noun
Refers to muscle tissue in the body. Common in compounds like 筋肉 (きんにく).
筋を痛めてしまった。
I've strained a muscle.
筋肉 is the more common, everyday word for 'muscle', while 筋 is often used in compounds or more technical contexts.
The kanji 筋 originally referred to sinews or tendons. The reading きん is the on'yomi, commonly used in medical and anatomical terms.