Exercise causes number of cell divisions to approach Hayflick limit faster? And hence shorten life expectancy?
Following damage to muscle (e.g., as a result of strenuous activity) the injured fibers are removed (by immune cells) and then replaced by new myofibers (individual muscle cells that join to create new muscle tissue). This is why for a few days after exercise your muscles can be sore - it is the tissues being initially cleared of damaged cells and repaired. 

Muscles, like other tissues, have resident stem cell populations ("satellite cells") that are used to replace damaged cells. 

Stem cells have potentially indefinite replicative potential, as they express telomerase and thus do not have a traditional Hayflick limit.




Volonte D, Liu Y, Galbiati F. The modulation of caveolin-1 expression controls satellite cell activation during muscle repair. FASEB journal : official publication of the Federation of American Societies for Experimental Biology. 2005 Feb;19(2):237–9.