Understanding Knee Joint Trajectory

When you pedal, your knee doesn't just move up and down. It follows a complex 3D path known as knee joint trajectory. Analyzing this path is the frontier of modern bike fitting.

The Ideal Trajectory

From a frontal view, the knee should track in a near-vertical line over the pedal spindle. Imagine a piston moving in an engine.

Common Deviations

1. Varus (Knees Out): The knee bows outward at the top of the stroke.
2. Valgus (Knees In): The knee collapses inward towards the frame during the power phase.

Why Analysis Matters

• Injury Prevention: Poor tracking increases shear forces on the knee joint, leading to tendonitis or patellofemoral pain.
• Power Efficiency: Lateral movement wastes energy. Straightening the trajectory ensures all force goes directly into the crank.

How to Analyze

While professional 3D motion capture (like Retül) is best, you can use a smartphone and a trainer:

1. Set up a camera directly in front of your bike.
2. Apply a small sticker to the center of your knee cap.
3. Record in slow motion and trace the sticker's path.

Conclusion

Pay attention to your knees. A straighter line means a longer, healthier cycling life.