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OpenSim::FiberForceLengthCurve Class Reference

This class serves as a serializable FiberForceLengthCurve, commonly used to model the parallel elastic element in muscle models. More...

#include <FiberForceLengthCurve.h>

Inheritance diagram for OpenSim::FiberForceLengthCurve:
OpenSim::Function OpenSim::Object

Public Member Functions

 FiberForceLengthCurve ()
 The default constructor creates a fiber-force-length curve using the default property values and assigns a default name. More...
 
 FiberForceLengthCurve (double strainAtZeroForce, double strainAtOneNormForce, double stiffnessAtLowForce, double stiffnessAtOneNormForce, double curviness)
 Constructs a fiber-force-length curve using the provided parameters and assigns a default name. More...
 
double getStrainAtZeroForce () const
 
double getStrainAtOneNormForce () const
 
double getStiffnessAtLowForceInUse () const
 
double getStiffnessAtOneNormForceInUse () const
 
double getCurvinessInUse () const
 
bool isFittedCurveBeingUsed () const
 
void setCurveStrains (double aStrainAtZeroForce, double aStrainAtOneNormForce)
 
void setOptionalProperties (double stiffnessAtLowForce, double stiffnessAtOneNormForce, double curviness)
 
double calcValue (const SimTK::Vector &x) const override
 Implement the generic OpenSim::Function interface. More...
 
double calcValue (double normFiberLength) const
 Evaluates the fiber-force-length curve at a normalized fiber length of 'normFiberLength'. More...
 
double calcDerivative (double normFiberLength, int order) const
 Calculates the derivative of the fiber-force-length multiplier with respect to the normalized fiber length. More...
 
double calcIntegral (double normFiberLength) const
 Calculates the normalized area under the curve. More...
 
SimTK::Vec2 getCurveDomain () const
 Returns a SimTK::Vec2 containing the lower (0th element) and upper (1st element) bounds on the domain of the curve. More...
 
void printMuscleCurveToCSVFile (const std::string &path)
 Generates a .csv file with a name that matches the curve name (e.g., "bicepsfemoris_FiberForceLengthCurve.csv"). More...
 
void ensureCurveUpToDate ()
 
Property declarations

These are the serializable properties associated with this class.

 OpenSim_DECLARE_PROPERTY (strain_at_zero_force, double,"Fiber strain at zero force")
 
 OpenSim_DECLARE_PROPERTY (strain_at_one_norm_force, double,"Fiber strain at a tension of 1 normalized force")
 
 OpenSim_DECLARE_OPTIONAL_PROPERTY (stiffness_at_low_force, double,"Fiber stiffness at the end of the low-force region")
 
 OpenSim_DECLARE_OPTIONAL_PROPERTY (stiffness_at_one_norm_force, double,"Fiber stiffness at a tension of 1 normalized force")
 
 OpenSim_DECLARE_OPTIONAL_PROPERTY (curviness, double,"Fiber curve bend, from linear (0) to maximum bend (1)")
 
- Public Member Functions inherited from OpenSim::Function
 Function ()
 
 Function (const Function &aFunction)
 
virtual ~Function ()
 
virtual void init (Function *aFunction)
 
Functionoperator= (const Function &aFunction)
 
virtual double calcDerivative (const std::vector< int > &derivComponents, const SimTK::Vector &x) const
 Calculate a partial derivative of this function at a particular point. More...
 
virtual int getArgumentSize () const
 Get the number of components expected in the input vector. More...
 
virtual int getMaxDerivativeOrder () const
 Get the maximum derivative order this Function object can calculate. More...
 
- Public Member Functions inherited from OpenSim::Object
virtual ~Object ()
 Virtual destructor for cleanup. More...
 
virtual Objectclone () const =0
 Create a new heap-allocated copy of the concrete object to which this Object refers. More...
 
virtual const std::string & getConcreteClassName () const =0
 Returns the class name of the concrete Object-derived class of the actual object referenced by this Object, as a string. More...
 
virtual const VisibleObjectgetDisplayer () const
 Methods to support making the object displayable in the GUI or Visualizer Implemented only in few objects. More...
 
virtual VisibleObjectupdDisplayer ()
 get Non const pointer to VisibleObject More...
 
bool isEqualTo (const Object &aObject) const
 Equality operator wrapper for use from languages not supporting operator overloading. More...
 
