Is this some kind of expression?

[kiyominai]

ぽんと膝を叩き気合いを入れて立ち上がり、キッチンに 行って湯を沸かし始める

I thought of it as:

"It's all in the knees, get into the kitchen and start boiling some water"

 

[SomeCallMeChris]

I don't know where you get 'It's all in the knees' from.
ぽんと膝を叩く is to slap the knee (in this case, presumably one's own knee)
気合を入れる is to get one's energy up (get psyched, get fired up)
立ち上がる is to stand up. It does have idiomatic uses, but in this case, among other reasons, the next verb is 行く so it's literal.

The second half is correct.