Thinking hard is physically exhausting

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You have probably experienced it many times; when you get to the end of a particularly long busy day at work and you feel absolutely pooped. But it's more than just mental tiredness, you often feel physically exhausted as well.

But why is that, because if you have only been exercising your mental muscles, surely all the other muscles of your body will still be in good shape to go out and do some exercise, right?

Well you may be saying, "wrong" - and you would be right!

A team at Bangor University undertook research to examine the impact of mental fatigue on cognitive and physical performance.

Mental fatigue was induced in the subject group by making them undertake 90 minutes of demanding cognitive work while the control group had to just sit and watch 90 minutes of emotionally neutral video material.

Both groups then had to undertake bicycle riding to exhaustion at 80% of their peak power output.
The results showed that the cognitively tired participants rated the physical effort as being significantly higher, they reached their maximal peak of exertion earlier and they terminated the exercise earlier than the control group.

Interestingly it was the perception that the physical task was harder than it actually was for the mentally tired group because there was no difference in the cardiovascular and metabolic response of heart rate, breathing rate or blood sugar levels.

This means that mental fatigue does indeed limit exercise tolerance but through perceived higher effort required rather than actual physiological mechanisms.

My exercise colleagues are always advising me that exercise is best done in the morning. This study would appear to support this because firstly you are fresher to undertake the exercise first thing and secondly after a long hard day at work you are not going to be nearly as motivated to want to put in the effort to go out and do the training.

Looks like I need to set the alarm to go off a bit earlier to ensure I get that exercise done before work tomorrow.

Samuele M. Marcora, Walter Staiano, and Victoria Manning (2009) Mental fatigue impairs physical performance in humans Journal of Applied Physiology vol. 106 no. 3 857-864

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