createMuscleSpecificSurrogateModel.m

% This function is part of the NMSM Pipeline, see file for full license.
%
% This function fits muscle tendon lengths and moment arms as a 
% function of joint angles
%
% Inputs:
% jointAngles (numberFrames x degreesOfFreedom)
% muscleTendonLengths (numberFrames x 1)
% momentArms (numberFrames x degreesOfFreedom)
% polynomialDegree (value)
%
% (2D Number matrix, Number array, 2D Number matrix, Number) -> 
% (Symbol array, 2D Symbol array, Number array)
%
% returns muscle-specific polynomial expressions with corresponding 
% coefficients that best fit experimental muscle tendon lengths and 
% moment arms.

% -----------------------------------------------------------------------

function [polynomialExpressionMuscleTendonLengths, ...
    polynomialExpressionMuscleTendonVelocties, ...
    polynomialExpressionMomentArms, coefficients] = ...
    createMuscleSpecificSurrogateModel(jointAngles, ...
    muscleTendonLengths, momentArms, polynomialDegree)

% Get polynomial expressions for muscle tendon length and moment arms
[polynomialExpressionMuscleTendonLengths, ...
    polynomialExpressionMuscleTendonVelocties, ...
    polynomialExpressionMomentArms, theta]  = ...
    getPolynomialExpressions(jointAngles, polynomialDegree);
% Get A matrix
matrix = getDataMatrix(polynomialExpressionMuscleTendonLengths, ...
    polynomialExpressionMomentArms, jointAngles, theta);
% Create b vector
vector = cat(1, muscleTendonLengths, reshape(momentArms, [], 1));
% Calculate coefficients using linear least squares regression
coefficients = matrix \ vector;
end