Hmm.. that sounds odd, but the context of the article might be helpful in determining what the authors intended to discuss. I can think of one distinction that is sometimes made between resultant and actual forces. A 'contact force' may describe an actual force that is measured (this has been done using instrumented joint replacements). A 'hip joint force' may refer to a resultant quantity which is calculated from inverse dynamics, but is not necessarily the actual force experienced by the joint.

agree.

Agree too

Thanks guys. Sean's distinction between two sounds good to me too. I think I got my answer.

Rıza,

David Winter made this issue clear in his pioneering book (Biomechanics and Motor Control of Human Movement, 2nd Edition, page 78-79).

“Considerable confusion exists regarding the relationship between joint reaction forces and joint bone-to-bone forces. The latter forces are the actual forces seen across the articulating surfaces and include the effect of muscle activity. The bone-to-bone force equals the active compression force due to muscle plus joint reaction forces.”

In your case, hip contact force equals to bone-to-bone forces. See the attached file for further details.

I have attached a 2010 paper which provides a good introduction to the topic.

For perspective, resultant forces tend to be on the order of 1-3 BW, depending on the activity. The actual joint contact forces include all of the stabilizing (internal) muscle forces, and are usually much larger. For example, Pavol et al, calculated ankle joint contact forces during running to be around 8-10 BW, whereas the resultant ankle forces were only around 2-3 BW. The joint contact forces are what the bone, cartilage etc actually experiences. However, they cannot be directly measured noninvasively. Instead, calcuation of these forces requires some assumptions to be made about muscle attachment points and activation levels.

The Joint reaction forces on a link segment model consider the resultant joint reaction forces on a joint AND the net effect of muscle/ligament activity at a joint (from agonist and antagonist muscles) as a net muscle moment. This is the reason why when calculating joint reaction forces on a joint, we consider 2 reaction forces per plane of analysis and an effective muscle moment.

The bone-one-bone forces (as the one on your figure) are the actual forces that are seen on the articulating surfaces, plus the effect of muscle activity and action of ligaments as (usually compressive or shear) forces working on the joint. Notice that there is no net muscle moment on the diagram.