April 22 (Bloomberg) -- Intense weight lifting to increase strength may stiffen heart muscle, Harvard University scientists said, urging more research into possible health consequences.

After 90 days of strength training including weight lifting, football players had thicker walls in a chamber of their hearts, decreasing the ability of the muscle to relax, according to a study released today by the Journal of Applied Physiology. Rowers, whose conditioning stresses endurance, had different changes.

More serious reductions in heart relaxation are often seen in people with a form of heart failure, said lead author Aaron Baggish, a Massachusetts General Hospital cardiologist. About 5 million people in the U.S. have heart failure, a condition in which the organ can't pump enough blood, according to the U.S. National Institutes of Health. Most patients are 65 and older.

``The question of whether strength training puts people at risk for heart failure needs more investigation,'' he said today in a telephone interview.

While athletes generally have strong hearts, studies hadn't established whether they were the result of exercise or genes. Baggish said his study is the first to look at changes in the structure and function of the heart as it adapts to vigorous, regular exercise.

The study used an imaging method called echocardiography to track changes in 20 female and 20 male rowers, and 35 male football players, from the beginning of their training for the fall season in 2006.

Muscle Growth

After three months, muscle growth in the left ventricle, the heart chamber that pumps blood into the aorta, was virtually the same for rowers and football players. Mass in the left ventricle grew 11 percent among rowers, and 12 percent among football players.

Subtle differences arose in how those changes appeared in the two groups, Baggish said. Endurance training led to more volume in the heart's left ventricle; strength training increased the size of the muscle wall in the same area, he said.

That increase in size makes the chamber stiffer and less able to relax, he said. In contrast, the changes brought about by endurance training may be protective against heart disease, he said.

``We have pretty significant data that suggests that not all training is equal,'' Baggish said. ``The implications for long- term heart health are still unknown.''

To contact the reporter on this story: John Lauerman in Boston at jlauerman@bloomberg.net.

Last Updated: April 22, 2008 17:35 EDT