Loanword from English, often used in fitness, bodybuilding, or casual contexts. Can refer to physical muscles or muscular strength. Compare with native 筋肉 (きんにく) which is more formal and anatomical.
See also: 筋肉 (きんにく)
He is building his muscles at the gym.
This protein is effective for gaining muscle.
outer muscles
Borrowed from English 'muscle'.