MadSci Network: Anatomy

Re: Can overstimulation of certain muscles produce temporary superstrength ?

Date: Mon Feb 20 09:17:30 2006
Posted By: Mitchell Maltenfort, Staff, Neurosurgery, Thomas Jefferson University
Area of science: Anatomy
ID: 1139939746.An

That "urban legend" of the mother lifting up a car has actually been
verified by Cecil Adams of "The Straight Dope": see
 However, it looks like
the car was only lifted a few inches -- not as dramatic as Superman's
feats, but enough for someone to get a car jack in (as in the Adams story)
or to wriggle out.

Muscle force is generated by the action of actin and myosin cross-bridges
in muscle fibers. Muscle contraction is actually temporary: the muscle
"twitches" and then relaxes, to twitch again.  There's a nice review of
this at

Muscle contraction seems smooth because each muscle can have up to a few
hundred motor units, and so the twitches blend into a near-constant force
the same way that many voices speaking at once blend into a dull rumble.  

To focus on your question: the maximum force that the muscle can generate
depends on the number of cross-bridges.  You can increase the number of
cross-bridges by building muscle, but you can't amplify the force by
electricity or adrenaline. What you can do with them is to increase the
excitation to the muscles.  

A large part of our muscle strength is only used for so-called "ballistic"
movements, like jumping or running.  These actions call on "fast" muscle
fibers, which are very strong but tire quickly.  Standing and walking
require "slow" muscle fibers, which are weaker but can maintain force for a
long period. The larger the excitation, the more units get recruited, and
the "size principle" of Edward Henneman (see
states that
larger units are last to be recruited. 

If you're at an institution with a subscription to the Journal of 
Neurophysiology where you can get the Henneman papers, also look for
Walmsley B, Hodgson JA, Burke RE.  Forces produced by medial gastrocnemius
and soleus muscles during locomotion in freely moving cats. J Neurophysiol.
1978 Sep;41(5):1203-16 (or at

Under an electrical stimulus, it is theoretically possible to get all of
the motor units contracting at the same time.  The force would drop off
quickly, though, as the stronger "fast" muscle fibers got fatgiued.  

How much  larger is this "peak" muscle force versus what we can call upon
normally?  I didn't see it in the literature, and I'm not surprised one
bit.  To apply electrical stimulation to a person's muscle just to "max
out" the force risks is a mischevious experiment, risking injury for litle
possible scientific gain.  

What a lot of people are trying to do right now is use electrical
stimulation to _reproduce_ normal (sub-maximal) muscle contraction in limbs
paralyzed by spinal cord or other central nervous system injury.  It's more
challenging than you think -- for one thing, the larger units which should
be recruited last are the _first_ to respond to electrical stimulus!

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