I've read every article on t-nation, I work out >20 hours a week, and I train for triathlons and marathons.
That being said, your statement is unfounded. If you're LIFTING, and lifting HEAVY, then yes, it doesn't make sense to go for more than 45 minutes, for exactly the reasons you stated.
However, cardio? People will see huge cardio benefits if they run for a long time. There's no "negative fitness effect."
Yes, you may burn muscle, but it's not like you're going to all together stop burning fat. In fact, when you hit glycogen depletion (the 'hitting the wall'), your body will switch energy burning modes entirely, and start burning a TON of fat.
I'm nit-picky about your statement because America, as a whole is in such a crisis. We look for any excuse to get out of exercise. Those idiots at the gym lifting 10 lb weights? Do you really think they're getting a good workout? They're not lifting heavy, and they can go far longer than 45 min doing 7 lb arm curls. What about those on the treadmill, walking for 45 minutes while talking on the cell phone?
As a society, we've become too used to everything being easy- we want an easy job with lots of money, we want to consume everything, and we want to be able to eat what we want, and just have the pounds come off.
Losing weight is hard. It takes discipline and focus. Now, especially in the two weeks of the Olympics, we should be able to collectively get off our fat asses and look at what some Americans have been able to accomplish with a ton of hard work and sacrifice.
Think Michael Phelps works out 45 min a day? All those gymnasts? Tyson Gay? So maybe you should step back, realize there's more to life than T-nation, and look at actual exercise science.
T-Nation is the equivalent of looking at some IT trade magazine talking about Java applets and their Web 2.0 capabilities. Yeah, it can be done. Is it the best tool for the job? No.
Most people that go to the gym are mainly trying to look better so those people should be doing heavy lifting. Even most training calls for time with heavy weights. Thus, the 45 minute or so rule. You point out a major caveat: Most people are not going anywhere near their limits. Around 45 minutes of high weight/volume training, one should naturally notice a loss of power not quit prematurely due to a stopwatch.
A (fat) person once told me she heard some advice that one should do cardio at a pace in which one will not experience heavy breathing and still be able to hold a conversation with the person next to them. What's next? Laying in bed as cardio?
Beginner advice: It's better to go out and just do something rather than overthinking it. Look at the people who are skinny, ripped, massive, fast, or whatever your goal is and do what they do. Many people apply premature optimization to working out. Diet matters but guys in prison become huge on three square meals a day. Sets and reps matter but Arnold wrote something to the effect of, "I don't know why beginners worry so much about details. If you are bad at pull ups, do as many pull ups as you can, rest, repeat." The key is to push yourself and measure your progress to make sure you are actually improving.
You don't "Switch" burning modes when you run out of glycogen stores. Your blood sugar drops because you can't process fat into sugar fast enough. You are always burning fat, and when you are running you are burning it pretty much as fast as you can. Thats why cyclists and long distance runners have to eat while they train longer than 1.5hrs to supplement their fat burning because the glycogen stores are out.
But you are not always burning fat, that's the problem. If the body needs energy like right now and it doesn't have glycogen on hand and cortisol levels are elevated, then it will cannibalize muscle. This is the point I was trying to make earlier. Powerade is strictly for amateurs; serious cyclists (et al) are drinking Perpetuum or Spiz, both of which contain BCAAs and other proteins to offset gluconeogenesis.
