You can probably feel your own tight iliacus muscles. (physiologicnyc.com)
Insertions: Psoas major unites with iliacus at the level of the inguinal ligament and crosses the hip joint to insert on the lesser trochanter. (dieselcrew.com)
But one of the important factors that contributes to a tight piriformis is the lack of hip internal rotation, often caused by lack of core strength, decreased iliacus firing and decreased arch strength. (fitnessandpower.com)