Knee and hip: valgus, varus, hyperextension, and intra-articular compressions

The knee is the point of convergence of forces arising from the hip and the foot. Pathological patterns follow a logical progression determined by intensification of muscular shortening: from initial hyperextension and internal rotation to flexion and external rotation in the more advanced phases. Vector analysis identifies the muscles responsible for each pattern and explains the mechanism of intra-articular compressions at the hip, knee, and ankle.

The attached PDF document, available for free download, develops the complete vector analysis with images and bibliographic references.

Hip: muscular dominances that change with loading

With the femur as the mobile point — unloaded — vector dominance is in hip flexion. With the femur as the fixed point — under load — the same muscles change the direction of their action: instead of bringing the femur toward the pelvis, they bring the pelvis toward the femur. The iliopsoas and rectus femoris, pulling through their pelvic insertions, determine anterior pelvic tilt and lumbar hyperlordosis. The hip extensors — hamstrings and gluteals — although opposing this action, are subdominant when axial loading increases the stabilisation required of the joint.

Adduction dominance is determined by the femoral adductors, gracilis, gluteus maximus (through the gluteal tuberosity), pectineus, quadratus femoris, and obturator externus. Rotatory dominance under load is in internal rotation — semitendinosus and semimembranosus, with adductor magnus and gracilis as co-agonists.

Piriformis syndrome: two opposite scenarios

The piriformis can be overloaded in two opposite scenarios. With internal rotation and abduction of the femur — as in knee valgus — the piriformis must activate at high intensity to contain the joint. With knee varus, the femoral head is "pushed" into the acetabulum, and the piriformis, with its insertions approximated, must work with increased basal tone to be effective as an active ligament. In both cases, the increase in Resistant Force reduces available Work Force, producing mechanical inefficiency and overload that may compress the sciatic nerve.

Knee: hyperextension under load

In upright standing, with the foot as the fixed point, the hamstrings pull the tibia posteriorly. Since the tibia cannot move backward, the traction is translated into a push of the knee toward extension. Similarly, the triceps surae, with its femoral insertion anterior relative to the heel as the fixed point, pulls the femur posteriorly. The true motors of extension under load are the hamstrings–triceps surae pair, not the rectus femoris, whose extensor action is expressed only in the portion between the patella and the tibial tuberosity.

Recurvatum also produces alterations at the hip and ankle: the G and R forces concentrate within the acetabular cavity instead of being distributed along the entire femur.

Differential test for recurvatum

By correcting femoral internal rotation through active or passive external rotation: if recurvatum is not modified, it is due to ligamentous laxity; if derotation produces knee flexion, recurvatum is induced by muscular shortening.

Progression of pathological patterns

The four patterns are associated and represent a progression: initially internal rotation and hyperextension, then flexion and external rotation. A knee that appears well positioned may truly be so, or may have exhausted the first two directions of movement and entered the following two.

In flexion, the hamstrings–triceps surae pair again becomes extensor, but the vertical vector components act at such intensity as to prevent extension itself. The result is a rigid knee with high energy expenditure.

Valgus and varus

Valgus is determined by the femoral adductors, tensor fasciae latae, triceps surae (a strong heel supinator), and tibialis posterior, all with the foot as the fixed point. The tensor fasciae latae, once valgus is established, reverses its action and contributes to fixation, although it is never the primary cause.

Varus is determined vectorially by the tibialis anterior and the fibularis longus and brevis. The hip abductor muscles, being monoarticular with short vectors, can only minimally influence the femorotibial relationship — with global shortening of less than 2–3%, their effect on knee geometry is negligible. They are, however, highly effective in stabilising the femoral head within the acetabulum.

The derotation test eliminates interference of the rotational component on the frontal plane, revealing the true varus–valgus relationship of the joint.

Patellar deviation

When the femur is in internal or external rotation due to hamstring action, the quadriceps repositions the patella through the vasti, creating a femoropatellar dissociation. Patellar deviation is therefore caused directly by the vastus medialis or lateralis, but as a consequence of limb rotation induced predominantly by the hamstrings.

Physical foundations of the model.
This article applies the AIFIMM biomechanical model.
Its physical foundations are developed in three sequential articles, best read in order:
1. How muscle shortening generates joint conflict — why muscles shorten and the Resistant Force / Working Force model
2. Do antigravity muscles really oppose gravity? — how segmental malalignment raises Resistant Force
3. Why joint conflict develops: vector analysis of muscular forces — how the responsible forces are identified and predicted

This topic is part of the online course Systemic and Segmental MSK Biomechanics.