In my teenage fantasies about keeping a girl prisoner in the cellar, one of the things I loved to imagine doing to her was to make her work out under the whip till she dropped in an exhausted heap, begging for mercy between gasps for breath.

Now that I can do it for real, I have a few gadgets to make it more fun. My favourite is an exercycle with the seat replaced by a bar topped with rounded spikes: I set the height so that the victim has to stand up on the pedals and it still hurts. It doesn't take much of that to tire out most people.