When we talk about knee pain—especially patellofemoral pain syndrome (PFPS)—most people imagine the patella “rubbing” or “grinding” against the femur.
But the truth is far more biomechanically interesting.
To understand why pain happens and how physiotherapy treats it, we must first break down how the patella actually moves, how it contacts the femur, and what compression forces look like during different activities.
Let’s simplify the science behind the patellofemoral mechanism.
1. Patellar Contact: How the Patella Engages the Trochlear Groove
The posterior patellar surface is made up of multiple facets—and it doesn’t perfectly match the shape of the femoral trochlea. Because of this, joint contact depends entirely on knee position.
°0 — Complete Knee Extension
- At full extension, the patella sits above the trochlear groove.
- There is minimal to no joint contact, meaning no patellofemoral compression.
- The femur and tibia are nearly parallel → little compressive force.
15° of Flexion
- The inferior pole of the patella starts to contact the superior part of the trochlear groove.
30°–60° of Flexion
- As the knee bends, the patella slides distally.
- Contact surface area increases, improving force distribution.
- This is also the range where joint reaction forces increase the fastest.
Past 60° & Beyond 90°
- Contact either increases, remains stable, or decreases—depending on anatomical variation.
- After 90°, the quadriceps tendon begins contacting the trochlea, sharing some of the load.
- The patella moves further inferiorly and deep into the groove.
More contact = more stability
More flexion = more load—but also more surface area to resist it.
2. Understanding Patellofemoral Compression Forces
Compression is not constant—it changes dramatically with knee position and type of movement.
✔ In Full Extension (0°)
- No significant compression—because the patella is not engaged.
- Quad and patellar tendon create a mild sagittal “bowstring,” but forces are low.
✔ 30°–60° Flexion
- Rapid increase in compression due to:
- Greater quadriceps demand
- Larger resultant force between quadriceps & patellar tendon
- This is also the range where many patients report pain during activities like stair descent.
✔ During Squatting
- Compression increases steadily until ~90°.
- After 90°, it levels off or reduces slightly because:
- The quadriceps tendon contacts the trochlea, absorbing part of the load.
This explains why deep squats sometimes feel easier for some people than half squats—they shift how forces are distributed.
✔ During Open-Chain Knee Extension (with weight at ankle)
- The highest patellofemoral joint stress occurs around ~30° flexion.
- This is due to the moment arm of the weight, not patellar compression alone.
- Physiotherapists often avoid heavy open-chain knee extensions in painful PFPS cases for this reason.
3. The Influence of the Q-Angle
The Q-angle affects patellar tracking.
- Increased Q-angle = increased lateral tracking
- More lateral tracking = more lateral facet compression
- This becomes more pronounced as the knee bends.
This is why:
- Hip strengthening
- Patellar taping
- Glute activation
- Foot alignment correction
…are so effective—they reduce lateral load.
4. Why This Mechanism Matters in Physiotherapy
Understanding patellofemoral mechanics allows physiotherapists to:
✔ Choose exercises with optimal load
E.g., closed-chain squats vs open-chain knee extensions depending on symptoms.
✔ Modify knee angles that aggravate pain
Avoiding the 30°–60° range temporarily during painful phases.
✔ Improve patellar tracking
By improving hip control, foot alignment, and quadriceps balance (especially VMO activation).
✔ Reduce excessive patellar compression
Through movement retraining, taping, soft-tissue work, and flexibility interventions.
A well-informed patient recovers faster because they understand why their knee hurts—and what each exercise is doing at the joint level.
Conclusion
The patellofemoral joint is a dynamic system—its contact, compression, and loading vary with every degree of knee movement.
Pain often arises not from “damage,” but from increased or poorly managed compression forces, altered biomechanics, and tracking issues.
By understanding this mechanism, we can design rehabilitation plans that are safe, specific, and scientifically guided.
At Physiogain, I focus on restoring optimal patellar mechanics, reducing excess load, and helping patients return to pain-free movement with confidence.
Movement heals—especially when backed by science.