I will be posting a video (hopefully today) on an exercise plus a stretch that targets this muscle.
The psoas muscles and the abdominal muscles are agonist and antagonist as well as synergists. Allow an imbalance between these two groups and you will run (literally) in to problems. Lower back pain is one of the more common issues from having a tight psoas. Kerrie Wlad to a white courtesy phone...
In the standing position, contracted psoas muscles move the pelvis backward; the abdominal muscles move the pelvis forward. (antagonists)
In running, the psoas muscles of one side initiate forward movement of that leg, while the abdominals bring the same-side hip and pelvis forward. (synergists)
The psoas major muscles pull the lumbar spine forward; the abdominal muscles push the lumbar spine back. (antagonists)
The psoas minor muscles pull the fronts of attached vertebrae (at the level of the diaphragm), down and back; the abdominals push the same area back. (synergists)
Unilateral contraction of the psoas muscles causes rotation of the torso away from the side of contraction; abdominals assist that movement.
Dr. Ida P. Rolf described the psoas this way:
Now.. as athletes we need to really let this quote sink in a bit. Legs follow the core.Let us be clear about this: the legs do not originate movement in the walk of a balanced body; the legs support and follow. Movement is initiated in the trunk and transmitted to the legs through the medium of the psoas.
Something else to ponder- When the thigh is farthest back, in running, the ankle is most dorsi-flexed. That means that the calf muscles and hip flexors are at their fullest stretch and primed by stretch receptors to contract. This is what happens in well balanced running: assisted by the stretch reflex, the plantar flexors of the feet put spring in the step, which assists the flexors of the hip joints in bringing the thigh forward.
NOW- Here's what makes all of this interesting: when the plantar flexors fail to respond correctly, bringing the thigh forward falls heavily upon the psoas and other hip joint flexors, which become conditioned to maintain a heightened state of tension and readiness to contract, and there we are: tight psoas and back pain. (Note that ineffective dorsi-flexors of the feet prevent adequate foot clearance of the ground, when running; the hip flexors must compensate by lifting the knee higher, leading to a similar problem.)
So we can now say- responsibility for problems with the psoas falls largely on the feet. No resolution of psoas problems can be expected without proper functioning of the lower legs and feet. Foot plant and lower leg strength are critical. I have posted (in the past) a video of jumping rope which I feel is one of the best (if not the best) exercise for teaching proper foot plant and also strengthening the lower leg.