Which Arm is stronger?

The reason for this is that the nervous system is wired up, so that you balance out movements. so that if a muscle gets made longer than it thinks it should be, you have an organ called a spindle, which is inside the muscle, which signals length of the muscle and if that gets stretched it feeds back on to the mototr nerve supplying the muscle and increases the firing of the motor nerve-making the muscle get a bit shorter. So if you have 2 antagonistic muscles, one hand and the other hand, pulling against the other then one's going to win a bit and that's going to make the muscle on the other arm get a bit stonger then that will pull back and that one will win a bit and make the muscle on that side pull harder and so on. And it will just do a tug-of-war so you won't go anywhere because the brain is set up so you're finger's in the middle.