What defines accessory joint motion?

Accessory joint motion refers to movements that are not under voluntary control and typically involve the way joint surfaces interact during movement. This type of motion is crucial for the proper function of joints because it facilitates the overall range of motion and stability. The key characteristic of accessory joint motion is that it occurs due to an external force, such as gravity or the movement of surrounding structures like muscles and tendons, rather than through the deliberate effort of the patient.

For example, when a person moves their arm, the rolling, sliding, or spinning of the joint surfaces that occur in conjunction with that movement are considered accessory joint motions. These motions are essential for ensuring that the primary movements can occur smoothly and effectively. Understanding that accessory joint motion is governed by these external forces helps clinicians assess joint function and diagnose potential issues related to joint mechanics.