Foot orthotic top cover friction

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...been wondering about how much "top cover friction" should affect material choice when choosing orthotic top cover material.

Obviously there are a truckload of top cover varieties on the market. And while I don't want - or expect to find - a full breakdown on the subject, some input or pointers from some of the arena veterans would go a long way...
- Conrad

 

Re: orthotic top cover friction

It depends on what you're trying to acheive. The level of friction affects the force vector your orthotic will exert. Short version, a higher friction top cover will give you a more vertical force vector (I think).

As always there is no "right" answer. Its a variable which can be manipulated just like all the others.

 

Re: orthotic top cover friction

Depends whether the foot is sliding up or down the orthotic plane or neither. Here's something I wrote about it a while back.

 

Re: orthotic top cover friction

So for the cases where the orthotic is required to produce a relatively large external moment, a higher friction material would be justified(?).

Thanks to you kind Sirs! It makes sense and gives me something to work with.

 

Re: orthotic top cover friction

No. Not that simple. Moment = force x perpendicular distance from axis. So lets assume that the foot is sliding down the slope of an orthotic, a higher co-efficient of friction might result in a more vertical net ground reaction force vector than with a top cover of lower coefficient of friction. This may bring the vector closer or further away from a given joint axis depending on where that joint axis is. Viz., it does not necessarily increase external moment, it may even reduce it. Understand?

 

Re: orthotic top cover friction

Understood....Oversimplification withdrawn.

This seems well writ:

 

Re: orthotic top cover friction

Try this Conrad. Sit with your legs crossed so your right ankle is on your left knee. Put your thumb under your navicular and push straight up in line with your shin. Assuming your st axis is normal ish your foot should supinate.

Now angle your push so you are aiming at the 5th toe. Push again and the foot will probably pronate.

How can this be when you are pushing medial to the axis? Well you're not. The axis actually lies not on the sole of the foot but in the air somewhere. Imagine a knitting needle passing from your thumb up in direction of the force. If the needle passes above the axis you are exerting supination. Below it, pronation.

Now imagine dipping your thumb in oil to make it slippy. Try pushing straight up again. harder because the thumb slides off the foot. The force vector becomes more sideways.

But superglue your thumb to your foot it gets easier. You can push straight up. More vertical vector, needle passed further from the axis, more lever arm, etc

that make more sense?

 

Re: orthotic top cover friction

Yes, brilliant. Luckily I am right-handed.

 

Re: orthotic top cover friction

Thanks Robert - great practical demonstration for those with no imagination like myself


Robin

 

Re: orthotic top cover friction

Ah Robert, where would we be without your brilliant analogies? Or I'm rather hoping that this is evidence based Naturally, conducted in Sweden (always willing to put their bodies on the line for science)


On a serious and vastly duller note, and no where near so technical, don't forget that increased friction increases heat, so not so good for the heat sensitive or sweaty foot, but could be ideal for the chilly foot. Practical reasons for cover material, not just mechanical.

 

Be an easy and fun study to do. Put a pair of orthotics in someones shoes with different covers, send em for a run, then get an infra red thermometer (like the ones you get from the chemist for kids) and measure the temperature of the feet.

Might have significant implications for wound healing / tissue viability...