Adrenal Fatigue Revisited [Recovery]

Several years ago, I wrote a post on the subject of adrenal fatigue. I don’t recall exactly what prompted the rant, but I’m sure it had something to do with a personal trainer or MD-turned-author trying to make a quick buck by pushing supplements. It really annoyed me (still does, really) that people who should know better would jump on bandwagons built on almost insultingly simplistic science. Capitalize on the general mistrust of mainstream medicine and you’ve got a set of passive income streams in the making.

I think that this is a topic worth revisiting. It appears that my original article, now almost five years old, garnered some attention in the last day or two, so I thought it’d be worth addressing some of the “interesting” replies. More importantly, my understanding of stress and fatigue has progressed since those days, so this post can serve not only to debunk “adrenal fatigue” claims, but to explain how stress actually works and what might be happening in lieu.

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How much can the CNS handle? [Stress]

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Awhile back, when I was talking about heavy daily training, I wrote a post about inflammation and how this contributes to the common feeling of ‘overtraining’ (which is probably better termed ‘staleness’). There’s a lot to be said about this topic. I’ve said a lot already, and there’s still plenty more to go.

Overtraining, overreaching, and the interaction between training and the stress response is a blurry area. Relating wider biological concepts, like stress, to specific instances, like workouts and training schedules, is no easy task. Contrary to popular belief, research doesn’t do that. Virtually all of our knowledge on ‘overtraining’ comes from observations in athletes or inference from neurological or biochemical effects.

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CNS vs. Peripheral Fatigue

The title is a topic that’s come up a lot over the years, and it’s been on my mind lately. I’ve written about this quite a bit in the past, on forums and in some detail in Maximum Muscle, but I think this is something that could use some elaboration for my blog audience and those of you that aren’t familiar with my older writings.

I also want to scoop all these upstarts that think they’re on to something. What I want to do is define “CNS Fatigue” and talk a little about fatigue in general, as it relates to strength training and exercise in a broader sense.

Firstly, just so we’re all on the same page, CNS is short for Central Nervous System. That’s the brain and the spinal cord, for you bio-illiterates.

Fatigue, at least in exercise-science terms, is a reduction in your ability to express physical fitness for a given task. Fatigue is a temporary reduction in your ability to perform at some activity, in other words. Note also that fatigue is fairly specific, although like everything else it can overlap with other things. Get tired from lifting weights and you may still be able to go for a run, as an example.

Fatigue can be both slow-acting and fast-acting, depending on the activity and the rest time allowed. Doing singles with 10 minutes rest between each rep will generate less fatigue than doing sets of 10 with 60 second rests between each set. Work out every day and you’ll accumulate more fatigue than working out once a week.

Fatigue is largely a function of the work:rest ratio, in other words. More work and less rest yields higher fatigue.

CNS fatigue (also known as central fatigue) is therefore a reduction in performance attributed to factors in the CNS, as opposed to the peripheral nervous system and neuromuscular system (peripheral fatigue; that is, the rest of the body besides the brain and spinal cord).

The question is, how much can you separate the two? It’s hard to distinguish central (CNS) action from peripheral (rest of the body) action because the CNS tends to influence everything, and is in turn influenced by everything. CNS fatigue will filter down through the rest of the body through hormonal feedback loops and similar mechanisms, so it’s not always so clear-cut.

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Fat Loss and Running

For some reason people seem to have the idea that running will create fat loss. There’s two ideas that probably contribute to this: the best runners are usually very lean, and the 1980s obsession with aerobics. The latter especially created a tradition so that “everybody knows” you go run or do aerobics classes to get in shape. Right?

As usual, it’s not that easy. There is some truth to the fact that steady-state aerobic exercise does tend to preferentially use circulating triglycerides as a fuel source (body fat, effectively). However as the current fad towards interval training will quickly remind us, the fuel source isn’t the entire story. There’s something to be said for shorter, more intense exercise. I won’t go into the details because they aren’t terribly important here, but maximal activation of anaerobic metabolism tends to chew up more calories and do favorable things for nutrient partitioning: calories go to muscle instead of fat storage, and more fat ends up being burned over a 24-hour period.

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