about the lactic acid build up in the muscles which means the muscle is overloaded and that the "burn" will make you stop lifting, and how you don't get this running. I get this when I run and when I cycle (been cycling for over 2 years and I still get it going up hills!!!) so does this mean that my muscles are being overloaded and if so what does this mean in terms of what my muscles are doing?

Very interesting thread BTW I've enjoyed reading it!

Yes going uphill you get this because there's a definite shift in resistance. You can get a burn, but if you're able to continue without stopping then lactic acid build up didn't peak. That means that muscular contraction is still going on (which is an indication of muscular endurance, which the slow twitch fibers are responsible for).

Thank you very much for your reply, very interesting - especially the top bit about running too much. Will need to give that some thought as currently I am running 3 miles plus 4 days a week and cycling every day (only 2 miles there/back to work, so not as much as it sounds). Cheers.

I like your comment. I do primarily cardio. Treadmill to be specific. I'm just getting back into the gym after a break. Sadly I'm one of those on again off again gym people. I joined here hoping it might keep me motivated so I'd keep going long enough to see results this time. I try to do 30 minutes 3-4 times a week. Then I try to do the gyms circuit area 1-2 times a week as well for some weight bearing activities. Hoping the cardio will help get me in some over all better shape. I'm not looking to build much muscle just to loss some of the fat.

So, not-so-hypothetical situation -- person spends 30 years sitting most of the time and walking only as needed for daily activity. Then, same person decides to start running 3 miles per day. Are you seriously suggesting that this running will not cause that person to gain muscle??

If that is the case, there MIGHT be a gain, but it would be insignificant. And if continued running is the preference to lose weight and you're on calories deficit, any "gain" would be deleted since every weight loss program will reduce some lean muscle. Again the misconception is that when people can actually see the muscle under less fat, that they "gained" it somehow.

In that situation, you didn't build up one leg, the other leg atrophied, then went back to normal. If you lose muscle mass due to an injury, then yes, it takes less effort to overload, but you can't "build muscle" without increasing resistance.