Objectoperator= (const Object &aObject)
 Copy assignment copies he base class fields, including the properties. More...
 
virtual bool operator== (const Object &aObject) const
 Determine if two objects are equal. More...
 
virtual bool operator< (const Object &aObject) const
 Provide an ordering for objects so they can be put in sorted containers. More...
 
void setName (const std::string &name)
 Set the name of the Object. More...
 
const std::string & getName () const
 Get the name of this Object. More...
 
void setDescription (const std::string &description)
 Set description, a one-liner summary. More...
 
const std::string & getDescription () const
 Get description, a one-liner summary. More...
 
const std::string & getAuthors () const
 Get Authors of this Object. More...
 
void setAuthors (const std::string &authors)
 Set Authors of this object, call this method in your constructor if needed. More...
 
const std::string & getReferences () const
 Get references or publications to cite if using this object. More...
 
void setReferences (const std::string &references)
 Set references or publications to cite if using this object. More...
 
int getNumProperties () const
 Determine how many properties are stored with this Object. More...
 
const AbstractPropertygetPropertyByIndex (int propertyIndex) const
 Get a const reference to a property by its index number, returned as an AbstractProperty. More...
 
AbstractPropertyupdPropertyByIndex (int propertyIndex)
 Get a writable reference to a property by its index number, returned as an AbstractProperty. More...
 
bool hasProperty (const std::string &name) const
 Return true if this Object has a property of any type with the given name, which must not be empty. More...
 
const AbstractPropertygetPropertyByName (const std::string &name) const
 Get a const reference to a property by its name, returned as an AbstractProperty. More...
 
AbstractPropertyupdPropertyByName (const std::string &name)
 Get a writable reference to a property by its name, returned as an AbstractProperty. More...
 
template<class T >
bool hasProperty () const
 Return true if this Object contains an unnamed, one-object property that contains objects of the given template type T. More...
 
template<class T >
const Property< T > & getProperty (const PropertyIndex &index) const
 Get property of known type Property<T> as a const reference; the property must be present and have the right type. More...
 
template<class T >
Property< T > & updProperty (const PropertyIndex &index)
 Get property of known type Property<T> as a writable reference; the property must be present and have the right type. More...
 
void setObjectIsUpToDateWithProperties ()
 When an object is initialized using the current values of its properties, it can set a flag indicating that it is up to date. More...
 
bool isObjectUpToDateWithProperties () const
 Returns true if no property's value has changed since the last time setObjectIsUpToDateWithProperties() was called. More...
 
void readObjectFromXMLNodeOrFile (SimTK::Xml::Element &objectElement, int versionNumber)
 We're given an XML element from which we are to populate this Object. More...
 
virtual void updateFromXMLNode (SimTK::Xml::Element &objectElement, int versionNumber)
 Use this method to deserialize an object from a SimTK::Xml::Element. More...
 
virtual void updateXMLNode (SimTK::Xml::Element &parent)
 Serialize this object into the XML node that represents it. More...
 
bool getInlined () const
 Inlined means an in-memory Object that is not associated with an XMLDocument. More...
 
void setInlined (bool aInlined, const std::string &aFileName="")
 Mark this as inlined or not and optionally provide a file name to associate with the new XMLDocument for the non-inline case. More...
 
std::string getDocumentFileName () const
 If there is a document associated with this object then return the file name maintained by the document. More...
 
void setAllPropertiesUseDefault (bool aUseDefault)
 
bool print (const std::string &fileName)
 Write this Object into an XML file of the given name; conventionally the suffix to use is ".osim". More...
 
std::string dump (bool dumpName=false)
 dump the XML representation of this Object into an std::string and return it. More...
 
void clearObjectIsUpToDateWithProperties ()
 For testing or debugging purposes, manually clear the "object is up to date with respect to properties" flag. More...
 
virtual bool isA (const char *type) const
 The default implementation returns true only if the supplied string is "Object"; each Object-derived class overrides this to match its own class name. More...
 
const std::string & toString () const
 Wrapper to be used on Java side to display objects in tree; this returns just the object's name. More...
 
PropertySetgetPropertySet ()
 OBSOLETE: Get a reference to the PropertySet maintained by the Object. More...
 
const PropertySetgetPropertySet () const
 

Additional Inherited Members

- Static Public Member Functions inherited from OpenSim::Function
static FunctionmakeFunctionOfType (Function *aFunction, const std::string &aNewTypeName)
 
- Static Public Attributes inherited from OpenSim::Object
static const std::string DEFAULT_NAME
 Name used for default objects when they are serialized. More...
 
