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Name: Cathy C Fraser
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I am a rock climber and have noticed that when you get to a particular hard part of a climb, your legs start shaking. This is what we call sewing machine legs. It would make more sense that in times of fright your body would want to conserve energy NOT use it so quickly. Why does our body do this?

Muscles work in pairs, the extensors and flexors each pulling oppositely on a movable bone. Ordinarily even when "resting" there is a small force a muscle exerts, your muscle "tone." Extensors and flexors are under nearly equal "tone" force, though, so there is no motion. Or almost none. In fact the slight variations from second to second of the muscle pull and other environmental factors (exact weight the limb is bearing, etc.), and the attempts of the nervous system to correct for these, will give a minute and constantly changing force imbalance, so that the limb will actually wiggle a little bit. If you try to hold your fingertips 1 mm apart you will see the wiggle. When you are frightened, one of the responses of your body (mediated by adrenaline, among other things), is to increase muscle tone. The wiggles get stronger and faster, and can exceed the nervous system's ability to control them. You can see the same thing by drinking loads of coffee, which mimics some of the chemistry of the fright response. Why increase tone under threat? I would guess because accurate limb motion is faster if you have the muscle force aimed and ready to go, being held back by an opposing muscle that only has to relax for the motion to occur, than if you need to contract and aim a relaxed muscle. Generally the body does not slow down in times of crisis, perhaps because during most of our evolution we were not under control of a conscious brain, for which thinking a little longer can generate a better solution to a problem than acting immediately. For a brain ruled by instinct the solution it already has is the only one it will ever have. There's no point to waiting, and lots of advantage in acting quickly.

Christopher Grayce

I buy the first part of the last answer (Christopher Grayce's) but the tension and aiming/loading of a muscle for the "extra response" under stress does not make complete sense in terms of control. I guess the trade off is strength and reaction time for finesse. None of this makes sense in terms of the martial arts, which summons super control under all types of stress. Here is a case where training can override rudimentary biochemical and nervous reactions. It would be interesting to see if sewing machine legs can be overcome via meditation and one of the best martial arts. If so, what would be the physiological mechanism?

Lou Harnisch

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