-----
Original Message -----
Sent:
Tuesday, May 01, 2007 5:44 AM
Subject:
Re: Ankle Joint Dorsiflexion - Are We Really Measuring the Ankle
Joint?
Dave and Colleagues:
Dave
wrote:
<<I believe you
have already coined the term by using ' stiffness and compliance' in
terms of a joints resistance to angular displacement by GRF.
The midtarsal joint is a mechanism
which appears to be designed so that in supination there are two
mechanism factors that increase the stiffness of the joint as a
whole.
1) Increasing the 2nd moment of area
(sometimes, but not correctly, known as moment of inertia). IE
increasing a beams resistance to bending by increasing the
effective thickness of a beam in the same plane as the direction of
force. The increased thickness increases the lever arm distance from
the centre of the beam to the edges..
2) Reducing the perpendicular
congruency of the axis of the CCJ and TNJ to the applied
vertical ground reaction force.
A third factor is the
mechanical internal reaction of the ligaments, muscles and
bone to resist external moments.So
the foot becomes more or less stiff, dependent of these three
factors.>>
I agree with number one but don't agree with
number two. The axes of the midtarsal joint are imaginary and
are a figment of a few clinician's imaginations (which, for some
unknown reason, podiatry schools around the world still continue to
teach even though all the latest research shows the midtarsal joint
axes to be moving axes). In the human body, the motion
determines the axes, the axes do not determine the motion.
However, I do agree with your third factor. Two out of three
isn't too
bad.
Sincerely,
Kevin
****************************************************************************
Kevin
A. Kirby, DPM
Adjunct Associate Professor
Department of
Applied Biomechanics
California School of Podiatric Medicine at
Samuel Merritt College
Private Practice:
107 Scripps
Drive, Suite 200
Sacramento, CA 95825
USA
Voice: (916) 925-8111
Fax: (916)
925-8136
****************************************************************************
David
Smith wrote:
Deat Kevin K
I believe you have already coined the
term by using ' stiffness and compliance' in terms of a joints
resistance to angular displacement by GRF.
The midtarsal joint is a mechanism
which appears to be designed so that in supination there are two
mechanism factors that increase the stiffness of the joint as a
whole.
1) Increasing the 2nd moment of area
(sometimes, but not correctly, known as moment of inertia). IE
increasing a beams resistance to bending by increasing
the effective thickness of a beam in the same plane as the
direction of force. The increased thickness increases the lever
arm distance from the centre of the beam to the
edges..
2) Reducing the perpendicular
congruency of the axis of the CCJ and TNJ to the applied
vertical ground reaction force.
A third factor is the
mechanical internal reaction of the ligaments, muscles
and bone to resist external moments.So the foot becomes more or less stiff, dependent of these
three factors.
To say the joint is 'locked' is more
an intuitive or laymans term, where deflection is difficult
to see or feel with increasing force, but can have no real meaning
in engineering terms since the joint will continue to deflect
until failure if enough force is available. This intuitive term
'locked' may be significant if there were an osseous block to the
motion of the joint, say like the olecranon process of the elbow
joint, and deformation previous to failure may be imperceptible to
the human eye. Even in this example there are soft tissue
components that will deform under stress. Whereas one might say
that the elbow is locked when it is fully extended would one
say it was locked if held by muscle power in an immoveable
position at 90dgs flexion with the same magnitude of applied force
. Is this the same for the midtarsal joint? can we say it is
locked if the resisting forces are muscular and ligament based. At
what point is it locked? ! ! When pushed by hand with with 25Nm
external moments or when pushed by GRF at 200Nm external
moment.
Let's assume the spring
analogy and GRF acting on the distal foot (and this is where
one needs to bear in mind Newtons 3rd law). The GRF can only apply
as much force as the resisting spring force allows. The
body may have a downward momentum which can apply
infinite force in a given direction if the acceleration in the
opposite direction is fast enough ie the upward
acceleration. The internal springs attenuate this force
(reduces the peak and extends the time) to that which is
managable by the tissues without injury. The maximum force applied
is that which is allowed by the most compliant (active) spring in
the system.
Therefore at any one instantaneous
point in time the system is in eqilibrium IE where the
input/external forces and moments equal the output/internal forces
and moments.
Can we saftely use the term
'Locked'? - From several dictionaries, in this context,
Locked means - to hold fast, become rigid, become immobile. A
joint can be held fast or rigid in terms of human perception or to
a given load but at what point can we define that as a
definitive clinical term?