- Protected Member Functions inherited from OpenSim::Function
void resetFunction ()
 This should be called whenever this object has been modified. More...
 
- Protected Attributes inherited from OpenSim::Function
SimTK::Function * _function
 

Detailed Description

This class serves as a serializable FiberForceLengthCurve, commonly used to model the parallel elastic element in muscle models.

The fiber-force-length curve is dimensionless: force is normalized to maximum isometric force and length is normalized to resting fiber length. The user can adjust the maximum strain at no load and the strain developed under 1 normalized unit of force using the fitted curve. Additionally, if desired, it is possible to directly set the low-force stiffness of the fiber, the stiffness of the fiber at 1 normalized unit of force, and the shape of the curve (its 'curviness'):

Properties

  • strainAtZeroForce
    The fiber strain at which the fiber starts to develop force. strainAtZeroForce = 0.0 means that the fiber will begin developing tension when it is at its resting length.
  • strainAtOneNormForce
    The fiber strain at which the fiber develops 1 unit of normalized force. strainAtOneNormForce = 0.6 means that the fiber will develop a tension of 1 normalized force when it is strained by 60% of its resting length or, equivalently, when it is stretched to 1.6 times its resting length.
  • stiffnessAtLowForce
    The normalized stiffness (slope of the curve) when the fiber is just beginning to develop tensile force.
  • stiffnessAtOneNormForce
    The normalized stiffness (slope of the curve) when the fiber develops a tension of 1 normalized unit of force.
  • curviness
    A dimensionless parameter between 0 and 1 that describes the shape of the curve: a value of 0 indicates a curve that is very close to a straight line segment and a value of 1 indicates a curve that smoothly fills the corner formed by the linear extrapolation of 'stiffnessAtOneNormForce' and the x-axis, as shown in the figure.

Note that we use the Cauchy or engineering definition of strain throughout: strain = (l-l0)/l0, where l is the current fiber length and l0 is its resting length.

Required Properties

  • strainAtZeroForce
  • strainAtOneNormForce

Optional Properties

  • stiffnessAtLowForce
  • stiffnessAtOneNormForce
  • curviness
fig_FiberForceLengthCurve.png

Conditions

strainAtZeroForce < strainAtOneNormForce
stiffnessAtOneNormForce > 1/(strainAtOneNormForce-strainAtZeroForce)
0 < stiffnessAtLowForce < stiffnessAtOneNormForce
0 <= curviness <= 1

The required parameters can be set using either the constructor or the setCurveStrains function; the optional parameters can be set using the setOptionalProperties function. Note that filling in one optional parameter but not the others will throw an exception when the curve is built. The optional parameters can be used to vary the shape of the curve from a close approximation of a line to a sharply-bent curve.

The advantage of this curve over the typical exponential curve used in the literature is that it is continuous to the second derivative; the usual linearly-extrapolated exponential curve is only C0-continuous. The improved smoothness of this curve makes the equations somewhat easier to simulate and, more importantly, permits the use of derivative-based numerical methods on the curve. In addition, the extra parameters in this curve formulation can be adjusted to match a wide variety of shapes, should it be desired to fit the curve to a different set of experimental data.

Default Parameter Values

If the optional parameters are not specified, the curve is fit to the experimentally measured fiber-force-length curves of Winters et al. (2010, Fig. 3a).

strainAtZeroForce .......... 0.0
strainAtOneNormForce ....... 0.7
stiffnessAtLowForce ........ 0.2
stiffnessAtOneNormForce .... 2.0 / (strainAtOneNormForce-strainAtZeroForce) = 2.86
curviness .................. 0.75

Example

// Make a fitted fiber-force-length curve.
fpeCurve1.setCurveStrains(0.0, 0.7);
double fpeVal1 = fpeCurve1.calcValue(0.1);
// Make a custom fiber-force-length curve by supplying all parameters.
FiberForceLengthCurve fpeCurve2(0.0, 0.7, 0.2, 2.86, 0.75);
double fpeVal2 = fpeCurve2.calcValue(0.02);
double dfpeVal2 = fpeCurve2.calcDerivative(0.02, 1);

Note that this object should be updated through the set methods provided. These set methods will take care of rebuilding the curve correctly. If you modify the properties directly, the curve will not be rebuilt, and upon calling a function like calcValue, calcDerivative, or printCurveToCSVFile, an exception will be thrown because the curve is out-of-date with its properties.

