tense up
     v 1: become tense or tenser; "He tensed up when he saw his
          opponent enter the room" [syn: {tense}] [ant: {relax}]
     2: make tense and uneasy or nervous or anxious; [syn: {tense},
        {strain}] [ant: {relax}, {relax}]
Source: WordNet® 2.0