They have done away with body building program in US prisons because of the same reason. But inmates are very creative and many have developed their own calisthenics (body weight only) exercise programs using very limited space with impressive results.
I also read that prisoners in US could make stabbing weapons out of rolled up newspaper. Criminal minds are dangerous
Changi also can.
Plastic bags are prized commodities that can be filled with water for weight training. That’s why every single rubbish bags must be accounted for in prison. Plastic bags can also be made into strings and ropes. Very useful.