From a reader:
When considering an energy source, such as oil, or human powered hamster wheels, you need to consider the total energy cost to generate the energy, figuring out the Energy Return on Investment. The EROEI for oil is pretty high… usually considered 30:1 or 25:1… It costs 1 barrel of oil to get 25 out of the ground. (You can see more at http://www.eroei.com/eval/net_energy_list.html) .
Sadly, the EROEI for things like most human food is less than one. It takes more than a corn cob’s worth of energy to raise and reap a corn stalk, which has some amount of energy (measured in calories).
Because humans need to eat food, and food is typically at EROEI of <1, and humans are also pretty weak, relative to say, a small internal combustion engine, the EROEI for humans is <1 and it's pretty pointless to suggest using them to generate energy.
(Which, by the way, is why the basic premise of The Matrix – that humans were being used for just that – is so stupid).