Thanks for that!
Okay. You've got a lot of attachments in there.
Some of the muscles, it looks like every single vertex is participating in an attachment.
This is what I would do.
Get every tissue-tissue attachment and turn it off. This will leave your origin and insertion attachments as they are. But hopefully you'll see your muscles loosen up. Then, one by one adjust the properties to get the right behaviour. You might have to manually turn off the ones that attach the belly of the muscle to the bones.
As a general rule you want to be fairly minamalist with the number of vertices participating in attachments. This leaves the system free to do it's thing.
Some code to select every tissue-tissue attachment:
import maya.cmds as mc
import maya.mel as mm
mc.select( cl=True )
for at in mc.ls(type='zAttachment'):
src = mm.eval( 'zQuery -as %s;' % at )
tgt = mm.eval( 'zQuery -at %s;' % at )
if mm.eval( 'zQuery -bt %s' % src ) == 'zTissue':
if mm.eval( 'zQuery -bt %s' % tgt ) == 'zTissue':
mc.select( at, add=True )