The Mystery of “Nonlocal” Fatigue: Why does arm exercise make your legs feel tired?

[Snow Leopard]

This is an article about fatigue in athletes. It has been shown that eight minutes of arm exercise (hand cycling), followed by a cycling test six minutes later resulted in 38% poorer performance than without the arm exercise. Yet, when tested with electrical stimulation, the legs responded as normal.

http://www.runnersworld.com/sweat-science/the-mystery-of-nonlocal-fatigue
The article in the Runners World Magazine concluded:

Instead, the best predictor of when the subjects would give up was their level of perceived effort, which reached the same level in both trials at the moment of exhaustion. Somehow, the effects of arm cycling lingered and influenced the overall perception of fatigue during the leg trial—which, of course, raises an even bigger question: what is effort? In this case, the study focused specifically on the sense of discomfort in the legs and breathing, but there’s plenty of debate about what contributes to the sensation of effort and how it should be defined.

Primary article:
http://www.ncbi.nlm.nih.gov/pubmed/26272315

Electrical stimulation does not really replicate an exercise task, as the cardiovascular demands for example are not matched. We can conclude that the neurolgical response in the leg muscles is not inhibited. But it does not necessarily follow that the lack of performance is only due to (mental) sensory perception.

This is a case of: pay attention to what you are directly measuring and try to avoid leaps of logic.

It is quite possible that the hypothesised "central fatigue" in this case may be mediated by cardiovascular feedback effects limiting performance peripherally during sustained exercise, despite receiving the same neurological input.

Lastly, it would be foolish to assume that those other feedback mechanisms that are limiting performance can or should be ignored. Those mechanisms exist for a reason.

 

[Hip]

I posted this elsewhere, but it is worth copying here:


Central Fatigue And Peripheral Fatigue

Central fatigue is the psychological feeling of fatigue generated in the brain, and contrasts to peripheral fatigue, which is defined as the physical running out of energy in the muscles, or the running out of other factors needed to make the muscles work. For a fuller description of the two types of fatigue, see here: Muscle weakness - Wikipedia

One study I read on this subject discovered that when you use your muscles a lot, you get a serotonin build-up around the motor neurons in your brain (the motor neurons that are activating your muscles). The more you use the muscles, the more this serotonin engulfs your motor neurons. This flood of serotonin then creates a psychological feeling of muscle fatigue. This is central fatigue.

This central fatigue is not a real loss of physical muscle energy, but a feeling created in your brain that the muscles are tired. The idea of deliberately creating this psychological feeling of muscle fatigue is presumably to protect you from overusing your muscles, which could lead to muscle damage.

 

[Snow Leopard]

One study I read on this subject discovered that when you use your muscles a lot, you get a serotonin build-up around the motor neurons in your brain (the motor neurons that are activating your muscles). The more you use the muscles, the more this serotonin engulfs your motor neurons. This flood of serotonin then creates a psychological feeling of muscle fatigue. This is central fatigue.

It is one potential hypothesis for central fatigue.

The key question for whatever proposed mechanisms is does the kinetics make sense for PEM in ME/CFS? What would cause the spillover of 5-HT 24-48 hours later? But it is these sorts of models that provide ideas on what could go wrong.