Progress Killers: Hard Cardio and CNS Fatigue

Facts About Central Nervous System Fatigue

What is central nervous system fatigue? What causes it? How can you avoid it? Here’s exactly what you need to know.

What is central fatigue? Is it true that hard cardio causes it more than heavy lifting? Good questions. Let’s jump right into it.

The Bullet Points

  • Much like overtraining doesn’t mean training too much, central fatigue has nothing to do with feeling fatigued. It’s simply a weakening of the central drive from the motor cortex to your muscle tissue.
  • To be brought into play, high-threshold motor units (fast-twitch fibers) require a strong excitatory signal. Central fatigue reduces that signal, making those fibers harder to recruit, even when sets are taken to failure. This is true for strength, speed, power, and even hypertrophy because fast-twitch fibers have a lot more growth potential.
  • We used to believe that heavy lifting was hardest on the nervous system. It’s not untrue. When you do heavy compound lifts, you need more central nervous system activity (motor skill complexity, higher tension, etc.)
  • Going to failure will also increase central fatigue. The last rep will feel like a max-effort rep.
  • But the main thing that will increase central fatigue is hard cardio. We’re not talking about long-distance running, but the kind of high-intensity energy systems work where you feel like you’re drowning and on fire at the same time: high-intensity intervals, metcons in CrossFit workouts, etc.
  • The main triggers of central fatigue are signals of pain and discomfort. The more signals sent to the brain, the more the brain shuts down to prevent damage. Central fatigue is a protective mechanism.
  • HIIT sends these signals. A tough HIIT session can be more damaging from a neurological perspective than a heavy lifting session.
  • The inhibitory signals that cause central fatigue can linger for 48 hours. A hard cardio session the day before lifting may affect the effectiveness of the lifting session.
  • I only use HIIT sessions maybe once per week at the end of a fat loss phase. (A longer calorie-burning session, such as walking with a weighted backpack, is less hurtful for the nervous system.) I’m not saying NOT to use high-intensity intervals, but don’t do them every day and keep central fatigue in mind.

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I guess this rugby team didnt know that cardio hurts muscle gains. So did many NFL guys also didnt get the memo i guess…

And CNS fatigue is just disgustingly over-used term, just like words “i love you”. Most garbage people say meaning “CNS Fatigue” has nothing to do with actual CNS.

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Is that a rugby league team ?

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This is a naive take at best and a disingenuous one at worse. Virtually all top level rugby and American football players are using copious amounts of steroids and have been doing so since their university days (every major college football program has systemic doping programs, this is widely known in college football circles). This isn’t a normative statement (I.e., steroids are bad), but a positive one (virtually all top athletes are using PEDs).

Using PEDs mitigates many of the issues of central fatigue that Thib speaks to. These athletes are constantly in an anabolic state in the off season and can recover at a supraphysiological rate. Hard training (be it weights or hard METCON) depletes our limited training resources. PEDs both create a larger fuel tank and fill the tank faster, so to speak. Thib isn’t railing against cardio or METCON (he had two excellent articles in 2021 that argue comprehensively for its necessity); he’s urging natural lifters to be judicious about their use of hard cardio in the context of a overarching goals and training programs to ensure they aren’t burning the candle at both ends.

Moreover, while central fatigue or CNS fatigue has been at times a buzzword or overused, it’s a scientifically validated phenomenon that has to be taken into account in every intelligent training program. Thib isn’t saying “cardio kills your gains bro” (quite the opposite if you are familiar with his work), he’s presenting a well-evidenced argument that HARD Metcon needs to be used in an intelligent and judicious manner in alignment with one’s goals given it’s neurological costs and the reality that training money is limited and needs to also be invested in hard lifting.

Muscle fatigue: general understanding and treatment | Experimental & Molecular Medicine.


Sorry. My mistake. I think it is a South African rugby union team.

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Ah, maybe. When i see a steroid using coach making posts i automatically assume we know whats going on. Even the coach himself doesnt bother to be natural so i assume natural stuff is not relevant here.

That is the South African Rugby team who have all been rumoured for a while to be using gear. That being said, although they are fit, they are rugby fit which is more about HIT, with sprints and other physical activities rather than steady state cardio fitness.

I don’t think rugby teams in general have wide spread steroid use though. It is in the game but not rife.

You obviously know your stuff. Is that the Stormers South African Rugby team ? I recognise Eben Etzebeth, but he has played for a few teams. Thanks.

Its the South African national team. I think Eben was playing his club rugby in France when this was taken.

Having met a lot of professional rugby players, yes they are strong (but only at certain things and not all of them), yes the top end ones are ripped but they aren’t massive in relative terms compared to being big in the BB world.

I know a load of people down the gym who are stronger than professional rugby players but they don’t have the cardio that rugby players do. A big thing as well is the hits, after i play a game of rugby (at amateur level) my shoulders and neck etc are in bits. It effects your gym performance and slowly weakens you in season.

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Thanks for the info.