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Table of Contents



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


The purpose of this tutorial is to demonstrate how musculoskeletal models can be used to study orthopaedic surgical techniques and to illustrate how muscle moment arm, optimal muscle fiber length, and tendon slack length influence the variation of muscle force with respect to changes in joint angle. In this tutorial, you will:


13. Write down the peak values of each curve (flexion moment, tendon force, moment arm), the joint angle at which the peak occurs, and describe the general shapes of the curves.

OpenSim's Property Editor allows you to examine and edit the muscle parameters used to estimate the force-length curve of this muscle.

  • To view the properties of a muscle, click on the muscle name in the Navigator. The muscle parameters are visible below the Navigator in the Properties window.


14. What is the optimal fiber length of the ECU_pre-surgery muscle?  

15. What is the optimal fiber length of ECU_post-surgery?

16. Calculate the ratio of optimal fiber length to peak moment arm for ECU_pre-surgery and ECU_post-surgery.   

17. Explain the differences in the isometric moment vs. wrist flexion angle plots for the ECU_pre-surgery and ECU_post-surgery muscles, based on the plots of force and moment arm and the ratio of optimal fiber length to peak moment arm.   

18. Specifically, what does the difference between the ratios of optimal fiber length to moment arm for the ECU before and after the tendon transfer tell you?

After answering these questions, close all of the plotter windows. 


The previous simulation illustrated how the moment arm and optimal fiber length of a muscle influence its isometric strength and the portion of the force-angle curve over which the muscle operates. Another factor in determining the isometric force vs. joint angle relationship is tendon slack length. Tendon slack length is the length at which the tendon begins to generate force.

  • Make three new plots in three separate plotter windows for the ECRB muscle: tendon force vs. flexion, muscle-tendon length vs. flexion, fiber length vs. flexion.
    Note: You can open multiple plotter windows simultaneously. Do not close any of the plots. 
  • Click on ECRB in the Navigator and examine its properties in the Properties Window.


19. What is the tendon slack length of the ECRB muscle?