Locked can describe a state of
function but cannot define a predetermined position of function
and gives the impression of absolute maximum deflection.
Maybe it would be more useful to say
the position 'maximum allowable compliance' for a given load.
Unfortunately this is a tautology since it impossible for a joint
under load to be anything other than at its maximum allowable
compliance, if you accept that in any mechanism or mechanical
system there is always equilibrium of forces and
moments.
I've tried my best, Cheers, Dave
Smith
-----
Original Message -----
Sent:
Saturday, April 28, 2007 2:05 PM
Subject:
Re: Ankle Joint Dorsiflexion - Are We Really Measuring the Ankle
Joint?
Dave and Colleagues:
Dave
Smith wrote:
<<I think the correct word to
replace dampening or shock absorption is attenuation. If a
force is applied then it must also have a time component. (the
time it takes for a finite amount of energy to transfer from one
segment to the next segment)A force/time curve ususally produces
an oscillating signal, which is in effect a sin wave when
plotted on a graph. During attenuation the sin wave amplitude
(where y = force) is reduced and time (x axis) or wave length is
increased. Therefore attenuation also decreases the signal
frequency and so bumps in a road feel smoother because of the
increase in time and reduced peak force. It is worth remembering
Newtons 3rd law 'every action has an equal and opposite
reaction' so therefore the applied force can only be as high as
the reaction force.The stiffer the spring the less attenuation
(reduction in signal amplitude) for a given force/time. And vice
versa the compliant spring dissipates the energy over a longer !
! time period and so the force peak cannot be so high.
Sorry to butt in on your interesting discussion, Vive la
Jiscmail!>>
Thanks for that clarification,
Dave. Always nice to have an engineer reading along to
offer help to a poor self-taught engineer. ;-)
I was
starting to see that the word "dampening" was not as precise of
a word as I wanted. I think attenuation is much more
accurate and describes the process better.
Now that I
have your attention here back on JISCmail, where my postings
probably will never be deleted, I can get you involved in this
discussion further.
Here's my question for you
Dave: What do you think of the term "midtarsal joint (MTJ)
locking" in regards to what this clinical term is trying to
describe mechanically and what do you think of its accuracy
biomechanically? In other words, what other term could we
use to describe the position of static equilibrium of the MTJ
and midfoot joints when the plantar forefoot is loaded by a
force which exerts a forefoot dorsiflexion moment? My
problem with "MTJ locking" is that this term implies that the
forefoot won't dorsiflex further with increasing magnitudes of
forefoot dorsiflexion moment which then gives the false
impression to the clinician as to actual mechanical nature of
the more spring-like actions of the MTJ and midfoot
joints. Should we be saying instead of "MTJ locking"
something like "position of longitudinal arch stability" or "MTJ
stability position" or "position of MTJ dorsiflexion
stability"? Maybe we should continue using "MTJ locki! !
ng" but since no one has been able to precisely define it for
me, then I say let's get rid of it!
Hope your project is
going well. Keep me informed of your
progress.
Sincerely,
Kevin
****************************************************************************
Kevin
A. Kirby, DPM
Adjunct Associate Professor
Department of
Applied Biomechanics
California School of Podiatric Medicine
at Samuel Merritt College
Private Practice:
107
Scripps Drive, Suite 200
Sacramento, CA 95825
USA
Voice: (916)
925-8111 Fax: (916)
925-8136
****************************************************************************
David
Smith wrote:
Dear Kevin K and Stanley
I
think the correct word to replace dampening or shock
absorption is attenuation. If a force is applied then it
must also have a time component. (the time it takes for a
finite amount of energy to transfer from one segment to the
next segment)A force/time curve ususally produces an
oscillating signal, which is in effect a sin wave when plotted
on a graph. During attenuation the sin wave amplitude (where y
= force) is reduced and time (x axis) or wave length is
increased. Therefore attenuation also decreases the
signal frequency and so bumps in a road feel smoother because
of the increase in time and reduced peak force. It is worth
remembering Newtons 3rd law 'every action has an equal and
opposite reaction' so therefore the applied force can only be
as high as the reaction force.The stiffer the spring the less
attenuation (reduction in signal amplitude) for a given
force/time. And vice versa the compliant spring dissipates the
energy over a longer time period and! ! so the force peak
cannot be so high.
Sorry to butt in on your
interesting discussion, Vive la Jiscmail!
