腰筋 is read as ようきん (youkin) and means psoas.
ようきん
youkin
だいようきん
psoas major muscle
しょうようきん
psoas minor muscle
ようつい
lumbar vertebra
こしくだけ
collapsing in the middle of a bout
こしなわ
leash
こしだかしょうじ
tall-paneled sliding door
こしべんとう
lunchbox hanging from one's waist
こししょうじ
shoji screen with a wooden panel in the bottom part
こしばり
sliding door's decorative paper skirt
すじ
(Edo period) counter for hundreds of mon (obsolete unit of currency)
はらすじ
sit-up (exercise)
てっきん
reinforced concrete