A unique characteristic of skeletal muscle is its diversity. This diversity is
created by its design, i.e., its fiber composition and the heterogeneity of
the individual fibers. Indeed, we now know that no one skeletal muscle
within an animal is identical with a second. Even homologous muscles exhibit
differences in fiber composition between species and strains. This heterogeneity
of muscle tissue reflects its high degree of functional specialization
and is the basis of its functional plasticity. The full extent of the diversity
exhibited by muscle fibers is only now being realized. Current and future
research is important for improving our understanding of the molecular
basis of phenotypic expression in skeletal muscle fibers during development,
during functional specialization, and under both normal and pathological
conditions.