Osteokinematics
The movement of bones in space around an axis and in a certain planes. We are describing changes in angulation as flexion, extension and so on.
Arthrokinematics
The movement of joint surfaces of a male or convex joint in relation to a female or concave joint or the other way round.
An example for a convex joint would be the head of the humerus on its concave joint partner the glenoid.
In arthrokinematics we are talking about combined movements of rotations and translations, which are further specified as spin, roll and glide, as well as traction and compression.
Roll
Rolling always occurs in the same direction as the angulating bone, whether the surface is concave or convex.
If rolling occurs alone, it causes compression on the surface on the side to which the bone is angulating and separation on the other side. In normally functioning joints, pure rolling does not take place without a spin or a slide. That’s logical, because otherwise the moving joint surface would just roll off the edge of its joint partner.
Glide
Also called sliding movement, is a translation, where one point on one of the surfaces is contacting new points on the opposing surface.
Spin
In case of a spin movement, one point of the moving joint partner stays in contact with the same point on the surface of the other joint partner. All other points of the moving joint rotate around an axis which is identical with the plumb line through the contact point of both surfaces.
Although spinning rarely occurs alone, an example of an impure spin would be during flexion and extension of the head of the humerus on the glenoid or the head of the femur in the acetabulum.
Traction
Traction is the distraction or separation of two joint partners. Be aware that traction does not always occur in the direction of the long axis of the bony partners. In the hip joint for example, traction in the direction of the axis of the femur would result in an inferior glide.
Instead, traction must happen perpendicular to the joint line of the concave joint partner.
Compression
Compression is the decrease of joint space between two partners. Compression occurs during weight bearing, provides stability to the joints and helps to move synovial fluid to maintain cartilage health.