Joint movement involves more than the simple bending or straightening we consciously perform. When a person moves a limb, smaller, unseen motions occur simultaneously within the joint. These subtle, internal movements are known as accessory movements, and they are fundamental to achieving a full and fluid range of motion. Accessory movements are mechanical interactions between joint surfaces, not muscle contractions, that enable the larger, visible actions of our limbs. Understanding this dual nature is essential for grasping how the body achieves mobility.
Defining Accessory Movements
Accessory movements, often called arthrokinematics, are the small, involuntary motions that happen between the articulating surfaces within the joint capsule. A person cannot consciously decide to perform them. These movements include distraction (separation of joint surfaces) or compression (pushing joint surfaces together). They are a prerequisite for normal joint function and prevent damage during larger, voluntary motions by maintaining proper contact and alignment. This internal movement, sometimes called “joint play,” is required for the full range of physiological motion without the joint jamming or sustaining injury.
Distinguishing Accessory from Physiological Movement
Total joint movement is divided into two categories: physiological and accessory. Physiological movements, or osteokinematics, are the large, visible movements of the limb segments that a person actively controls, such as flexion, extension, abduction, or rotation. These motions move the bone around an axis and can be easily measured. Accessory movements are the small shifts and glides occurring directly at the joint surfaces, which are involuntary.
When lifting the arm overhead, the voluntary movement of the upper arm bone (humerus) is the physiological action. For the head of the humerus to stay centered and avoid hitting the socket rim, it must simultaneously perform a small, involuntary downward slide and spin within the shoulder socket. If this necessary accessory slide is restricted, the larger physiological movement becomes limited and often painful. Unrestricted accessory motion is a requirement for the full-range physiological motion to be possible.
The Three Primary Types of Accessory Movement
Accessory movements are composed of three fundamental mechanical actions describing how one joint surface moves on another: roll, glide (or slide), and spin. Roll is the motion where new points on one joint surface contact new points on the opposing surface, similar to how a car tire rolls across the road. This action results in the angular motion of the bone.
Glide, also called slide, occurs when a single point on one joint surface moves across multiple points on the other, visualized as a tire skidding on ice. The direction of this glide is dictated by the shape of the joint surfaces—specifically, whether the moving surface is convex (rounded) or concave (hollow). Spin is the rotation of a joint surface around a stationary mechanical axis, like a top spinning in place. While pure roll, glide, or spin rarely happen in isolation, they combine to create the complex, coordinated movements required for fluid joint action.
Clinical Significance in Joint Health and Therapy
Understanding accessory movements is fundamental to diagnosing and treating musculoskeletal conditions. When these involuntary joint motions are restricted, it leads to joint stiffness, limited range of motion, and pain, even though the primary physiological movement may appear mostly intact. The loss of joint play can be caused by various factors, including injury, inflammation, or contracture of the joint capsule and surrounding connective tissues.
Physical therapists use this knowledge through specialized techniques called joint mobilization and manipulation. These are passive, manual movements applied by the clinician to restore the lost accessory movement, or “joint play,” by applying precise, controlled forces to the joint surfaces. Performing distraction or gliding movements stimulates the production of synovial fluid, which nourishes the avascular cartilage, and helps relieve muscle guarding and pain. Restoring normal accessory movement is often the fastest way to improve a patient’s overall functional range of motion and reduce discomfort.