Posted: Monday, February 17, 2014, 7:00 AM

SUNDAY, Feb. 16, 2014 (HealthDay News) -- As millions of aging Baby Boomers know, muscle tone and strength declines with advancing age, regardless of gym workouts. Now scientists say they might have uncovered a clue as to why that happens -- and new cell targets to help reverse it.

In studies in aging mice, researchers at Stanford University found that, over time, the stem cells that help repair damaged muscle cells after injury are less able to do so.

This helps explain why regaining strength and recovering from a muscle injury gets more difficult with age, the researchers said in work published online Feb. 16 in the journal Nature Medicine.

"In the past, it's been thought that muscle stem cells themselves don't change with age, and that any loss of function is primarily due to external factors in the cells' environment," study senior author Helen Blau, director of Stanford's Baxter Laboratory for Stem Cell Biology, said in a university news release.

"However, when we isolated stem cells from older mice, we found that they exhibit profound changes with age," said Blau, a professor of microbiology and immunology at the university. "Two-thirds of the cells are dysfunctional when compared to those from younger mice, and the defect persists even when transplanted into young muscles."

The research also revealed, however, that there is a defect specific to old muscle stem cells that can be corrected, allowing scientists to rejuvenate the cells.

"Most exciting is that we also discovered a way to overcome the defect," Blau said. "As a result, we have a new therapeutic target that could one day be used to help elderly human patients repair muscle damage."

The muscle stem cells in 2-year-old mice are the equivalent of those found in 80-years-old humans. In conducting the study, the researchers found that many muscle stem cells from these mice had increased activity in a certain biological pathway that interferes with the production of the stem cells.

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Scientists Get Closer to Rejuvenating Aging Muscles