References

  • Thelen, D.G. (2003) Adjustment of muscle mechanics model paramters to simulate dynamic contractions in older adults. ASME Journal of Biomechanical Engineering 125:70–77.
  • Winters, T.M., Takahashi, M., Lieber, R.L., and Ward, S. (2010) Whole muscle length-tension relationships are accurately modeled as scaled sarcomeres in rabbit hindlimb muscles. Journal of Biomechanics 44:109–115.
Author
Matt Millard

Constructor & Destructor Documentation

OpenSim::FiberForceLengthCurve::FiberForceLengthCurve ( )

The default constructor creates a fiber-force-length curve using the default property values and assigns a default name.

OpenSim::FiberForceLengthCurve::FiberForceLengthCurve ( double  strainAtZeroForce,
double  strainAtOneNormForce,
double  stiffnessAtLowForce,
double  stiffnessAtOneNormForce,
double  curviness 
)

Constructs a fiber-force-length curve using the provided parameters and assigns a default name.

See class documentation for the meaning of these parameters, each of which corresponds to a property.

Member Function Documentation

double OpenSim::FiberForceLengthCurve::calcDerivative ( double  normFiberLength,
int  order 
) const

Calculates the derivative of the fiber-force-length multiplier with respect to the normalized fiber length.

Parameters
normFiberLengthThe normalized length of the muscle fiber.
orderThe order of the derivative. Only values of 0, 1, and 2 are acceptable.
Returns
The derivative of the fiber-force-length curve with respect to the normalized fiber length.
double OpenSim::FiberForceLengthCurve::calcIntegral ( double  normFiberLength) const

Calculates the normalized area under the curve.

Since it is expensive to construct, the curve is built only when necessary.

Parameters
normFiberLengthThe normalized length of the muscle fiber.
Returns
The normalized area under the curve, which corresponds to the normalized potential energy stored in the fiber. To calculate the potental energy stored in the fiber in units of Joules, multiply the returned quantity by normForce*normLength (where normForce is the number of Newtons represented by a normalized force of 1.0 and normLength is the number of meters represented by a normalized length of 1.0).
double OpenSim::FiberForceLengthCurve::calcValue ( const SimTK::Vector &  x) const
inlineoverridevirtual

Implement the generic OpenSim::Function interface.

Reimplemented from OpenSim::Function.

double OpenSim::FiberForceLengthCurve::calcValue ( double  normFiberLength) const

Evaluates the fiber-force-length curve at a normalized fiber length of 'normFiberLength'.

void OpenSim::FiberForceLengthCurve::ensureCurveUpToDate ( )
SimTK::Vec2 OpenSim::FiberForceLengthCurve::getCurveDomain ( ) const

Returns a SimTK::Vec2 containing the lower (0th element) and upper (1st element) bounds on the domain of the curve.

Outside this domain, the curve is approximated using linear extrapolation.

