Recently I wrote about how movement is driven, and I went through how the bone movements lead you to what the joints feel. Remember: bones move, joints feel, muscles react. The example I used showed how we determine what the joint is feeling even when the proximal and distal bones are moving in the same direction. In that scenario at the knee we determine which is moving faster by asking which one is closer to the driving force of the movement.
The one thing I didn’t explain was how we name a joint movement. In the example I used, the knee felt internal rotation with both movements. However, in the first movement all the bones were moving left and in the second all the bones move right, but you get internal rotation with both. The reason this is possible is because in one case the driver is coming from below and in the other the driver is from above.
As we know, there are 5 ways of getting a joint motion. The above are just two of the ways of getting internal rotation. The other three are proximal still/distal moving, distal still/proximal moving and both moving in opposite directions. But, how do we name the joint movement?
Well, in anatomical terms, a joint movement is named by what the distal bone does on the proximal bone. So, in the case of the knee, if you imagine that the femur (proximal bone)is still, it is the tibia movement that will determine the name of the joint movement.
If we stick with the example of right knee internal rotation and we keep the femur still, it’s what the tibia does in the transverse plane that we need to consider. If in this scenario the tibia rotates right, the knee will feel external rotation, and if the tibia rotates left the knee will feel internal rotation. Simple as that!
It becomes more complicated when you are trying to look at functional movement with both bones moving in all three planes, but in my mind I always bring it back to the simple bone motions and then decide which motion the joint is feeling.