  We present a continuum model for the mechanical behavior of the skeletal
muscle tissue when its functionality is reduced due to aging. The loss of
ability of activating is typical of the geriatric syndrome called sarcopenia.
The material is described by a hyperelastic, polyconvex, transverse isotropic
strain energy function. The three material parameters appearing in the energy
are fitted by least square optimization on experimental data, while
incompressibility is assumed through a Lagrange multiplier representing the
hydrostatic pressure. The activation of the muscle fibers, which is related to
the contraction of the sarcomere, is modeled by the so called active strain
approach. The loss of performance of an elder muscle is then obtained by
lowering of some percentage the active part of the stress. The model is
implemented numerically and the obtained results are discussed and graphically
represented.
