Biomechanics of standing

1
Biomechanics of standing
Chap. 4
2
Objectives

• Understand how the ground reaction force arises
• Know how the external joint moment is calculated
  from the ground reaction force
• Understand how the external moment is balanced by
  an internal muscle moment
• Know how to infer muscle action from the location
  of the ground reaction force
• Understand normal ankle and knee action during
  quiet standing

3
The ground reaction force

• The ground produces a reaction force equal and
  opposite to their body weight a consequence of
  Newtons Third Law
• The location of the centre of pressure (CoP)
  marks the line of action the GRF
• The location of CoP in normal quiet standing is
  about 5cm anterior to the ankle joint

4
Ankle movement

• Force is applied some distance away from a joint
  or fulcrum, it will tend to rotate the joint in
  the direction of the force? joint moment
• External dorsiflexor moment due to the GRFAnkle
  Moment GRF X Moment Arm of GRF / 2
  mg X d / 2 80 X
  10 X 0.05 / 2 20Nm(body mass 80kg,
  acceleration of gravity 10m/s, moment arm of
  GRF 5cm)

5
Tendon and muscle forces

• No movement in quiet standing? require equal
  apposite opposing moment
• Internal plantarflexor moment produced by tension
  (force) in the Achilles tendonAnkle Moment
  Tendon Force X Tendon Moment ArmTendon Force
  Ankle Moment / Tendon Moment Arm
  20 / 0.04 500N(moment arm of the Achilles
  tendon 4cm)

6
Control of standing

• The GRF isnt always 5cm anterior to the ankle
  joint.- GRF move forwards increased Achilles
  tensionAnkle Moment GRF X Moment Arm of GRF /
  2 mgd / 2
  80 X 10 X 0.07 / 2 28NmTendon Force
  Ankle Moment / Tendon Moment Arm
  28 / 0.04 700N- GRF move posteriorly
  tibialis anterior tension

7
Proximal joint moments

• Very small joint moment or very little muscle
  contraction required in quiet standing.
• Squatting posture

Knee moment Force X Moment arm of GRF at knee
(mgd)/2
For symmetrical standing, m80kg(800N), GRF
passing 10cm posterior to the knee
Joint moment 800 X 0.1 / 2 40 Nm
(external flexor moment)
Resisted by internal extensor moment by the
contraction of the quadriceps

• Rule of thumb the active muscle is always the
  one on the opposite side of the joint to the GRF.

8
Proximal joint moments

• Rule of thumb the active muscle is always the
  one the opposite side of the joint to the GRF
• What happens if the GRF passes in front of the
  knee joint?The knee cannot extend beyond
  0because the strong posterior capsule ligaments
  become taut at this angle and so prevent any
  hyperextension ? no muscle action is necessary

9
Static and Dynamic Postures

• Static standing, lying, or sitting
• dynamic walking, running, jumping, throwing, and
  lifting
• Maintenance of erect bipedal stance is unique to
  humans.
• Crutches, canes or other assistive devices
• Erect bipedal stance (humans)
• Increases the work of the heart
• Increases stress on the vertebral column, pelvis,
  and lower extremities
• Reduces stability
• In humans COG at S2
• The instability caused by small base of support
  and a high COG
• Very little energy expenditure in muscle
  contraction
• Bone, joints and ligaments provide major torques
  needed to counteract gravity.

10
Postural Control

• Maintenance and/or control of posture depends the
  integrity of the CNS, the visual system, the
  vestibular system, the musculoskeletal system and
  inputs from receptors.
• Altered inputs results in altered posture.
• Ex) Absence of gravitation force, decreased
  sensation in lower extremities
• Postural response is task-specific and vary in
• (1) size of the supporting surface
• (2) direction of motion of the supporting surface
  (AP or ML)
• (3) Location of the perturbing force
• (4) the magnitude of the applied force
• (5) initial posture at the time of perturbation
• (6) Velocity of perturbation

11
Postural Control

• Forward motion of the platform
• Posterior movement of the body
• Displacement of the bodys COG posterior to the
  base of support
• Employment of ankle strategy
• Activation of dorsiflexors, hip flexors,
  abdominals, and neck flexors
• TA contributes restoration of stability by
  pulling the tibia anteriorly.

