Thought expt on distal to proximal effects of foot orthoses.

My take is that we should not extend the GRF vector proximally to calculate joint moments because this approach does not take into account the segmental masses nor accelerations of the segments between the ground and the point of interest. As we move proximally this becomes a bigger problem due to the increasing segmental masses. In fatties the problem will be worse; in runners the problem will be worse than in walkers.

 

Kevin, how do we calculate the external moment acting on the foot at the ankle joint axis during swing?

 

Here is a quote from one of David Winter's colleagues at the University of Waterloo who used Winter's data for his analysis.

http://www.rehab.research.va.gov/jour/81/18/1/pdf/wells.pdf

 

Yep, already got that from Winter himself. It also becomes problematic when gait is not "smooth",e.g. in the pathology / elderly, according to Winter. Slow, smooth walking- OK to the knee; faster or unsteady walking/ running= not good = avoid.

I think this is an important question and shouldn't be lost:

 

Agreed. In our paper, http://www.ncbi.nlm.nih.gov/pubmed/16988168 Jo and I couldn't say whether the medial shift in the centre of pressure was representative of an increased external supination moment from the foot orthoses, or whether the medial shift in the centre of pressure was due to increased pronation of the foot.

 

What Winter tells us is that we first need to consider the internal forces which are causing the reaction forces. Additionally, I have suggested for some time now, we cannot simply look at the position of the point of application of a net ground reaction force relative to the position of a joint axis to infer the polarity of the moment.

 

Looks like we will need to agree to disagree for now.

I need to focus on my upcoming lectures and also make sure Griff doesn't stay out all night drinking and partying (on Simon's advice???) before his big lecture tomorrow morning ...:drinks

 

So, you think the polarity of the moment can be infered from the CoP in isolation, Kevin?

Enjoy your meal. Griff doesn't drink much (nor eat much) before a big lecture. Weirdo.

 

I think I'm getting a grip on this. There are two contributors to the measured ground reaction force vector. There is the portion of the vector that is resistance to the force of gravity acting on the body and there is the portion of the vector that is the result of muscular contraction. The muscle contraction will change the measured ground reaction force vector from what it would have been without muscle contraction.

As Simon pointed out, it seems that Winter is aware of these problems. His above statement does seem to be ignoring the resistance to gravity portion of joint forces.

Eric