Mental fatigue impairs our physical performance.

This study by Marcora et al. compared subjects who first either watched a movie for 90 min., or did computer exercises requiring concentration, memory and reaction speed - following which all exercised on a stationary bicycle until they were exhausted — that is, unable to maintain a cadence of 60 revolutions per minute. There were no significant differences in physiological measures (heart rate, cardiac output and others) under the two conditions, but bicyclers consistently tired about 15 percent more quickly after the mental exercise than after watching the movies. Apparently their poorer exercise results after mental effort were not caused by reduced performance of their bodies, but because mental fatigue limits exercise tolerance through higher perception of effort.