Matt Perryman Matt Perryman

CNS vs. Peripheral Fatigue

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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.

I’ve done quite a bit of research into this topic, and consequently I’ve got a whole ton of journal articles on the matter which I’m not going to cite here. If you want a reference, email me or hit me on Twitter @ImpulseStrength or something and I’ll tell you where to look. Otherwise, this is far too in-depth a topic to cite everything.

To understand this, I need to talk a little about how the CNS regulates movement. In the brain, there’s two important regions that govern movement: the motor cortex and the cerebellum. What we know is pretty simple: wherever CNS fatigue occurs, it happens “upstream” of these areas. That is, the thing(s) getting fatigued are deeper regions of the brain, not so much the areas that control movement.

Now, when you lift a weight, your muscles are being turned on by impulses sent from the brain, which are sent from those two regions. The more impulses are sent, and the faster those impulses are sent, the more force a muscle contraction generates. That’s why so-called “grinding reps” are so hard, and why they require so much mental focus.

Think of how you feel when you lift a 1RM weight, or do a high-rep set that’s just brutally difficult. How do those reps feel? They’re slow, and mentally challenging because you have to focus and really push yourself through them. That’s a grinding rep.

When you’re grinding out reps like that, you’re also sending a lot of nerve impulses (this flow of nerve impulses is called neural drive, just for future reference).

In the past I’ve compared central (CNS) fatigue to flipping a circuit-breaker. That is, when you do too many grinding reps like that, you signal some backlash in the brain. I’m of the opinion that the brain will inhibit its own output as a protective measure, to stop you from hurting yourself with too much exertion. There’s also the strong possibility that you actually deplete some areas of the brain, and it simply can’t keep up the neural drive.

This all adds up to a lot of evidence that CNS fatigue actually happens deeper regions of the brain, those areas involved with mood, emotion, and psychological arousal – which also implies that neural drive actually originates in these same areas. This is why being psyched up hard can help performance in maximal efforts; mental focus and “push” can actually increase neural drive, which is useful in lifting a maximum weight or grinding through a set of maximum reps.

We also see a strong relationship with the activity of dopamine and serotonin networks, which are responsible for the bulk of motor control and behavioral regulation. There’s some convincing evidence that the ratio of dopamine to serotonin governs central fatigue, and this has implications in training: burn yourself out in the gym and you’re going to see your mood and motivation affected too.

This is why you can hit a maximum lift and feel completely exhausted after the fact even if you didn’t do very much. You wiped yourself out across the board; not only did you fatigue yourself in the sense of lifting a weight, but you did it in terms of your mental state as well. Not only can your brain not keep up, physically, but it makes you not want to lift things.

There’s another element to this which I call the competition effect. For those of you that haven’t been there, lifting in a competition is a whole different matter from lifting in the gym. There’s a constant rush of adrenaline and mental arousal that you just can’t duplicate in the gym. On top of that, you’re encouraged to take very taxing lifts, which will definitely raise your heart rate and otherwise increase stress levels.

Great for setting PRs, but the toll on your body can be substantial. It’s not unheard of for powerlifters to have fatigue after-effects for weeks after a meet, all from nine reps. It’s not the weight that does it. It’s all the mind-body stress that revolves around competition.

Russian weightlifting coaches were known for distinguishing between training maxes and competition maxes for this very reason. A training max is a lift that could be done without any psychological or emotional stress, which they measured by increased heart rate. A competition max was done with all-out mental wind-up, and it’s been noted that there’s as much as a 10% difference between the two – which is nothing to sneeze at. A 10% difference is the difference between a 400 lb lift and a 440 lb lift.

Vasily Alexeyev, the legendary Russian weightlifter, has written that he always stayed away from any emotionally-challenging attempts in the gym. He left all his mental energy for the platform. Similarly, you always hear the old-time golden-age lifters talk about not “training on nerve”. These guys always left reps in the tank and left some excess in the gym, instead of wiping themselves out every time they got within eyesight of a barbell.

That’s contrary to the wisdom in some circles, which would have you believe that if you aren’t exhausted in body and mind, you haven’t had a good workout. It would seem that you can and should leave something in the gym, for best results (at least when pure strength is the goal, though the case could be made for most every sport).

