Accessory motion forces refer to external forces applied to a joint that help assess or improve arthrokinematic motion (like glide, roll, and spin). These motions cannot be performed voluntarily and are typically applied by a clinician during manual therapy or joint mobilization.
There are three primary types of accessory motion forces:
🔹 1. Traction (Distraction)
- Definition: A force applied perpendicular to the joint surface that pulls the joint surfaces apart.
- Purpose: To reduce joint compression, increase joint space, and relieve pain.
- Used in: Joint mobilization, especially to prepare the joint before applying gliding techniques.
🧠 Example:
Gently pulling the humerus away from the glenoid cavity in the shoulder joint.
🔹 2. Approximation (Compression)
- Definition: A force applied perpendicular to the joint surface that pushes the joint surfaces together.
- Purpose: To enhance joint stability by activating stabilizing structures, and assess joint integrity.
- Can occur naturally: Like when jumping or weight-bearing.
🧠 Example:
Pushing the femur and tibia together during a standing squat compresses the knee joint.
🔹 3. Shear
- Definition: A force applied parallel to the joint surface that causes gliding of one surface over the other.
- Purpose: To improve arthrokinematic glide, test joint movement quality, and increase joint mobility.
- Often used in manual therapy to restore movement or reduce stiffness.
🧠 Example:
Applying a glide to the tibia in a forward direction while the femur is stabilized (anterior tibial shear at the knee).
📊 Summary Table
| Accessory Force | Direction Relative to Joint Surface | Main Effect | Example Use |
|---|---|---|---|
| Traction | Perpendicular (pulling apart) | Increases joint space, reduces pain | Shoulder distraction in therapy |
| Approximation | Perpendicular (pushing together) | Increases stability, tests joint integrity | Knee compression during squats |
| Shear | Parallel | Promotes glide, increases mobility | Anterior glide of tibia on femur |