Cheers Dave
----- Original Message ----- From: "Stanley Beekman" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Saturday, April 28, 2007 6:30 AM
Subject: Re:
Ankle Joint Dorsiflexion - Are We Really Measuring the Ankle
Joint?
Kevin,
As a self taught
mechanic prior to going to podiatry college, I can tell you
that springs do not dampen, shocks do.
I still think it
is an interesting point about the spring effect of the
joints, which would make some sense for efficiency. My
understanding is that the connective tissue in the muscle is
responsible for the spring effect. This is what is developed
in pliometric exercises.
Did the study you quote discuss
muscles vs. ligaments? In John Jesse's book Hidden
Causes and Treatment of Athletic Injuries, he discusses an
unpublished study on foot exercises and its effect on
vertical jump and 100 meter times on fifth graders. For a
spring effect to occur in the joints, the ligaments
would have to have elastic and possibly contractile
properties.
Could you expand on your concept?
Regards,
Stanley
Kevin Kirby wrote:
Kevin M:
You wrote:
<<In all of my cadaver work, I have seen the
spring action to which you refer. However this
action does not seem to dampen or dissipate force it seems
to be a force transfer mechanism, which would follow the
work of Gracovetsky very closely. Is this what you
mean? In any case, perhaps, you might re-think what
you are saying about midtarsal joint locking. Without
midtarsal joint, locking the soft tissue structures of the
midfoot could not store and return energy. So, the
question is does the foot store and return energy or
simply dissipate it.>>
I don't think we are
in disagreement . When I said dampen, I was using
the analogy of the leaf springs of a truck which dampen
the shock of an uneven road. However, dampening is
also a force transfer mechanism in that the force is still
transferred but at a slower rate and with decreased peak
magnitudes.
By the way, no need to rethink "midtarsal
joint locking", I have been thinking about midtarsal joint
locking for over 20 years. "Locking" implies that no
further movement is allowed with increases in external
loading forces, which is not what a foot or leaf spring in
a truck does while functioning. Is it desirable in a
truck to have a spring that "locks"?? Not really since we
call that a "bottoming out" of the springs and causes a
sudden vertical acceleration of the truck axle and an
uncomfortable ride for the passenger. In the same
way, we don't want a midtarsal joint that "locks" and
suddenly stops forefoot dorsiflexion motion while walking
or running or jumping down from a height....we rather want
a longitudinal arch that functions like a leaf spring that
will deform under increasing loads so that lower extremity
accelerations are kept to a minimum during weightbearing
activities.
When it comes to the question of
storage and release of energy in the longitudinal arch of
the foot, the classic study was done 20 years ago (Ker RF,
Bennett MB, Bibby SR, Kester RC, Alexander RMcN: The
spring in the arch of the human foot. Nature, 325:
147-149, 1987). Ker et al showed that the human foot
stores 17 joules of energy in the compliant elements (i.e.
ligaments) of its longitudinal arch during running.
However, the foot both stores and releases energy and also
dissipates this energy as heat since the ligaments of the
are arch are not perfectly elastic and have a hysteresis
loop.
Sincerely,
Kevin
****************************************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California
School of Podiatric Medicine at Samuel Merritt College
Private Practice:
107 Scripps Drive, Suite
200
Sacramento, CA 95825 USA
Voice: (916)
925-8111 Fax: (916) 925-8136
****************************************************************************
Kevin Miller wrote:
Kevin K,
I a long
moment that I was about to agree with you. Perhaps
I still do agree with you however a little explanation
would be in order. In all of my cadaver work, I
have seen the spring action to which you refer. However
this action does not seem to dampen or dissipate force
it seems to be a force transfer mechanism, which would
follow the work of Gracovetsky very closely. Is
this what you mean? In any case, perhaps, you
might re-think what you are saying about midtarsal joint
locking. Without midtarsal joint, locking the soft
tissue structures of the midfoot could not store and
return energy. So, the question is does the foot
store and return energy or simply dissipate it.
Cheers,
Kevin M.
----- Original
Message ----
From: Kevin Kirby <[log in to unmask]>
To: [log in to unmask]
Sent: Friday, April 27, 2007 9:15:37 AM
Subject:
Re: Ankle Joint Dorsiflexion - Are We Really Measuring
the Ankle Joint?
Neil and Colleagues:
This idea that both the ankle joint and
midtarsal-midfoot joints are "spring-like" is very
important mechanically. What does this concept of
"spring-like" mean? ......It means that there is not a
hard "end-point" or "locked" position for either the
ankle joint or midtarsal joint or midfoot joints.
