The rule tells us how the sliding (gliding) motion of joint surfaces behaves when one bone is fixed and the other moves.
✅ The Two Parts of the Rule:
1. **If the moving surface is concave:
- The glide happens in the same direction as the bone’s movement.
Example:
- In the knee joint, the tibia (concave) moves on a fixed femur (convex) during extension → the tibia glides forward, in the same direction as the leg moves.
2. **If the moving surface is convex:
- The glide happens in the opposite direction of the bone’s movement.
Example:
- In the shoulder joint, the humeral head (convex) moves in the glenoid cavity (concave) during abduction → the humeral head glides downward, while the arm moves upward.
🧠 Why It Matters:
- It helps physical therapists and other clinicians understand joint play, apply manual therapy, and improve mobility without causing injury.
- It’s essential in designing rehab exercises and understanding joint dysfunction.
📊 Summary Table
| Moving Surface | Glide Direction | Example |
|---|---|---|
| Concave | Same as bone movement | Tibia on femur (knee) |
| Convex | Opposite bone movement | Humerus on glenoid (shoulder) |