I know that ~てやる is to do something for someone.

I also know that やるもんじゃない means "shouldn't do"

I have a feeling I should ignore ~てやる because て is a part of つけて.

However, I don't know what つけて is supposed to mean after 助走

Anyway, for context, I'm reading about a guy asking a girl to stretch. He says:

じゃあ背中押してもらえるかな

助走つけてやるもんじゃないから柔軟は体硬くてごめんね

I'm mostly confused what つけて's role is and how the rest of やるもんじゃない combines with it.

Not to mention the word 助走 doesn't seem to have a good translation to English?

I'm having a tough time deconstructing the meaning behind all the parts

I know that ~てやる is to do something for someone.

I also know that やるもんじゃない means "shouldn't do"

I have a feeling I should ignore ~てやる because て is a part of つけて.

However, I don't know what つけて is supposed to mean after 助走

Anyway, for context, I'm reading about a guy asking a girl to stretch. He says:

じゃあ背中押してもらえるかな

助走つけてやるもんじゃないから柔軟は

体硬くてごめんね

I'm mostly confused what つけて's role is and how the rest of やるもんじゃない combines with it.

Not to mention the word 助走 doesn't seem to have a good translation to English?

I'm having a tough time deconstructing the meaning behind all the parts