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."
Full story at the Telegraph