COM backward
W
W
W
W
ankle
ankle
Platform moves forward
Balance recovery
12
Postural Control

• Backward motion of the platform
• Anterior movement of the body
• Displacement of the bodys COG anterior to the
  base of support
• Employment of ankle strategy
• Activation of plantarflexors, hip extensors, back
  and neck extensors
• TA contributes restoration of stability by
  pulling the tibia anteriorly.

COM forward
W
W
W
W
ankle
ankle
ankle
Platform moves backward
Balance recovery
13
Postural Sway

• The body sways back and forth like an inverted
  pendulum, pivoting about the ankle, at quiet
  stance

14
The relationship of COG and COP during quiet
stance

• If the COP ahead the COG, a CCW moment (Ia) is
  present at the ankle joint, resulting in backward
  trunk rotation and the balance is regained.
• If the COP behind the COG, a CW moment is present
  at the ankle joint, resulting in forward trunk
  rotation of the trunk and the balance may be lost
  and possibly fall forward.

15
COM parameters

• absolute position of the COM in the AP and ML
  positions
• excursion of the COM
• linear acceleration of the COM equals to
  (COP-COM).
• where k constant           a linear
  acceleration of the COM

so
16
Center of Pressure (COP)

• two-force-platform method measurement the COP
  with one foot standing on one force plate and the
  other foot on the second force plate

17
Postural balance recovery against horizontal
perturbation

• Importance in postural balance against falling
  injuries especially in
• elderly or disabled people
• Falling (slipping, tripping, stumbling)
• Major cause of work-related injuries
• Leading cause of injury, death, and disabilities
  among people older than 65 year
• Head trauma, Fractures of the hip, spine,
  pelvis, hand and ankle
• Nearly 9,000 people in this population died from
  falls in 1997
• Postural Balance
• Maintained by making postural adjustments to
  recover the center of mass(CoM)
• over the base of support (BoS)
• Activates appropriate muscles to produce force
  and relocate the body mass

18
Perturbation Conditions

• Forward perturbation
• Acceleration phase(AP), constant-speed
  phase(CP), deceleration phase(DP)
• 2 cases with different speeds (0.1m/s, 0.2m/s)

19
Perturbation System
rodeless
guide
rodeless
guide
AC motor
force plate
AC motor
force plate
motor controller
serial communication
20
Perturbation Experiments
21
Overall Procedure
Data Acquisition
Perturbation
Synchronization
Bioware Program
Motor controller
Synchronized Box
Serial interface
Accelerometers (Acceleration)
EMG GM,BF,RF,GCM,TA (Onset time)
Forceplate (GRF, CoP)
Motion (Jt. angles)
Platform perturbation
Analysis

• Accelerations of force plate, heel, and sacrum
• Joint angles
• Muscle onset time
• Patterns of GRFs and CoP movements

22
Results
Joint Angles
23
CoP Patterns

• Case 1 Backward CoP (ankle plantarflexion)
  during the early CP - Forward CoP joint (flexions
  of the ankle,
• the knee and the hip) - Slight
  backward movements (ankle plantarflexion) during
  the DP
• Case 2 Similar to those in Case 1
• Forward and backward CoP movements
  continued to maintain balance even after the
  perturbation
• After the perturbation, CoP
  movements in Case 2 were significantly larger
  than those in Case 1

24
Anteroposterior GRF Patterns

• Case 1 Slight backward GRFs (ankle
  plantarflexion) during the first half of the AP -
  Forward GRFs
• (joint flexions) until the early
  CP - Backward GRFs occurred until the end of the
  CP
• - Anteroposterior GRFs disappeared
  gradually after perturbation
• Case 2 Slight backward GRFs (ankle
  plantarflexion) during the first half of the AP -
  Forward GRFs
• (joint flexions) until the middle
  of the DP - Backward GRFs (ankle plantarflexion
  and knee extension)
• after perturbation

25
Correlations of EMG Onsets with Joint Motions
26
EMG Onset Joint Motions
27
Conclusions
(1) The sequence of lower extremity motions to
regain balance against the expected forward
perturbation is ankle-knee-hip response. (2)
Joint flexions in the lower extremity are
important in the balance recovery against the
forward perturbation. (3) The accelerometer
appears to be a promising tool for understanding
dynamic postural control against the falls of the
elderly and the disabled. (4) A more thorough
understanding of balance recovery mechanism of
abnormal subjects or persons including upper
extremities may be helpful in reducing falls and
the resulting injuries.