Differences between brain and muscle fatigue as we become trained (how exercise increases our ability to handle all stress)

Posted on July 26, 2017

(Last Updated On: July 26, 2017)

Charlie Francis was a sprint coach, a consultant to many different sports, and one of the best sport coaches we’ve had. He’s easily the greatest sprint coach. He popularized the notion of central nervous system (CNS) fatigue, also known as neural fatigue.

The way I describe this to clients is some days everything feels off. The most obvious time is when you don’t sleep well. In that case, you have the intuition your brain is tired. Exercising that day isn’t harder for your whole body because your quads are fatigued. It’s harder because you have a general body, neural fatigue.

Charlie was a huge proponent of increasing concern around CNS fatigue. That we place too much focus on how muscles or joints are feeling, peripheral fatigue, and not enough on how the general athlete / person is doing. Because of this he routinely asked his clients how their lives were going. If an athlete was having relationship issues, or merely they had a rough time with traffic before getting to practice, that could be grounds for adjusting the workout that day. At a minimum, it was justification for being more cautious. This is also why he was huge on having his athletes be full-time. “Part-time athletes get part-time results” was a line of his. Daily life stress, like a regular job, impedes performance training.

-> I wrote about this previously where we can see injuries go up in college football players during exam weeks.

Charlie was awesome, but we’ll end up challenging him some as we dissect:

Endurance capacity and neuromuscular fatigue following high- vs moderate-intensity endurance training: A randomized trial

We’ll also see how exercise can be good for handling general stress.

We have two groups, a high intensity interval training group, and a non-interval group. They were both matched for volume of work. The authors wanted to then assess differences in VO2 max, Lactate Threshold, and where the subjects got tired- the central nervous system or the muscles.

How the hell do we measure central and peripheral fatigue?

This amazingly started in 1954.


That researcher measured the maximal force a person could generate at their thumb.

He then assessed whether that force was the same when he added an electrical stimulation to the nerve innervating the thumb (ulnar nerve). At the time scientists, and still to this day coaches / trainers, believed people can’t maximally stimulate their own muscles. The initial idea being if the person’s maximal voluntary contraction was different than the superimposed twitch, then we knew people weren’t giving as much as they theoretically could.

That was found to be false. People can maximally contract their muscles as much as when a twitch is added. The additional twitch doesn’t add anything to a single maximal contraction.

Then he assessed performance during fatigue. If as someone’s maximal contraction degraded, the superimposed twitch did not add any force, then the fatigue could be deemed local / peripheral. He backed this up by showing if you did not let the area get blood flow after the fatiguing contractions, the area did not regain any force. Illustrating a lack of local oxygen was causing fatigue.

We’ve since found we can more accurately describe the maximal strength of an area if we potentiate it. If we perform some maximal contractions, then twitch the muscle at rest. That’ll be a better barometer of our maximal voluntary contraction. Potentiation is like warming up. It takes us a second to get going.

-> I believe the theory behind twitching the muscle rather than having the person maximally contract is to standardize the data. Humans mess things up. The less subjectivity, the better.

  • Superimposed twitch: adding an electrical stimulus on top of an already maximally contracting muscle
  • Potentiated twitch: performing a twitch after some maximal contractions, but not while contracting


We now have an equation for assessing voluntary activation:

(1 – superimposed twitch / potentiated twitch) * 100

If our superimposed twitch is the same number as our potentiated twitch, where it adds nothing on top of the potentiated twitch, then we have:

(1 – 0 / 1) * 100 = 100%

Meaning there would be no decrement in our voluntary activation. We’re at 100%. If we start fatiguing where our maximal twitch is still the same as our superimposed, but both have declined 50%:

(1 – 0 /0.5) * 100 = 100%

We still can blame all the fatigue on local sources. We want to see if our superimposed twitch is greater than our non-superimposed. Say our non-superimposed is 90% of our max, but if we superimpose we get to 100%.

(1 – .1 / .90) * 100 = 89%

Then we know we have 11% we aren’t generating due to central factors. Somewhere between the brain and the nerve innervating the muscle we aren’t generating as much as we do if we provide extra stimulation.

