Breathing heavily, the brigade of stroller-pushing mothers lunge, stretch and jump their way through Manhattan's Central Park. In the lead: 43-year-old parent of three Elizabeth Trindade. "I have not walked into a gym in 10 years," she declares proudly, "because being a mom is a gym."

Trindade is the founder of Strollercize, a program that rallies mothers into getting fit using mom-friendly exercises such as stroller lunges and biceps curls using baby-formula cans. The program, which last year grossed $250,000, is expanding to other cities, and a book is due next spring. Among the 4,000 or so converts: Manhattan software salesperson Kristen Brewer, 32, who enrolled in classes last year after the birth of her daughter Claire. "I have much more energy," says Brewer. "Mentally I'm a little more grounded."

Trindade knows how important that can be. A personal trainer, now divorced, she says she was "depressed and lonely" after giving birth to her first child, Tatiana, in 1990. Nightly stroller walks lifted her spirits and helped her lose weight, and in 1995 she began recruiting mothers to Strollercize with her. How does Trindade envision herself when her children are grown? "A hot grandma," she says, "with a whole bunch of grandmas behind me strolling their grandchildren."