Another CNS thread

Ive read most of the threads regarding the nervous system and the effect certain training has on the CNS. I have also read many books and articles on the topic. I have searched in scientific journals as well. However, I still have yet to come to an understanding of what exactly is stressed within the CNS. Dr. Fred Hatfield once described it as “emotional stress.”

So my question is what exactly is being stressed?

Is it only the endocrine system?

Is it a decrease in the motor neuron speed?

Im sure there are more factors, but I feel as though there needs to be some sort of main stage, where it stems from. Every coach is quick to say certain things are too stressful for the body, but i want to know what is, and why, and how?

I always wondered the same thing. Regarding the endocrine system, that is a whole different topic if you are only talking about CNS stress. If you read about the stress response, the endocrine system plays a major role, and there is a release of so many different hormones and what not, but I always forget the exact details.
I think you are right that there is a decrease in the motoneuron speed. From what I have read, after stressful exercise there is a few “phases” of the CNS. Depending on the exercise, first there is an excitation of the CNS for a few minutes after (which is the idea behind complex training), and then an inhibition phase, where there is a decrease in excitability of the spinal motoneuron. I don’t know all the details, but I am assuming the whole mechanism of CNS fatigue has something to do with the action potentials and the different ion concentrations in the neurons.

What has me confused is that I also read that motoneuron excitability was completely recovered after 8 to 12 minutes after the exercise. This obviously goes against what Charlie has said about CNS fatigue, so maybe he can chime in.

The term CNS is used by almost all ppl I know extremely loosely.

  1. CNS acronym stands for “Central Nervous System”
  2. Central Nervous System seats of fatigue are only 2
    • activation of motor cortex
    • the neural drive to motoneurons

Anything else, including (but not limited to) activated motoneurons status, neuromuscular propagation , excitation-contraction coupling
are not seats for central fatigue.

It is interestingly to look at activation of motor cortex. It was shown that lack of motivation causes a marked decrease of activation in the motor cortex. IMO, similar effects can be cause by particular hard workouts, high level competitions , emotional stress, incorrect management of training loads which can cause a small , but cumulative reduction of activation of motor cortex and so on.

  1. So, to resume: motoneuron excitability is not “CNS fatigue”. Motoneurons are not CNS. Endocrine system is not CNS.

  2. There are many factors which can cause a decrease in the motor cortex and supraspinal motor drive. And contrary to the popular belief, they cant be simply categorized as “high intensity causes CNS fatigue”.

Generally, central fatigue, mainly expresed as activation of motor cortex, is (subsequently) exhibited when a subject uses maximum motivation to accomplish a task. This can be lifting a competition load in Weightlifting, or a sprint close to competition times, or geenrally, loads high enough to cause emotional stress… you got the idea. Motor tasks which do not require a lot of motivation from the athlete part appear to cause no significant CNS stress.

The other side of the medal is incorrectly managing training loads (not only HI training loads)can lead to “overtraining” like responses in the body. Between other factors, motivation will decrease. A decrease motivation will cause a inadequate activation of motor cortex. This can be seen also as “CNS fatigue”.

I wonder if we get into trouble because of the CNS moniker, but we need a name for the phenominon and: “that which is not muscular” seems a bit cumbersome.

To add to what Dan stated, it must be understood that the CNS is linked to the autonomic system which governs non-voluntary physiological functions (respiration, immune function, organ function, and so on) via its primary branches (parasympathetic and sympathetic)

thus, upon excessive overload of CNS intensive efforts a whole series of biological functions may adversely be affected.

here’s a flash card representation of related factors that may serve to clarify the issue:

Thanks Dan. I am still kind of confused though… So motoneuron excitability is not CNS fatigue…
I was just thinking that, because the CNS controls neural drive to the motoneuron, then an inhibited motoneuron from stressful exercise could indicate a reduced drive from the CNS, which could mean CNS fatigue…?

Also, with the statement about motoneuron excitability being completely recovered after 8-12 minutes, let me get this straight…
So the actual motoneuron excitability is back to normal, but the activation of the motor cortex is not necessarily recovered yet, so central fatigue could still be there? How do they test motoneuron excitability?

Finally, if motivation is the key factor in central fatigue, how is it then that something like hill sprints are any less taxing than flat ground sprints? The speeds will not be the same but the motivation to reach the highest possible speed could be the exact same couldn’t it?

NMF: Non-muscular fatigue

The determinant factor is the actual seat of fatigue. Correlation doesnt mean causation.

IIRC, the way this is done in laboratories is to create M-waves in 3 steps, before, after, and during a muscular contraction. A decline in M-wave amplitude indicates impairment in the processes which convert axon potential into sarcolemmal action potentials. This in effect is a beautiful measurement for neuromuscular propagation.

There are lab tests and experiments for central factors, excitation contraction coupling, and so on.

“Motivation” should be understood in this context as the will to generate maximal maximum speed, or maximum speed-strength.

Motivation is a key factor, but not enough. In addition to be motivated to generate max speed or max speed-strength, you must actually generate them. Cleaning 70% of your max will not cause significant CNS fatigue, even if you are totally motivated to lift it.

A pre-competition clean with a maximal weight which was not used before will cause fatigue. If you lack motivation in such a lift you will miss it. And also, such a lift would probably cause emotional distress even in seasoned lifters.

I think reading Charlie’s books will give you a good grasp on the practical side of CNS fatigue. It’s basically a all you need to know in practice. And also, that many trainers will be fast to yell CNS fatigue at anything which appear not easy to pinpoint, but this doesn’t mean they re right.

Also what James said about the CNS and control of body functions is very important.