A few years back we came across these electromyographic muscle sensors sold at SparkFun:
They have three electrodes that you attach around a muscle that can detect the amount that the muscle is flexing. It does this by measuring the filtered and rectified electrical activity of a muscle; feeding in positive and then negative voltage and measuring amount of of voltage loss across the muscle. That output voltage will depend the degree of contraction in the selected muscle. Stick on a few electrodes attach to an Arduino and read the analog value and flex some muscles.
We completely forgot that we had these sitting in a box but then while looking for something else, we came across them and after remembering what they actually were thought we had to do something with them. Since we have a two we had to make a game out of it. Two players go head to head, seeing who can flex the hardest for 60 seconds, with a power bar and a cumulative score and, of course, some flashing and heavy metal.
It is surprisingly hard to flex for 60 seconds! The version of the sensors that we’re using has been discontinued but there’s a new and improved sensor here that you should check out.