Exercise: Does it help or hurt telomere length and aging?

There seems to be conflicting evidence on the effects of exercise on telomere length.  For some people, telomeres appear to shorten dramatically, presumably due to the stress on cells to repair damage caused by exercise.  For other people, however, telomeres actually seem to get longer with exercise.  The medical community say that exercise is good, and this message is constant and clear.  But if some people age prematurely as a result of exercise, doesn't it make sense to know why?  If we can identify those people whom it will hurt, we can tell them not to exercise.

Scand J Med Sci Sports. 2009 Sep 17