– > This is where central nervous system vs peripheral fatigue can be misleading, as nerves travel peripherally. Neural vs a specific muscular area fatigue is a better description. The study did a lot more to assess differences in fatigue, such as electrically stimulating the brain. While quite cool, it is not easy reading, so we’re going to stick with what we’ve gone over and ignore the other methods. Nervous vs muscular system is good enough.

What the study found

After the training period the subjects performed two trials. One at the same absolute intensity they started the study at. Another at the same relative intensity. This intensity was halfway between VO2 max and lactate threshold. If after the training you improved your VO2 max, then the relative trial would be at a greater intensity than the absolute.

“Compared with pre-training, HIIT increased the level of potentiated quadriceps twitch reduction (-34% vs -43%, respectively) and attenuated the level of voluntary activation reduction (-7% vs -3%, respectively) following the Time To Exhaustion trial at the same relative intensity. There were no other training effects on neuromuscular fatigue development. “

Translating that academese: after the study, in the high intensity group, in the relative trial, the quadriceps could not be twitched as much, but the voluntary activation was better. The nervous system could produce more, but the quadriceps could produce less. The nervous system less tired; the muscular system more tired.

Where after a six weeks of high intensity training a person became more susceptible to local fatigue and less susceptible to general fatigue. As the authors then say,

“This suggests that central fatigue resistance contributes to enhanced high-intensity exercise endurance capacity after HIIT by allowing greater performance to be extruded from the muscle.”

By not getting as tired centrally, we can do more peripherally. But there is a cost to that- more muscular fatigue!

More fatigue means more injury risk.

This is where we can think of athletes who always say “No, I feel good. I want to do more. I’m ready to go.” etc. As people get more trained they’re less likely to have general fatigue.

-> This is where we’re pushing back on Charlie from the introduction. However, Charlie did not work with newbie cyclists, nor did he discount the importance of the rest of the body. He was training people at the highest level (results may differ for the elite) and saying more focus should be given to the nervous system, not the periphery should be ignored.

Mentally, they are more often going to feel ready to go than they do peripherally. But peripheral fatigue often doesn’t manifest until someone starts doing an activity. You don’t notice your legs are tired until you try to use them. Hence, many athletes who say they’re ready to go, then go do more, then come limping off.

Finally, this provides more credence for everyday people to exercise with at least with some high intensity. Basically, putting yourself through pain helps you better handle pain, mentally.

Before the high intensity lovers get too excited

This is one of those studies many high intensity, or many interval fans, love to jump on. However, the non-interval group improved just as much in VO2 max and lactate threshold!

If you stand, run, stand, run somewhere, compared to only jog, that running needs to be at a faster pace than the jog in order to generate the same amount of work. (You’re both going the same distance, so the only metric you have available to manipulate is force: Work = Force * Distance.) Or if you’re not concerned with a specific distance, depending on the runner’s pace, if you want the jogger to match their workload they can go for a greater distance to make up for their lesser force.

The HIIT group started with 6 sets of 5 minutes, 30 minutes, to do as much work as the non-interval group did in a continuous 59 minutes. With 29 less minutes it’s no surprise the interval group had greater power output (they did the same work in less time: Power = Work / Time):

If you’re working at that much more of a power output, then you do a time to exhaustion trial, it makes sense you do better considering you’ve improved your ability to produce power. The interval group got the benefits of VO2, lactate threshold, and power, whereas the continuous group didn’t get the power benefit. The interval group worked harder, but not longer, as reflected in their heart rate numbers.

What do we know about working harder? It makes you more tired. In fact, we can see it will increase your risk of peripheral injury.

Practical application

  • As people become better trained they become less likely to be fatigued mentally and more likely to be fatigued muscularly.
  • One should throw some harder sessions in regularly, but you shouldn’t interval train every day. Thus, we have what athletes do- hard and easy days. Hard days tend to be interval oriented; easy days aerobically oriented, if not an off day (which is still an aerobic day).
  • However, the reason harder sessions improve our ability to handle neural stress is because it more significantly stresses the nervous system. Therefore, if a trainee comes in already tired, “I didn’t sleep well, my boyfriend is a dick, I wish I never had kids,” it’s not a good move to do something high intensity ala interval training. A lesser intense, or an aerobic day, makes more sense.

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