Not here, James, the two verbs which are accepted are 'détendre' and 'relaxer ses muscles'.

Take a look at the different meanings of 'relâcher' and how it might be used on the following page -

https://www.linguee.fr/francais-anglais/traduction/rel%C3%A2cher.html#:~:text=loosen%20v%20(loosened%2C%20loosened)

Bonne Continuation !

Hi James, 

Yes, it is a strange one but having checked with a colleague it just doesn't work in the sentence given.

You might use 'relâcher un muscle' in the context of a physio session, being the opposite of 'contracter un muscle'. 

Hope this helps!