I get a pain in my lower back when I bend down.
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means yield, and when you do that to someone else you tend to lean over or to bow to show them respect.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
Who do you need to stoop and to bow before? Takeshi Kaga (かが), that's Chairman Kaga to you! Before you can start cooking for him in Iron Chef, you must bow! Otherwise you'll be disqualified for being rude.