Healthy older men who given large doses of testosterone show increases in muscle mass and power, but no improvement in physical function, a new study shows.

This was likely because the men in the current study were "unusually fit for their age," Dr. Thomas W. Storer of Boston University School of Medicine and colleagues say, meaning that the strength and muscle they gained didn't affect their already-excellent physical function.

Future investigations of testosterone therapy should be conducted in people who do have functional limitations, the researchers say, "so that there is room for demonstrable improvement in function with increased muscle strength."

Giving men extra testosterone can build muscle, but studies investigating its effects on performance and function have had mixed results, Storer and his team note in the Journal of the American Geriatrics Society.

To better understand the hormone's effects on physical function, the researchers blocked normal testosterone production in 44 60- to 75-year-old men and then administered testosterone shots. They received 25, 50, 125, or 300 milligrams of the hormone every week for 20 weeks. Some men had initially been getting a 600-milligram dose, but this portion of the study was halted because they developed leg swelling and excessive red blood cell production.

The men in the top two dose groups showed significant increases in skeletal muscle mass. While each group showed an increase in the amount of weight they could press with their legs, this was only statistically significant in the men on the highest testosterone dose. On average, there was no improvement in the number of leg presses the men could do without becoming fatigued.

And when the researchers tested the men's physical function using six different tasks involving walking, stair climbing, and rising from a sitting position, there was no difference in performance between baseline and after 20 weeks on testosterone for any of the groups.

When considering the use of testosterone to improve physical function, Storer and his team note, the hormone's risks should be taken into account; these range from acne and breast enlargement and tenderness to possibly promoting aggressive prostate cancer growth. Extra-high doses, they add, may be associated with a greater likelihood of side effects.

SOURCE: Journal of the American Geriatrics Society, November 2008.