This work is concerned with the biological functioning and mechanical behavior of muscles and tendons. A general three-dimensional continuum constitutive model for the mechanical behavior of skeletal muscles and tendons when the materials are subjected to finite strains is proposed. Muscles and tendons are considered as ccomposite materials that consist of fibers, connective tissues, and biofluids surrounding the fibers. The model accounts for all the essential information about the microstructure of the materials. Also, it is constructed in such a way that different material properties of different muscles can be altered easily when the behavior of a whole group of muscles is to be studied in the same time. Furthermore, a methodology to handle numerically the resulting non-linear and anisotropic constitutive equations in the context of finite element method is developed. The constitutive model is, then, implemented in a general-purpose finite element program. The developed model is used to study the behavior of several musculoskeletal structures.