Hi there,
A normal human should be able to sustain a constant output of 200-300W over a reasonable amount of time.
Now convert that from mechanical force to electrical force and allow for losses of approx 60% then your electrical output would be in the region of 120W give or take.
This could in theory be delivered to the grid via a Grid Tie Inverter with more losses so at best you could deliver 100W continuously back to the grid.