Brain vs. Brawn: How mental fatigue affects physical performance.

What if I told you that I could influence your exercise performance just by making you read a book or play a computer game? Well, research suggests that I could.

In a previous article (http://www.thesportinmind.com/articles/what-limits-endurance-exercise-performance/) I presented some evidence that endurance performance is limited by psychological factors, rather than physiological mechanisms. Originally this viewpoint was quite controversial and according to some sports scientists, it still is. Controversy aside, it is an important discovery that has been expanded upon by an interesting body of research surrounding mental fatigue.  These studies all used cognitively demanding computer or reading tasks to induce mental fatigue in athletes before different types of exercise. The athletes also performed the same exercise tests following a non-fatiguing control treatment and the results were compared.

OK, so they weren’t just playing Super Mario or reading Harry Potter, but the mental tasks didn’t involve any physical effort, so they shouldn’t influence physical performance. Should they? Well, perhaps the best way to summarise the findings of this research is with the title of an article published in the Journal of Applied Physiology (1): Mental fatigue impairs physical performance in humans. Below is a brief outline of the findings of this, and several similar studies:

• Mental fatigue decreased cycling time to exhaustion by nearly 2 minutes (16% decline in performance). Athletes cycled at 80% of peak power for 12:34 (12 minutes and 34 seconds)  in the control condition but only 10:40 when mentally fatigued (1).

• Mental fatigue decreased power output during two 10-min self-paced cycling bouts. When mentally fatigued, athletes produced by 17.5% less power when cycling at a fairly light pace and 8% less power at a hard pace (2).

• Mental fatigue decreased time to exhaustion in a sub-maximal contraction of the knee extensors by an average of 36 seconds (13% decline in performance). Athletes were able to maintain the contraction for 4:26 in the control condition but only 3:50 when mentally fatigued (3).

• A difficult reading task impaired 5-km running time trial performance by an average of 5.5% (1 minute and 18 seconds). On average, athletes took 23:06 to run 5 km in the control condition, compared to 24:24 in the difficult reading task condition (4).

• Giving long lectures or examinations decreased muscular endurance of the index finger (5). It’s a study from the 1800’s so I don’t have the data; sorry.

• Recently, we have found that mental fatigue reduces running velocity during intermittent running designed to simulate the demands of team sport.

• We have also seen impaired performance on soccer specific skills tasks in mentally fatigued players. You can find out more when these studies are published.

Overall it is pretty clear that being mentally fatigued is detrimental for several different types of physical performance. Practically speaking, this means it is important for athletes to avoid mentally fatiguing activity immediately before competition and perhaps even taper mental workload in a similar way to physical workload.

But what exactly is it that makes a task mentally fatiguing? Firstly, mentally fatiguing tasks lead to feelings of tiredness and lack of energy. Secondly, they are prolonged (at least 30 min in the studies mentioned above). Thirdly, they are boring but require focussed attention. Finally, they usually require response inhibition (stopping your natural response to a stimulus, before responding appropriately) like in the Stroop task. Here is a link to an example of the Stroop task, try it out and let me know how you went (my best score for the second round is just under 15 seconds): https://faculty.washington.edu/chudler/java/ready.html

Now imagine doing this task for at least 30 min and then going for a run. I’m sure you can see that you might not perform as well as usual. But what if athletes repeated this, or similar tasks a few times a week for a couple of months? Could we actually improve performance just like we do with physical training? This “Brain Endurance Training” is currently being investigated and the preliminary results seem promising, but I will save that for another post.