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Hip flexor - Does it really flex your hip?

Hip flexor - Does it really flex your hip?

Naming muscles according to their function should be viewed critically. Assuming that the hip flexor flexes the hip is correct, but it only reflects its role very superficially. Moreover, this insinuation is only one-dimensional. And as we know, all of our muscles function in all three axes of motion. In childhood and adolescence, the iliopsoas has considerable length, or extensor capacity. This characteristic falls victim over the years, to excessive sedentary behavior and a lack of exercise. In competitive sports, this often leads to a one-sided overload. Under these circumstances, this muscle group loses its mobility and functionality in all three movement axes. 

The Basics

In functional everyday life, i.e. in a standing or walking position, flexion of the hip joint is given. The hip flexor does not produce the flexion through a concentric contraction. It is gravity that forces the upper body to lean forward. In movement, in addition, the intrinsic mass and its acceleration have a considerable influence. In many sporting activities, the iliopsoas has a great influence on movements in the frontal and transverse direction, even when the hip is flexed.
PATband Stand Scale 2 | PAT.fit
Hip flexion with gravity and PATband

The hip flexor controls extension, internal rotation and adduction & abduction.

The iliopsoas is also interesting because it is not a superficial muscle. It is located in the deep anterior trunk and lumbar region. For its part, the most important neighbors are the abdominal muscles, rectus femoris and adductors. 

The iliopsoas consists of the psoas major and the iliacus. The former originates from the twelfth thoracic vertebra to the fourth lumbar vertebra, the transverse processes of the first to fifth lumbar vertebrae. The iliacus begins at the inner surface of the ilium. Both muscles unite and twist medially to attach to the lesser trochanter of the femur.

If you believe the anatomy books, the hip flexor causes flexion and external rotation in the hip joint. The psoas adducts and iliacus abducts in the hip joint. Without a closer look, the muscle function is very poorly described. From this concentric view, the muscle work takes place in a different environment. Namely, in the supine position. In my observation, this insufficient anatomical basis is still the foundation in many training institutes, also for the area of functional training.

Athletiktraining Rotation Bein Körper 1 | PAT.fit
Functional loading of the right hip side

Interesting from a functional perspective:

The iliopsoas is able to eccentrically control extension, internal rotation and both movements (adduction and abduction) in the frontal plane. All these movements activate and lengthen this muscle. In functional observation, i.e. on a living object, not in the supine position but walking upright, the following picture emerges for the iliopsoas:

When stepping forward with the left foot at the exact moment the left heel hits the ground, the following movements occur in the hip joint: Flexion (Sagittal), Adduction (Frontal) and Internal Rotation (Transverse). If we now keep in mind the functions described above, we find that the hip flexor should work eccentrically in the frontal and transverse planes but concentrically in the sagittal plane. This is just another example of the terminology of eccentric muscle work.
| PAT.fit

This means that a muscle works in different movement axes in different modes (concentric/eccentric/static) at the same time. However, it is still unclear here whether the tension caused by elongation of the muscle in the frontal plane slows down a movement or, due to its origin, rotates the lumbar vertebrae in the same direction as the pelvis. In the frontal plane, the pelvis tilts to the left side when stepping with the right. The iliopsoas of the right side of the hip counteracts a left lateral flexion of the spine by muscle elongation (tension). If the left foot swings forward, the pelvis begins to rotate spatially to the right. This rotation causes even more significant internal rotation of the right thigh at the hip joint. This motion causes extension and effectively brings the right foot backward by overtaking it from its left neighbor. The pelvis simultaneously shifts from right to left on the frontal plane. 

It is precisely these three movements that stretch the iliopsoas in all three axes of motion. This three-dimensional eccentric work makes it such a powerful and important muscle in the hip joint. The energy (load) stored by lengthening is mainly converted into hip flexion, then adduction and external rotation (explode). The acceleration of the forward movement of the thigh is what makes it possible to lift the heel off the ground when walking. Conversely, however, this means that limited dorsiflexion in the ankle - possibly supported by an overly stiff big toe - significantly slows down a forward step and therefore has a negative influence on the stride length and thus the extension in the hip joint.

Conclusion

This three-dimensional approach results in significant differences in terms of training and therapy techniques compared to the classical approach. The focus should not be the use of the muscle exclusively in leg lifting exercises, because a voluntary concentric operation of this muscle is anything but functional. The goal of any functional training is to provide the body with stimuli that are as authentic as possible, which can be quickly understood, integrated and implemented by the body.

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Patrick

Functional Therapist, Head of PAT.fit

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