  The mathematical modeling of the contraction of a muscle is a crucial problem
in biomechanics. Several different models of muscle activation exist in
literature. A possible approach to contractility is the so-called active
strain: it is based on a multiplicative decomposition of the deformation
gradient into an active contribution, accounting for the muscle activation, and
an elastic one, due to the passive deformation of the body.
  We show that the active strain approach does not allow to recover the
experimental stress-stretch curve corresponding to a uniaxial deformation of a
skeletal muscle, whatever the functional form of the strain energy. To overcome
such difficulty, we introduce an alternative model, that we call mixture active
strain approach, where the muscle is composed of two different solid phases and
only one of them actively contributes to the active behavior of the muscle.
