Note: This is the third blog post article in a series of 11 articles on Psoas Major Function. See below for the other articles in this series on psoas major function.
The hip joint is a triaxial joint that allows motion in all three cardinal planes. Therefore we can examine the effect of the psoas major in each of the three cardinal planes. Further, we need to consider the open-chain motions of the thigh relative to the pelvis at the hip joint; and the closed-chain motions of the pelvis relative to the thigh at the hip joint.
In the sagittal plane, there is little or no controversy over the potential action of the psoas major at the hip joint. It clearly crosses the hip joint anteriorly, passing anterior to the mediolateral axis of motion (see Figure 4A); therefore it flexes the hip joint. If we are in an open-chain position, the thigh flexes at the hip joint. If we are in a closed chain position the pelvis anteriorly tilts at the hip joint (Figure 5).
Figure 5. Flexion at the hip joint. A, Open-chain flexion of the thigh at the hip joint. B, Closed-chain anterior tilt of the pelvis at the hip joint. Reproduced with kind permission from Muscolino, J. E., Kinesiology: The Skeletal System and Muscle Function (3rd ed.). (2017) Elsevier.
Sagittal Plane: Thigh Flexion
All sources concur that the psoas major is a flexor of the hip joint. In fact, most sources state that hip flexion is its primary function (3, 5, 9). Stuart McGill goes as far as to state “The role of the psoas is purely as a hip flexor.” (12). And many sources go on to describe the psoas major’s hip flexion role rather effusively. Janet Travell and David Simons described the psoas major as a “major muscle of hip flexion” (27); and its hip flexion role has been described by others as “strong” (5), “powerful” (6), or “dominant” (19). Carol Oatis specifically points out that the psoas major is a “strong hip flexor” because it has a large physiological cross sectional area (20). Sometimes authors discuss the psoas major along with the iliacus as the iliopsoas. In these cases, it can be difficult to determine what to ascribe to the psoas major versus the iliacus, but the iliopsoas as a whole is often stated to be the prime mover (in other words, the most powerful mover) of hip joint flexion (4).
Although no source contests the ability of the psoas major to create flexion at the hip joint, not every source is as convinced of the power of its hip flexion ability. One study asserts that the psoas major’s hip flexion is relatively weak at the beginning and end ranges of motion, and that it is strongest between 45 and 60 degrees of flexion (31). In fact, many sources believe that the primary role of the psoas major is not to actually move the bones at the hip joint by concentrically contracting, but rather to stabilize the bones of the hip joint by isometrically contracting (2, 21, 26). They point out that the moment arm of the psoas major is smaller than the moment arm for most of the other hip flexors because the muscle’s line of pull passes so close to the mediolateral axis of motion (Figure 6) (19, 20).
Figure 6. Right lateral view demonstrating lines of pull for flexors and extensors at the hip joint. A, Flexors shown with solid lines; extensors shown with dotted lines. B, Moment arms drawn in for the iliopsoas (psoas major) and the rectus femoris. Reproduced with kind permission from Joseph E. Muscolino. Modeled from Neumann, Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation (2nd ed.). Elsevier.
Note: Strength of a Muscle’s Contraction
Determining what joint action a muscle can create is a factor of the line of pull of the muscle relative to the joint’s axis of motion. However, other factors must be looked at to determine the strength that the muscle will have when creating this motion. These factors can be divided into internal and external factors. The major internal factor is the internal strength of the muscle, which is essentially determined by the number of sarcomeres, or more specifically the number of myosin-actin cross-bridges within the muscle. Because the architectural arrangement of the muscle fibers affects this equation (whether the muscle is pennate or non-pennate in arrangement), the measure of a muscle’s internal strength is effectively determined by the physiological cross sectional area of the muscle. The external factor that determines a muscle’s strength is its leverage force, or moment arm, at the joint crossed. In effect, the farther the muscle’s line of pull is from the axis of motion, the greater is the leverage/moment arm, and therefore the stronger is the effect of the muscle’s contraction force; the closer the line of pull is to the axis, the weaker is the muscle’s contraction force. A moment arm is the measure of the distance from the axis of the joint along a line that meets the muscle’s line of pull at a perpendicular angle (see Figure 6).
