The Limits In Your Head (CNS Fatigue)

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“The CNS recovers in 12-24 hours after a workout”. What does that even mean? What’s recovering? What got tired in the first place? Nobody talking about the trendy subject of “CNS fatigue” ever seems to know, and being skeptical as I am of the outrageous-sounding, my suspicion is that the shroud of mystery is hiding voodoo — or just plain old ignorance. We already know that “fitness people” typically have a grasp of biology somewhat less than what you’d expect from a middle-school science education, which lets them speak of “toxins” hiding away in your body, or with a belief that genes “evolve for” certain types of foods found only in organic supermarkets.

Unfortunately even many who come through exercise science programs come out thinking of the human body as a Mr. Potatohead, just a bunch of pieces that happen to stick together and do stuff. Biology is not a rigid machine obeying a clear set of formal rules. Think storm. Think global economy. Complex, nonlinear, exponential.

Central fatigue is nevertheless a real and observable phenomenon, and I was recently pointed at an article, The Race Against Time, which neatly sums up how it applies to sport.

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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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