Stanley,

Force=mass x acceleration

body weight = mass x acceleration due to gravity

Try standing on some bathroom scales, then bend your knees to squat or jump
up and down, you will find that the force measured changes so that sometimes
you will be "heavier", this is because your acceleration changes. This
explains why we see the classic double hump for vertical load in force plate
data during walking, i.e. force is not constant but dependent upon
accelerations. The first hump occurs because the CoM is accelerating
downwards (this hump is usually greater than body weight), the trough that
follows occurs because the CoM is accelerating upwards away from the ground,
force is reducing, the second hump occurs during propulsion because the CoM
is once again accelerating downwards. Any biomechanics text will explain
this.

Moreover, the position of the centre of force and it's line of action on the
foot changes with time during the gait cycle. Both the magnitude and
position of the centre of force appear to be influenced by FFO's. Thus
moments about the joints of the foot may also be altered.

Best wishes,
Simon

----- Original Message -----
From: "Dr. Stanley Beekman" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, December 16, 2004 7:10 AM
Subject: Re: EVA vs Plastic/Carbon Fibre FFO's: Discuss


> Eric,
>
> You wrote:
>
> >There can be a change in R, and moment, without a change in position.  I
> >have always liked the nutcracker analogy.  Place a nut so that it touches
> >one arm of a nutcracker.  Now slowly move the other arm so that it also
> >touches the nut.  (This is analogous to the maximally pronated
> >position.)  Now squeeze harder.  The position of the nut has not changed
> >relative to the arms of the nutcracker.  The force, and stress, on the
nut
> >has changed.
>
> Thanks for explaining this to me. But I am now further confused. My
limited
> knowledge on the subject is RXF.
> Radius times Force. Since we are not changing Radius, we must be changing
> Force, which is what I seem to understand from your analogy. Since force
is
> a function of body weight, and body weight has not changed (unless too
many
> nuts have been eaten), where exactly does this increase in force come
from.
>
> I will stop eating nuts until this is explained to me.
>
> Sincerely,
>
> Stanley
>
>
>
>
>
> >Good eating,
> >
> >Eric Fuller
> >
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