Computational mechanisms underlying the dynamics of physical and cognitive fatigue, 2023, Matthews et al

[Andy]

Abstract

The willingness to exert effort for reward is essential but comes at the cost of fatigue. Theories suggest fatigue increases after both physical and cognitive exertion, subsequently reducing the motivation to exert effort. Yet a mechanistic understanding of how this happens on a moment-to-moment basis, and whether mechanisms are common to both mental and physical effort, is lacking.

In two studies, participants reported momentary (trial-by-trial) ratings of fatigue during an effort-based decision-making task requiring either physical (grip-force) or cognitive (mental arithmetic) effort. Using a novel computational model, we show that fatigue fluctuates from trial-to-trial as a function of exerted effort and predicts subsequent choices. This mechanism was shared across the domains. Selective to the cognitive domain, committing errors also induced momentary increases in feelings of fatigue. These findings provide insight into the computations underlying the influence of effortful exertion on fatigue and motivation, in both physical and cognitive domains.

Open access, https://www.sciencedirect.com/science/article/pii/S0010027723002378

 

[Andy]

In Study 1, we showed that fatigue induced by physical exertion (grip-force) is best explained by a computational model in which fatigue has both recoverable and unrecoverable components. That is, exerting effort increases fatigue, and taking a rest reduces it, but there is also a gradual increase in fatigue which cannot be recovered by short breaks. This model accounted for fluctuations in both effort-based decisions and in ratings of fatigue, with ratings also predictive of choices on the next trial.

In Study 2, we showed that self-reported fatigue and its effect on effort-based decisions are similarly increased by the exertion of cognitive effort (mental arithmetic) and are also explained by a model with recoverable and unrecoverable components. However, this is supplemented by an error-driven mechanism that monitors trial outcomes. These results highlight that fatigue: (1) can fluctuate on a momentary basis due to either cognitive or physical effort, (2) impacts the willingness to exert effort for reward, and (3) is sensitive to errors in a cognitive task.

 

[Andy]

A major debate in research on fatigue has been the extent to which physical and cognitive tasks induce similar sensations and whether they similarly impact on motivation (Boksem & Tops, 2008; S. M. Marcora et al., 2009). Our results suggest that there are some striking similarities in the mechanisms that underlie the development of fatigue, even if the underlying task is different. In particular, we found that similar effort-driven processes influenced sensations of fatigue and effort-based decisions in both tasks.

In both, a model containing recoverable and unrecoverable components of fatigue that increased with the amount of effort exerted and partially recovered after rest best explained people's ratings. This suggests similarity between how fatigue develops during physically and mentally demanding tasks, with both induced (at least in part) by effort costs. Moreover, it raises the possibility that previous evidence of recoverable and unrecoverable components of fatigue induced by physically demanding tasks is not driven by peripheral or muscular fatigue, but instead more directly relates to the mental fatigue induced by cognitive tasks that influences the valuation and choice to exert effort

 

[Creekside]

I like that it's questioning everything that's "known" about fatigue. We don't really "know" anything about fatigue and fatigue-like states, so starting from scratch would be a good idea. I do feel that my "fatigue-like state" is derived from the neural/glial valuation and choice to exert, rather than a lack of ATP or other such physical limitations.

 

[rvallee]

Is this not all stuff known for decades? Centuries even? Millennia?!

Because aside from the silly New age biopsychosocial stuff making fatigue about lack of motivation, all of this has been well-known for decades.

I guess there's the cognitive effort being similar that is not fully accepted, but again that's mostly a function of holding on to silly psychobehavioral stuff that would somehow have mental energy be its own separate thing from any other kind of effort. Which makes no sense. The body is an organism, it's not a fully separate set of organs operating in isolation. If this stuff is so obvious to me that I've been repeating it for years, damn, I don't know anymore. This profession is so amazing on some things, and completely messed up at other times.

All knowledge out of physical training, and it doesn't even have to be scientific, makes all of this pretty obvious about muscular recovery, and how training has its limits, how humans aren't machines and those processes can't be supplemented with simple chemical solutions, something that's been tried for millennia. How this isn't about willpower and motivation at all. The 20th century is filled with attempts at creating "super soldiers", all of which have failed miserably.

I really thought all this stuff was basic knowledge. But the prominence of BPS belief systems really does make it questionable. It's probably a good thing on the whole that basic stuff like this is being reaffirmed, but it shows that for all intents and purposes, medicine has basically made zero progress on the notion, definition and mechanism of fatigue, with most of this stuff already known back in Antiquity.

I guess the less is stop fooling around with psycho-mumbo-jumbo, but that's not about to happen any time soon, there's still quite a few years more with silly BPS nonsense left before they let go of it.