The concept that prisoners should become slaves doesn't mean that they should be used inefficiently. There is much talent in prison that would be better used and more financially taken advantage of in light of their potentials beyond being muscle machines.

There is not enough energy to be harvested to be worth setting up the program.

But it's a great way to keep prisoners unhappy. That way, when they finish serving their sentence and discover they cannot get jobs and have lost their power to vote they are not only mean, they are strong and can cause real trouble so that they are soon back in prison. And all those prisons in the USA that need prisoners to keep the working guards working and making profits for the private security companies will find more reasons to build more prisons.

It was either the National Geographic channel or the Discovery channel where they showed how this group in Africa is putting merry-go-rounds in parks that generate power for fresh water pumping and filtering.

Unfortunately the amount of power consumption in the Western World is thousands time more than what would be generated by this.

Two comments:

1. A fit, regularly exercising adult male human (not an elite athlete) can sustain an output of 0.15 hp (113 watts) for 5 hours until complete exhaustion. Of course other time/effort combinations are possible but this is just about the optimum for maximum energy output per day. 113 watts for 5 hours is 0.565 kilowatt hours per day, which is worth somewhere on the order of 4 cents per day, depending on the local cost of electricity. In other words, totally and utterly worthless. Simply put, whatever are person's creative potential we are lousy as sources of raw power which is perhaps why modern socities flourished through the abolition of slavery and replacing raw muscle with powerful machines.

2. Four words: "cruel and unusual punishment". A century before people put them in gyms, many prisons did had treadmills for the punishment of convicts sentenced to "hard labour". Two hours per day was the basic level of punishment and was considered roughly equivalent to "stone breaking" (i.e. spending all day in a quarry breaking up rocks with a sledgehammer.) Especially vicious or recalcitrant prisoners might get 6 hours per day on the wheel, but not many survived that for long. Most treadmills were removed in the 1870s if not before, being seen as barbarous. Your odds of getting it past the Supreme Court are hence very, very slim.

What if a con refuses to participate ?