Simulation-based design to reduce metabolic cost

Overview: Lecture + Hands On Exercise 1. Generating and evaluating a muscledriven simulation of walking 2. Metabolics 101 3. Designing and evaluating devices to reduce metabolic cost 4. Next Steps: a. Heavily loaded walking simulations b. Finding help and resources

10 Gait Cycle Simulation Created By Chand John Muscle Activation 0 fully deactivated 1 fully activated

Elements of a Musculoskeletal Simulation

Research Grade Musculoskeletal Model 12 body segments 29 degrees of freedom 1,2 92 musculotendon actuators 1,2 Arms 3 driven by torque motors 1 Delp, 1990 2 Anderson and Pandy, 2001 3 Holzbaur et al., 2005

Simple Model for the Exercise Torso + Right & Left Femur, Tibia, Foot 10 degrees of freedom 18 musculotendon actuators No arms

Experimental Data Experimental Data Collection Full body motion capture measures kinematics Force plate treadmill measures ground reaction forces Electromyography (EMG) measures muscle activity Subject Specs Speed: 1.2 m/s Height: 1.8 m Weight: 75 kg Data collected by Chand John and Jill Higginson at the University of Delaware Neuromuscular Biomechanics Lab

Generating the Simulation Scale IK RRA CMC F v resi F v grf Scale the Generic Model 1 Inverse Kinematics Residual Reduction Algorithm 2 Computed Muscle Control 3 1 Hamner et al., J Biomech, 2010. 2 Delp et al., IEEE Trans Biomed Eng, 2007. 3 Thelen and Anderson, J Biomech, 2006.

Computed Muscle Control Algorithm x

Residual and Reserve Actuators Residual Actuators Reserve Actuators Lumbar MZ FX Hip Knee Ankle

Muscle Driven Simulation of Walking fully deactivated fully activated

Part I: Explore the Model 10 minutes 1. How many degrees of freedom does the model have? How many muscles? How many bodies? 10 dof, 18 muscles, 12 bodies (feet welded) 2. Do any muscles cross the lumbar joint? No we ll probably need reserve moments at the lumbar joint 3. Which model (generic or subject-specific) do you think has a lower BMI (body mass index)? The subject-specific model it is taller and weighs less

Part II: Simulate Unassisted Walking 15 minutes 1. Which coordinate had the biggest tracking errors? Max Knee Angle Tracking Error < 1 degree 2. What is the maximum value of the residual forces and moments? Why only OK forces? FY Max = 21 N No arms, Large CMC Time Window Peak Power = 4 Watts (120 Watts for muscles) 3. Why is the lumbar extension reserve so much larger than the reserves for the hip, knee, and ankle? No muscles cross the lumbar joint

Part II: Simulate Unassisted Walking 4. When do plantarflexor forces peak? What about the dorsiflexors?

How can we analyze metabolic cost? Measure oxygen consumption from human experiments Physiologically accurate Limited to available prototypes Facility and labor intensive Only gives a bulk measure of cost Evaluate metabolic cost using musculoskeletal simulations Fast and inexpensive Iterate and optimize design parameters Explore general principles Requires a sophisticated simulation environment

Calculating Energy Consumption E = h A + h M + h SL + w CE [W/kg muscle mass] h h h A M SL w CE ( u t), a( t), r, S) ( ST activation heat rate due to transport of calcium ions ( u t), a( t), r, S) ( ST due to actomyosin interaction ( u t), a( t), r, S, v ( t) ) ( ST CE : : maintenance heat rate : separatecalculationsfor fast- and slow-twitch fibers shortening/lengtheningheat rate : mechanical work rate of thecontractile element 1. Umberger, B.R., Gerritsen, K.G.M., and Martin, P.E. (2003) A Model of human muscle energy expenditure. Computer Methods in Biomechanics and Biomedical Engineering, 6(2):99 111. 2. Umberger, B.R. (2010) Stance and swing phase costs in human walking. Journal of the Royal Society Interface, 7(50):1329 1340.

Calculating Energy Consumption Key variables: Activation Muscle mass Fast/slow twitch fiber ratio Aerobic vs. Anerobic Fiber velocity

Metabolic Probes in OpenSim Model CMC States Probe Set Probe Reporter Probe Results Variable to set when adding new probes Slow/Fast Twitch Ratio Use defaults for everything else Probes work with Forward Tool and Analyze Tool (value only)

Part III. Explore Metabolics of Unassisted Walking 15 minutes 1. What is the metabolic energy consumed for one walking trial? 960 Joules; 9.8 J/kg/s or Watts/kg 2. For which parts of the gait cycle is the total rate of metabolic energy consumption highest? Early Stance/Push Off 3. Why are there differences between force production and metabolic cost? Soleus is acting concentrically (doing positive work) in late stance, which increases energy consumption

Part III. Explore Metabolics of Unassisted Walking

Example Assistive Device: Ankle Spring Spring Torque vs. Gait Cycle Plantarflexion Torque (Nm) 75 0 0 100 % Gait Cycle k = 10 Nm when dorsiflexion > 5 degrees

Assistive Device: Path Spring 625 Path Spring Tension vs. Gait Cycle Tension (N) 175 0 % Gait Cycle 100 k = 10,000

Part III. Simulate Walking with Assistive Devices 15 minutes 1. Which device reduced metabolic cost and by how much? Path Spring; 2.6% Reduction 2. How do the devices affect the ankle muscles in early to mid stance (gastroc, soleus, tib ant)? 3. How do the devices affect iliopsoas and soleus in late stance and swing? 4. Are there any significant changes in residuals, reserves, or tracking errors?

Soleus Metabolic Rate

Tibialis Anterior Metabolic Rate

Iliopsoas Metabolic Rate

Next Steps: Modeling Your Device OpenSim Model Structure Model Body Joint Constraint Force Controller

Modeling Your Device: Forces Force Prescribed PathSpring Bushing Contact Actuator External Force function of time function of state PathActuator PointActuator function of control TorqueActuator CoordinateActuator Muscle

Why the #!%@ isn t OpenSim working?

OpenSim Resources Key Online Resources: Support Portal with Search Box Users Guide Examples and Tutorials User Forum Best Practices Guides Doxygen GUI Resources: Context-Sensitive Help in the Tools Property Editor for Property Descriptions XML Browser Scripting Help: Examples Online Documentation methodsview() Developer Help: Developer s Guide Developer s Wiki Explore the Source