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Shivering when nervous or anxious is a common thing.

But, shivering or trembling sometimes also occurs when we are performing a work which requires high accuracy. In such case, our whole body doesn't shivers but a part (usually hand) which is involved with that work shivers. The most common example is threading a needle. It requires great accuracy and you might have noticed that when you are doing so, your hands are continuously shaking which prevents you from accomplishing that task flawlessly. Even when we aren't in a state of nervousness, our hands tremble. It's un-resistable.

Yet another quick example - Try to stretch your one arm and try to keep it straight (have it loose, don't try to force it straight), you might notice that your arm is actually shaking a little bit. You can't hold it exactly straight without moving even a little bit.

It's also noticeable when we try to pin-point a small spot with our hand(finger) without actually touching it. (that finger and the hand as a whole starts trembling)

So, what's the reason? It isn't anxiousness as we shiver even when we are calm. Perhaps, it's gravity which is pulling our hand down and we are trying to lift it continuously which creates a imbalance? But, then we should shiver always?

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If you thread needles often - like a dozen times a day for many days - you will eventually be steady as a rock, because practice improves the process (you will also unconsciously maneuver to steady both hands.) It's not inevitable that all people have involuntary muscle movements with very fine motor skills; it's a matter of ability/practice for most.

The steadying of larger muscle groups like the upper arms - especially against gravity - is less controllable (e.g. threading a needle with arms fully extended), but not even closely outside the realm of possibility. Think of what great athletes, dancers, craftsmen are able to do that is "impossible" for mere mortals.

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    $\begingroup$ ..and what surgeons can do! +1 $\endgroup$ – AliceD Feb 24 '16 at 20:22
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    $\begingroup$ @Christiaan - D'oh! Great example! $\endgroup$ – anongoodnurse Feb 24 '16 at 20:37
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    $\begingroup$ A trivial example : applying an eyeliner flawlessly :P $\endgroup$ – biogirl Feb 26 '16 at 6:54
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Norepinephrine (noradrenalin) is a neurotransmitter and hormone released by the body in situations which, among other things, require focus. This may (partially) explain the shaking hands when focusing.

"The general function of norepinephrine is to mobilize the brain and body for action. Norepinephrine release is lowest during sleep, rises during wakefulness, and reaches much higher levels during situations of stress or danger, in the so-called fight-or-flight response. In the brain, norepinephrine increases arousal and alertness, promotes vigilance, enhances formation and retrieval of memory, and focuses attention; it also increases restlessness and anxiety."

The shaking occurs because the body is increasing burning of energy. There's not many good links online for this (unsurprisingly google searches of "noradrenalin shaking hands" pretty much only returns 'medical' websites trying to tell people they have a brain tumor), but I think what happens is the muscles twitch as a result of the excess energy release.

"When you suffer from anxiety, your body rushes with adrenaline - a hormone that gives your body a tremendous amount of energy, which - when unused - leads to physical agitation. That causes your hands and legs to shake, often visibly."

Beta-blockers are known to reduce symptoms of noradrenalin and are frequently (if questionably and irresponsibly) used to treat anxiety. We may shake less when we practice a process multiple times possibly because with practice anxiety associated with such tasks (I imagine a surgeon is quite nervous the first time they open up a patient) and the level of focus required (intricate tasks become easier with practice thus requiring less focus) both reduce, therefore reducing the amount of focus-related and anxiety-related physiological changes.

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  • $\begingroup$ While I would certainly agree that doing fine motor activity under intense focus might be stressful (and cause cortisol and norepinephrine release), intensively focused individuals - as @Christiaan mentioned - can perform fine-motor actions without trembling. While I'm not impressed with his political views, Ben Carson was amazing in the operating theater. Intense focus doesn't necessarily cause norepinephrine release, and inability to perform fine-motor actions smoothly is more likely a muscle control issue overcome with practice. Examples are innumerable. $\endgroup$ – anongoodnurse Feb 24 '16 at 23:13
  • $\begingroup$ It's (I believe) more related to neuroplasticity, for example demonstrated by the ability of stroke patients in regaining gross and then fine motor function with intensive therapy. Practice. $\endgroup$ – anongoodnurse Feb 24 '16 at 23:18
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    $\begingroup$ Perhaps as one gets more practiced they need less intensive focus and becomes less stressful - I know from my own lab work at the start I needed to focus a lot, now I can do it with my eyes closed! Practice makes most things much easier, thus requiring less intense focus (and meaning less noradrenaline release) $\endgroup$ – rg255 Feb 24 '16 at 23:19
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    $\begingroup$ I considered that as well. I think it's likely a combination of stress and ability in differing ratios under different circumstances. :) $\endgroup$ – anongoodnurse Feb 24 '16 at 23:22
  • $\begingroup$ @anongoodnurse and rg255 , both of your answers were pretty well written and answer my question to a satisfactory extent but both of you are holding slightly different reasons as the cause. Which one should I consider as the actual cause? Ability to control muscle movement or Norepinephrine release? or perhaps both of them? It's hard to decide the correct answer for me :P $\endgroup$ – Sarthak123 Feb 25 '16 at 6:34

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