# Why does flicking a finger generate so much more force than extending it quickly?

Flicking a finger (holding it back with e.g. the thumb while building up "pressure" against the thumb, then releasing) is much more powerful than just uncurling the finger quickly.

I tried to do the same with other body parts, like flexing my arm and holding it back with my hand as I tried to straighten it, but the force from releasing that doesn't seem to be more powerful than simply punching out the arm flexed to extended without holding it back in any way.

Are fingers special?

What causes the body to be able to generate extra power like this?

It makes me think of shooting rubber bands.

• No, the fingers are not special. Well, they're a miracle, actually, like the entire body is, but no more special than the foot or arm or anything else. – anongoodnurse Aug 23 '19 at 1:33

Your body in this case is generating more "power" by having something to push against.

When flicking your finger, your finger is pushing against your thumb and that pressure you feel is the force of both your finger against your thumb and your thumb against your finger. This is important as you can only apply so much force to a mass before it gives out and moves. Your thumb has much more mass than air so you can build up a lot of force on your thumb before releasing your finger and sent it accelerating. However, when extending your finger in the open air, it is only pushing against that, and air quite obviously has little mass to build off of.

You can experience this in another way; pushing against the wall creates builds up more force than shoving the air. If you were to somehow instantaneously make the wall disappear, your arms would extend faster than you can shove.

Using your hands to hold back your arm is correct, but the force you generate in that method is too little compared to the mass of an entire arm. Think about the mass:force ratio between a finger and the force between a flick, and an arm and the force holding back. (Edit:) Also, when letting your arm go, your hand stays stuck to the arm for a much longer period than your thumb against the tip of your finger.

• You've not explained how it works anatomically, nor is your answer very scientific at all. I still don't understand what the mechanics behind the flick are . – theonlygusti Aug 22 '19 at 19:51
• @theonlygusti - This answer is correct, and quite scientific. It involves physics: friction, force, mass, etc. If you cannot understand this answer, one delving into the molecular phenomena (actin-myosin coupling and uncoupling building up more force) will be of no value to you at all. – anongoodnurse Aug 23 '19 at 1:29
• @MrMineHeads - Thank you for a nice (and correct) explanation and first answer. Welcome and +1! – anongoodnurse Aug 23 '19 at 1:33
• @anongoodnurse what does "build up force" mean? – theonlygusti Aug 23 '19 at 10:10
• @theonlygusti What I meant by a "build up of force" is that you can push much harder against your thumb than you can extending your finger because extending your finger has no opposite (very little) force pushing back (Newton's 3rd law). At that point, the only way your finger can gain velocity is by your muscles contracting quickly. Compared to pushing against your thumb where your muscles are much better at exerting their potential, the force your muscles in your forearms is smaller. If you'd like I can draw a rough FBD of exactly what is going on, but I feel like the explanation is robust. – MrMineHeads Aug 23 '19 at 10:20

I tried to do the same with other body parts, like flexing my arm and holding it back with my hand as I tried to straighten it, but the force from releasing that doesn't seem to be more powerful than simply punching out the arm flexed to extended without holding it back in any way.

The answer above is spot on. But let me respond (in kind, i.e. like the answer above) to this aspect of your question, hoping you can relate it to the answer above.

The arm question is exactly like the finger question; you're just looking at it kind of in the wrong way.

If you bend your elbow, say, to 90° at a distance of 1 inch from a pane of glass, then try to extend your arm as forcefully as you can with someone holding your arm back at the wrist, if they suddenly let go, your arm may well have enough force built up to break the glass. (This depends somewhat on the strength of your triceps, not an overly strong muscle in non-athletes.)

However, bend your elbow, say, to 90° at a distance of 1 inch from a pane of glass, then extend your arm as forcefully as you can into the glass, It probably won't break, because you can't build up enough force in your triceps over the course of that inch without resistance.

This is exactly analogous to the finger example explained above, except for the size and appearance of the fingers. It is analogous even to the extent that we do not use our finger extensors against resistance very often, much like our triceps.

You can apply this to any muscle-joint group in the body. The outcome will be roughly the same.