Fatigue

  I just read an interesting review (in The New Scientist) of recent research on fatigue.  I’ve always considered feelings of fatigue to be based only on depleted energy supplies and accumulated “waste” chemicals in the muscles.  Recent research suggests it is more complicated; several brain systems are involved. In addition to the interoceptive reports on available energy and depletion of energy, reward systems assess the potential benefits of continuing the activity. A separate system balances the energy supply with the anticipated rewards in an on-going “cost/benefit analysis.”  The experience of being fatigued is the result of the brain’s assessment that the potential rewards of continued physical or mental effort do not justify the energy expenditure.  Anything that reduces the assessed reward value of the activity (such as depression or boredom) increases fatigue; anything that increases it (such as social encouragement – a cheering squad or the offer of monetary rewards) decreases the feeling of fatigue.  This helps explain why depression, which reduces the subjective pleasure of almost everything, is so enervating.

I have often noticed that the last mile of a hike seems especially long and tiring, whether it comes after three miles or fifteen miles.  The rewards of the hike (pretty views, birdsong, etc.) are all past, and apparently the reward of reaching the car and resting feels about the same regardless of how long the hike was.