The most common and most likely answer to this crossword clue is the 11 letter word ANTAGONISTS
I've seen this crossword clue in The Daily Telegraph
A
N
T
A
G
O
N
I
S
T
S
Buddy explains!

The straight part
We know that "muscles" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case muscles = antagonists.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ANTAGONISTS mean?
ANTAGONISTS noun- An opponent or enemy.
- One who antagonizes or stirs.
- A chemical that binds to a receptor but does not produce a physiological response, blocking the action of agonist chemicals.
- (authorship) The main character or force opposing the protagonist in a literary work or drama.
- A muscle that acts in opposition to another.