In other words, the greater the magnitude of GRF loading
the plantar forefoot, then the greater will tend to be
the dorsiflexion of the forefoot on the rearfoot and the
talus on the tibia, assuming no muscular contractile
forces are causing forefoot plantarflexion moments
and/or ankle joint plantarflexion moments. Therefore, to
use the term, "midtarsal joint locking" is not only
erroneous but also is biomechanically misleading since
we don't want our midtarsal joint to "lock" we want the
midtarsal and midfoot joints to rather "dampen" forefoot
dorsiflexion moments. This is analagous to the
leaf spring of a truck suspension functioning to dampen
the vertical oscill! ! ations of the truck axle when the
truck tires have suddenly increased magnitudes of GRF
when driving over a bump in the road.
Sincerely,
Kevin
****************************************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California
School of Podiatric Medicine at Samuel Merritt College
Private Practice:
107 Scripps Drive, Suite
200
Sacramento, CA 95825 USA
Voice: (916)
925-8111 Fax: (916)
925-8136
****************************************************************************
Neil Frame wrote:
Dear Kevin
I can
relate to this 'spring-like' analogy that you use and
I think it does convey more realistically what occurs
within the foot during movement. I do feel,
however, that we can complicate matters even further
by taking into account the other factors that may just
dictate the loading of your 'springs'.
Biomechanics never was that straight forward ;-)
One persons forefoot may dorsiflex to a far greater
extent than that of anothers and then we will be
noting that individuals ability to compensate for
these 'interrelating, variably loaded springs'.
To summarize; understand the 'spring-like' bit, but
don't forget they are 'variably loaded and variably
resistant' and compensation will occur in a multitude
of ways throughout the body during locomotion.
p.s. Who prefers examination/measurement of
the ankle joint prone and who prefers supine and why
and is there any difference? .................. neural
tension?
Best wishes
Neil
------------------------------------------------------------------------
From: /Kevin Kirby <[log in to unmask]>/
Reply-To: /A group for the
academic discussion of current issues
in podiatry <[log in to unmask]>/
To:
/[log in to unmask]/
Subject: /Re: Ankle Joint
Dorsiflexion - Are We Really Measuring
the Ankle Joint?/
Date: /Thu, 26 Apr 2007
21:08:32 -0700/
Jeff,
Howie, Kevin, Stanley and Colleagues:
My, my, my. I didn't
expect such numerous and rapid responses
to my first posting to JISC
podiatry mailbase in many months. It is gratifying to
see that the old gang is indeed back in
action again. Maybe we
can dig a few other fossils out of the
woodwork to join in on these
discussions. ;-)
The
purpose for my posting is to emphasize, not that our
old
measurement techniques are
wrong or useless, but rather that
when we say that a patient has,
for example, 10 degrees of ankle
joint dorsiflexion, that we are
not truly measuring ankle joint
dorsiflexion alone but rather also measuring
dorsiflexion of the
forefoot on
the rearfoot. This may not seem like an
important
point until one
realizes that the forces that we use on the
forefoot to manually execute
the ankle joint dorsiflexion
measurement are probably at least 10 times less in
magnitude
than the ground
reaction forces that will be present on the
forefoot during late midstance
and early propulsion. In
addition, as the force is increased on the plantar
forefoot not
only will the
talo-tibial joint continue to dorsiflex but also
the midtarsal and midfoot
joints will also continue to dorsiflex
relative to the rearfoot.
I like to describe these mechanical
interactions as being rather
than as being "end-ranges of
motion" since if increased force if applied to the
forefoot,
such as would be
present during a lunge-test, the apparent
dorsiflexion of the plantar
foot to the tibia is nearly always
increased versus what would be
apparent during manual ankle
joint dorsiflexion testing. It is probably best
that we think
of both the
talo-tibial joint and midtarsal-midfoot joints as
spring-like joints with varying
stiffnesses that mechanically
interact to determine not only the foot posture during
relaxed
stance but also during
dynamic function. It is the mechanical
interaction of these
spring-like joints with each other that may
also be very important in
determining the gait function and also
the pathologies we see on a
daily basis in our patients.
Sincerely,
Kevin
****************************************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied
Biomechanics
California School
of Podiatric Medicine at Samuel Merritt College
Private Practice:
107 Scripps Drive, Suite 200
Sacramento, CA
95825 USA
Voice: (916) 925-8111
Fax: (916) 925-8136
****************************************************************************
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