See: https://github.com/csu-hmc/GaitAnalysisToolKit/blob/master/gaitanalysis/octave/2d_inverse_dynamics/leg2d.m#L198
See: https://github.com/csu-hmc/GaitAnalysisToolKit/blob/master/gaitanalysis/octave/2d_inverse_dynamics/leg2d.m#L198