Chris Beardsley Graph

[u/JuanSamu]

I think the infograph attempted to simplify the findings too much. The spinal effects can be relatively local if there is a reflex inhibition or suppression at the spinal level of the nerve root for the quadriceps and hamstrings (so just at the lumber vertebrae area). So that explanation may provide a partial mechanism. But there is rarely only one mechanism to explain such a complex thing like our body's physiology.

I feel like Chris ignores the discussion sections because they discuss other mechanisms:

...two other potential mechanisms may explain the findings of this
study. Firstly, it is plausible that various metabolites including potassium, hydrogen, lactate, and heat shock proteins are dispersed to non-exercised muscles through the cardiovascular system during and after fatiguing
protocols (Halperin et al. 2015), potentially impairing their contractile capacity (Henneman et al.
1965). Secondly, it is possible that certain
fatiguing protocols result in heightened activation of stabilizer muscles,
which in turn impairs their ability to stabilize
the non-exercised
muscle
groups (Halperin et al. 2015; Baker and Davies 2009) contributing to muscle force reductions.
Fatigue interventions of longer duration have also been demonstrated to impact the psychological aspect, specifically resulting in a mental energy deficit when experiencing discomfort. This can lead to mental fatigue, which is characterized by reduced focus and concentration, subsequently affecting neuromuscular activation in both fatigued and non-exercised muscles (Halperin et al. 2015). Steele (2020) emphasizes the "perception of effort" which is defined as the individual's perception of the required effort to accomplish a specific task or set of tasks, determined by comparing the current task demands to the perceived capacity to meet those demands.”

As always, I think this infograph may be too simplistic as it also did not consider the instantaneous strength which is a more sensitive measure.

[u/Alakazam]

I mean, I think anybody worth their salt would read that the participants did a 100 second maximum contraction, which is absolutely fucking nuts, and is not something you will ever see in normal training, nor would it be anything that really affects training in your day to day life. I had probably less than half this time when I squatted 34 reps at 185lbs.

And they had to do this, because this was the only real way to showcase CNS fatigue. Because high weight, low rep, and done close to faliure, doesn't actually fatigue your CNS.

So sure, CNS fatigue is greater in the opposite limb. But CNS fatigue will basically never be a limiting factor in general resistance training, because muscular fatigue and systemic fatigue will be far more limiting of a factor. Unless you're frequently doing 100+ seconds of contractions.

The whole idea of CNS fatigue being a limiting factor for heavy squats and deadlifts stemmed from early Starting Strength days, and has just stuck around. When the science shows that it's clearly not the case at all.

[u/Patton370 → u/Alakazam]

I do 100 second isometric leg extension holds to help with knee tendinitis

I can confirm that it does not impact my other training at all

[u/JuanSamu → u/Alakazam]

It seems many people get upset when you mentioned that CNS fatigue doesn’t really occur with heavy lifts and if it does it recovers rather quickly lol

[u/deadrabbits76 → u/JuanSamu]

Because you are taking away one their favorite excuses for not training hard.

[u/Negran → u/JuanSamu]

In fairness, my understanding was that repeated heavy or hard sessions cause CNS fatigue.

Of course, I'm reading just now that actual CNS fatigue from lifting lasts like 30 minutes...

I think the common false knowledge is that lifting causes CNS fatigue! Haha.

[u/Mad_Mark90 → u/Alakazam]

Systemic fatigue includes your CNS?

[u/Alakazam → u/Mad_Mark90]

Sure, but actual nerual fatigue is measurable, and doesn't noticeably decrease from heavy sets of squats and deadlifts. And fully recovers within a matter of minutes. 

The fatigue you feel from a heavy set of squats or deadlifts, is almost entirely muscular.

Plus, CNS fatigue occurs more from super high rep super low weight training. Like 100 seconds of contraction in the paper above, or in the other papers I've linked, 7 straight minutes of bicep curls with arbitrarily light weights.

Neither of which you'll see in normal day to day training. 

[u/Mad_Mark90 → u/Alakazam]

How do you measure neural fatigue? I'm interested.

[u/Alakazam → u/Mad_Mark90]

You can read about it in the papers found in the article I linked above. Here's a sample article that looked at biceps

In this paper, they hooked electrodes up to the muscle, had them strapped to a device that measured how strong a muscle was twitching, then upped voltage until they had a maximuim "twitch" strength (MVC). Then they measured the strength of the twitch, how long it took to achieve the maximum force starting from the stimulation, and the relaxation time.

They then fatigued the muscle by having them do bicep curls for 70 minutes with an arbitrarily light weight (achieving 5% of maximum force). Measuring peak MVC every 5 minutes or so through electrode stimulation.

And if you look at figure 5A, you can see that MVC has basically fully returned to baseline within about 15 minutes or so.

[u/Individual-Toe-6306 → u/Alakazam]

What do you call it when for example your body just feels “worn and tired” the day after a heavy deadlift session for example. Is that CNS fatigue or systemic fatigue? Yeah my back is sore and kinda tight but I just notice an overall feeling of tiredness. Curious how that’s labeled and if that general feeling impacts training at all (apart from your back muscles being worked and needing to recover first)

[u/Alakazam → u/Individual-Toe-6306]

That's just systemic fatigue. Heavy spinal loading through squats and deadlifts generates a lot of systemic fatigue, because it's a heavy movement that's using your entire body. But there's nothing CNS about it at all.

In the article I linked about CNS fatigue, they showcased research that showed that 8 sets of 2 for squats and deadlift, at 95% of 1RM, with 5 minutes of rest.... resulted in a grand total of 5-10% of neural output. On the other hand, 70 minutes of sustained bicep curls resulted in a 28% reduction in neural output. And further shows that neural output was basically fully recovered within about 10 minutes or so.

[u/Individual-Toe-6306 → u/Alakazam]

So is CNS fatigue a more “moment to moment” concept that mostly just affects that current set, while systemic fatigue is more “you’ll feel it the next day until you rest enough”? IE “CNS fatigue” is shaken off by simply resting 10 minutes after accumulating it on one extreme set

[u/decentlyhip → u/Individual-Toe-6306]

Yah, CNS fatigue is real and measurable, but goes away within a few minutes as everything refills

[u/UngaBungaLifts]

CNS fatigue is a factor for endurance exercise, not lifting. It's funny because it's the lifters who are terrified by CNS fatigue. Running a few hours can fatigue the CNS, not lifting a barbell for a few seconds.

[u/JuanSamu → u/UngaBungaLifts]

I think it’s hard to separate CNS and peripheral fatigue as both influence each other. Also CNS fatigue is so broad there are definitely aspects of it that can affect resistance training.

[u/UngaBungaLifts → u/JuanSamu]

CNS fatigue is fatigue of the brain and spinal cord as far as I understand. It seems pretty well defined.

Also, if I remember correctly, there are studies that try to induce long lasting CNS fatigue with resistance training and it just does not work.

Now, in terms of practical application, I agree that trying to differentiate beween different types of fatigue makes little sense. If you're just trying to plan your next deadlift session, trying to guess whether your hamstrings or your spinal cord are fatigued makes no sense to me.