YOU SAID:

They were clapping their hands.

INTO JAPANESE

彼らは彼らの手を叩いていた。

BACK INTO ENGLISH

They were knocking on their hands.

INTO JAPANESE

彼らは彼らの手をノックしていた。

BACK INTO ENGLISH

They knocked at their hands.

INTO JAPANESE

彼らは彼らの手をノックした。

BACK INTO ENGLISH

They knocked at their hands.