"Just" Cardio

Mcrow is 100% right in here. All professional (or even regular) athletes do lift weights, even runners. Look, I learned this in Biology.

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.

The thing is, cardio can help you gain and maintain muscle if you do it right and in the right amounts.

RUNNING can help you build and maintain muscle.

CARDIO is purely about heart rate.

That is the distinction most people seem to miss on this site.

Ummm.......no.

Seriously, I have to go around and around with you on everyone of these subjects. No offense but if that's what you think, then you don't know as much as you think.

While I will say that those who say cardio/running burns muscle are wrong as well. You burn muscle if you are not eating enough and are in too large of a deficit for too long. Simply running a few times a week for a half and hour to an hour isn't going make you lose muscle if you eat right. However, you are not likely to be building muscle either.

Running is a type of cardio and of course is centered around HR, particularly if you are a competitive runner.

What I am saying is how can there be such a things as pure "cardio"? I can't think of any exercise that raises your heart rate with no muscular force. And muscular force WILL build muscle.

Not true. It takes muscular force to lift a coffee cup. To wink your eyes. To type on a computer. How much muscle are you building there?
To build muscle you need to OVERLOAD it over a duration of time. Running doesn't overload it since your weight doesn't change, with the exception of going down, which would UNLOAD more weight.

I think we should cut our losses with these guys, they will not believe anything we said even when it's backed up with science and is being taught as such at practically every University in the country.

Your body will adapt to the stress being placed on it. Running stresses your legs, but at some point it's the same stress over and over. Your legs will reach some sort of equilibrium.

Even with weights, you need to lift heavier and heavier weights to get bigger.

Even your skin. Spend 15 minutes in the sun every day and you'll reach a level of tan that stays the same. If you want to get darker, you have to spend more time in the sun. (I stole this from Starting Strength btw)

For me, (mostly) cardio exercises are not an substitute in any way for strength training.

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.

From what I've read you have to put a load on the muscle equal to or greater than 60% of the maximum load it can handle to recruit a significant amount of FT muscle fibers to see a significant amount of hypertrophe.

Running (outside of sprints) will not do that.

I would have thought that the average force a runner produces with their legs when they hit the ground would easily reach 60% of max.

The force that the foot hits the ground would be, but that's not what moves you forward, it's the push-off with the back foot, after the front foot hits that moves you forward. When you are already un motion the push off to keep moving isn't that much and that's why acceleration happens.

For instance, I can easily do a 600 pound leg press, if I weight 200 pounds the first step when I start to run might be close to 60% of my max of one leg, maybe. However each step after that the amount of force used and needed is less until you reach max speed or your distance running speed. For sprinters, they are going max effort most of the time they are running so it's a different thing.

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.

RUNNING can help you build and maintain muscle.

If you're a sprinter maybe. But then again their regimens are considered anaerobic.
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reply to wrong post

Lactic acid is an indication if a muscle is being overloaded. The "burn" you feel when you lift weights will cause you to stop. You don't get that burn running unless there's resistance. Like running up a hill or stairs, or in the sand at high speed.

I would say that it is true with the exception of an untrained individual. If you have not run for years and go out and run 3 miles your legs will probably be killing you. In that case I think it's a conditioning thing and not so much about overloading due to weight.

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.

There is no activity that your body does that doesn't involve your muscles so nothing is purely "cardio". You won't grow your muscles from just running, but you will make them stronger. Look at it this way, your heart is a muscle. When you run, you are working your heart and the more you run, the stronger your heart gets to the point where you are able to run the same speed and distance at a lower heart rate. But your heart didn't get bigger. It got stronger. The muscles are already there in your body. Doing cardio will make them stronger, but will not make them bigger, especially at a caloric deficit. Even if you are hardcore strength training you can't build muscle if you don't eat enough. Just because you have been only doing cardio and can now see your muscle doesn't mean the cardio built or grew the muscle. It strengthened the muscle and burned the layer of fat that was covering the muscle, but the muscle was already there.