He is writing a letter.

The baby is waving its hands and laughing.

He waved his hand.

He held a new book in his hand.

My opponens pollicis muscle hurts.

He underwent surgery due to an injury to his extensor digitorum communis.

He is doing special exercises to strengthen his flexor digitorum superficialis.

He is doing special exercises to strengthen his flexor digitorum profundus.

He is doing a special exercise to strengthen his extensor indicis.

The intertransversarius muscle is an important part of our body, especially part of the back muscles.