Question: Don't the electric shocks you give hurt people?

Simon Marchant answered on 22 Jun 2015:

Good question, my young padawan (this looks like a class, but I’m not sure what the plural of padawan is). It can hurt. The “sensory threshold” of electrical current that you can feel is below the “motor threshold” where the current can make your muscles contract. So if we move your muscles, you can probably feel it. But all is not lost! When we fit the device, we slowly ramp up the power until we get the amount of movement that we want. We ask the person to tell us if it hurts, and if it hurts before we get the movement we want then we will try other things (like moving the place we put the electrical current, or changing how the voltage rises and falls). So we can usually get the result we want without hurting our patient! Not always though, sometimes we have to give up. The human body is weird and nobody really understands it.

Interestingly, you can apply a magnetic field instead of an electrical one, which does the same thing (because of Faraday’s laws, in case you’re interested) but can go deeper under your skin. It can even penetrate your skull and make your muscles move if you put the magnet over the right area (I had this done to me once by the inventor of transcranial magnetic stimulation – also known as magnets on brains). There are no pain receptors in your brain, so it will never hurt! Unfortunately, it also takes MASSIVE amounts of power: the inventor says that making a leg move enough to walk takes so much power from the mains that it makes lights dim all over his building!