Kevin responded to my post

>
>   <<First ray and medial forefoot pressure.  I think this is a good
illustration
>   of the problems that can arise when you attempt to use joint position
rather
>   than forces.  I don't think it matters whether or not the ray is
>   plantarflexed.  It matters how much force the 1st metatarsal can apply to
the
>   medial side of the ski in the position that it is in.  I would agree with
>   Phils statement about the problem of having the ray plantarflexed is that
it
>   has range of motion to dorsiflex.  Yes, the peroneus longus may hold the
met
>   plantarflexed in that position (while everting the foot and ski) but it
>   doesn't get the help of the ligaments in holding down the first met when
the
>   met is in its maximally dorsiflexed position.>>

Kevin's reply
>
>  I don't exactly agree with your description of peroneus longus function
here.
>   If a first metatarsal is maximally dorsiflexed in a foot with a pes cavus
>  deformity and the peroneus longus contracts, this will produce more
>  plantarflexion moment on the first metatarsal than in a foot with a pes
>  planus deformity where the first metatarsal is maximally plantarflexed.
The
>  magnitude of first metatarsal plantarflexion moment produced by peroneus
>  longus contraction is dependent mostly on the height difference of the
medial
>  longitudinal arch to the lateral longitudinal arch.
>

I don't think that we are disagreeing about the need for a plantarflexion
moment from peroneus longus to resist a dorsiflexion moment acting on the met
from the medial aspect of the ski.

However, I do have a disagreement about the difference in medial and lateral
arch height effecting the moment from peroneus longus acting on the first
metatarsal.  I assume that you are basing your assumption on the diagrams in
Normal and Abnormal Function in the Foot by Root, Orien and Weed. (Normal and
Abnormal)  (Sorry, I don't have the page numbers of the diagrams my copy is
not with me.)  Those diagrams use a frontal plane analysis of the pull of the
peroneus longus muscle.  The problem I have is that this analysis does not
use the concept of a force couple for creating a moment at a joint.  The
forces shown in that diagram should flatten the arch because the base of the
metatarsal is pulled down at a time when ground reactive force is pushing
upward on the head.  This is not what is seen so further explanation of how
peroneus longus works is needed.

Force couple: In the sagittal plane, a downward force at the base and an
upward force at the head will create a dorsiflexion moment acting on the
metatarsal.  Another example, At the 1st MPJ when there is a proximal pull at
the base of the phalanx from all the structures that attach to the sesamoids
and there is a distal push from the metatarsal acting on center of the base
of the phalanx then there will be a force couple causing a plantar flexion
moment on the phalanx.  These forces, for the MPJ, are both horizontal but
not directly aligned.  The anterior to posterior force is plantar to the
other force.  The magnitude of the moment is equal to the magnitude of one of
the forces times the distance between the forces.  (If there is a proximal
pull at the base, and the phalanx does not accelerate posteriorly then there
must be a distal push from somewhere [met] to keep the net horizontal force
on the phalanx at zero.  F = ma.  When a = 0 then the net F = 0)

A force couple also must occur at the 1st met cuneiform joint.  In the
frontal plane analysis of the forces there is no upward force, hence there is
no upward push that would counter act the downward pull of the tendon.  (An
upward pull from the cuneiform acting on the metatarsal would cause an equal
and opposite downward pull from the met acting on the cuneiform.  So the arch
is still flattened.)

Transverse plane analysis.  Also in Normal and Abnormal there are diagrams
that show the transverse plane and sagittal plane analysis.  In the
transverse plane, the peroneus longus, as it runs from the met around the
cuboid, runs from distal medial to proximal lateral.  The medial to lateral
pull on the base of the metatarsal is very interesting, because I'm not sure
where the counter force is that is acting on the metatarsal.  (I've heard
people say that this is a cause of 1st met rotation away from the second met
in bunions. My hunch is that it does not.)   Now, the important part of the
force in terms of 1st ray plantar flexion is the distal to proximal pull on
the plantar base of the metatarsal.

Sagittal plane analysis: The proximally directed pull from the tendon is
counteracted by a distally directed push from the cuneiform.  These forces
create a plantar flexion moment acting on the metatarsal.  I don't think that
this moment will be significantly effected by arch height.

It is important to analyze forces in all three cardinal planes.  This problem
with single plane analysis occurs multiple times in Normal and Abnormal.

Peroneus longus is a fascinating muscle.  As it runs from the cuboid to the
lateral maleolus it has an upward component of force that will tend to
pronate the STJ.  Also, as curves around the maleolus it will tend to cause
internal leg rotation.  It is really hard to determine its purpose.  It
pronates the STJ to increase load on the medial forefoot and it plantarflexes
the metatarsal to also increase load on the medial forefoot.  It seems like
it is a very important muscle for skiing, but I don't think that is how it
developed evolutionarily.  Has anyone looked at peroneus longus in apes?

Cheers,

Eric Fuller


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%