Componentry - Feet
Feet appear pretty simple things
while we’re using them. It’s only
when we try to copy them that we
realise how complicated they are.
With 26 bones, 20 internal muscles
plus the calf and shin muscles
acting on it, and a complex network
of blood vessels and nerves the
human foot is an astonishingly
effective piece of anatomy. Trying
to match the function of such a
masterpiece artificially is a big ask,
especially given that a prosthetic
foot must be lightweight, reliable
and cheap.
Before trying to do this we first have
to think about what we require our
feet to do. There are three main
functions of the foot; support,
balance and locomotion.
Support is pretty straightforward.
A foot must support our weight. In
the body this is the function of the
skeleton, with a big supporting role
from ligaments and muscles.
Balance in the anatomical foot is
a dynamic process. Balance on
one (anatomical) leg and feel the
constant small movements and
muscle contractions in your leg and
foot – that is going on all the time,
completely unconsciously as our
bodies works to keep us upright.
Constant interaction between
nerves, muscles and brain keep us
on our feet while we move over our
base of support, while the surface
we are on changes in angle,
roughness or consistency, and
while changing speed, direction or
doing any number of activities.
Locomotion is where the real
complexity starts. Entire university
departments are dedicated to
examining exactly how we move
as efficiently as we do. For our
purposes we’ll simplify things
drastically. Firstly, the body must
be able to move over the foot
in order to move smoothly and
effectively. Secondly, we actively
push ourselves forward. This push
is generated by our calf muscles
and carried out by our feet. Actually
in everyday walking this push
is pretty subtle, but it becomes
more apparent in brisk walking or
running.
For an amputee we can see a few
problems. No calf muscles to push
us forward, no muscles and joints
to realign us to different surfaces,
and no nerves to sense where our
feet are or what we are walking on.
Prosthetic feet are unable to do
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Componentry - Feet
all the things that human feet do.
Instead they try to allow walking
through by much simpler means.
Traditionally prosthetic feet are
divided into three categories; solid
ankle cushioned heel (SACH),
multiaxial and dynamic response.
However these categories are a bit
arbitrary and some feet do not fit
neatly under any one label.
SACH feet have been around
the longest. Progression occurs
initially by compressing a foam heel
bumper, and then by deforming
the internal keel of the foot as
your body weight moves forward.
However this progression often
does not feel smooth, and if the
foot is too stiff it may be quite
difficult. This can cause more
pressure on the stump as you
try to push over an unyielding
foot, and there is little adjustment
possible. People who have poor
balance may find the fixed ankle
to be an advantage as it gives a
solid, predictable support . For this
reason they are often the foot of
choice for people who tend to only
walk indoors or limited distances.
They also have the advantage of
being cheap, low maintenance and
durable as they have no moving
parts.
SACH feet
Multiaxial feet are often considered
a good mid-range option for most
amputees. They allow movement
between the foot and the rest of
the prosthesis, usually by some
combination of joints connected by
bumpers or bushings. Progression
is easier as the joints allows the
body weight to move forward
easily, which often makes walking
feel smoother. The joints also allow
side to side movement so when
uneven ground is encountered the
foot can adapt to it without trying
to throw the user off their preferred
upright position. In most of these
feet the bumpers can be changed
to suit people’s different needs or
walking styles.
Multiaxial foot
Like all mechanical things, once
there are moving parts there is
maintenance to be considered.
Bumpers wear out, squeaks
develop and so forth. Also the
very movement that most people
like can feel like a disconcerting
‘wobble’ to people with poor
balance.
While multiaxial feet help with
balance and the progression part
of locomotion, they do little for
propulsion. All the momentum of
the body is lost in squashing the
bumpers of the foot and so is not
available for pushing us forward.
Dynamic response feet attempt
to harness the momentum of the
body and return at the end of the
step to provide some propulsion.
At the extreme end they include
the strange J-shaped legs worn
by Paralympic sprinters. They use
carbon fibre struts as leaf springs
which store energy by bending
as your weight comes down onto
them, and then unbending to
release the energy as you step off
and so (in theory) actively pushing
you forward. Various combinations
of struts and curves are used to
provide a natural feel for everyday
use.
Dynamic feet claim to make
walking more energy efficient by
creating this active push forward.
Laboratory testing, however,
indicates that energy consumption
is only improved at brisk walking
and running, not under normal
walking. Nonetheless, most people
who have used the feet do like
their springiness. People with low
mobility will get no advantage
from dynamic feet over the simpler
options.
As these feet rely on bending a
spring it is important that they have
the right degree of springiness.
This means these feet are generally
ordered specifically for each user
with weight, activity and type
of amputation being taken into
account. Only limited adjustment
can be done after they have been
supplied.
Dynamic feet meet requirements for
balance also. They usually do this
by having a lengthways split which
allows the inside and outside of the
foot to move independently, and so
adapt to uneven ground.
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Componentry - Feet
Dynamic feet tend to be much
more expensive than multiaxial or
SACH feet, so are not available on
the government limb schemes.
these needs can be met, just not
all at the same time. Take the time
to discuss foot selection with your
prosthetist. Certainly cost is an
issue, especially if you are on the
government limb scheme. If you are
thinking about a more expensive
foot you may be able to try one out
before committing definitely.
Geoff Hill
The extreme variation of the
dynamic foot for athletes
No prosthetic feet do all of the
things that anatomical feet do. The
important thing is to find one that
does most of what you want to do,
knowing it will be limited in other
areas. Some people have strong
preferences for one particular
feature, which overrides others.
For example, people who strongly
want to be able to change from flat
to high heeled shoes may choose
adjustable heel height feet, though
many feel this feature makes the
foot less functional in other ways.
Similarly other feet can be made to
lock into a swimming position, or
made completely flat for barefoot
walking. A few amputees still find
the extremely old fashioned ‘peg
leg’ arrangement works best for
them in some situations (walking
in mud, apparently, is one). All of
First Published 2006
© Limbs 4 Life Inc.
www.limbs4life.org.au
1300 78 2231
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