A new review article in the British Journal of Sports Medicine, from researchers in Norway, looks at the question of blood pressure in athletes by pooling the results of 51 previous studies. It's well known that regular exercise lowers blood pressure in the general population (the figure quoted in the paper is that it lowers blood pressure by 4-9 mmHg), but does the same hold true for athletes training intensely?
The main point highlighted by the authors is that most blood pressure studies of athletes have been poorly done. Getting a reliable blood pressure reading involves taking a numbers of steps to make sure the subject is relaxed, averaging multiple readings, using the appropriate cuff size (a particular problem with muscular athletes), and so on. Few of the studies took the necessary steps to get reliable readings.
That said, the overall conclusion was that there isn't a major systematic difference between athletes and non-athletes. Of the 16 studies that compared athletes to non-athlete controls, athletes had higher blood pressure on average in seven of the studies and lower in nine of them. If you break the results down further, you find that strength-trained athletes have slightly higher blood pressure than endurance-trained athletes. There may also be a slight effect where training more than 10 hours a week produces higher blood pressure, but this difference wasn't statistically significant.
One question that remains open is whether adaptations to the heart that occur with prolonged training, like an enlarged left ventricle, may either contribute to or result from elevated blood pressure. For now, the most unambiguous conclusion we can draw from this data is that exercise, no matter how much you do, doesn't give you a free pass from hypertension. You've still got to pay attention to other aspects of lifestyle, particularly diet.