There is as it respects the duration of the contractions a remarkable
difference in the muscles, according as we excite these contractions
artificially or naturally.
When upon a living animal or one recently killed, we excite the muscle
itself, or we stimulate its nerves, the relaxation succeeds almost
suddenly the contraction; neither state is ever lasting, though we
continue for a long time the action of the stimulant; the effect which
it has produced is immediately exhausted. When galvanism, mechanical or
chemical agents are used in our experiments, the phenomenon is the same.
On the contrary, when the will directs the contraction, it can sustain
it for a very long time. The support of burthens, standing, &c. clearly
prove this fact. When even during life, a morbid irritation is directed
upon the nerves, the contraction can be very permanent, of which we
have terrible proofs in tetanus.
The permanence of the muscular contraction fatigues the muscle much
more than alternate relaxation and contraction. Hence why when we are
standing long, we contrive by turns to carry the weight of the body
more upon one limb than the other.