When he hit 50, Tim Carrigan’s lower back started hurting so badly he could barely walk.

The injury, which dated to a childhood accident, had caused only occasional pain until Mr. Carrigan lost muscle tone with age. The pain dogged him for several years, but last year the Quincy, Mass., insurance-company treasurer started strength training twice a week on a circuit of a dozen machines.

Not only did his back improve, “I feel better. I feel stronger. I sleep better,” says Mr. Carrigan, now 54. He adds that his stronger back has held up while shoveling during Boston’s historic snowstorms.

While old-school wisdom held that older adults were too frail to pump iron, a growing body of research is showing that strength training helps stave off age-related disability, preserve bone mass in women and even boost brainpower.

“It’s way more dangerous to not be active as an older adult,” says Miriam Nelson, professor of nutrition at Tufts University’s Friedman School of Nutrition Science and Policy in Boston.