助走つける take a run up

助走つけてやるもの something you take a run up for

助走つけてやるもんじゃない not something you need to take a run up for

つける means to “add” the run up for whatever it is you’re taking the run up for

In this case it seems to be that he’s asking someone to help stretch by pushing his back. They put too much effort in so he says “you don’t need a run up for stretching”

Look on YouTube for examples of 助走