BackScratcher

Mythbusters did an episode on this. C4 can't be set off with anything short of a blasting cap, you could hit it with a sledgehammer and it still wouldn't go off. It's actually pretty awesome how stable C4 is.

It should never be about wanting to go the the gym. You have to go, treat it like a second job. Only excuse is being sick or injured. If you only go when you feel like it you cannot build a habit.