Biomechanical Evidence: Biomechanical Evidence: The complexity of human standing and its The complexity of human standing and its associated compensatory mechanisms associated compensatory mechanisms Virginie Lafage PhD Virginie Lafage PhD Frank Schwab MD Frank Schwab MD Jean-Pierre Farcy MD Jean-Pierre Farcy MD NYU Hospital for Joint Diseases, New York, NY NYU Hospital for Joint Diseases, New York, NY
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Biomechanical Evidence:Biomechanical Evidence:The complexity of human standing and its The complexity of human standing and its
NYU Hospital for Joint Diseases, New York, NYNYU Hospital for Joint Diseases, New York, NY
Human Standing Position
BalanceBalancestate of
equilibrium; equal
distribution of weight
PosturePosturerelative
disposition of the body’s parts
StabilityStabilitystate or quality of
maintaining equilibrium, self-
restoring
http://pmgagey.club.fr/http://pmgagey.club.fr/
BalanceBalance
“[In a state of equilibrium ...] the body is positioned between two aligned, equal and opposite forces”.[Brisson MJ, 1803]
COMCOM
COPCOP
Human BodyHuman Body– Gravity Force => Gravity Force => Center of mass (COM) Center of mass (COM)– Ground Reaction => Ground Reaction => Center of Pressure (COP) Center of Pressure (COP)
Balance = COM & COP aligned on Balance = COM & COP aligned on the same verticalthe same vertical
Biospace / ENSAM
StabilityStability
COMCOM
COPCOP
Body = moving partsBody = moving parts– Perfect Alignment almost never reachedPerfect Alignment almost never reached– Sophisticated Control System (NCS)Sophisticated Control System (NCS)
MaintainMaintainStabilityStability
Two stabilization Strategies [Gagey]– COM Maneuver
• Body parts motion• Localize COM above COP
– COP Maneuver• Feet pressure distribution• Localize COP below COM
Biospace / ENSAM
Dynamic MeasurementsDynamic Measurements
• Stabilometry platformStabilometry platform– Ground reaction measurements (COP)Ground reaction measurements (COP)– Over time (sample rates)Over time (sample rates)– Travel over timeTravel over time
• Pressure data obtainedPressure data obtained• Gravity Line (GL) = COP in static condition Gravity Line (GL) = COP in static condition [Zatiorsky, 1998]
• Foot pressure distribution => foot locationFoot pressure distribution => foot location
Static MeasurementsStatic Measurements
Force Plate vs. X-rays Analysis
• X-raysX-rays• Global alignment Global alignment • Spinal parametersSpinal parameters• PelvisPelvis
Kyphosis increases (Sf group vs. the 2 others) Lordosis decreases
Spinal parameters
35 ± 1036 ± 1038 ± 9Sacral Slope
21 ± 816 ± 610 ± 7Pelvic Tilt
56 ± 1152 ± 948 ± 10Incidence
Sagittal Forward (Sf)
Neutral (N)Sagittal
Backward (Sb)
With increasing SVAWith increasing SVA
No significant differences in Sacral SlopePelvic Tilt increases
Higher Pelvic Incidence (Sf group vs. the 2 others)
=> Differences in pelvic morphology and orientation
Pelvic parameters
S. BackwardS. Backward NeutralNeutral S. ForwardS. Forward
GL vs. Heels = constant
Pelvis shifts posteriorly
Forceplate parameters
Gravity LineGravity LineHeels LineHeels Line
Young Adult
Gravity Line vs. Heel = ConstantGravity Line vs. Heel = Constant
Required to keep our standing position
Age and spinal deformity do not affect this constraint
By definition, the whole body mass is equally distributed around the gravity line
Conclusions
How to maintain balance if the trunk inclines How to maintain balance if the trunk inclines forward ?forward ?
=> Body mass distribution should compensate trunk inclination to keep the GL within foot imprint
What do we know ?What do we know ? Pelvis shifts posteriorly
Pelvic tilt increases Feet are fixed in standing position
Adaptation of lower Adaptation of lower extremitiesextremities Hip flexion ?
Knee flexion ? Ankle regulation ?
Conclusions
Young Adult
Permits essential quantification of foot positionPermits essential quantification of foot position– Relationship feet / pelvisRelationship feet / pelvis– Offset of Gravity Line (GL)Offset of Gravity Line (GL)
If GL-heel offset is fixed….then a balance formula exists
Further quantification of spino-pelvic relationship necessary
Pelvic Incidence ?
Force Plate Analysis
Clinical ImpactClinical Impact
X-rays offer only part of X-rays offer only part of the storythe story