Ask A Scientist© Biology Archive

Question:  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?
Cathy C Fraser

Answer 1:
   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

Answer 2: 
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