Central Nervous System Fatigue

By Skyler Zarndt MS, ATC, CSCS

As I was talking to one of my co-workers today after his heavy deadlift session, the topic of Central Nervous System (CNS) fatigue came up as a topic.  What prefaced this topic was his horrendous attempts at weights near 90% of his 1 Rep Max.  And when I say “horrendous,” I’ll say that he made the lifts.  But he felt terrible.  And if you’re reading this, then I think we all know the feeling.  The struggle of getting TO the gym is real.  The struggle of “getting going” is real.  The struggle of pushing around iron is real.  So why does this happen?

Well, its obvious that many factors account into why we have bad lifts.  Maybe we are tired, stressed, over-trained, sick, whatever…pretty much anything that could mess with our body and take it out of homeostasis could be considered stress and therefore affect our lifts.  It should be noted that “stress” in a general term isn’t always bad.  Stress is absolutely necessary for growth.  Physical, mental, emotional growth.  It doesn’t really matter. From a physical standpoint, its how we get stronger.  It’s PROGRESSIVE OVERLOAD.  I’ve talked about this principle before and how it’s necessary to getting stronger, gaining mobility, getting faster.  You could probably apply it to math problems.  If you try to teach someone calculus before they can add and subtract, it’s gonna end poorly.  Actually, it probably won’t even start.  But I digress.  By definition, it’s “the gradual increase of stress (load, weight) placed on the body during exercise.”

I think it is important to note before we go too much further, that as mentioned above, stress is necessary for growth.  We get into trouble when we cannot balance the stress we face on a daily basis with proper recovery techniques.  There is a fine balance between training and recovery that we constantly seek.  Too much training and not enough recovery = over-training or CNS fatigue.  Too much recovery and not enough training = no growth or lack of growth.  Pretty simple stuff (in theory).

So let’s define the Central Nervous System.  The CNS is made up of the brain and the spinal cord.  It runs the show in regards to how we move, how well me move, performance, whatever.  It’s the boss and everything else acts according to its directions.  The CNS is so named because it integrates information it receives from, and coordinates and influences the activity of, all parts of the body.

So what exactly is CNS fatigue?  Well, I don’t know if we exactly know.  We have an idea.  At least I think we do.  It’s a phenomenon that obviously affects the CNS, and we know that the CNS basically tells our body what to do, so we don’t necessarily want to be fatiguing that, do we?  What we do know, is that CNS fatigue is usually a result of INTENSE training at very high workloads.  It’s a little more specific than say, over-training syndrome, because it doesn’t actually affect the muscles itself.  Just how the brain and spinal cord communicate with the muscles.  Some symptoms of CNS fatigue include:

  • Decrease in performance
  • Increased resting heart rate
  • Slowed Reflexes
  • Irritability
  • Loss of motivation
  • Mental sluggishness

So the symptoms might initially scream “over-training,” but I would be quick for someone to challenge (in this case) the reasons behind the symptoms.  Over-training is going to be more “peripheral,” or it’s going to affect the parts of the body that ARE’NT part of the CNS.  This might include unusually sore-muscles, sluggishness, etc.  CNS fatigue may not have that many physical signs, except when you go to perform at a high level.  That’s perhaps the biggest difference between CNS fatigue and over-training syndrome, in my opinion.

So WHY does our CNS get fatigued?  And also, HOW does it get fatigued?  And what exactly does it mean when it IS fatigued?  Well, to start, let’s discuss the why’s.  Like stated before, whenever we don’t balance the workload we place on our bodies with the amount of recovery we have, we run the risk of fatiguing ourselves.  But to be more specific to CNS fatigue, let’s say that whenever we don’t balance neural excitation with neural inhibition, is when we find ourselves in trouble.  We talked about CNS fatigue usually coming with too many INTENSE, HEAVY workouts.  That is, lifting near maximal levels.  Heavy singles.  All the time.  THIS is what fatigues our nervous system.

I think its important to understand not only why the CNS gets fatigued, but what exactly is going on from a physiological standpoint when we lift super heavy (>90% 1 RM).  So in order to lift really heavy weights, we need a lot of muscles to be working in accord to pick up, push, or pull the weight.  There are two ways our body uses more muscle to perform a task.

  1. Activation or recruitment of MORE motor units
  2. Recruiting LARGER motor units
  3. Firing frequency of said Motor Units

So whenever we choose to lift weights that are near or at maximal levels, we have to employ ALL of these tactics to complete the lift.  And since all of these tactics involve a neural component (that is, the CNS sends signals to fire more/larger muscles), then it makes sense that we would become fatigued in a NEURAL sense.  We are going all out, all the time.  This takes a toll on our signaling processes.

So when the CNS is fatigued, that’s when we see the signs and symptoms listed above.  We just don’t function at a level that maybe we think we should.  We feel, overall, not bad.  Different than we would if we were over-trained.  We just don’t have the extra juice in the tank.  We miss lifts.  Our overall strength is down.  We might feel a little tired or stressed.

So what can we do to combat this?  Well, for one, we need to recover.  We need rest.  We need hydration.  We need PROPER nutrition.  We may need to gradually increase our work capacity from a cardiovascular standpoint to be able to do more work in a given time frame.  All of these are probably issues that need to be addressed.  But I think the most important thing is that we need to have a plan.  We need to figure out what are goal is, make an educated plan, and then follow that plan.  Don’t always push past the tired, drained feeling.  Listen to your body.  PLAN de-load weeks (another topic).  Let your body balance all the training you’ve been throwing at it with a little recovery.  You’ll find yourself more motivated, more energized, and more importantly, making better progress than you would by going all-out every session.

And the reason my buddy that we talked about at the beginning of this post struggled with his lifts was because he got only 2 hours of sleep on Saturday night.  So he wasn’t over-trained by any means.  He just went a little too hard over the weekend!


2 thoughts on “Central Nervous System Fatigue

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