Members of Gymbox in Bank, central London, can choose to lift any of five differently sized "human weights".
Wearing Lycra catsuits which label their weight, they sit on specially adapted machines and shout words of encouragement.
The "dumbbells" include two dwarfs – 32-year-old Arti Shah, who weighs just 4.5 stones (30kg) and 64-year-old Mike Edwards, who weighs 8 stones (55kg).
At the other end of the scale for those with stronger muscles is 24 stones (155kg) Matt Barnard, 37.
Gymbox owner Richard Hilton said: "A lot of our members felt that lifting metal weights was boring and not especially motivating.
"They said that they had no idea what they were lifting.
"Experts in sports psychology agree that visualisation is a significant factor in improving performance and human weights are as visual as it can get.
"Our human weights will, at the gym-goers request, shout encouragement like 'Come on, lift me harder and faster' and 'You're doing great. Look at those muscles building'.
"Otherwise, they will just keep quiet and keep still as they are lifted."
The gym already runs a "Chav fighting" self-defence class, "boob aerobics" to help women increase their bust size and "WAG workouts" aimed at making female members more attractive to footballers.
