Immediately after a prolonged bout of exercise, the blood pressure falls to levels below its resting value. This can occur after any type of exercise which uses large muscle groups, such as walking, running, bicycling, and swimming. The lowest level is seen about 30 minutes after the end of the exercise and may be as much as 20 mm Hg (systolic) below the resting level. Over the next three to four hours, it gradually returns to baseline.
This fall of blood pressure occurs in virtually everybody, particularly after more prolonged periods of exercise (20 minutes or longer), but the changes are more pronounced in people with higher blood pressures-- the pressure falls more, and stays down longer, sometimes as much as 12 hours.