A TECHNICAL DESCRIPTION OF HOW THE PSOAS PARTICIPATES IN NORMAL WALKING:
What this statement means is that movement forward starts in the trunk (as a slight swaying forward). That slight swaying forward starts as a shifting of weight onto one foot and a subtle lifting of the toes and/or front of that foot, which decreases support, so that you slightly sway forward. When you have swayed far enough forward, you spontaneously bring your other leg forward to catch your forward weight (knee movement forward initiated by the psoas). Your leg comes forward, your foot comes down and supports your weight as it comes forward; then your other leg comes forward. The movement is: trunk, foot, hip, knee, foot, in a cycle.
A casual interpretation of this description might be that the psoas initiates hip flexion by bringing the thigh forward. It's not quite as simple as that.
By its location, the psoas is also a rotator of the thigh. It passes down and forward from the lumbar spine, over the pubic crest, before its tendon passes posteriorly (back) to its insertion at the lesser trochanter of the thigh. Shortening of the psoas pulls upon that tendon, which pulls the medial aspect of the thigh forward, inducing rotation, knee outward.
In healthy functioning, two actions regulate that tendency to knee-outward turning: (1) the same side of the pelvis rotates forward by action involving the iliacus muscle, the internal oblique (which is functionally continuous with the iliacus by its common insertion at the iliac crest) and the external oblique of the other side and (2) the gluteus minimus, which passes backward from below the iliac crest to the greater trochanter, assists the psoas in bringing the thigh forward, while aligning thigh rotation so the leg (optimally) swings directly in the line of travel (not commonly seen, but then idiosyncratic muscular tensions and inefficient movement are more common than well-organized movement -- so common that they are taken as "normal"). The glutei minimi are internal rotators, as well as flexors, of the thigh at the hip joint. They function synergistically with the psoas.
This synergy causes forward movement of the thigh, aided by the forward movement of the same side of the pelvis. The movement functionally originates from the somatic center, through which the psoas passes on its way to the lumbar spine. Thus, Dr. Rolf's observation of the role of the psoas in initiating walking is explained.
Interestingly, the abdominals aid walking by assisting the pelvic rotational movement described, by means of their attachments along the anterior (front) border of the pelvis. Thus, the interplay of psoas and abdominals is explained.
A final interesting note brings the center (psoas) into relation with the periphery (feet). In healthy, well-integrated walking, the feet assist the psoas and glutei minimi in bringing the thigh forward. The phenomenon is known as "spring in the step."
Here's the description: When the thigh is farthest back, in walking, 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, in those muscles, to contract. This is what happens in well-integrated walking: 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.
Here's what makes it particularly interesting: when the plantar flexors fail to respond in a lively fashion, ones feet lack spring and the burden of 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 (lifters of the fronts of the feet) lead to tripping over ones feet, when walking; to avoid tripping over ones own feet, the hip flexors must compensate by lifting the knee higher, leading to a similar problem. The answer to this problem, by the way, is not usually to strengthen the muscles of the shin (dorsiflexors), but to free the muscles of the calf, which are usually too tight, commonly due to previous injury to a foot or ankle or habitual "swayback" (weight swayed too far forward).
Thus, it appears that the responsibility for problems with the psoas falls (in part, if not largely) upon the feet. No complete resolution of psoas problems should be expected without proper functioning of the lower legs and feet.