Saturday, Nov 01, 2014

Mental fatigue weighs heavily on exercise

Written by New York Times | Posted: October 5, 2013 3:08 am

Gretchen Reynolds

Tire your brain and your body may follow,a remarkable new study of mental fatigue finds. Strenuous mental exertion may lessen endurance and lead to shortened workouts,even if,in strict physiological terms,your body still has plenty of energy reserves.

Scientists have long been intrigued by the idea that physical exertion affects our ability to think,with most studies finding that short bouts of exercise typically improve cognition. Prolonged and exhausting physical exercise,on the other hand,may leave practitioners too worn out to think clearly,at least for a short period of time.

But the inverse possibility — that too much thinking might impair physical performance — has received far less attention. So scientists from the University of Kent in England and the French Institute of Health and Medical Research,known as INSERM,joined forces to investigate the matter. For a study published online in May in Medicine & Science in Sports & Exercise,they decided to tire volunteers’ brains with a mentally demanding computer word game and see how well their bodies would perform afterward.

Fatigue is a complex,multifaceted condition. Exercise science usually concentrates on bodily fatigue,meaning a reduction in our ability to contract muscles and stay in motion. Run,cycle,lift weights or just stand,and a small army of muscles contract,burning fuel and eventually tiring. This fatigue occurs both within the individual muscles and at the level of the nervous system,a condition known as central fatigue.

Our minds tire,too,although the causes are difficult to pin down. Neurons may run low of fuel,and other processes probably also are involved. But it is clear,as many of us know from personal experience,that concentrating intensively on an intellectually demanding project for hours typically leaves you feeling mentally dull.

The researchers found that mental fatigue significantly affected volunteers’ endurance. They tired about 13 percent faster after the computer test than after watching “Earth”,a serene 90-minute documentary. They also reported that the workout felt far more taxing.

In simpler terms,exercise simply feels harder when your brain is tired,so you quit earlier,although objectively,your muscles are still somewhat fresh.

This finding has multiple implications for how we combine ratiocination and sweat. It suggests,for instance,that the morning of an important race or challenging training session may not be the ideal time to finish your taxes,since overthinking could lead to underperforming physically.

Inversely,the results also suggest that “training our brain to avoid or limit mental fatigue” could be a hitherto untapped means of improving physical performance,said Romuald Lepers,a professor at the INSERM research laboratory at the University of Burgundy in France and,with Samuele M. Marcora and Benjamin Pageaux of the University of Kent,co-author of the study.

Training yourself to speed through crossword puzzles,in other words,might improve your workouts,by subtly altering how mind and muscles communicate and making your brain less likely to consider your muscles easily enfeebled.

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