|MadSci Network: Anatomy|
That "urban legend" of the mother lifting up a car has actually been verified by Cecil Adams of "The Straight Dope": see http://www.straightdope.com/columns/060120.html. 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 http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Muscles.html. 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 http://www.the-aps.org/publications/classics/originals.htm) 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 http://jn.physiology.org/cgi/reprint/41/5/1203). 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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