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Components of an OpenSim Model

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What is a musculoskeletal model?

1.

Skeleton:

Bones are rigid bodies

Joints permit motion between bodies

Constraints limit motion

2.

Muscles : Specialized forces

3. Other forces:

PrescribedForce BushingForce

CoordinateActuator

Contact

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OpenSim Model File

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Tree Topology of Multibody Models

Each body is connected by ONE joint to create a chain or open tree structure.

Body

Ground

BodyTorso

FreeJoint

Body

RightHand

PinJoint

Body

LeftHand

PinJoint

BodyHandle

WeldJoint

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5.00

0.0 0.0 0.00.1...

0.0

Defining a Body and its Joint

BP

Bo

Po

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childbody B

Boparentbody P

Po

BP

Joint

Body and Joint Reference Frames

B specified by joint locationand orientation

P specified by joint locationInParentand orientationInParent

Joint coordinates specify the kinematics of B relative to P

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Defining a Body and its Joint

ground

0.0 0.0 0.0

0.0 0.7853981633974483 0.0 0.0 -0.1 0.0 0.0 0.7853981633974483 0.0

translational 0.00000000-10 10false

...

B

P

Bo

Po

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Tree Topology of Multibody Models

Each body is connected by ONE joint to create a chain or open tree structure.

Body

Ground

BodyTorso

FreeJoint

Body

RightHand

PinJoint

Body

LeftHand

PinJoint

Constraint is required to form a closed loop

BodyHandle

WeldJointWeldConstraint

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Kinematic Constriants

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Available Joints and Constriants

: No qs, adds body frame to parent

: One q, rotation about common Z

: One q, translation along common X

: Three qs, rotation about body-fixed X, Y, Z

: Six qs, rotations like Ball and 3 translations

: User-defined SpatialTransform,1 to 6 qs

: frames on two bodies are fixed

: points on two bodies are fixed

: qdep= F(qind)

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Types of Forces in OpenSim

Force

Ligament Bushing Actuator

PointActuator

TorqueActuator

CoordinateActuator

Muscle

Prescribed ...

function of time function of state

function of control

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Muscle Actuator Example

-0.00240000 -0.15330000 0.00710000humerus_r

0.00000000 0.03100000 -0.00530000r_ulna_radius_hand

...

972.000000000.08580000

0.053000000.000000.010000000.0400000010.00000000

...

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Adding a joint to the model increases/decreasesthe number of degrees of

freedom. Adding a constraint increases/decreases

the number of system equations.

How are actuators different from other

types of OpenSim forces?

Hands On Example:Add a Body and Actuator to the Model