The Functional Status of Dystrophic Muscles and Functional Recovery by Skeletal Muscles Following Myoblast Transfer

The diversity of movement in mammals requires three different types of muscle contraction: isometric (fixed end), miometric (shortening), and pliometric (lengthening). Consequently, an adequate evaluation of the functional status of dystrophic muscles and of the recovery of dystrophic muscles after myoblast transfer requires measurements during each type of contraction. Although subject to some criticism as a model for Duchenne muscular dystrophy (DMD) in human beings, the lack of expression of dystrophin and the ongoing degeneration of fibres in striated muscles provide strong support for the mdx mouse as a suitable model. The diaphragm muscles of mdx mice show a progressive loss of mass and maximum and normalized force throughout the life span, whereas during adulthood the limb muscles of mdx mice appear to be larger but weaker than those of age-matched control mice. Muscle power of mdx mice is impaired compared with control mice to an even greater extent than force development, due to lower optimum velocities for power. The limb muscles of mdx compared with control mice appear to be more susceptible to injury during pliometric contractions, but muscles in young mdx mice show remarkable powers for recovery. Following transfer of myoblasts, survival and fusion of myoblasts is minimal and no evidence exists to support the conclusion that myoblast transfer into limb muscles provides any functional improvement in the severely impaired muscles of teen-age boys, or to the less impaired muscles of mdx mice.