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1. To become suddenly very tense in one's body, especially in anticipation of or reaction to something unexpected or undesired. I tensed up when the teacher passed by, hoping that she wouldn't comment on the quality of my work. You need to stay loose—if you tense up, you won't be as quick to avoid your opponents punches. The cat began to tense up as the dog approached, rearing up its hindquarters and hissing.
2. To flex or tense the muscles in some part of one's body. In this usage, a noun or pronoun can be used between "tense" and "up." The pain from the needle made me tense my arm up, which made it hurt even more. He tensed up his back as he bent over, and the kids stood on top of it like a stepping stool.
tense up (for something)
to become tense, anxious, and ready for something. Liz tensed up for the game and was very nervous. He tensed up and that made it hard to give him the injection he needed.
1. To become tense: The deer tensed up when it heard my footsteps.
2. To make something tense: I tensed up my wrist just before making contact with the ball. The muscle in my calf hurts whenever I tense it up.