Gretchen Reynolds via a kind John Dickert email::

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.