The psoas major is the main muscle of walking, an action ideally initiated deep in the core of the trunk. The psoas connects the spine to the legs attaching on the lower spine (T12, L1-4), before crossing the rim of the pelvis to attach on the back half of the inner thigh. The psoas if properly aligned is capable of supporting and moving us through space. Its downward pull on the front of the lower spine allows many muscles at the back of the body to lengthen and tone upwards providing support for the head at the top of the spine. When walking it is the work of the two psoas muscles moving in opposition that stimulates a natural and healthy rotation in the pelvis and throughout the spine. Most people use their big thigh muscles (quadriceps) to pull them through life but if everything works according to design the psoas is generating our movement and many overworked muscles get to relax.
The diaphragm, a dome shaped muscle at the base of the ribcage, is the essential muscle of breathing. When we inhale the diaphragm is meant to descend allowing air to be drawn into the expanding lungs. At the base of the pelvis three layers of muscle support the weight of the organs. These pelvic muscles and the diaphragm work synergistically. With each inhalation the diaphragm and pelvic floor should lower and with each exhale they should rise back up moving the contents of the trunk with them. Add to this up and down movement the rotational possibilities of the psoas and every breath and every step we take can tone and massage the entire contents of the trunk.
There are also literal connections between the psoas and the diaphragm. There are two tendons for the diaphragm called the crura that extend down and connect to the spine right alongside the psoas connection. One of the ligaments of the diaphragm (medial arcuate) wraps around the top of the psoas. Finally the diaphragm and the psoas connect through fascia, the webbing that encases the body in both safe and sorry ways. There is a particular fascial grouping that connects the diaphragm, psoas and other hip muscles.
The body is a machine designed to work very specifically. The relationship between these two titans of walking and breathing cannot be understated. Health in one can and will encourage health in the other.