It’s important to note that CNS fatigue is almost always going to be associated with local/peripheral fatigue factors, too. There’s a whole lot of things that can get tired and damaged in and around the muscle itself, and this can often be mistaken for central fatigue. If you’ve ever trained HIT-style, with higher-rep sets to a point of failure, you’ve undoubtedly experienced this – a feeling of exhaustion in the muscle, the pump, and so on.

But it’s rarely that clear-cut. The local muscular factors can influence CNS fatigue as well. There are afferent cells in the muscles which constantly relay feedback to the CNS. These can have a powerful inhibiting effect on neural drive if they detect excessive fatigue in the muscles or excessive loads. If you’ve ever tried to pick up something too heavy and automatically dropped it, almost like a reflex, you’ve experienced this.

And if you think that CNS fatigue can happen just by lifting heavy things, think again. Even endurance athletes can suffer from this, albeit in a roundabout way. There’s a class of compounds called inflammatory cytokines that are released in response to exercise. The nasty ones are TNF-alpha, IL-6, and IL-1beta, if you care, and all three of those are known to mediate effects of fatigue from the muscles to the brain.

The brain of course picks up on these things and modifies itself accordingly. In fact, one prominent model of overtraining is that these cytokines induce fatigue in the brain as a result of “too much” exercise. The flu-like effect of this is not unlike what happens after competitive strain. You feel almost sick; look at the laundry-list of “overtraining symptoms” and these will invariably match up.

Different avenues, but the destination is the same. The brain affects the body, the body affects the brain.

Any way you swing it, if you do too much work of any kind, you can cause CNS fatigue. It doesn’t matter if you’re a powerlifter or a marathoner.

I’d also note that it isn’t necessarily a bad thing that this happens. The mechanism is in place for a reason, and that’s to stop you from working yourself into paste for no reason. I realize that exercise addicts get caught up in the idea that more is better, but that’s simply not the case. Overworking yourself for the sake of “doing exercise” is not helping you.

That said, I do think there’s something to be said for testing the limits from time to time, and there’s definitely a degree of adaptation that can go on, in terms of how much we can tolerate. You just can’t be stupid about it; you have to gradually increase workloads, and make sure that the work you are doing has a point instead of just being there to be there.

As far as programming goes, this has led to a few features worth keeping in mind:

  • High-Low Programming – This is based on the idea that CNS-intensive training methods, which includes maximum weights, plyometrics, or any activity with maximal power output (hi there, HIIT!), require at least 48 hours of rest between sessions. Summed up, this principle would have you consolidating as much CNS-intensive work as possible on the same days, and separating them with either rest days or non-taxing methods (like low-intensity cardio or easier bodybuilding workouts). This is why you hear people telling you to do HIIT on the same day(s) you train your legs.
  • Auto-regulation – I’ve talked about this one in greater detail before. Auto-regulation is important to manage fatigue by adjusting your daily workloads – both intensity and volume – towards what you can handle for any given workout. You can pick the right weights and you can regulate volume simply by gaging how fatigued you are at any point in the workout. The example of “grinding reps” I gave earlier is a prime example – for an easy session, stay un-psyched and avoid grinders. For an all-out workout (which should be rare, IMO), turn it up to 11 and grind out reps all you want. The middle ground lies in between the extremes.
  • Long-term Adaptation Process – This is the underlying goal of any periodization system, to build you up over time by gradually (and cyclically) increasing the amount of work you do. Instead of just jumping in to train six days a week with no real goal, you’ll want to consider the process in both the short-term and the long-term. In simpler terms, you want to handle more in a year’s time than you do right now, both in tonnage and intensity; I’m not so concerned with volume, as in “doing more reps”, but rather in the sense of “lifting more weight per session”. But you need to do this with a plan in mind, preferably in some cyclical manner with peaks and valleys so that you don’t bring down all the fatigue nasties.

Or if you like the KISS wisdom: think about what you’re doing and be mindful of your condition at any given point in time. If you feel like crap, chronically, there’s a reason for it, and it’s probably not a good one (unless you’re doing Sheiko or Smolov or something, which is designed to make you feel like that for a greater long-term purpose).

As far as that goes, there is something to be said for concentrated loading and deliberate overreaching, but that’s a whole different post.