So, here's an interesting question. I was at the shooting range today, and I noticed that when trying to aim, and when my dad was trying to aim, there was some shaking, making it just a little bit difficult. It seems hard to have a muscle evenly acting for a few seconds - simply put, there was a little bit of shaking/trembling. You can also see it when weight-lifters are straining to lift a very heavy object. Now, I'm not looking for "low blood sugar" or "arthritis" or such as to "what causes shaking", but the root fundamental cause. One thought that crossed my mind is perhaps the signal from the neurons is a bit noisy - but that doesn't really make sense because you can feel shaking, which means you're receiving a sensory signal that's not as noisy as the signal that's triggering the muscles to contract. So all the way down to the physics-level, why is it that muscles shake? Another thread I looked up on spasms said that they occur because of buildup of ions that can cause nerves to fire without receiving a command from the brain (I assume as a small batch of ions at one end of a synaptic gap disperse over time until they bridge a gap and allow a neuron to fire?). Perhaps it's similar to this somehow?