The study involved 167 runners, with an average age of 50, who participated in the 2006 and 2007 Berlin Marathon races.
All participants had completed at least one 26-mile race.
Doctors examined them 10 days before the race, as soon as they crossed the finish line, and two weeks after the race.
Knebel says their heart rates jumped from an average of 62 beats per minute before the race to 88 beats per minute by the end of the marathon.
Also, more than half of the runners showed increases in levels of troponin T and/or N-terminal pro-B-type natriuretic peptide, both markers of heart damage, after they crossed the finish line.
"Two weeks after a marathon, these key parameters were all back to normal levels," Knebel says.
"Concerns about marathon running causing sustained damage to the heart appear to be unfounded," he says. Echocardiogram testing performed in each runner showed normal results two weeks after the marathon.
Dehydration Blamed for Transient Heart Changes
The most likely cause of the temporary changes in heart function was dehydration, Knebel says.
"Dehydration is probably the No. 1 reason for health problems in marathon runners," says American Heart Association spokesman Ray Gibbons, a professor of medicine at the Mayo Clinic in Rochester, Minn.
No matter what your age, dehydration can lead to dangerously rapid heartbeats, he tells WebMD.
"You really have to work at drinking at all those water stops along the way, which most runners don't want to do," Gibbons says.