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 ForceF0 = Max Isometric Forcev = Contraction Velocitya = Coefficent of Shortening Heatb = 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