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Using the muscle model
Structure of the SIMULINK block
Once you have created your Muscle_Database, you can create blocks of physiologically
functioning muscle from the BuildMuscles function. The SIMULINK blocks are based on a
modified Hill-type muscle model, which is described in detail in Brown et al. 1996. Although it
is not necessary to understand the internal details of the muscle block, they are summarized here
briefly. If you are not interested in the internal details, proceed to the next section.
In summary, each SIMULINK muscle block contains a contractile element (which is composed
of an active element and a parallel visco-elastic element) and a series elastic element. For
modeling purposes, a muscle mass is interposed between the contractile and series elastic
elements to prevent unrealistically large, instantaneous accelerations and instabilities that occur
if the velocity dependent contractile element is connected directly to the series elastic element.
Descriptions of SIMULINK subsystems used to model each of the elements are given below:
• Recruitment block: This element represents the motor pool (for natural recruitment) or
stimulators (for FES). A single activation input (plus a frequency input for FES) is
transformed into frequency outputs for each motor unit of that muscle. The combined vector
is sent to the CE + PE element to calculate active force.
• Contractile element + Passive element subsystem: This element represents the fascicles in
the muscle belly. Its output, force, is determined by three inputs; activation, fascicle length
and fascicle velocity. Note that the fascicle length input to this subsystem is different from
the entire musculotendon path length value which is input to the whole- muscle block. It is
assumed that pennation angle is negligible for this model. Fascicle velocity is computed
within the SIMULINK block by integrating the calculated acceleration. Also note that this
element calculate the active and passive forces for the fascicles.
The block representing the contractile element subsystem consists of one sub-block for each
fiber-type within that muscle. Each of those fiber-type specific blocks then contains blocks
that calculate those muscle properties which are the same for all motor units (of that fibertype) as well as one sub-block for each motor unit as defined in the BuildMuscles function.
The force blocks for each motor unit sum together to produce a force output for each fiber
type block, which then sum to produce a single force output for the entire contractile
element.
• Series elastic element subsystem: This element represents the effective length of the
internal and external tendons; aponeurosis elements should be included. The force produced
by this element is dependent only on length, and has been shown to have no significant
velocity dependence at physiologically relevant frequencies. For this reason, changes in the
musculotendon path length are made to act directly on the series elastic element.
A single function block provides all the calculations for this element. Note that the length of
this element is calculated by subtracting the length of the contractile element from the
musculotendon path length.
• Muscle mass subsystem: The muscle mass was included to prevent the system from
becoming unstable as the series elastic element and contractile element act on each other.
02/23/ 01
User’s Manual - Virtual Muscle 3.1.5 Documentation.doc
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