godan verb (-ru), intransitive verb
become sinewy; become stringy; become stiff (physically)
Describes muscles, meat, or other physical things becoming tough, fibrous, or stiff.
この肉は焼きすぎて筋張ってしまった。
This meat has become tough and stringy from being overcooked.
運動不足で体が筋
My body has become stiff from lack of exercise.
godan verb (-ru), intransitive verb
become stiff (attitude/conversation); become formal; become awkward
Used for situations, attitudes, or conversations that become rigid, overly formal, or uncomfortable.
彼が来てから、会話が急に筋張ってしまった。
After he arrived, the conversation suddenly became stiff.
初対面の人と話すときは、どうしても筋張ってしまう。
When talking with someone I'm meeting for the first time, I can't help but become formal.