Returns
The minimum and maximum value of the domain, x, of the curve y(x). Within this range, y(x) is a curve; outside this range, the function y(x) is a C2-continuous linear extrapolation.
double OpenSim::FiberForceLengthCurve::getCurvinessInUse ( ) const
Returns
A dimensionless parameter between 0 and 1 that describes the shape of the curve: a value of 0 indicates a curve that is very close to a straight line segment and a value of 1 indicates a curve that smoothly fills the corner formed by the linear extrapolation of 'stiffnessAtOneNormForce' and the x-axis, as shown in the figure in the class description. This property is set to 0.75 by default.
double OpenSim::FiberForceLengthCurve::getStiffnessAtLowForceInUse ( ) const
Returns
The normalized stiffness (slope of the curve) when the fiber is just beginning to develop tensile force. This property is set to 0.125 by default.
double OpenSim::FiberForceLengthCurve::getStiffnessAtOneNormForceInUse ( ) const
Returns
The normalized stiffness (slope of the curve) when the fiber develops a tension of 1 normalized unit of force. This property is set to 5.0 by default.
double OpenSim::FiberForceLengthCurve::getStrainAtOneNormForce ( ) const
Returns
The fiber strain at which the fiber develops 1 unit of normalized force. strainAtOneNormForce = 0.6 means that the fiber will develop a tension of 1 normalized force when it is strained by 60% of its resting length or, equivalently, when it is stretched to 1.6 times its resting length. This property is set to 0.6 by default.
double OpenSim::FiberForceLengthCurve::getStrainAtZeroForce ( ) const
Returns
The fiber strain at which the fiber starts to develop force. strainAtZeroForce = 0.0 means that the fiber will begin developing tension when it is at its resting length. This property is set to 0 by default.
bool OpenSim::FiberForceLengthCurve::isFittedCurveBeingUsed ( ) const
Returns
True if the optional properties are empty and the fitted curve is being used, false if the optional properties are filled and are being used to construct the curve.
OpenSim::FiberForceLengthCurve::OpenSim_DECLARE_OPTIONAL_PROPERTY ( stiffness_at_low_force  ,
double  ,
"Fiber stiffness at the end of the low-force region"   
)
OpenSim::FiberForceLengthCurve::OpenSim_DECLARE_OPTIONAL_PROPERTY ( stiffness_at_one_norm_force  ,
double  ,
"Fiber stiffness at a tension of 1 normalized force"   
)
OpenSim::FiberForceLengthCurve::OpenSim_DECLARE_OPTIONAL_PROPERTY ( curviness  ,
double  ,
"Fiber curve  bend,
from linear(0) to maximum bend(1)"   
)
OpenSim::FiberForceLengthCurve::OpenSim_DECLARE_PROPERTY ( strain_at_zero_force  ,
double  ,
"Fiber strain at zero force"   
)
OpenSim::FiberForceLengthCurve::OpenSim_DECLARE_PROPERTY ( strain_at_one_norm_force  ,
double  ,
"Fiber strain at a tension of 1 normalized force"   
)
void OpenSim::FiberForceLengthCurve::printMuscleCurveToCSVFile ( const std::string &  path)

Generates a .csv file with a name that matches the curve name (e.g., "bicepsfemoris_FiberForceLengthCurve.csv").

This function is not const to permit the curve to be rebuilt if it is out-of-date with its properties.

Parameters
pathThe full destination path. Note that forward slashes ('/') must be used and there should not be a slash after the last folder.

The file will contain the following data:

column: 1 | 2 |     3 |       4
  data: x | y | dy/dx | d2y/dx2

Samples will be taken from the zero-force region, through the curve, out to strains beyond which the fiber generates passive normalized forces greater than 1. The curve is sampled quite densely: the fiber-force-length .csv file will have 200+20 rows.

Example To read the .csv file into Matlab, you need to set csvread to ignore the header row. Since csvread is 0-indexed, the following example will begin reading the .csv file from the first column of the second row:

data = csvread('bicepsfemoris_FiberForceLengthCurve.csv', 1, 0);
void OpenSim::FiberForceLengthCurve::setCurveStrains ( double  aStrainAtZeroForce,
double  aStrainAtOneNormForce 
)
Parameters
aStrainAtZeroForceThe fiber strain at which the fiber starts to develop force. strainAtZeroForce = 0.0 means that the fiber will begin developing tension when it is at its resting length.
aStrainAtOneNormForceThe fiber strain at which the fiber develops 1 unit of normalized force. strainAtOneNormForce = 0.6 means that the fiber will develop a tension of 1 normalized force when it is strained by 60% of its resting length or, equivalently, when it is stretched to 1.6 times its resting length.

Conditions

strainAtZeroForce < strainAtOneNormForce
void OpenSim::FiberForceLengthCurve::setOptionalProperties ( double  stiffnessAtLowForce,
double  stiffnessAtOneNormForce,
double  curviness 
)
Parameters
stiffnessAtLowForceThe normalized stiffness (slope of the curve) when the fiber is just beginning to develop tensile force.
stiffnessAtOneNormForceThe normalized stiffness (slope of the curve) when the fiber develops a tension of 1 normalized unit of force.
curvinessA dimensionless parameter between 0 and 1 that controls the shape of the curve: a value of 0 will create a curve that is very close to a straight line segment and a value of 1 will create a curve that smoothly fills the corner formed by the linear extrapolation of 'stiffnessAtOneNormForce' and the x-axis, as shown in the figure in the class description.

Conditions

stiffnessAtOneNormForce > 1/(strainAtOneNormForce-strainAtZeroForce)
0 < stiffnessAtLowForce < stiffnessAtOneNormForce
0 <= curviness <= 1

The documentation for this class was generated from the following file: