The psoas (pronounced so-as) is your core muscle and an integral part of a centered and functional body. The psoas muscle is the most complex muscle in the body. It connects the lower body to the pelvic area, the pelvic area to the lower lumbar spine and the lower lumbar spine to the upper body. As a major player in back issues, knee issues and tight hip sockets, it is often an exhausted psoas that disrupts range of motion, as well as digestion and bladder functioning.