The Limits In Your Head (CNS Fatigue)

Like this post? You might also be interested in my book which covers this subject in much more detail.

Get your copy of Squat Every Day.

“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.

This article is a wonderful demonstration of what central (or CNS) fatigue really is, as discussed by one of the scientists who coined the whole idea. Tim Noakes’s research into what he calls the “central governor” was one of the first legitimate discussions of “neural” fatigue I came across years ago. The idea is that your brain is constantly monitoring the physical work done by your muscles and cardiovascular system, and that monitoring process is what we experience as a sensation of effort. Your brain, says Noakes, has a safety margin which is intended to stop you from hurting yourself by pushing to true physical exhaustion. When you reach that point, the brain effectively pulls the plug on motor drive — the nerve impulses that make you go — and you experience this as a feeling of tiredness and exhaustion.

While his ideas remain controversial, in the sense that the “governor” may or may not exist as Noakes describes it, virtually no one questions the idea that there’s a bottleneck operating in the nervous system. Those of you familiar with the late coach Charlie Francis will know that he spoke of training in “highs” and “lows”. You have some days where you’re on fire, and other days where you just can’t make it go. Charlie’s whole methodology centered on the alternation of high days and low days. He’d discovered that when his sprinters came in and worked near PR levels one day, that usually meant they were going to pay for it for the next day (or three). So he built a system where each “high” day was balanced by a “low” day for restorative purposes.

Charlie’s thinking makes a lot of sense for any “high output” activity, anything that makes demands on neurological output. This is obvious in sprinting, throwing, other assorted track events, and you’ve probably made the connection with maximum efforts in lifting. A PR deadlift or clean and jerk or even a new 10RM squat are all “high output” activities, in that they place a large neurological demand on your body (as well as the demands on the muscles and heart and lungs).

The connection between the central governor and anaerobic modes of training like those hasn’t been given a lot of attention in the research so far, but I feel confident in drawing inferences. We’ve all had those days where you’re lifting the same weight, but you have a bad morning and now it feels 10kg heavier. Or you show up to lift with the powerlifting team and now it feels 10kg lighter. The perception of the weight matters insofar as how it feels, and that feeling determines what you’ll actually do with it. These changes can happen fast, far too fast for it to be anything to do with muscle tissue, so it has to be neurological.

The “inverted U” pacing strategy that Noakes described in the article, in which we see the best performances at the beginning and end of an event, shows up repeatedly in human psychology. We come with one ruler, and everything we measure is resized to fit it. Work expands to fill the time allotted, willpower scales to the (perceived) difficulty of the challenge at hand, and now we see that even physical effort is substantially dependent on psychological conditions.

It’s not hard to reconcile this with strength sports. If you see a PR weight and get scared of it, then you’ve scaled that weight in your mind and your output — your strength — scales with it. This is not a tremendous effect, but it can mean the difference in six reps and 10 reps, or hitting that new 1RM and missing it halfway.

This is why training on stimulants is so popular. For years I’d always lift with 25mg of ephedrine in my system and the loudest metal I could find blasting in my ears. Stimulants and psyching up with music are ways of artificially elevating neural-mental arousal, and that arousal level translates to automatically higher output. It doesn’t make you “stronger”, but it does take the parking brake off and help you do the best you’re capable of doing. The weights literally feel lighter and that translates to higher neural output.

I don’t care for that any more. I’m finding that training is far more effective by relaxing and taking as much of that “effort” out of it as I can. I’ve written recently about Baumeister’s willpower research, and while I’ll spare you the in-depth science, I’m convinced that our rational selves — the self-regulatory capacity or “will” — are more closely related to day-to-day performance than we realize. This willpower capacity is limited, easily depleted when overused, and probably related to many of the same neural circuits governing both motor drive and perception of effort.

The analogy I’ve often used is a long car trip or a day of hard concentration. You didn’t “do” anything, but you feel wiped out anyway. Sap that mental capacity, through whatever activity, and you feel wiped out. You aren’t experiencing “CNS fatigue”. You’ve temporarily exhausted your capacity to “switch it on” and focus yourself, potentially leaving your weights for the day that much “heavier” (at least within the 5-10 percentage points that motor drive accounts for).

The insight here is that nothing is actually “fatigued”. The “fatigue” thinking is the same half-hearted quasi-science that gets us vague terms like “toxin” and the New Agey quack treatments meant to fix said ailments. In this case the “exhaustion” is meant to be taken metaphorically — it’s a feeling, something you experience mentally, but there is no “fatigue” happening in the biological sense. The brain’s activity changes; it does not fatigue.

The trick is to learn how to scale. A depressing number of people seem to have no gears under the hood — they’re either going at maximum or sitting on the couch. A cynical observer would consider this a built-in flaw in the character of exercisers, but I’m going to be more upbeat and assume that the lack of effort-grading ability is a failing of the culture and the people doing the teaching. Very few even realize this is an issue, let alone a skill that can be taught.

Most folks treat exercise with aggression. It’s a challenge to be conquered, a threat to be overcome. Stimulants and aggressive psych-up are meant to mask this understood need to “declare war” on the weights, filling in for “natural energy” on days when you don’t feel so great. You’ve created an antagonistic relationship before you even touch the thing, and that brings us back to training relaxed.

Treating the weight as your adversary means that, by definition, it’s an emotional challenge. Emotion depletes willpower, and emotion triggers physical stress.

Learn to be cool. When you can just lift, without all the mental arousal and emotional energy, you’ve changed the perception. You aren’t competing with the weight and, as Noakes found, you’re no longer scaling your performance to the expectation. Being around others and competing against them (whether you realize it or not) is good. Battling your training weights is not.

Leaving the emotion out helps scale back physical stress. Much of what people naively call “CNS fatigue” is really just feeling bad after a hard workout. There are reasons for this, having to do with certain feedback loops between your brain and the immune system, but this is not “CNS fatigue” in any real sense. You just feel bad because your body is trying to cope with what it perceived as a threat.

There’s no harm in using the occasional “high” day to let some adrenaline out and see what you can do. Even here, though, it should be more about focusing your energy into the lift, rather than trying to “beat” the bar.

What the central governor tells us is that perception is a large part of performance. Most of the time, it’s better to stay cool and “just lift” instead of competing with the weights. Take your mind out of the process, listen to some soothing chillout tracks, and just do the thing.