Usage patterns
筋肉をつけるto build muscle
I do strength training every day to build muscle.
筋肉を鍛えるto train/strengthen one's muscles
I do sit-ups every night to strengthen my abs.
筋肉が張るfor muscles to feel tight or stiff
After exercising, the muscles in my calves feel tight.
筋肉を使うto use a particular set of muscles
This movement really uses the muscles in your back.
筋肉が落ちるfor muscle to waste away or weaken
After I stopped exercising because of an injury, I lost muscle.