Are мускул and мышца synonymous?
Non-interchangeable use of
мышца/ы in (about) medical situations with verbs:
болеть – to ache, мышцы болели (after training)
сводить – to get cramp, свело мышцу [судорогой]
потянуть – to pull/strain, потянул мышцу
порвать – to tear, порвал мышцу
мышечная масса – muscle(s) mass
Non-interchangeable use of
мускул with verbs:
дрогнуть – to move/jerk, ни один мускул не дрогнул на его лице – his face stayed still, without moving a muscle
I don’t think it’s practical to learn cases when the two words are interchangeable but maybe have some secondary and tertiary shades of difference. I’m sure it’s a matter of serious linguistic research, evidently not done. Unconscious remembering of use cases can be of more help.
Nevertheless: one more guess about trends of
мышца appears to be a base word of traditional medical/anatomical terminology about muscles,
мускул seems to be a newer word which is more active in contexts/styles perceived as more modern and progressive, e. g. concerning ‘культура тела’ which is not limited to body-building. (These contexts arose in Russia a century ago, I suppose.)
In Russia, if I’m not mistaken, a physical force was traditionally attributed to sinews, not to muscles. I think that for some time
мускул was used to reflect new (not of 'common' people of the past) views about it and marked the adeptness to 'new world' of reason and so on (I perceive it so in some texts).
Мышца is 'deeper' Russian than мускул, a child probably first learns a word мышца, then – мускул (true for me).
Making the difference farcical: a simple country guy has мышцы, a towny builds мускулы.
Yes they are.
Мышца is a CS calque of Latin musculus "little mouse". They can be used interchangeably in Russian.
Some derived terms like внутримышечно "intramuscularly" or мускулистый "muscular, having big muscles" use only one of the roots, though.
Also, мышца is used in established compound names of separate muscles: широчайшая мышца спины "lat", икроножные мышцы "calves" etc. In non idiomatic descriptions like мышцы пресса "abs" мускул can be used as well.
These words are often interchangeable. But in general the difference is like this:
мышцыis what everyone has from birth, biologically. If you go to Wiki, you can see that all 640 muscles are called
мышцы, because it's the preferred word for tiny muscles unlike biceps e.g. When we say
мышцыwe often mean flesh.
мускулыis associated with physical power often acquired through training. Biceps, triceps, but not abs! Abs are called:
мышцы пресса, мышцы живота(although Google found ~4K matches for
мускулы пресса, but check out 386К for
мышцы пресса). So another criterium:
мускулis a muscle which is visibly big/long and expands/contracts by a wide amplitude.