Stress, which can tighten muscles, is probably a cause of some low back pain.

He tightened his muscles to try and fight off the sensation.

He tightened his abdominal muscles against the power of the feeling within him.

Many people unconsciously tighten their back muscles when they are under stress.

Andrew felt himself trembling, and tightened his muscles to prevent it.

The tension returned to him then, tightening his sore muscles.

Perhaps he should take care lest it someday tighten muscles and cut his breath.

Tighten your lower abdominal muscles as if you are going to have a bowel movement.

That's what you just did when you tightened your facial muscles.

Like any man so effected, he had unconsciously tightened his muscles.