The phrase "to tense up" means to become rigid or stiff, often as a response to stress, anxiety, fear or discomfort. It can also refer to tensing one's muscles involuntarily due to nervousness or anticipation of something unpleasant.
If you worry it can make your body tense up which stops the contractions do their job and is almost like you fighting against your own body. (health-and-parenting.com)
The difference is not perception but rather performance when risk is involved — the muscles tense up. (scientificamerican.com)