With his enormous muscles bulging beneath a small T-shirt, Martin Luther King Addo guided one of his most dedicated clients through a squat exercise inside his tiny Manhattan gym. “You can do it, Shirley,” he said.

Shirley Friedman, a silver-haired 90-year-old standing 4 feet 9 inches, shifted into another gear, bending at the knees for multiple repetitions.

“I never did this stuff before, but he gives you the confidence that you can do it, if you’re up to it,” Mrs. Friedman said afterward. “He’s not a phony. Got me?”

Mr. Addo, who honed his muscles using a mango tree as a pull-up bar and concrete blocks for dumbbells, is a two-time former winner of the Mr. Ghana bodybuilding championship. Years ago, his chiseled physique, bowling ball biceps and camera-ready smile brought him fame across his homeland in West Africa. Today Mr. Addo uses his imposing muscles and hard-won expertise to help frail seniors like Mrs. Friedman restore their balance, mobility and strength.