Download Link Segment Model & Inverse Dynamics

Link Segment Model
Joint Reaction Forces
Muscle Moments
Joint Power
Newton’s Laws of Motion
I.
II.
III.
Law of Inertia: A body at rest stays at rest
and a body in motion stays in motion
unless acted upon by a force or torque.
Law of Acceleration: The acceleration a
body experiences is directly proportional to
the force or torque, inversely proportional
to the mass or moment of inertia and
occurs in the direction of the force or
torque.
Law of Reaction: For every action there is
an opposite reaction.
Newton’s First Law: Law of Inertia
Law of Inertia: A body at rest stays at rest and a body in
motion stays in motion unless acted upon by a force or
torque.
Mass is the measure of an object’s resistance to linear
change.
Moment of inertia is the measure of an object’s resistance
to angular change.
ΣF = m a
ΣT = Iα
To solve for forces and torques, we need both mass
and moment of inertia.
Moment of Inertia (I)
ICG= 3.5 kg·m2
I CG   md
ICG= 6.5
kg·m2
ICG= 15
kg·m2
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ICG= 83 kg·m2
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2
The Moment of Inertia (I) represents
an objects resistance to angular change
about some axis.
Moment of inertia is a sum of the
product of mass times the squared
distance of the mass about the axis.
When body parts are moved closer to
the axis of rotation the moment of
inertia (resistance to spin) is reduced.
Link Segment Model Assumptions
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Each segment has a fixed mass located at its center of mass
The joints are considered as hinge joints
The moment of inertia is fixed during movement
The length of each segment is constant
Input Information to
Solve for Joint Forces
and Torques

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
Center of Pressure
gives the location of
external forces acting
on the body.
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
Mass & moment of inertia
of each segment.
Location of each segments
Center of Mass (CM).
Position, velocity,
acceleration of each
segments CM (x, y, Vx, Vy,
Ax, Ay).
Angular position, velocity,
acceleration about each
segments CM (Θ, ω, α).
External forces for all
contact points.
Center of pressure or
location of external forces
acting on the body.
Inverse Dynamics: Finding Forces & Torques from Position Data.
[working backward to solve for forces & torques]
Anatomical Relationship
Forces & Torques Acting On Each Segment
Joint Moments