Introduction to Muscle Modeling

Muscles back

Muscles are the actuators of living bodies. They are activated by the central nervous system (CNS) by a complicated electro-chemical process. Determining the activation that realizes a desired movement requires an extremely intricate control algorithm. The CNS is superior to any computer man has made in this respect. AnyBody mimics the workings of the CNS by computing backwards from the movement and load specified by the user to the necessary muscle forces in a process known as inverse dynamics. To do so, the system must know the properties of the muscles involved, and this is where muscle modeling comes into the picture.

AnyBody contains four different muscle models ranging from simple to more complicated physiological behavior. The simplest model just assumes a constant strength of the muscle regardless of its working conditions. The more complicated models take such conditions as current length, contraction velocity, fiber length, pennation angle, tendon elasticity, and stiffness of passive tissues into account. Please refer to the AnyScript Reference Manual (Find it in AnyBody->Help) for concise information about the available muscle models:

  1. AnyMuscleModel - assuming constant strength of the muscle

  2. AnyMuscleModel3E - a three element model taking serial and parallel elastic elements into account along with fiber length and contraction velocity

  3. AnyMuscleModel2ELin - a bilinear model taking length and contraction velocity into account.

  4. AnyMuscleModelUsr1 - a custom user-defined strength that can be an explicit function of muscle variables, such as length and contraction velocity.

The muscle models can be linked to different types of muscles:

  1. AnyViaPointMuscle - a muscle that passes through any number of nodes on segments on its way from origin to insertion

  2. AnyShortestPathMuscle - a muscle that can wrap over geometries such as cylinders and ellipsoids and even CAD-defined surfaces. Please beware that this muscle type is very computationally demanding and requires careful adjustment.

  3. AnyGeneralMuscle - a more standard actuator-type muscle that can be attached to a kinematic measure.

Please refer to the AnyScript Reference for more information, or proceed to the basics of muscle modeling in Lesson 1.