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The tutorial below is designed for use with OpenSim version 4.0 and later. A version of the tutorial compatible with OpenSim version 3.3 is available here. |
I. Objectives
Purpose
The purpose of this tutorial is to demonstrate how OpenSim solves an inverse kinematics and dynamics problem using experimental data. To diagnose movement disorders and study human movement, biomechanists frequently ask human subjects to perform movements in a motion capture laboratory and use computational tools to analyze these movements. A common step in analyzing a movement is to compute the joint angles and joint moments of the subject during movement. OpenSim has tools for computing these quantities:
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II. Generic Musculoskeletal Model
In this tutorial, you will be using a generic musculoskeletal model with 23 degrees of freedom and actuated by 54 muscles entitled 3DGaitModel2354. To load the generic musculoskeletal model into OpenSim:
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III. Scaling A Musculoskeletal Model
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Subject-specific modeling involves (i) scaling a generic musculoskeletal model to modify the anthropometry, or physical dimensions, of the generic model so that it matches the anthropometry of a particular subject and (ii) registering the markers placed on the model to match the locations on the subject. Scaling and Registration are the most important steps in solving inverse kinematics and inverse dynamics problems because IK and ID solutions are sensitive to the accuracy of the scaling and registration. To scale the generic model and register the markers:
This Scale Setup file is an xml file that contains pre-configured settings to scale and register the generic gait2354 musculoskeletal model to the dimensions of a subject that we have experimental data for. A detailed explanation of the Scale Tool can be found on the Scaling page of the documentation. | |
Model ScalingIn OpenSim, the scaling step adjusts both the dimensions of the body segments, as well as the mass properties (mass and inertia tensor). Scaling can be performed using a combination of two methods: (1) Manual Scaling: Scaling that allows the user to scale a segment based on some predetermined scale factor. Manual scaling is sometimes necessary when suitable data are not available, or if the scale factors were determined using an alternative algorithm. (2) Measurement-based Scaling: Scaling that determines scale factors for a body segment by comparing distance measurements between specified landmarks on the model, known as model markers, and the corresponding experimental marker positions. Marker RegistrationIn OpenSim, the registration step adjusts the location of model markers to match the location of markers on the subject. To do this, you must first estimate a pose for the model that closely resembles the pose of the subject during the experimental static trial. | |
To complete the scale step:
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Questions
1. Based on information in the Scale Tool dialog, what is the mass of the generic musculoskeletal model? What was the mass of the subject?
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3. Click on the Scale Factors tab. Which body segments were scaled manually?
4. In the Scale Factors Tab, click on the Edit Measurements Set. Which marker pairs are used to determine the Right Femur scaling? Is this a good assumption? When is this not a good assumption?
5. Click on the Static Pose Weights tab. Which markers are used to determine the pose of the model?
IV. Inverse Kinematics
Kinematics is the study of motion without considering the forces and moments that produce that motion. The purpose of inverse kinematics (IK) is to estimate the joint angles of a particular subject from experimental data. In this section, you will estimate a subject's joint angles during walking by performing an IK analysis using the subject scaled model and experimentally collected walking data.
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To set up an inverse kinematics analysis:
subject01_Setup_IK.xml contains pre-configured settings for the inverse kinematics tool. Notice the text boxes in the dialog window are now are filled with values. A detailed explanation of the Inverse Kinematics Tool can be found on the Inverse Kinematics page of the documentation. | |
Navigate to the Weights tab.
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To perform inverse kinematics:
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Questions
4. In the inverse kinematics Tool Inverse Kinematics Tool dialog window, click the Weights tab and scroll through the list of markers in the top half of the weights tab. Which markers have weighting values less than one? Why?
Hint: Think about joints that have not been modeled.
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9. What are the maximum magnitudes of the residual forces? Using the mass of the subject from Question 1, what fraction of body weight are the maximum residual forces?
Acknowledgments
The experimental gait data were collected by Jill Higginson and Chand John in the Neuromuscular Biomechanics Lab at the University of Delaware [8]. The data include marker trajectories and ground reaction forces for an adult male walking at a self-selected speed on an instrumented split-belt treadmill. Please note that the data distributed with OpenSim is from a different subject than the one described in the paper. Data collection protocols were the same for both subjects.
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