OpenSim Workshop March 2013 Moment Arms for Curved Muscle Paths and Multi-Joint Systems Anita Vasavada, Washington State University Liying Zheng, University of Pittsburgh Workshop Goal: -How do the methods used to describe muscle curvature and complex kinematic systems affect the estimate of moment arm in OpenSim? -Better understanding of the “effective torque” method. Approaches: -3 simple models -Compare moment arms -Compare results of inverse and forward dynamics

OpenSim Workshop March 2013 (A) 2 gencoords, independent (B) 1 gencoord 2 joints coupled (C) 1 joint with equivalent center of rotation Equivalent kinematics: In (A) and (B), the upper joint rotates by an angle which is 3x the lower joint. (C) represents the equivalent center of rotation for (A) and (B), which is located at 75% of the lower segment. *

OpenSim Workshop March 2013 (A) 2 gencoords, independent (B) 1 gencoord 2 joints coupled (C) 1 joint with equivalent center of rotation Equivalent kinematics: Video *

OpenSim Workshop March 2013 (A) 2DOF independent (B) 1DOF 2 joints coupled (C) 1DOF Equivalent center of rotation Moment Arm: (A) has a moment arm at each joint, (B) and (C) have only one. (B) and (C) have the same moment arm (at 0°, upright posture). *

OpenSim Workshop March 2013 (A) 2DOF independent (B) 1DOF 2 joints coupled (C) 1DOF Equivalent center of rotation Inverse Dynamics: For a static posture, (A) calculates the appropriate moments at each joint, but (B) does not … in OpenSim (OK in SimBody) *

OpenSim Workshop March 2013 (A) 2DOF independent (B) 1DOF 2 joints coupled (C) 1DOF Equivalent center of rotation Forward Dynamics: Apply a torque to the endpoint, resist it with a muscle (path actuator). (A) and (B) generate equivalent moment arms (and moments) *