To brace oneself; to take a defensive stance or get ready (physically or mentally) in anticipation of something.
相手が近づくと、彼は自然と身構えてしまった。
When the opponent approached, he naturally braced himself.
大事な発表の前に、みんなが身構えている雰囲気だった。
There was an atmosphere of everyone bracing themselves before the important presentation.
犬が吠えるとすぐに身構えた。
He immediately braced himself when the dog barked.