-----
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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