Instead of visiting the gym alone, Joe Duffy and his wife exercise twice a week with a group of seven to nine people and a personal trainer.

Though it sounds like an oxymoron — personal training in a group? — it is a growing trend at large health clubs and small gyms. A survey by the IDEA Health and Fitness Association, an organization for fitness professionals, showed that 71 percent of personal trainers had two or more clients per session in 2005, up from 50 percent in 1999; 44 percent said they had groups of three to five clients, up from 43 percent nine years ago.

Besides the significant cost savings — depending on the size of the group, it can cost as little as $20 a session, compared with as much as $100 for individual training — the group dynamic can foster camaraderie and lure more people into the club. While trainers and patrons say that’s a lot of fun, group training sessions can also feel a bit like being back in the proverbial sandbox.