This article is an overview of some of the OpenSim models we use in this blog. The intention is to help you better understand the capacity and limitations of OpenSim modeling.
OpenSim is an open source biomechanics software platform for simulation and data analysis for biomechanics. It contains a variety of tools for evaluation of human movement patterns and is built on a robust physics engine, Simbody. Maintained by Stanford University, it can:
- Model the human body with a high degree of accuracy
- Allow for movement of joints in anatomically correct ways
- Reproduce movement from 3D camera data of a human
- Simulate surgery outcomes
- Complete inverse kinematics and inverse dynamic calculations
- Estimations of muscle forces and joint reaction loads
- and much more…
Gait2392 is a low extremity model with 23 degrees of freedom designed by Darryl Thelen, Ajay Seth, Frank C. Anderson and Scott L. Delp. This model focuses on the lower body and contains 92 musculotendon actuators across 76 muscles. Perfect for cycling! The biomechanics of the model have been verified by a few research papers and this model is considered highly accurate with regards to the correct movement pattern of the lower extremity. With this model, we have full control of the important joints of the pedal stroke: the hips, knees and ankles. The model can adduct, abduct, flex, extend, rotate and this allows us to generate the pedal stroke.
The next model is similar to the Gait2392 model, but includes ‘simple arms’. The shoulder joint is a simple ball and socket with one dimensional elbows and some wrist rotation. This model is a better representation of a real skeleton, containing all necessary bones. It was originally developed to simulate running for the whole body, but can be used in some situations where there is some arm movement. There is no spinal or shoulder blade movement in this model, so evaluations of the back or chest are not realistic. Muscles appear in dark blue when a movement is loaded, but when the model is in free motion they appear red.
RoadOne is the first model we have developed for simulations of cycling biomechanics. This model uses the Full Body Running Model, but includes the important features of a bicycle and some constraints. Specifically, a saddle, handlebars and a functional crankset are added. The model’s feet are constrained to sit on the pedals while the models hands and pelvis are restricted to the handlebars and saddle respectively. The joint range of motion for the model was also restricted to realistic human values to ensure realistic motion. This model accurately represents the lower body for cycling biomechanics under the assumption that the sit bones are static relative to the pelvis. Back curvature is not possible with all bending occurring in the lumbar spine. With that being said, there is still opportunity for movement in the lower back relative to the pelvis. Changes in joint angles of the arms are also possible, with the model adapting to maintain all constraints.
If you have questions about the models or have suggestions on potential simulations, let us know in the comments!