godan verb (-mu), intransitive verb
strain oneself; bear down; exert oneself; try too hard
Physical or mental effort where one tenses up, pushes too hard, or forces strength. Often implies unnecessary strain or trying too hard, sometimes with a negative nuance of overexertion.
重い荷物を持ち上げようと力みすぎて、腰を痛めた。
I strained too hard trying to lift the heavy luggage and hurt my back.
試験中、力んでしまって普段の実力が出せなかった。
During the exam, I tensed up and couldn't perform as well as usual.
godan verb (-mu), intransitive verb
put on a bold front; swagger; bluff; boast
Acting tough, showing off strength, or bluffing to impress others. Often used when someone is trying to appear stronger or more confident than they really are.
彼はみんなの前で力んで見せたが、内心は不安だった。
He put on a bold front in front of everyone, but inside he was anxious.
そんなに力まなくても、ありのままでいいよ。
You don't have to swagger like that; just be yourself.