In the 19th century they used to put prisoners on the treadmill. they would be on the mills for hours at a time, with no actual benefit or productive result. Could we not build huge treadmills attached to the National Grid, and put prisoners to work on them in order to produce man power? There is no shortage of prisoners, and they would be paying back to society and keeping fit at the same time. There could be reductions in sentences for the most productive prisoners. Any reduction in the use of fossil or nuclear energy would benefit our world and it seems a shame to waste all that manpower.