My opponens pollicis muscle hurts.

She opened the book with her thumb.

My head hurts.

He and she were standing against each other due to a disagreement.

There is a serious opposition between them.

He underwent surgery due to an injury to the opponens digiti minimi muscle.

He injured his extensor pollicis longus.

He underwent surgery due to an injury to his extensor pollicis brevis.

My flexor pollicis brevis hurts.

He is doing special exercises to strengthen his flexor pollicis longus.