muscle force is not a function of muscle activation !!

[Sina Porsa]

Hi, does anyone know what am I missing in the following script.
I am using the default tugOfWar.osim model. I assign two different muscle activations to the muscle and I expect to have two different muscle forces.
here is the script:

Code: Select all

Model OsimModel ("tugOfWar.osim");
SimTK::State& system = OsimModel.initSystem();
const Set<Actuator>& actuatorSet = OsimModel.getActuators();
Thelen2003Muscle* myMusc1 = dynamic_cast<Thelen2003Muscle*>( &actuatorSet.get(0) );
myMusc1  ->setActivation	(system, 1.0);
myMusc1  ->setFiberLength (system, 0.108865924);
Thelen2003Muscle* myMusc2 = dynamic_cast<Thelen2003Muscle*>( &actuatorSet.get(1) );
myMusc2  ->setActivation	(system, 0.0);
myMusc2  ->setFiberLength (system, 0.108865924);
OsimModel.getMultibodySystem().realize(states, Stage::Acceleration);

//PRINTING SOME MUSCLE PARAMETERS
printf("Fibre  Length %f -- %f\n", myMusc1->getFiberLength(states), myMusc2->getFiberLength(states));
printf("Fibre  NormVel %f--  %f\n", myMusc1->getNormalizedFiberVelocity(states), myMusc2->getNormalizedFiberVelocity(states));
printf("Muscle Activation %f -- %f\n", myMusc1->getActivation(states), myMusc2->getActivation(states));
printf("Fiber  Force %f -- %f\n", myMusc1->getFiberForce(states), myMusc2->getFiberForce(states));
printf("Active FiberForce %f -- %f\n", myMusc1->getActiveFiberForce(states),	 myMusc2->getActiveFiberForce(states));
printf("Pass   FiberForce %f -- %f\n", myMusc1->getPassiveFiberForce(states), myMusc2->getPassiveFiberForce(states));
printf("Pass   Force Multiplier %f -- %f\n",	myMusc1->getPassiveForceMultiplier(states), myMusc2->getPassiveForceMultiplier(states));
printf("Actv   Force Multiplier %f -- %f\n",	myMusc1->getActiveForceLengthMultiplier(states),myMusc2->getActiveForceLengthMultiplier(states));
printf("Force  Vel Multiplier %f -- %f\n", myMusc1->getForceVelocityMultiplier (states), myMusc2->getForceVelocityMultiplier (states));

and here is what is printed on the screen:

Fibre Length 0.108866 -- 0.108866
Fibre NormVel -0.958064 -- 0.000000
Muscle Activation 1.000000 -- 0.010000
Fiber Force 24.880154 -- 24.880154
Active FiberForce 9.844103 -- 9.844103
Pass FiberForce 15.036052 -- 15.036052
Pass Force Multiplier 0.015036 -- 0.015036
Actv Force Multiplier 0.984402 -- 0.984402
Force Vel Multiplier 0.010000 -- 1.000008

As you can see the two muscles have the same fiber lengths and two different activations. Based on that, I expect to have equal passive forces but two different active forces. But as you can see, two muscles have the same active fiber force and active force multipliers. The only difference is the contraction velocities. I was wondering if I am missing sth in my script? Or does it mean that muscle force is not a function of muscle activation? !!
Thanks
Sina

[Ajay Seth]

The Thelen2003Muscle is an equilibrium muscle model. The muscle fiber force is always in equilibrium with the tendon force. The tendon force is dependent only on its length. Thus, the output force of this muscle is determined by the muscle's fiber length state (and total length)- not activation. Activation affects the rate at which a fiber will lengthen and shorten and this is calculated from the equilibrium condition. The behavior seems consistent with the muscle model type. If you want the muscle to apply active fiber force directly then please try the RigidTendonMuscle, which has no fiber length state. Alternatively, the Millard2012EquilibriumMuscle has the option to ignore_tendon_compliance.