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.
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 visualization 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."