Page "Exercise physiology" Paragraph 20

At rest, the human brain receives 15 % of total cardiac output, and uses 20 % of the body's energy consumption.

The brain is normally dependent for its high energy expenditure upon aerobic metabolism.

The brain as a result is highly sensitive to failure of its oxygen supply with loss of consciousness occurring within six to seven seconds, with its EEG going flat in 23 seconds.

If it affected the oxygen and glucose supply to the brain, the metabolic demands of exercise could therefore quickly disrupt its functioning.