Robert Hooke showed the Royal Society microscopical observations of the motion and composition of muscles

May 5, 1678

Birch, History, vol. III, p. 402, 25 April 16768 (O.S.) in London:

After this a discourse was occasioned concerning the motion and fabric of muscles. And Mr. Hooke shewed an experiment in order to the explanation there of, which was a chain of small bladders fastened together, so as that by one pipe the whole series might be filled; which they would be successively, one after another, that, which was next the pipe, being first filled, and then the next successively. Now it was supposed, that the globules of the fibres of the muscles, which seemed like a necklace of pearl, might be some fabric; as this of bladders, in which might be included a certain portion of air or other very agile matter; which air being included in so exceedingly small and very thin skins, was very easily wrought on by heat and cold, and other agitating properties of the liquors, that pass between them; and thereby they might be presently filled by the said included air being rarified and emptied by the condensation of the same from the want of that heat continued: and so by the successive rarefaction and condensation of the same air included in the aforesaid chain of bladders the string thereof was made either shorter or longer, each of which was so much the more, by how the rarefaction or the condensation was the greater.

Upon this an occasion was taken, to discourse of the causes of the motion of the muscles; and how far the air taken in by the lungs might contribute towards muscular motion. And it was thought, that it was of great necessity for that very purpose.