Muscle
Common expressions
build up / develop your muscles
relax / tense your muscles
massage (tense) muscles
pull a muscle
(a bath) soothes tired muscles
muscles feel stiff
Exercise
Common expressions
Try not to tense the muscles in your stomach so much. Breathe out slowly and try to relax them.
I use weights in the gym to build up my muscles.
Our best player won’t be able to play on Saturday because he's pulled a muscle.
After a hard day at work, I get my wife to massage the tense muscles in my back.
My leg muscles usually feel stiff for days after I've run a marathon.
After the football match, I had a long, hot bath to soothe my tired and aching leg muscles.
Note
Note this expression:
How can the guards at the palace gates stand for hours without moving a muscle?