I feel pain in my lower back/hip area.
I do exercises to strengthen the muscles of the lower back and hips.
Long hours of sitting work put strain on the lower back.
She went to the hospital because of lower back pain.
This belt is designed to support the lower back.
He hurt his lower back lifting a heavy load.
臀部 refers specifically to the buttocks, whereas 腰部 covers the broader lower-back and hip region.
Compound of 腰 (こし, 'waist, lower back') and 部 (ぶ, 'part, region'). The reading ようぶ uses the on'yomi of both characters, forming a technical anatomical term.