[Ed. note: Last Saturday night, at UFC 156, behemoth Dutch kickboxer Alistair Overeem got knocked out standing up by Antonio "Bigfoot" Silva after battering Silva for the the better part of two rounds. By the beginning of the third round, it was clear Overeem, who has been denying accusations of steroid abuse for years, was completely exhausted, that his muscular body was betraying him, that he was incapable of fighting off anyone, much less a 260-pound mauler like Silva. We talked to Dr. Michael Kelly, a sports-medicine specialist, part-time ringside doctor, and the author of the book Fight Medicine, to explain how someone who looks like the comic-book portrait of health and vitality could collapse so suddenly and with so little resistance.]

Dr. Michael Kelly: The initial explosive energy with muscle – the anaerobic burst of energy -- is usually fueled by glucose. or glucose in the muscle called glycogen. But it’s a very short-lived source of energy. There are slow-twitch and fast-twitch muscle fibers. People tend to spend a lot of time building the size of the muscle; often what they’ll do is build up the fast-twitch fibers and sacrifice the slow-twitch fibers, or smooth muscle versus striated muscle. What happens is the striated muscle can use glucose very effectively for a short period of time but then it tends to burn out real quick, whereas the smooth muscle, the slow-twitch fibers, they tend to use other sources of fuel very effectively, what we call oxidative phosphorylation, or aerobic metabolism.

If you look at athletes where their body looks more like a bodybuilder’s, like Overeem's, it’s because they built up these fast-twitch muscle fibers at the expense of the slow-twitch. In the muscle itself, the proportion of muscle fibers that can use that slower-burning energy is much smaller.

In effect, the explosive muscle fibers don’t have a lot of endurance. They tend to burn out quick because the percentage of energy used is predominately glucose over fat in your blood stream. Glucose goes away faster because it’s a very inefficient way of utilizing energy. Your muscles only have so much glycogen stored in them, and it takes time to replenish that. So it’s a combination of limited stores and very inefficient use, which results in the glucose burning very quickly and burning out faster.

And a bi-product of that inefficient metabolism, of an anaerobic process without oxygen, is lactic acid in the muscle. That burn in your muscle when you exercise is lactic acid building up. It’s a bi-product of burning the glucose. It’s like if you have an engine and it’s running very inefficiently, it tends to blow out a lot of smoke and pollutants out of the exhaust pipe. Eventually the liver can detoxify the lactic acid and put that back into the circulation, but that takes time. When you see these athletes who spend more time lifting weights and building muscle than building endurance and skills, they tend to burn out faster.

The cause of the actual sensation of burning in the muscle has been debated. The pH does change slightly, so it does become a slightly acidic environment, but there’s also the phenomenon where the nerve cells in the muscle become very irritated so they’re more sensitive to stimulus caused by those chemicals. It’s a combination of the two.

As for the issue of steroids and the possible role they played in Overeem’s performance: Athletes who use steroids tend to get more focused in their training on muscle strength and size as opposed to endurance. The other thing is, your body only has limited amounts of energy, of fuel, and if you increase the size of the engine you’re not going to last long. You have so much glucose circulating in your body to build up more and more fast-twitch muscle fibers, now you have a bigger engine using the same resources. It’ll burn them up quicker. You can almost see it when fighters get in the ring. There are some guys whose muscular development is so pronounced, you can almost guess they’re going to gas out immediately.

Think of it like this: If you have a tank full of gas and you have 250-cc engine, you’re probably going to go longer on that tank of gas than if you have a 1000-cc engine. It’s always a ratio: The body doesn’t ever only burn just fat or just glucose. In round one, it may be where the athlete is burning 70% glucose, 30% fat, and by the end of round three it may be the inverse -- maybe 30% glucose and 70% fat.

And if Overeem was in fact coming down off steroids after all those years, that would without question affect his energy levels. The effect of steroids on the endocrine system is very pronounced. When you’re using exogenous steroids a lot of times you’ll deplete your body’s own feedback mechanism, so when you come off the steroids, not only do you feel horrible because you have less growth hormone or testosterone in your system, the amount of cortisol in your system plummets. When you drop the cortisol you’re completely exhausted. Everything hurts, you have no energy, you have no ability to adapt to physiologic stressors. Your blood sugar levels are low, your blood pressure is low, you can’t stay as hydrated. You body can’t operate at a high level.

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