Therefore it would make sense that these other hip flexor muscles with greater moment arms would more efficiently pull the hip joint into flexion. Evan Osar believes that the major role of the psoas major at the hip joint is to stabilize and center the head of the femur in the acetabulum as other hip flexors contract (21). He uses the term “centration” to describe this idea. Gibbons also believes that the primary role of the psoas major at the hip joint is stability. He points out that the fiber architecture of the psoas major is not fusiform; rather it is unipennate (2, 31). Pennate muscles are designed to produce greater force over a shorter distance, whereas nonpennate muscles are designed to produce a greater range of motion. Therefore, “…the ability of the muscle to shorten is less than believed. This calls into question its efficiency as a hip flexor.” (2).
However, it should be noted that these comparative flexion moment arms are at anatomic position. If the thigh were first in flexion, the moment arm of the psoas major would increase, and therefore its strength and potential role in creating flexion motion at the hip joint would increase (as previously mentioned, a study found the psoas major to be strongest between 45 and 60 degrees) (Figure 7).
Figure 7. The moment arm and therefore leverage force for thigh flexion of the psoas major increases when the thigh is first flexed. A, Anatomic position. B, 50 degrees of flexion. Reproduced with kind permission from Joseph E. Muscolino. Art work by Giovanni Rimasti.
What to conclude from this discussion? There is no doubt that the psoas major’s line of pull is anterior to the hip joint and that its contraction creates a force of flexion at the hip joint. The only question seems to be whether this hip flexion force is more important for motion or for stabilization. These concepts, however, do not need to be mutually exclusive as a muscle can have a stabilization role as well as a role in motion.
Generally, it is true that deeper muscles at a joint tend to function more for stabilization than for motion, and looking at the psoas major’s location does show it to be a deep muscle. Further, given all the other hip flexor muscles that exist with greater moment arms, it is likely that they would more efficiently act toward creating hip flexion motion. This all points to the psoas major acting primarily as a stabilizer of the hip joint when we are in anatomic position and/or when lesser hip flexion force is necessary. But the psoas major is a large and powerful muscle and it would make sense that if a greater hip flexion contraction force were needed, then the psoas major would be recruited to assist in the creation of this motion. This is especially true if the hip joint were already flexed, because of the increased moment arm leverage.
Sagittal Plane: Pelvic Anterior Tilt
Regarding closed-chain sagittal plane motion of the pelvis at the hip joint, the line of pull of the psoas major would pull the pelvis into anterior tilt at the hip joint (14, 19, 25, 29). This assumes that the pelvis is fixed to the trunk as the trunk is pulled anteriorly. Closed-chain position in the lower extremity usually occurs when the foot is planted on the ground. For this reason, psoas major closed-chain function is especially important for standing posture. If the baseline tone of bilateral hip flexor musculature, including the psoas major, is tight, it will create an increased anterior tilt of the pelvis (4, 5, 19). Note: This will have important ramifications for the spine when discussing the effects of the psoas major upon the spine later in this article.
Note: This is the third blog post article in a series of 11 articles on Psoas Major Function.
The 11 articles in the series are:
- Introduction & Muscle Biomechanics
- Biomechanics of the Psoas Major (Overview)
- Psoas Major Hip Joint Actions – Sagittal Plane
- Psoas Major Hip Joint Actions – Frontal Plane
- Psoas Major Hip Joint Actions – Transverse Plane
- Psoas Major Spinal Joint Actions – Frontal and Transverse Planes
- Psoas Major Spinal Joint Actions – Sagittal Plane
- Stabilization of the Spine by the Psoas Major
- Psoas Major and the Sacroiliac Joint
- Psoas Major and Fascial Pulls
- Summary of Psoas Major Function & Further Research
Note: This article is modified from an article originally published in the massage therapy journal (mtj): Psoas Major Function: A Biomechanical Examination of the Psoas Major. Spring 2013 issue.