Fig 1. Scheme of the constitutive model applied to fibres and bundles.

Separated meshes can better describe both intra- (PE, red finite element regions) and extra- (P_ECM, blue finite element regions) sarcomeric components of the passive forces in muscle bundles through an extension of the classical Hill’s three-element model. The experimental characterization of the constitutive laws for PE and P_ECM can be made separately, passively stretching single fibres and small bundles. ECM passive properties can be deduced in an almost physiological situation subtracting from the whole passive tension the single fibre component (see text).