Biomechanics Of Feet
Sven Zarling CPO, Strategic Technology Management
Biomechanics Workshop 17.06.2013
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1C40 C-Walk® 1E56 Axtion 1E57 LoRider®
1C30/31 Trias/+ 1M10 Adjust
1C60-63 Triton
1D35 Dynamic Motion
Which foot for which patient?
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Function matrix – Classification guide of Otto Bock
feet
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Function matrix – Classification guide of Ottobock
feet
• Overview of Ottobock feet
characteristics in relation to MGs
• Every foot is described by main and
side criterions, e.g.
 Heel stiffness
 Forefoot dynamics
 Length of heel lever
 Midstance flexibility
• Order number 646F307=GB-02-1005
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Function matrix – Structure
• Split into primary and secondary properties
• Basic explanation of foot functions
• Characteristics are described in relation to MG
Walking on level ground Special needs of the amputee
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Gait Cycle
Important sections of a prosthetic foot
Initial contact Midstance Preswing
Loading response Terminal stance
Heel Midfoot Forefoot
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Different tasks at different situations during gait
cycle
Initial contact Loading response
Heel
• Main objectives are…
 Acceptation of body’s weight
 Shock absorption
 To move forward (with COM)
• Important behaviors of the feet are…
 Heel stiffness (softness and
comfort at first weight bearing)
 Length of heel lever
 Fastness of the shortening heel
lever
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Heel Facts
• Heel stiffness is something which does not change and is
independent from the foot position
• Length of heel lever is something can be influenced by the
position of the foot
• Influence on knee bending/forward motion can be
influenced by changing stiffness and position of the foot
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How To Influence Heel Behavior
• Shifting of the foot:
 Shifting forward => shorten heel lever
 Shifting backward => increase heel lever
 Shifting does influence the aggressivity on stance flexion
 Plantarflexion => reducing time on heel
 Dorsiflexion => increase time on heel
 Plantarflexion and Dorsiflexion does not influence the
aggressivity on stance flexion!
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Positioning Of The Heel
• Foot is the same and will not be changed
• With bench alignment the foot has a defined
influence on the knee bending and on the forward
movement also
• Knee bending
• Forward movement (accelleration)
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Positioning Of The Heel
• When the foot is shifted more backwards,
the influence on knee bending and
forward movement will be higher
• Knee bending
• Forward movement
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Positioning Of The Heel
• To shift the foot backwards can help
to overcome the prosthesis easier if:
• The knee supports stance stability
with:
 Hydraulic
 Polyzentric
 Lock
• The patient has enough hip extensors
to compensate the flexion tendency of
the knee and if the patient wants to
walk fast the hole time.
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Positioning Of The Heel
• For AK patients you have to find a
compromise and the patient will only be
able to influence the position of the foot
by step length which is a compensation
• This compromise is normaly set by the
bench alignment, but can be changed
depending to the behavior of the knee
and the ability of the patient to control
movements by hip extension or flexion
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Positioning Of The Heel
• For BK patients you have to find a
compromise and the patient will be able
to influence the position of the foot by:
 Flexing the knee more for higher gait
speed to increase the heel lever
 Extending the knee more for lower gait
speed to decrease the heel lever
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Heel Position And Influence On The Toe
• Starting position of bench alignment
• Shifting backwards of the heel will
 Decrease toe length
 Help to flex the knee for pre swing
 Lower stability at terminal stance
 Shorten the step range in front
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Heel Position And Influence On The Toe
• Starting position of bench alignment
• Shifting the foot forward will
 Increase toe length and decrease heel
length
 Lower tendency to push stance flexion
 Increase stability at terminal stance
 Stabilize the knee before pre swing
 Make step range forward bigger
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Biomechanics Of the Feet
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Different tasks at different situations during gait
cycle
Main characteristics are…
• Foot is standing flat on the floor
during the whole section
• Moments change direction (PF ->
DF)
Important behaviors of the foot are…
• Transportation of the COP
(depending to the MG!)
• Degressive / linear / progressive
behavior
• ROM Range Of Motion (DF angle
before foot lifts off the ground)
Mid stance
Midfoot
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Different tasks at different situations
Terminal stance Preswing
Main characteristics are…
• The heel leaves the ground
• 2nd peek of the vertical force
increases to maximum
• At the end of TS a large DF
moment is created
• In PS the foot moves from DF
back to PF
Important behaviors of the foot are…
• Support the body’s weight as the
COM moves further forward
• Stiffness / length of the (fore)foot
Influence on swing phase initiation
Forefoot
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Biomechanics Of Feet
• The forefoot has the focus when the heel lifted
up
• The focus is more on two things. Is the forefoot
more:
 Deflecting and with springy energy return
 Less deflecting with energy return while
lifting the COM
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Biomechanics Of Feet
• Deflection with springy energy return
(1C40,1C30,….)
• The foot returns the energy from
terminal stance to pre swing in flexion
direction of the knee
• This can increase the flexion angle
during swing which could be helpful
for:
 Slow walking speeds to get
clearence during swing
 Short stumps because of no
need to pull stump so hard into
flexion to increase knee flexion
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Stability of COP position during double limb stance
• Check prosthetic foot characteristics
during relaxed double limb standing
 Only prosthetic side on L.A.S.A.R
posture (loaded with 35-50% of
BW)
 Check GRF referred to SRP, knee
center of rotation and foot
 Does COP position sway?
• Load and unload prosthetic foot
alternately
 Does COP position sway?
 Influence is depending on foot type
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Standing Stability
The Adjust Prinzip:
• The load line normaly falls close to the
middle of the foot.
• The Triton has this point where heel and
connection spring are in contact.
The load line stays arround this point
during vertical loading and unloading
which makes the foot so stable and
balanced especially for standing.
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Behavior of the 1E56 Axtion
• Shorter heel lever
 COP runs forward quickly
(less control necessary for stance
flexion)
 More comfortable for short AK or Hip
EX and short BK (attention - long
forefoot lever)
• Softer heel
• Stiffer Rollover
• Long forefoot lever (COP runs forward
very fast so Preswing is more difficult)
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Behavior of the 1C30 Trias
• Short heel lever
 Good shock absorption
 Less knee bending moments
 Smooth transition between heel
and midfoot
• Soft heel and easy rollover
• Soft forefoot
 Slow transition of COP because
of flexible forefoot
 Easy initiation of swing phase
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Biomechanics Of Feet
All this information is necessary for…
• Understanding the role the foot
is playing in alignment
• Choosing the right foot for the
right amputee
• Knowing the procedure how
we categorize different feet
• Handling bad influences from
the foots perspective
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Wrap-up
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Questions
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Discover what moves us!
Sven Zarling
zarling@ottobock.de
+49 5527 848 1690
www.ottobock.com