system
Given the following post, respond as if you're commenting on Reddit:
human
Why do hands shake from nervousness or anxiety?

Would there have been any evolutionary benefit, or is it a side-effect of the normal functions of other bodily systems? Are there comparable reactions to fear in other species?
gpt
Everyone has some degree of tremor from the way that your brain and muscles work together. The normal tremor is called the physiologic tremor. Tremors are measured in hertz (Hz) or beats per minute. Physiologic tremors occur anywhere from 6-12 Hz (as opposed to Parkinsonian tremors which are 3-10 and essential tremor which is 4-8 Hz). Under stress (which could be psychological or mechanical like holding your arm out for a period of time), this tremor becomes exaggerated and is called an exaggerated physiologic tremor. The tremor is exaggerated because the muscle groups start to fire in groups. Adrenaline (aka epinephrine) enhances the effect. So interestingly, athletes can use beta blockers (like for your heart) to block the effect and reduce the tremor (that is why beta blockers are considered performance enhancing drugs in archery and rifle shooting).