The topics covered in this section include:
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Overview
An OpenSim model represents the neuromuscular and musculoskeletal dynamics of a human or animal that is of interest to study within a computer simulation. The OpenSim model is made up of components corresponding to parts of the physical system that combine to generate or describe movement. These parts are: reference frames, bodies, joints, constraints, forces, contact geometry, markers and controllers.
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An OpenSim Model File (arm26.osim). The file below was opened in the XML editor Notepad++, which provides the color syntax highlighting. The sections have been collapsed to highlight the model components. Clicking on the + icon to the left of a section would expand it, displaying the relevant tags for that section.
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Use the OpenSim GUI's XML Browser, under Help to find the XML mark-up for any OpenSim model component described below. |
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<Body name="r_humerus"> <attached_geometry>...</attached_geometry> <WrapObjectSet>...</WrapObjectSet> <mass>1.8645719999999999</mass> <mass_center>0 -0.18049599999999999 0</mass_center> <inertia>0.01481 0.0045510000000000004 0.013193 0 0 0</inertia> </Body> |
Every model comes with a Ground body, which exists as a property of a Model, not in the BodySet.
Geometry
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OpenSim currently supports three types of built-in constraints: PointConstraint, WeldConstraint, and CoordinateCouplerConstraint. A point constraint fixes a point defined with respect to two bodies (i.e., no relative translations). A weld constraint fixes the relative location and orientation of two bodies (i.e., no translations or rotations). A coordinate coupler relates the generalized coordinate of a given joint (the dependent coordinate) to any other coordinates in the model (independent coordinates). The user must supply a function that returns a dependent value based on independent values. The following example implements coordinate coupler constraint for the motion of the patella as a function of the knee ankle angle and also welds the foot to ground.
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<!--Apply a force at a point in the direction specified in the body frame --> <PointActuator name="FY_residual"> <!--Name of Body to which this actuator is applied.--> <body> pelvis </body> <!--Location of application point; in body frame unless point_is_global=true--> <point> 0 0 0</point> <!--Interpret point in Ground frame if true; otherwise, body frame.--> <point_is_global> false </point_is_global> <!--Force application direction; in body frame unless force_is_global=true.--> <direction> 1 0 0</direction> <!--Interpret direction in Ground frame if true; otherwise, body frame.--> <force_is_global> true </force_is_global> <!--The maximum force produced by this actuator when fully activated.--> <optimal_force> 8.0 </optimal_force> </PointActuator> <!--Apply an equal and opposite torque on two bodies about the axis defined in the the first body --> <TorqueActuator name="MZ_residual"> <optimal_force> 1000.0 </optimal_force> <bodyA> ground </bodyA> <axis> 0.000 0.000 -1.000 </axis> <bodyB> pelvis </bodyB> </TorqueActuator > <!--Apply a generalized force along (force) or about (torque) the axis of a generalized coordinate. Positive force increases the coordinate --> <CoordinateActuator name="knee_reserve"> <optimal_force> 300.0 </optimal_force> <coordinate> knee_angle_r </coordinate> </CoordinateActuator> |
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The Muscle Actuator
There are several muscle models in OpenSim. All muscles include a set of muscle points where the muscle is connected to bones (bodies) and provide utilities for calculating muscle-actuator lengths and velocities. Internally muscle models may differ in the number and types of parameters. Muscles typically include muscle activation and contraction dynamics and their own states (for example activation and muscle fiber length). The control values are typically bounded excitations (ranging from 0 to 1) which lead to a change in activation and then force. Below is an example of a muscle model, as described by Thelen (2003), from an OpenSim model.
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