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Musculoskeletal Modeling Colin Smith A method for Studying Movement • Things we can measure in movement: – Kinematics (using motion capture) – Output Forces (Using Force Plates) – Muscle Activation Timing (EMG) • What about in vivo? We need a model – Muscle Forces – Bone contact forces Musculoskeletal Models • Approach the body like a Machine • Model Bones as Rigid Bodies • Model Muscles as actuators Inverse Kinematics • Collect Motion Data • Scale Model to match subject • Use least squares method to match position of markers to model segments • We now know the kinematics of each body segment Inverse Kinetics • Solving the Equations of motion for each segment, we can find the forces and moments between each segment Muscle Forces • If we know the moments at a joint, we can find the muscle forces needed to create that moment • Distribution Problem – Multiple Muscles cross each joint – Bi-articular Muscles – Ligaments – Bone-Bone Contact • Set up optimization problem – Solve to get Muscle forces Forward Dynamic Model • Give muscle excitations – Find kinematics, resultant forces • How do we model muscle forces? – Hill Muscle Model F = Muscle Force F0 = Max Isometric Force v = Contraction Velocity a = Coefficent of Shortening Heat b = a* v0 /F0 v0 = Max velocity (F=0) What can we do with a Musculoskeletal Model? • • • • • Simulate Surgeries Diagnose causes of atypical gait Study Neuromuscular Coordination Analyze Athletic Movements Compute Internal Forces – Wear on Knee Replacements Software Demo How do we know the model is correct? • Must Validate! – Use Inverse Model to compute muscle activations • Compare against EMG data – Measure something in vivo • Bone contact force The eTibia A parameter study • What are the most important parameters to measure in a Subject Specific Model • Muscle Properties • Length • Cross sectional area • Pennation Angle • Origin and Insertion locations • Tendon Slack Length • Bone Properties • Dimensions • Joint kinematics • Segment Properties • Mass • Inertia • Joint constraints • Error in Marker Locations? The Coolest Models The End • Download Open Sim: www.simtk.org