Science Questions

Which Arm is stronger?

Sun, 25th Oct 2009

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Question

John Kay asked:

When I link my fingers together so my arms are connected, and I try and push or pull them against each other, to find out which arm is strongest, my hands stay in the middle and my stronger arm never seems to get anywhere. Why is this?

Answer

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.

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jonK asked the Naked Scientists: Dear naked scientists. This is a bit of a stupid and random question but its been bugging me for a while. When I link all my fingers together so that my arms are connected and I try to push or pull them against each other (as if to find out which arm is stronger) my hands stay in the middle and my stronger arm never seems to get anywhere. Why is this? If this is something to do with the brain is there any evolutionary benefit in humans having this. Or maybe I am just being stupid and not taking into acount that i am locking my arms into a shape that isn't very flexible. =) Thanks, I also really enjoy your show/podcast and am happy that I found it when I was just randomly going through iTunes. =) What do you think? jonK, Tue, 28th Apr 2009

I think perhaps its a bit like tickling yourself and not feel ticklish? Chemistry4me, Wed, 29th Apr 2009

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