Which Comes First, Cardio or Weights_ - Alex Hutchinson [23]
This doesn’t mean that fatigue is imaginary. Your body really does have physical limits—but, if the central governor theory is correct, your brain rarely permits your body to actually reach them. The simplest example of this phenomenon is the finishing sprint that is a nearly universal phenomenon across endurance sports, from novices to world-record holders. No matter how hard you thought you were going, you suddenly find as you approach the finish that your legs can move faster after all. Nothing has changed physiologically—but your central governor allows you to speed up now that the finish line is in sight.
In contrast, if you put subjects in a hot room and ask them to pedal an exercise bike as hard as they can, their power output will be lower than in cool conditions—right from the first pedal stroke. The slowdown happens long before any of the physical effects of heat could be relevant—further evidence that the brain is quietly enforcing a safe “maximal” effort.
This debate between peripheral and central models of fatigue is perhaps the most controversial topic in current exercise physiology. No definitive conclusions are in sight, but there’s broad recognition that the brain plays a larger role than previously acknowledged. This role is unconscious, so you can’t simply “decide” to push through to your true physical limits—which is probably a good thing. What you can do, though, is gradually teach your brain what your body is capable of. For example, training at your goal race pace not only increases fitness, but also allows your mind to become familiar with the accompanying physiological feedback. You can’t turn your central governor off—but with patience you can adjust its settings.
Does lactic acid cause muscle fatigue?
Conventional wisdom says that lactic acid is the root of all athletic evil. When you’re exercising hard enough to go into “oxygen debt,” it’s lactic acid that makes your muscles burn and eventually forces you to stop, and it’s leftover lactic acid that makes you stiff and sore the next day—or so exercise physiologists believed for nearly a century. We know now that this is wrong. Lactic acid is in fact a crucial fuel for your muscles, not a painful waste product. (Actually, an ion called “lactate” is what’s typically found in the body, which can combine with a proton to form lactic acid.)
The roots of the lactic acid myth go back to experiments with isolated frog muscles in 1907. When researchers applied a shock to the muscles (which were disconnected from the frog’s bloodstream and thus had no source of oxygen), they found that lactate was produced. However, when they repeated the experiment with oxygen supplied, the lactate disappeared. Over the next few decades, physiologists developed the hypothesis that muscles produce lactate when they’re forced to contract without oxygen and that the accumulating acidity is what causes muscular fatigue.
These ideas weren’t challenged until a series of experiments in the 1970s by George Brooks of the University of California, Berkeley, and it wasn’t until the past decade that his views gained widespread acceptance. Brooks showed that you don’t produce lactate only when you’re in oxygen debt. In fact, you’re constantly converting your carbohydrate stores into lactate, even when you’re at rest. About half of this lactate is then immediately converted into ATP, the basic fuel for muscular contractions. The proportion of lactate used in this way hits 75 to 80 percent when you’re exercising, since it doesn’t require oxygen. The rest goes into the bloodstream and is used to fuel the heart or is converted by