FROM MY PT SCHOOL NOTES · FUNCTIONAL ANATOMY & BIOMECHANICS · CHAPTER 4
How Can Bones Move One Way While Joint Surfaces Move Another?
Saying ‘the knee bends’ describes a large movement, but not what the articular surfaces do as the bones move.
Describe the bone motion first
The FABIO principles distinguish osteokinematics, or recognizable bone motion in anatomical planes, from arthrokinematics, motion between contacting joint surfaces. Before describing either, name which bone moves relative to which other bone and whether the task is open or closed chain.
A knee extending in seated open-chain movement and a knee extending while rising from a chair may involve different moving segments. The motion name is similar, but the reference frame and loading are not.
Identify the joint surfaces
Synovial joints have articular cartilage, a capsule and synovial lining, with ligaments and other structures providing support. The joint-classification lecture contrasts hinge, pivot, ellipsoid, saddle, plane and ball-and-socket types.
Roll, glide/slide and spin describe simplified contact-surface motions. They do not mean a healthy joint follows one perfectly rigid, uniform path. Anatomical geometry, load and tissue constraints influence how surfaces move.
Use concave/convex rules carefully
Class materials introduce the concave-on-convex and convex-on-concave rule to help predict relative roll and glide in simple models. It can help organize introductory reasoning, but it is not an all-purpose direction prescription for every joint or technique.
A better study habit: draw both surfaces, label the moving segment and explain the assumptions. If the model and the observed motion disagree, revisit the geometry rather than forcing the diagram to fit.
Quick check: A student says the humerus abducts, so the glenohumeral joint has only one type of surface movement. What’s missing?
The statement describes osteokinematics but does not account for the joint-surface roll, glide or spin components and their relation to the scapula and load.
Try the mechanics on a real task
Example: A student describes seated knee extension and standing up from a chair as the same motion because the knee extends in both tasks.
Work through the reasoning
Answer: The broad osteokinematic name is similar, but the moving segment, contact conditions, joint-surface behavior and load differ. Specify tibia-on-femur versus femur-on-tibia assumptions before predicting arthrokinematics.
Common trap: Treating a single concave-convex mnemonic as a universal clinical prescription. Remember: Name the moving bone and the task before describing contact mechanics.
Sources: my original FABIO lectures: FABIO Principles I & II. Explanatory examples are newly written. This is a conceptual mechanics lesson, not a treatment instruction. Publisher or lecture figures are not reproduced.
