On the Movement of Animals, Part One

PROPOS. VII.

Muscles exert a twofold force: proper and instrumental.

It is established by experience that muscles exert a twofold force. One is the proper force of the fiber, depending on the very natural structure of the fibers; the other, however, arises from an externally arriving cause which makes use of muscles and organs to lift huge weights. For we see that the fibers are not entirely inert, but possess some contractile force; for in cadavers, shortly after death, severed muscles contract themselves just as they do in living beings, in whom such contraction is exercised from elsewhere without any laborious effort, and it is similar to the contraction which is effected by the plucked strings of a cithara, and therefore it depends upon the structure of the small machines from which the fibers are composed.

That the degree of such motive virtue of those small machines is small is evident from the short contraction of a muscle after the severance of one of the opposing ligaments, or after the cutting or removal of the Antagonist—a force which, if a violent convulsion does not supervene, can be overcome by the weight of a few pounds.

When an articulation is flexed as much as possible, the muscle placed in the concave part of it remains slack, and therefore can exert no contractile force; and at that time, a muscle placed in a connected position, without the opposition of an Antagonist, could exert its full force, yet we see it to be so weak that it is not able to overcome the weight or impediment of the articulation itself, since it is not able to direct it of its own accord. This is evinced by the fact that...