Arthrokinematics describes the small, precise movements that happen between joint surfaces when you move. These aren’t the big, visible motions like bending your elbow or lifting your arm. They’re the subtle rolling, sliding, and spinning of one bone’s surface against another inside the joint itself. Understanding these micro-movements explains why joints sometimes feel stiff, grind, or lose range of motion, and it’s the foundation for how physical therapists restore normal joint function.
Arthrokinematics vs. Osteokinematics
These two terms describe movement at completely different scales. Osteokinematics refers to the gross, observable movements of bones: flexion, extension, abduction, adduction, and rotation. When you swing your leg forward while walking, that’s osteokinematics. It’s what you can see from the outside, and it’s measured in degrees of motion around a joint axis.
Arthrokinematics, by contrast, is what’s happening at the joint surface while that larger movement takes place. The bone ends are covered in smooth cartilage, and those cartilage surfaces interact through tiny movements that allow the bigger motion to happen smoothly. You can’t see arthrokinematic motion from the outside. It’s the mechanical engine running beneath every movement you make.
The Three Types of Joint Surface Motion
Every joint movement you perform is produced by some combination of three arthrokinematic motions: roll, slide, and spin.
- Roll: One bone surface rotates against the other, like a tire rolling along pavement. New points on one surface contact new points on the other surface as the movement progresses.
- Slide (or glide): One joint surface translates across the other, the way a hockey puck moves across ice. A single point on one surface contacts multiple new points on the opposing surface.
- Spin: One bone rotates in place on the other surface, like a top spinning on a table. The contact point stays roughly the same while the bone turns around it.
In real movement, these three motions almost always happen together. Pure rolling, pure sliding, or pure spinning in isolation is rare. When you straighten your knee, the shin bone both rolls and slides on the thigh bone simultaneously. This combination is what keeps the joint surfaces aligned and distributes pressure evenly across the cartilage.
The Convex-Concave Rule
One of the core principles in arthrokinematics is the convex-concave rule, which predicts the direction that rolling and sliding occur based on the shape of the joint surfaces. Most joints pair a rounded (convex) surface with a cupped (concave) surface.
When the convex surface is the one moving, roll and slide happen in opposite directions. The shoulder is a classic example. During abduction (lifting your arm out to the side), the rounded head of the upper arm bone rolls upward on the shoulder socket while simultaneously sliding downward. That downward slide prevents the ball from jamming into the top of the socket.
When the concave surface is the one moving, roll and slide happen in the same direction. During knee extension in an open chain (like kicking a ball), the concave top of the shin bone rolls forward on the thigh bone and also slides forward.
This rule matters because the slide component essentially corrects for the roll. Without it, a rolling bone would eventually roll right off the edge of its partner surface. The paired slide keeps the contact point centered in the joint, maintaining smooth, full-range motion.
What Happens When These Mechanics Break Down
When the normal roll-slide relationship is disrupted, joints stop moving cleanly. Research on knee joints after periods of immobilization found that the quality of arthrokinematic motion declines measurably. Capsular contracture (tightening of the joint capsule from disuse) increases compressive stress between surfaces, which raises friction. This aligns with what many patients report after prolonged bracing or bed rest: popping, snapping, or grinding sensations, particularly in the front of the knee.
At the shoulder, the consequences are even more intuitive. If the humeral head fails to slide downward during arm elevation, it migrates upward into the narrow space beneath the bony arch above it. Over time, this compresses the rotator cuff tendons and bursa, contributing to impingement and pain with overhead movements. The visible motion (raising the arm) looks the same from outside, but the invisible arthrokinematic failure underneath is what produces the problem.
Close-Packed and Loose-Packed Positions
Joint surfaces don’t maintain the same amount of contact throughout their range of motion. At certain angles, the two surfaces fit together as tightly as possible, the ligaments become taut, and the bones essentially lock together as a single rigid unit. This is called the close-packed position. For the knee, it’s full extension. For the shoulder, it’s roughly 90 degrees of abduction with full external rotation.
In every other position, the joint is considered loose-packed. The surfaces have less contact, the capsule and ligaments are relatively slack, and the joint has a small amount of accessory play. This play is exactly what physical therapists assess and treat. If you’ve ever had a therapist wiggle or glide a joint in a specific direction while you relax, they were working in a loose-packed position to restore normal arthrokinematic motion.
How Physical Therapists Use Arthrokinematics
Joint mobilization, one of the most common manual therapy techniques, is built directly on arthrokinematic principles. A therapist applies an oscillatory force to a joint surface in a specific direction, chosen based on which glide is restricted and what the convex-concave rule predicts for that joint. For example, if shoulder external rotation is limited, the convex-concave rule suggests the humeral head needs to slide anteriorly (forward) during that motion. A therapist would apply a forward glide to the humeral head to restore that component.
Mobilizations are typically graded on a I-to-IV scale. Grades I and II are small and large oscillations performed before reaching tissue resistance, primarily aimed at reducing pain. Grades III and IV push into resistance, with the goal of increasing available motion. Grade III uses larger amplitude movements into the resistance, while Grade IV uses smaller, more targeted oscillations at the end of the available range. The choice depends on whether pain or stiffness is the primary barrier.
This grading system exists because arthrokinematic restrictions don’t all have the same cause. A joint that’s painful but not structurally tight responds to gentle oscillations that stimulate pain-gate mechanisms. A joint that’s genuinely stiff from capsular shortening needs sustained pressure into the restricted glide to lengthen the tissue. The arthrokinematic assessment tells the therapist which problem they’re dealing with and which direction to push.
Why It Matters Beyond the Clinic
You don’t need to memorize roll-slide directions to benefit from understanding arthrokinematics. The practical takeaway is that joint mobility isn’t just about muscle flexibility. You can stretch a muscle to full length and still have restricted joint motion if the surfaces aren’t gliding properly. This is why some people plateau in their flexibility work, or why a stiff shoulder doesn’t always respond to stretching alone.
It also explains why prolonged immobility is so damaging to joints. Cartilage relies on regular compression and decompression to receive nutrients from joint fluid. When arthrokinematic motion stops, the cartilage loses its nutrition source, the capsule tightens, and friction increases. Resuming movement early after injury, within safe limits, preserves these surface mechanics and reduces the grinding and stiffness that can otherwise persist for months.