Tuesday mornings in the Cambridge Public Library's lecture room aren't reserved for the pursuit of scholarly knowledge these days.

Instead, 40 senior citizens take a break to twist and bend their bodies to patriotic marching tunes and the instructions of Sylvia Piltch, a retired physical therapist.

"Forward and hold...three, four, five...and back...Don't forget to breathe in because it's good for the heart," says Piltch, who considers herself a "young senior citizen."

Piltch, a life-long Cantabridgian who began teaching aerobics to seniors upon retiring as Cambridge Hospital's director of physical therapy in July 1987, says the exercise has revitalized many of the seniors.

"There is no question that these classes have been successful," Piltch says. "We have been able to help them feel good about themselves and keep them moving. More of them can do things about the house and outdoors, go walking more often and even go swimming."

Florence Renna, a two-year veteran of Piltch's guidance, says the aerobics have helped her immensely.

"These classes are wonderful. I had a lot of aches and pains when I started exercising here, but the class makes the pains go away. My posture is better, and I'm more conscious of how I walk."

Piltch says the elders, who range in age from 65 to 87 years, have attended her sessions at the library for more than two and a half years. She says she began similar programs at the North Cambridge Senior Center and a war veterans home in Cambridge three years ago, as well a class at a Cambridge apartment complex in the past year.

"I could be all over the city," Piltch says, "But I'm limiting myself to the four classes at this point."

In their sessions, which last between 30 and 45 minutes, the seniors stretch and swing most all parts of their body, performing everything from chair pushups and leg kicks to neck rotations and finger exercises.

"We try to work every muscle of the body, and even some they didn't know they had," Piltch says. "The one thing we won't do is get down on the floor, because someone might have to pick us up."

Members of the class say that Piltch has been the difference in the program's success, including Margaret J. Sullivan '29, who spent her college years at Radcliffe before teaching high school mathematics in Cambridge.

"Sylvia was my pupil in high school math, and now I'm her pupil in physical education," says Renna, who participates in the exercises with her two sisters. "She's a much better teacher."

As for Piltch, she says she is content to help people turn back the clocks as they grow older.

"I haven't experienced any problems with the programs," Piltch says. "Everybody is ship-shape, and as they get older they get better."